| Type: | Package |
| Title: | Tools for Simulation of Evapotranspiration of Field Crops andSoil Water Balance |
| Version: | 1.0.3 |
| Date: | 2023-07-21 |
| Description: | Supports the calculation of meteorological characteristics in evapotranspiration research and reference crop evapotranspiration, and offers three models to simulate crop evapotranspiration and soil water balance in the field, including single crop coefficient and dual crop coefficient, as well as the Shuttleworth-Wallace model. These calculations main refer to Allen et al.(1998, ISBN:92-5-104219-5), Teh (2006, ISBN:1-58-112-998-X), and Liu et al.(2006) <doi:10.1016/j.agwat.2006.01.018>. |
| License: | GPL (≥ 3) |
| Encoding: | UTF-8 |
| LazyData: | true |
| Imports: | dplyr, ggplot2, ggpmisc, ggpubr, lubridate, magrittr, plyr,rlang, stringr, tidyr |
| RoxygenNote: | 7.1.2 |
| NeedsCompilation: | no |
| Packaged: | 2023-08-19 14:18:43 UTC; ymhua |
| Author: | Minguo Liu [aut, cre], Huimin Yang [dtc, fnd] |
| Maintainer: | Minguo Liu <liumg15@lzu.edu.cn> |
| Depends: | R (≥ 3.5.0) |
| Repository: | CRAN |
| Date/Publication: | 2023-08-19 14:40:02 UTC |
simET: Tools for Simulation of Evapotranspiration of Field Crops and Soil Water Balance
Description
Supports the calculation of meteorological characteristics in evapotranspiration research and reference crop evapotranspiration, and offers three models to simulate crop evapotranspiration and soil water balance in the field, including single crop coefficient and dual crop coefficient, as well as the Shuttleworth-Wallace model. These calculations main refer to Allen et al.(1998, ISBN:92-5-104219-5), Teh (2006, ISBN:1-58-112-998-X), and Liu et al.(2006) <doi:10.1016/j.agwat.2006.01.018>.
Author(s)
Maintainer: Minguo Liuliumg15@lzu.edu.cn
Other contributors:
Huimin Yanghuimyang@lzu.edu.cn [data contributor, funder]
Pipe operator
Description
Seemagrittr::%>% for details.
Usage
lhs %>% rhsArguments
lhs | A value or the magrittr placeholder. |
rhs | A function call using the magrittr semantics. |
Value
The result of calling 'rhs(lhs)'.
A example dataset of alfalfa under flood irrigation
Description
A example dataset of alfalfa under flood irrigation
Usage
FIalfalfaFormat
A data frame with 161 rows and 22 variables
Adjust the recommended Kc values at the middle and late stages
Description
Adjust the recommended Kc values at the middle and late stages
Usage
Kcb_adj_for_DualKc(Kcb_table, u2, RHmin, h)Arguments
Kcb_table | Recommended value of KC in FAO 56 at the middle and late stages |
u2 | wind speed at 2 m |
RHmin | Minimum relative humidity |
h | Plant height |
Value
A value for adjust Kc at middle and late stages
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Simulation of evapotranspiration using dual crop coefficient method
Description
Simulation of evapotranspiration using dual crop coefficient method
Usage
Model_DualKc(data, param)Arguments
data | A data box. Contains the daily data required by the model. You can refer to the function create_modelData() |
param | A list. Contains additional parameters.list(Kini,Kmid,Kend,fw,rootDepth,Dei_start,Dri_start,FCe,WPe,Ze,REW,TAW,p,FCr,CR_param) |
Value
A list for the model result including a data frame of daily model result ,a list of plots, A data frame of summary data
Note
The stages of data should include all four stages.If a crop has multiple growth cycles, each cycle should include all four stages.
Examples
library(simET) data("FIalfalfa") names(FIalfalfa) #--Model parameter Dparam_FI<-list(Kini=0.3,#Kcb for initial stage Kmid=1.15,#Kcb for mid-season stage Kend=1.1,#Kcb for late season stage DI=FALSE,#Is it drip irrigation? fw=1,# The fraction of the surface wetted rootDepth=1.2,#Maximum root depth Dei_start=0,#Initial depletion of evaporation layer Dri_start=35,#Initial depletion of root layer FCe=0.22,#Field capacity of evaporation layer WPe=0.15,#Wilting point of evaporation layer Ze=0.15,#Depth of the surface soil layer REW=6,#Readily evaporable water TAW=297,#Total available soil water of the root zone p=0.55,#Evapotranspiration depletion factor FCrmm=430,#Field capacity of root layer CR_param=c(430,-0.32,310,-0.16,-1.4,6.8,1.11,-0.98) #Capillary rise model parameters ) #--Run model Model_re_FI<-Model_DualKc(data = FIalfalfa,param = Dparam_FI) #--The Result data Model_re_FI$Result Model_re_FI$Plot #--The goodness Of Fit estimate_goodnessOfFit(Sim = Model_re_FI$Result$Sim_SoilWater, Obs = Model_re_FI$Result$SoilWater)Simulation of evapotranspiration using Shuttleworth-Wallace model
Description
Simulation of evapotranspiration using Shuttleworth-Wallace model
Usage
Model_SW(data, param)Arguments
data | A data box. Contains the daily data required by the model.You can refer to the function create_modelData() |
param | A list. Contains additional parameters. |
Value
A list for the model result including a data frame of daily model result,a list of plots, A data frame of summary data
Note
The stages of data should include all four stages.If a crop has multiple growth cycles, each cycle should include all four stages.
Examples
library(simET) #--Data preparation data("FIalfalfa") #--Parameter preparation param_SW<-list( plant=list( #the canopy extinction coefficient for net radiation kRn=0.3, alpha_plant=0.3,#Canopy reflectance w=0.01,#Leaf width Lmax=10,#Maximum leaf area index a1=10,# Leaf stomatal resistance coefficients a2=0.005,# Leaf stomatal resistance coefficients p=0.5,#the param of reduction factor for T rootDepth=1.2 #Maximum root depth ), Soil=list( zs0=0.04,#The soil surface roughnesslength (m) tau2=2,#Soil tortuosity PHI_p=2, #Soil porosity #The soil pore-size distribution index #from the Brooks-Corey equation. lambda_p=0.18, l1=0.02,#Depth of the surface soil layer (m) l2=1.2,#Depth of the root layer (m) #Saturation water content of evaporation layer THETA_v_sat_1=0.36, THETA_v_sat_2=0.40, #Saturation water content of root layer THETA_start_1=0.2,#Initial water content of evaporation layer THETA_start_2=0.36,#Initial water content of root layer THETA_wp1=0.15,#Wilting point of evaporation layer THETA_wp2=0.15,#Wilting point of root layer #Empirical coefficient of evaporation layer. #13 for homogenous soil alpha1=14, alpha2=14,#Empirical coefficient of root layer. #Saturated hydraulic conductitity of evaporation layer Ksat_1=13.52, Ksat_2=0.02,#Saturated hydraulic conductitity of root layer #Capillary rise model parameters CR_param=c(430,-0.32,313,-0.16,-1.4,6.8,0.5,-0.98) ), Mete=list( nu=2,#The wind speed extinction coefficient nK=2, #The eddy diffusivity extinction coefficient(taken as 2) zr=2, #The reference height (m) ##The vapor diffusion coefficient in air (24.7 10-6 m2 s-1) Dm_v=24.7*10^(-6), deltaT=1 #Time step difference (day) ) ) #--Run model Re_SW<-Model_SW(data = FIalfalfa, param = param_SW) #--The Result data Re_SW$Result Re_SW$Plot #--The goodness Of Fit estimate_goodnessOfFit(Sim = Re_SW$Result$Sim_SoilWater, Obs = Re_SW$Result$SoilWater)Simulation for evapotranspiration using single crop coefficient method
Description
Simulation for evapotranspiration using single crop coefficient method
Usage
Model_single_Kc(data, param)Arguments
data | A data box. Contains the daily data required by the model. You canrefer to the function create_modelData() |
param | A list. Contains additional parameters. |
Value
A list for the model result including a data frame of daily model result ,a list of plots, A data frame of summary data
Note
The stages of data should include all four stages.If a crop has multiple growth cycles, each cycle should include all four stages.
Examples
library(simET) #--Data preparation data("FIalfalfa") #--Parameter preparation param_SingleKc<-list(Kc_mid=1.2,#Kcb for mid-season stage Kc_end=1.15,#Kcb for late season stage rootDepth=1.2,#Maximum root depth #The soil type used for calculating #Kc for initial stage soil_type="coarse soil textures", Dr_start=40,#Initial depletion of root layer TAW=290,#Total available soil water of the root zone p=0.55,#Evapotranspiration depletion factor Field_capacity=420,#Field capacity of root layer fw=1,#The fraction of the surface wetted #Capillary rise model parameters CR_param=c(420,-0.32,303,-0.16,-1.4,6.8,1.11,-0.98) ) #--Run model Re_SingleKc<- Model_single_Kc(data = FIalfalfa, param = param_SingleKc) #--The Result data Re_SingleKc$Result Re_SingleKc$Plot #--The goodness Of Fit estimate_goodnessOfFit(Sim = Re_SingleKc$Result$Sim_SoilWater, Obs = Re_SingleKc$Result$SoilWater)A example dataset of alfalfa under subsurface drip irrigation
Description
A example dataset of alfalfa under subsurface drip irrigation
Usage
SDIalfalfaFormat
A data frame with 161 rows and 22 variables
Calculating actual vapour pressure for hourly time step
Description
Calculating actual vapour pressure for hourly time step
Usage
cal_ActualVapourPressure_for_hourly(Thr, RHhr)Arguments
Thr | is average hourly temperature (degrees Celsius). |
RHhr | is average hourly relative humidity [%]. |
Value
A vector for average hourly actual vapour pressure [kPa].
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating actual vapour pressure derived from RHmax
Description
When using equipment where errors in estimating RHmin can be large,or when RH data integrity are in doubt, then one should use only RHmax.
Usage
cal_ActualVapourPressure_from_RHmax(Tmin, RHmax)Arguments
Tmin | daily minimum temperature (degrees Celsius). |
RHmax | maximum relative humidity (%). |
Value
A vector for actual vapour pressure
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Actual vapour pressure derived from RHmax and RHmin
Description
The actual vapour pressure can also be calculated from therelative humidity. Depending on the availability of the humidity data,different equations should be used.
Usage
cal_ActualVapourPressure_from_RHmaxAndRHmin(Tmax, Tmin, RHmax, RHmin)Arguments
Tmax | daily maximum temperature (kPa). |
Tmin | daily minimum temperature (KPa). |
RHmax | maximum relative humidity %. |
RHmin | minimum relative humidity %. |
Details
For periods of a week, ten days or a month, RHmax and RHmin areobtained by dividing the sum of the daily values by the number of daysin that period.
Value
A vector for actual vapour pressure
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating actual vapour pressure derived from RHmean
Description
In the absence of RH max and RHmin, it can be used to estimateactual vapour pressure.
Usage
cal_ActualVapourPressure_from_RHmean(RHmean, Tmax, Tmin)Arguments
RHmean | mean relative humidity(%). |
Tmax | daily maximum temperature (degrees Celsius). |
Tmin | daily minimum temperature (degrees Celsius). |
Value
A vector for actual vapour pressure
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Actual vapour pressure derived from dewpoint temperature
Description
As the dewpint temperature is the temperature to which the airneeds to be cooled to make the air saturated, the actual vapour pressureis the saturation vapour pressure at the dewpoint temperature.
Usage
cal_ActualVapourPressure_from_dewPoint(Tdew)Arguments
Tdew | dew point temperature(degrees Celsius). |
Value
A vector for actual vapour pressure
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Actual vapour pressure (ea) derived from psychrometric data
Description
The actual vapour pressure can be determined from the differencebetween the dry and wet bulb temperatures, the so-called wet bulb depression.
Usage
cal_ActualVapourPressure_from_psychrometricData(Twet, Tdry, P, type)Arguments
Twet,Tdry | wet bulb depression, with Tdry the dry bulb and Twet the wet bulb temperature (degrees Celsius). |
P | is the atmospheric pressure (kPa). |
type | psychrometer type ("Asmann type","natural ventilated","non-ventilated"). |
Value
A vector for Actual vapour pressure (ea)
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating deep percolation
Description
Calculating deep percolation
Usage
cal_DP_for_singleKc(P, I, ETa, Dri_start)Arguments
P | Precipitation |
I | Irrigation |
ETa | Actual evapotranspiration |
Dri_start | The depletion of root layer |
Value
A value for deep percolation
Deep percolation loss from the topsoil layer
Description
Deep percolation loss from the topsoil layer
Usage
cal_DPe_for_DualKc(P, I, Dei_start, fw)Arguments
P | Precipitation |
I | Irrigation |
Dei_start | Depletion in the topsoil layer |
fw | Fraction of soil surface wetted by irrigation, 0.01-1 |
Value
A value for deep percolation loss from the topsoil layer
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Deep percolation loss from the root layer
Description
Deep percolation loss from the root layer
Usage
cal_DPr_for_DualKc(P, Irrigation, ETa, Dri_start)Arguments
P | Precipitation |
Irrigation | Irrigation |
ETa | Actual evapotranspiration |
Dri_start | Depletion in the root layer |
Value
A value for deep percolation loss from the root layer
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating Deep percolation
Description
Calculating Deep percolation
Usage
cal_DeepPercolation(Wa, Wfc, a, b, t)Arguments
Wa | actual soil water storage in the root zone (mm) |
Wfc | soil water storage to maximum root depth (Zr ) at field capacity (mm) |
a | A water storage value comprised between WFc and Wa at saturation. |
b | b<-0.0173 for soils draining quickly.Otherwise b>-0.0173. |
t | time after an irrigation or rain that produced a storage above field capacity (days) |
Value
A vector for deep percolation(mm/day).
References
Liu Y, Pereira L S, Fernando R M. Fluxes through the bottom boundary of the root zone in silty soils:Parametric approaches to estimate groundwater contribution and percolation[J].Agricultural Water Management, 2006, 84(1):27-40.
calculating the depletion in the topsoil layer at the end of the day
Description
In fact, it performs water balance in a day
Usage
cal_Dei_for_DualKc(Dei_start, P, I, E, Dep, TEW)Arguments
Dei_start | Depletion in the topsoil layer |
P | Precipitation |
I | Irrigation |
E | Evaporation on day i, mm |
Dep | Deep percolation loss from the topsoil layer on day iif soil water content exceeds field capacity, mm |
TEW | Maximum cumulative depth of evaporation (depletion) from the topsoil layer |
Value
A value for the depletion in the topsoil layer at the end of the day
calculating reference evapotranspiration from Penman-Monteith method
Description
The FAO Penman-Monteith method is maintained as the sole standardmethod for the computation of ETo from meteorological data.
Usage
cal_ET0_from_PM(delta, Rn, G, gamma, Tem, u2, es, ea)Arguments
delta | slope vapour pressure curve (kPa °C). From cal_slopeOfSaturationVapourPressureCurve() |
Rn | net Radiation at the crop surface [MJ m-2 day-1]. From cal_netRadiation() |
G | soil heat flux density [MJ m-2 day-1]. |
gamma | psychrometric constant (kPa °C). |
Tem | air temperature at 2 m height [°C]. |
u2 | wind speed at 2 m height [m s-1]. |
es | saturation vapour pressure [kPa]. |
ea | actual vapour pressure [kPa]. |
Value
A vector for reference evapotranspiration [mm day-1].
Note
Ten-day or monthly time step :
Notwithstanding the non-linearity in the Penman-Monteith equation and some weatherparameter methods, mean ten-day or monthly weather data can be used to compute the meanten-day or monthly values for the reference evapotranspiration. The value of the referenceevapotranspiration calculated with mean monthly weather data is indeed very similar to theaverage of the daily ETo values calculated with daily average weather data for that month.
When the soil is warming (spring) or cooling (autumn), the soil heat flux (G) for monthlyperiods may become significant relative to the mean monthly Rn. In these cases G cannot beignored and its value should be determined from the mean monthly air temperatures of theprevious and next month.
Daily time step:
Calculation of ETo with the Penman-Monteith equation on 24-hour time scales will generallyprovide accurate results.
As the magnitude of daily soil heat flux (G) beneath the reference grass surface is relativelysmall, it may be ignored for 24-hour time steps.
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating reference evapotranspiration from Penman-Monteith fordaily
Description
Based on lat, z, J, Tmax, Tmin, n, RHmax, RHmin, windSpeed parameters,reference evapotranspiration was calculated by Penman-Monteith.
Usage
cal_ET0_from_PM_for_daily(Latitude, Altitude, J, Tmax, Tmin, Rs, RHmean, Wind)Arguments
Latitude | latitude (radian), positive for the northern hemisphere andnegative for the southern hemisphere. |
Altitude | station elevation above sea level [m]. |
J | is the number of the day in the year between 1 (1 January) and 365 or366 (31 December). |
Tmax | daily maximum air temperature (degrees Celsius). |
Tmin | daily minimum air temperature (degrees Celsius). |
Rs | Solar radiation [MJ m-2 d-1]. |
RHmean | daily mean relative humidity %. |
Wind | wind speed at 2 m height [m s-1]. |
Value
A vector for reference evapotranspiration (mm/day)
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Examples
library(simET) data("FIalfalfa") names(FIalfalfa) Result_data<- dplyr::mutate(FIalfalfa, ET0=cal_ET0_from_PM_for_daily(Latitude=Latitude, Altitude=Altitude, J=Julian, Tmax=Tmax, Tmin=Tmin, Rs=Rs, RHmean=RHmean, Wind=Wind)) names(Result_data)Calculating reference evapotranspiration from Penman-Monteith methodfor hourly time step
Description
Calculating reference evapotranspiration from Penman-Monteith methodfor hourly time step
Usage
cal_ET0_from_PM_for_hourly( slopVapourPressureCurve, netRadiation, soilHeatFlux, psychrometricConstant, meanHourlyTem, windSpeed, saturationVapourPressure, actualVapourPressure)Arguments
slopVapourPressureCurve | saturation slope vapour pressure curve at Thr [kPa °C]. |
netRadiation | net radiation at the grass surface [MJ m-2 hour-1]. |
soilHeatFlux | soil heat flux density [MJ m-2 hour-1]. |
psychrometricConstant | psychrometric constant [kPa °C]. |
meanHourlyTem | mean hourly air temperature [°C]. |
windSpeed | average hourly wind speed [m s-1]. |
saturationVapourPressure | saturation vapour pressure at air temperature Thr [kPa]. |
actualVapourPressure | average hourly actual vapour pressure [kPa]. |
Details
In areas where substantial changes in wind speed, dewpoint orcloudiness occur during the day, calculation of the ETo equation usinghourly time steps is generally better than using 24-hour calculationtime steps. Such weather changes can cause 24-hour means to misrepresentevaporative power of the environment during parts of the day and mayintroduce error into the calculations. However, under most conditions,application of the FAO Penman-Monteith equation with 24-hour data producesaccurate results.
Value
A vector for reference evapotranspiration [mm hour-1].
Note
With the advent of electronic, automated weather stations, weatherdata are increasingly reported for hourly or shorter periods. Therefore,in situations where calculations are computerized, the FAO Penman-Monteithequation can be applied on an hourly basis with good results. When applyingthe FAO Penman-Monteith equation on an hourly or shorter time scale, theequation and some of the procedures for calculating meteorological datashould be adjusted for the smaller time step.
For the calculation of radiation parameters, see P74-75
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
An upper limit on the evaporation and transpiration from any cropped surface
Description
It is imposed to reflect the natural constraints placed onavailable energy represented by the energy balance difference Rn - G - H
Usage
cal_Kc_max_for_DualKc(u2, RHmin, h, Kcb)Arguments
u2 | The wind speed at 2 m |
RHmin | Minimum relative humidity |
h | Plant height |
Kcb | Basal crop coefficient |
Value
A vector for the upper limit on the evaporation and transpiration from any cropped surface
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Crop coefficient for the end of the late season stage
Description
Typical values for the crop coefficient at the end of the late season growth stage, Kc end, arelisted in Table 12 for various agricultural crops.
Usage
cal_Kcend_for_singleKc(RHmine, u2e, Ktable, he)Arguments
RHmine | mean value for daily minimum relative humidity during the mid-seasongrowth stage , for 20 <= RHmine<= 80 |
u2e | mean value for daily wind speed at 2 m height over grass during the mid-season growth stage (m/s), for 1 <= u2e <= 6 |
Ktable | value for Kc mid taken from Table 12 |
he | mean plant height during the mid-season stage (m) for 0.1 m < h < 10 m |
Value
A value for Kcend value
Note
only applied when the tabulated values for Kc end exceed 0.45. Theequation reduces the Kc end with increasing RHmin. This reduction in Kc end is characteristicof crops that are harvested 'green' or before becoming completely dead and dry (Kc end >= 0.45).
No adjustment is made when Kc end (Table) < 0.45 (Kc end = Kc end (Tab)). Whencrops are allowed to senesce and dry in the field (as evidenced by Kc end < 0.45), u2 andRHmin have less effect on Kc end and no adjustment is necessary.
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating Kcini value
Description
Calculating Kcini value
Usage
cal_Kcini_for_SingleKc(Pmean, ET0, tw, type, fw)Arguments
Pmean | is the average depth od infiltrated water per wetting events(mm) |
ET0 | mean ET0 during initial period(mm/day) |
tw | is the mean interval between wetting events(days) |
type | soil type:"coarse soil textures" and "medium and fine soil textures" |
fw | the fraction of surfaces wetted by irrigation or rain (0-1) |
Value
A value for Kcini value
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Crop coefficient for the mid-season stage
Description
Typical values for the crop coefficient at the end of the late season growth stage, Kc end, arelisted in Table 12 for various agricultural crops.For specific adjustment in climates where RHmin differs from 45 % or where u2 is larger orsmaller than 2.0 m/s.
Usage
cal_Kcmid_for_singleKc(RHmine, u2e, Ktable, he)Arguments
RHmine | mean value for daily minimum relative humidity during the mid-seasongrowth stage , for 20 <= RHmine <= 80 |
u2e | mean value for daily wind speed at 2 m height over grass during the mid-season growth stage (mls), for 1 <= u2e <= 6 |
Ktable | value for Kc mid taken from Table 12 |
he | mean plant height during the mid-season stage [m] for 0.1 m < h < 10 m |
Value
A value for Kcmid value
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Dimensionless evaporation reduction coefficient
Description
It dependent on the soil water depletion(cumulative depth of evaporation) from the topsoillayer (Kr = 1 when De,i-1 is equal or lesser than REW)
Usage
cal_Kr_for_DualKc(TEW, REW, De)Arguments
TEW | maximum cumulative depth of evaporation (depletion) fromthe soil surface layer when Kr = 0 (TEW = total evaporable water) |
REW | cumulative depth of evaporation (depletion) at the end of stage 1(REW = readily evaporable water), mm |
De | cumulative depth of evaporation (depletion) from the soil surfacelayer at the end of day i-1 (the previous day),mm |
Value
A value for evaporation reduction coefficient
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating Solar radiation from actual duration of sunshine
Description
Calculating Solar radiation from actual duration of sunshine
Usage
cal_Rs_from_Na(as = 0.25, bs = 0.5, Na, Latitude, J)Arguments
as | regression constant, expressing the fraction of extraterrestrialradiation reaching the earth on overcast days (n = 0).Default is 0.25. |
bs | as+bs is fraction of extraterrestrial radiation reaching the earthon clear days (n = N). Default is 0.50. |
Na | actual duration of sunshine [hour]. |
Latitude | latitude (angert). |
J | is the number of the day in the year between 1 (1 January) and 365or 366 (31 December). |
Value
A vector for solar radiation(MJ m-2 d-1)
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
calculating total evaporable water
Description
maximum depth of water that can be evaporated from the soilwhen the topsoil has been initially completely wetted.Estimated TEW for Kr calculation
Usage
cal_TEW_for_DualKc(FC, WP, Ze)Arguments
FC | Soil water content at field capacity, m3 m-3 |
WP | Soil water content at wilting point, m3 m-3 |
Ze | Depth of the surface soil layer that is subject to drying by way of evaporation, 0.10-0.15 m. |
Value
A value for total evaporable water
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
calculating the mean daily air temperature
Description
calculating the mean daily air temperature
Usage
cal_TemMean(Tmax, Tmin)Arguments
Tmax | the daily maximum.The temperature is given in degree Celsius or Fahrenhei. |
Tmin | the daily minimum.The temperature is given in degree Celsius,or Fahrenhei. |
Details
It is only employed in the FAO Penman-Monteith equation to calculatethe slope of the saturation vapor pressure curves and the impact of meanair density as the effect of temperature variations on the value of theclimatic parameter is small in these cases. For standardization, Tmean for24-hour periods is defined as the mean of mean of the daily maximum andminimum temperatures rather than as the average of hourly temperaturemeasurements.
Value
A vector for the mean daily air temperature
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating water stress coefficient
Description
Calculating water stress coefficient
Usage
cal_WaterStressCoef(Dr, TAW, p)Arguments
Dr | root zone depletion(mm). |
TAW | total available soil water in the root zone(mm). |
p | fraction of TAW that a crop can extract from the root zone withoutsuffering water stress. |
Value
A value for water stress coefficient which is a dimensionless transpiration reduction factordependent on available soil water
Calculating the volumetric water content after redistribution
Description
Calculating the volumetric water content after redistribution
Usage
cal_afterRedistribution(THETA_v_sat, alpha, Ksat, deltaT, L, THETA11_1)Arguments
THETA_v_sat | soil saturation water content (m3 m-3) |
alpha | empirical coefficient. 13 for homogenous soil, 13-16 for heterogeneous soil |
Ksat | saturated hydraulic conductitity |
deltaT | time step difference (day) |
L | the thickness(m) of soil layer i |
THETA11_1 | the volumetric water content before redistribution (m3 m-3) |
Value
A value for the volumetric water content after redistribution(m3 m-3)
Calculating air vapor pressure deficit at the mean canopy
Description
Calculating air vapor pressure deficit at the mean canopy
Usage
cal_airVaporPressureDeficit_meanCanopyflow( D, r_a_a, rho_cp = 1221.09, DELTA, A, gamma = 0.658, lambda_ET)Arguments
D | the vapor pressure deficit (mbar) |
r_a_a | the aerodynamic resistance between the mean canopy flow and reference height (s m-1) |
rho_cp | the volumetric heat capacity for air (1221.09 J m-3 K-1) |
DELTA | is the slope of the saturated vapor pressure curve (mbar K-1) |
A | is the total energy available to the system (W m-2 ground) |
gamma | psychometric constant (0.658 mbar K-1 ) |
lambda_ET | the total latent heat flux (W m-2 ground) |
Value
A vector for the vapor pressure deficit at the mean canopy flow (mbar)
Note
Knowing D0 is essential because this value is used to calculate the latent and sensible heat fluxes for the soil andcrop components.
Calculating anger from south
Description
A parameter used to determine the position of the sun relativeto the observer (the other one is solar inclination).
Usage
cal_angerFromSouth(latitude, solar_altitude, solar_declination)Arguments
latitude | is the latitude data (Radian). |
solar_altitude | It can be calculated from pi/2-cal_solarinclination. |
solar_declination | is solar declination anger. It can be calculated from cal_solardeclination() |
Details
The minus and positive signs are taken before and after solar noon,receptively. The reason for having the positive-and-negative signs ismerely an artificial convention so that we are able to distinguish betweenthe sun lying westwards (posite angles and after solar noon) and eastwards(negative angles and before solar noon).
Value
A vector for anger from south (Radian)
References
Teh CBS.Introduction to mathematical modeling of crop growth:How the equations are derived and assembled into a computer model.Brown Walker Press, 2006.
Examples
cal_angerFromSouth(latitude=0.52,solar_altitude=-0.715,solar_declination=-0.2974005)Calculating atmospheric pressure
Description
The atmospheric pressure, P, is the pressure exerted by theweight of the earth's atmosphere.
Usage
cal_atmosphericPressure(elevation)Arguments
elevation | elevation above sea level (m) |
Details
Assuming 20°C for a standard atmosphere. Evaporation at high altitudesis promoted due to low atmospheric pressure as expressed in thepsychrometricconstant. The effect is, however, small and in the calculation procedures,the average value for a location is sufficient.
Value
A vector for atmospheric pressure (Kpa)
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Examples
cal_atmosphericPressure(100)Calculating bulk boundary layer resistance
Description
Calculating bulk boundary layer resistance
Usage
cal_bulkBoundaryLayerResistance(nu, u_h, w, L)Arguments
nu | the wind speed extinction coefficient (taken as 2) |
u_h | the wind speed at the canopy top (i.e., at plant height h) (m s-1) |
w | is the mean leaf width (m) |
L | leaf area index |
Value
A vector for bulk boundary layer resistance (s/m)
The canopy penetration probability for net radiation
Description
The canopy penetration probability for net radiation
Usage
cal_canopyPenetrationProbabilityForNetRadiation(kRn, L)Arguments
kRn | the canopy extinction coefficient for net radiation (taken as 0.3) |
L | the leaf area index (m2 leaf m-2 ground) |
Value
The vector for canopy penetration probability for net radiation
Calculating canopy resistance
Description
Calculating canopy resistance
Usage
cal_canopyResistance(a1, a2, It, L, Lmax)Arguments
a1,a2 | are empirical coefficients, dependent on the crop type. |
It | the total hourly solar irradiance (W m-2 ground) |
L | the leaf area index (m2 leaf m-2 ground) |
Lmax | the maximum total leaf area index (m2 leaf m-2 ground) |
Value
A vector for canopy resistance(s/m)
Calculating canopy temperature
Description
Calculating canopy temperature
Usage
cal_canopyTem(Hc, r_c_a, Hs, r_a_a, rho_cp, Tr)Arguments
Hc | crop sensible heat fluxes (W m-2) |
r_c_a | the bulk boundary layer resistance (s m-1) |
Hs | soil sensible heat fluxes (W m-2) |
r_a_a | the aerodynamic resistance between the mean canopy flow and reference level (s m-1) |
rho_cp | the volumetric heat capacity for air (1221.09 J m-3 K-1) |
Tr | Tr is the air temperature at reference level (Celsius degree). weather station. |
Value
A vector for the canopy (foliage) temperature (Celsius degree)
Calculating capillary rise
Description
Calculating capillary rise
Usage
cal_capillaryRise( a1, b1 = -0.17, a2, b2 = -0.27, a3 = -1.3, b3, a4, b4, Dw, Wa, LAI, ETm)Arguments
a1 | Soil water storage to maximum root depth at field capacity(mm). |
b1 | A parameter. |
a2 | storage above the average between those at field capacity and the wilting point(mm). |
b2 | A parameter. |
a3 | A parameter. |
b3 | A parameter.6.7 for clay and silty clay loam soils, decreasing to 6.2 for loamy sands |
a4 | A parameter.4.6 for silty loam and silty clay loam soils,decreasing to 6.2 for loamy sands. |
b4 | A parameter.-0.65 for silty loam soils and decreasing to -2.5 for loamy sand soils. |
Dw | Groudwater depth below root zone(m). |
Wa | actual soil water storage in the root zone. |
LAI | Leaf area index. |
ETm | potential crop evaporanspiration (mm/day),usually ETm=ETc(mm/d). |
Value
The value for capillary Rise (mm/day).
References
Liu Y, Pereira L S, Fernando R M. Fluxes through the bottom boundary of the root zone in silty soils:Parametric approaches to estimate groundwater contribution and percolation[J].Agricultural Water Management, 2006, 84(1):27-40.
Calculating the crop roughness length
Description
Calculating the crop roughness length
Usage
cal_cropRoughnessLength(h)Arguments
h | the plant height (m) |
Value
A vector for the crop roughness length(m)
Calculating Daylight hours
Description
Calculating Daylight hours
Usage
cal_daylightHours(sunsetHourAngle)Arguments
sunsetHourAngle | is the sunset hour angle in radians from cal_sunsetHourAngle(). |
Value
A vector for day light Hours
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating eddy diffusivity at the canopy top
Description
Calculating eddy diffusivity at the canopy top
Usage
cal_eddyDiffusivity_Canopytop(k = 0.4, u_, h)Arguments
k | the von Karman constant (0.4) |
u_ | the friction velocity (m/s) |
h | the plant height (m) |
Value
A vector for eddy diffusivity at the canopy top(m2/s)
Calculating eddy diffusivity at height z
Description
Calculating eddy diffusivity at height z
Usage
cal_eddyDiffusivity_heightZ(Kh, nK, z, h)Arguments
Kh | eddy diffusivity at the canopy top(m2/s) |
nK | the eddy diffusivity extinction coefficient (taken as 2) |
z | height(m) |
h | the plant height(m) |
Value
A vector for eddy diffusivity at height z (m2/s)
Calculating extraterrestrial radiation for daily periods
Description
The extraterrestrial radiation, Ra, for each day of the year andfor different latitudes can be estimated from the solar constant, thesolar declination and the time of the year.
Usage
cal_extraterrestrialRadiation_for_daily(J, lat)Arguments
J | is the number of the day in the year between 1 (1 January) and 365or 366 (31 December). |
lat | latitude (Radian), positive for the northern hemisphere andnegative for the southern hemisphere. |
Value
A vector for extraterrestrial radiation for daily(MJ m-2 day-1)
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating extraterrestrial radiation for hourly or shorter periods
Description
Calculating extraterrestrial radiation for hourly or shorter periods
Usage
cal_extraterrestrialRadiation_for_shorter(lat, J, t, lz, lm, t1)Arguments
lat | latitude (radian), positive for the northern hemisphere and negativefor the southern hemisphere. |
J | is the number of the day in the year between 1 (1 January) and 365or 366 (31 December). |
t | standard clock time at the midpoint of the period (hour). For examplefor a period between 14.00 and 15.00 hours, t = 14.5. |
lz | longitude of the centre of the local time zone (degrees west of Greenwich).For example, Lz = 75, 90, 105 and 120° for the Eastern, Central, Rocky Mountainand Pacific time zones (United States) and Lz = 0° for Greenwich, 330°for Cairo (Egypt),and 255° for Bangkok (Thailand), radian. |
lm | longitude of the measurement site (degrees west of Greenwich) radian. |
t1 | length of the calculation period (hour) |
Value
A vector for extraterrestrial Radiation (MJ m-2 hour-1)
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating friction velocity
Description
Calculating friction velocity
Usage
cal_frictionVelocity(k = 0.4, zr = 2, u_zr, d, z0)Arguments
k | the von Karman constant (0.4) |
zr | the height of the weather station(m). |
u_zr | the wind speed(m/s) at the reference height zr (m). |
d | zero plane displacement height (m) |
z0 | the crop roughness length(m) |
Value
A vector for friction velocity(m/s)
Calculating hour angle
Description
Calculating hour angle
Usage
cal_hourAngle(th)Arguments
th | is the local solar time. |
Value
A vector for hour angle (Radian)
References
Teh CBS.Introduction to mathematical modeling of crop growth:How the equations are derived and assembled into a computer model.Brown Walker Press, 2006.
Examples
cal_hourAngle(12)Calculating inverse relative distance Earth-sun
Description
Calculating inverse relative distance Earth-sun
Usage
cal_inverseRelativeDistance_Earth_sun(J)Arguments
J | is the number of the day in the year between 1 (1 January) and 365or 366 (31 December). |
Value
A vector for inverse relative distance Earth-sun (Radian)
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating latent heat fluxes for crop
Description
Calculating latent heat fluxes for crop
Usage
cal_latentHeatFluxesForCrop(DELTA, Ac, rho_cp, D0, r_c_a, gamma, r_c_s)Arguments
DELTA | the slope of the saturated vapor pressure curve (mbar K-1) |
Ac | energy available to the crop (W m-2 ground) |
rho_cp | the volumetric heat capacity for air (1221.09 J m-3 K-1) |
D0 | the vapor pressure deficit at the mean canopy flow |
r_c_a | is the bulk boundary layer resistance (s m-1) |
gamma | is the psychometric constant (0.658 mbar K-1) |
r_c_s | the canopy resistance (s m-1) |
Value
A vector for latent heat fluxes for crop (W m-2 ground)
Calculating latent heat fluxes for soil
Description
Calculating latent heat fluxes for soil
Usage
cal_latentHeatFluxesForSoil( DELTA, As, rho_cp = 1221.09, D0, r_s_a, gamma = 0.658, r_s_s)Arguments
DELTA | the slope of the saturated vapor pressure curve (mbar K-1) |
As | energy available to the soil (W m-2 ground) |
rho_cp | the volumetric heat capacity for air (1221.09 J m-3 K-1) |
D0 | the vapor pressure deficit at the mean canopy flow |
r_s_a | the aerodynamic resistance between the soil and mean canopy flow (s m-1) |
gamma | the psychometric constant (0.658 mbar K-1) |
r_s_s | soil surface resistance, (s m-1) |
Value
A vector for soil latent heat fluxes (W m-2 ground)
Calculating local solar time
Description
Local solar time is different with local time.
Usage
cal_localDolarTime(td, t, gamma, gamma_sm)Arguments
td | The day of year. |
t | is the local time. |
gamma | is the local longitude (Radian). |
gamma_sm | is the standard longitude (Radian). |
Value
A vector for local solar time(Hour)
References
Teh CBS.Introduction to mathematical modeling of crop growth:How the equations are derived and assembled into a computer model.Brown Walker Press, 2006.
Examples
cal_localDolarTime(td=1,t=12,gamma=0.52,gamma_sm=2.09)Calculating mean canopy flow to reference level
Description
Calculating mean canopy flow to reference level
Usage
cal_meanCanopyFlowToReferenceLevel(k = 0.4, u_, zr, d, h, nK, z0)Arguments
k | the von Karman constant (0.4) |
u_ | the friction velocity (m s-1) |
zr | is the reference height (m).the height of the weather station(m). |
d | zero plane displacement height (m) |
h | the plant height(m) |
nK | the eddy diffusivity extinction coefficient (taken as 2) |
z0 | the crop roughness length (m) |
Value
A vector for mean canopy flow to reference level
Calculating mean saturation vapour pressure
Description
Due to the non-linearity of the above equation, the mean saturationvapour pressure for a day, week, decade or month should be computed as themean between the saturation vapour pressure at the mean daily maximum andminimum air temperatures for that period.
Usage
cal_meanSaturationVapourPressure(Tmax, Tmin)Arguments
Tmax | the daily maximum air temperature(degrees Celsius). |
Tmin | the daily minimum air temperature(degrees Celsius). |
Details
Using mean air temperature instead of daily minimum and maximumtemperatures results in lower estimates for the mean saturation vapourpressure. The corresponding vapour pressure deficit (a parameter expressingthe evaporating power of the atmosphere) will also be smaller and the resultwill be some underestimation of the reference crop evapotranspiration.Therefore,the mean sauration vapour pressure should be calculated as themean between the saturation vapour pressure at both the daily maximumand minimum air temperature.
Value
A vector for mean saturation vapour pressure (es)
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating net longwave radiation Rnl
Description
Calculating net longwave radiation Rnl
Usage
cal_netLongwaveRadiation(TKmax, TKmin, ea, Rs, Rso)Arguments
TKmax | maximum absolute temperature during the 24-hour period [K]. |
TKmin | minimum absolute temperature during the 24-hour period [K]. |
ea | actual vapour pressure [kPa]. |
Rs | measured or calculated solar radiation [MJ m-2 day-1].From cal_solarRadiation(). |
Rso | calculated clear-sky radiation [MJ m-2 day-1].From cal_skySolarRadiation_withas_bs() or cal_skySolarRadiation_withas_elevation(). |
Value
A vector for net outgoing longwave radiation [MJ m-2 day-1]
Note
The rate of longwave energy emission is proportional to the absolutetemperature of the surface raised to the fourth power. This relation isexpressed quantitatively by the Stefan-Boltzmann law. The net energy fluxleaving the earth's surface is, however, less than that emitted and givenby the Stefan-Boltzmann law due to the absorption and downward radiationfrom the sky. Water vapour, clouds, carbon dioxide and dust are absorbersand emitters of longwave radiation. Their concentrations should be knownwhen assessing the net outgoing flux. As humidity and cloudiness play animportant role, the Stefan-Boltzmann law is corrected by these two factorswhen estimating the net outgoing flux of longwave radiation. It is therebyassumed that the concentrations of the other absorbers are constant. Anaverage of the maximum air temperature to the fourth power and the minimumair temperature to the fourth power is commonly used in the Stefan-Boltzmannequation for 24-hour time steps. The term (0.34-0.14*sqrt(ea)) expressesthe correction for air humidity, and will be smaller if the humidity increases.The effect of cloudiness is expressed by (1.35 Rs/Rso - 0.35). The termbecomes smaller if the cloudiness increases and hence Rs decreases. Thesmaller the correction terms, the smaller the net outgoing flux of longwaveradiation. Note that the Rs/Rso term in Equation 39 must be limited so thatRs/Rso <= 1.0. Where measurements of incoming and outgoing short and longwaveradiation during bright sunny and overcast hours are available, calibrationof the coefficients in Equation 39 can be carried out.
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating net radiation Rn
Description
The net radiation (Rn) is the difference between the incomingnet shortwave radiation (Rns) and the outgoing net longwave radiation (Rnl).
Usage
cal_netRadiation(Rns, Rnl)Arguments
Rns | incoming net shortwave radiation. From cal_netSolarRadiation(). |
Rnl | outgoing net longwave radiation. From cal_netLongwaveRadiation(). |
Value
A vector for net radiation
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating net radiation available to the crop
Description
Calculating net radiation available to the crop
Usage
cal_netRadiationForCrop(pRn, Rn)Arguments
pRn | canopy penetration probability for net radiation |
Rn | the net radiation (W/m2 ground).see cal_hourlyNetRadiation() |
Value
A vector for net radiation available to the crop (W/m2 ground)
Calculating net radiation available to the soil
Description
Calculating net radiation available to the soil
Usage
cal_netRadiationForSoil(pRn, Rn, G)Arguments
pRn | canopy penetration probability for net radiation |
Rn | the net radiation (W/m2 ground).see cal_hourlyNetRadiation() |
G | the soil heat flux(W/m2 ground) |
Value
A vector for net radiation available to the soil (W/m2 ground)
Calculating net radiation available to the system(soil and crop)
Description
Calculating net radiation available to the system(soil and crop)
Usage
cal_netRadiationForSystem(Ac, As)Arguments
Ac | net radiation available to the crop (W/m2 ground) |
As | net radiation available to the soil(W/m2 ground) |
Value
A vector for net radiation available to the system(soil and crop) (W/m2 ground)
Calculating net solar (shortware radiation) Rns
Description
The net shortwave radiation resulting from the balance betweenincoming and reflected solar radiation.
Usage
cal_netSolarRadiation(alpha, Rs)Arguments
alpha | albedo or canopy reflection coefficient, which is 0.23 for thehypothetical grass reference crop [dimensionless]. |
Rs | the incoming solar radiation [MJ m-2 day-1]. From cal_solarRadiation() |
Value
A vector for net solar or shortwave radiation [MJ m-2 day-1].
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating percolation for excess water
Description
Calculating percolation for excess water
Usage
cal_percolationForExcessWater(THETA_i_t0, Pe_i_t1, THETA_sat_i)Arguments
THETA_i_t0 | the water amount of the day before in soil layer i (mm) |
Pe_i_t1 | the percolation of previous soil layer(mm) |
THETA_sat_i | soil saturation water amount (mm) |
Value
A value for percolation for excess water (mm)
Calculating psychrometric constant
Description
Calculating psychrometric constant
Usage
cal_psychrometriCconstant(atmospheric_pressure)Arguments
atmospheric_pressure | atmospheric pressure (kPa). |
Value
A vector for Psychrometric constant (kPa/degree Celsius)
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Examples
cal_psychrometriCconstant(100.1235)Calculating reduction factor for evaporation
Description
Calculating reduction factor for evaporation
Usage
cal_reductionFactorForE(THETA, THETA_sat)Arguments
THETA | the water amount of the day before in stop soil layer (mm) |
THETA_sat | soil saturation water content in stop oil layer (mm) |
Value
A value for reduction factor for evaporation
calculating reduction factor for transpiration
Description
calculating reduction factor for transpiration
Usage
cal_reductionFactorForT(THETA_v_wp, p, THETA_v_sat, THETA_v)Arguments
THETA_v_wp | soil water content at wilting point (m3 m-3) |
p | a coefficient. 0.5 for C3 and 0.3 for C4 plant |
THETA_v_sat | soil saturation water content (m3 m-3) |
THETA_v | the water amount of the day before in root layer(m3 m-3) |
Value
A value for reduction factor for transpiration
Calculating relative humidity
Description
The relative humidity (RH) expresses the degree of saturation ofthe air as a ratio of the actual (ea) to the saturation (eo(T)) vapourpressure at the same temperature (T).
Usage
cal_relativeHumidity(ea, e0)Arguments
ea | actual saturation vapour pressure. From cal_ActualVapourPressure_for_* |
e0 | saturation vapour pressure. From cal_saturationVapourPressure() |
Details
Relative humidity is the ratio between the amount of water the ambientair actually holds and the amount it could hold at the same temperature.It is dimensionless and is commonly given as a percentage. Although theactual vapour pressure might be relatively constant throughout the day,the relative humidity fluctuates between a maximum near sunrise and aminimum around early afternoon (Figure 12). The variation of the relativehumidity is the result of the fact that the saturation vapour pressure isdetermined by the air temperature. As the temperature changes during theday, the relative humidity also changes substantially.
Value
A vector for relative humidity %
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating saturation vapour pressure
Description
saturation vapour pressure at the air temperature T.
Usage
cal_saturationVapourPressure(Tem)Arguments
Tem | air temperature (degrees Celsius). |
Value
A vector for saturation vapour pressure at the air temperature T (kPa).
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating sensible heat fluxes for crop
Description
Calculating sensible heat fluxes for crop
Usage
cal_sensibleHeatFluxesForCrop(gamma, Ac, r_c_s, r_c_a, rho_cp, D0, DELTA)Arguments
gamma | is the psychometric constant (0.658 mbar K-1) |
Ac | energy available to the crop (W m-2 ground) |
r_c_s | the canopy resistance (s m-1) |
r_c_a | is the bulk boundary layer resistance (s m-1) |
rho_cp | the volumetric heat capacity for air (1221.09 J m-3 K-1) |
D0 | the vapor pressure deficit at the mean canopy flow |
DELTA | the slope of the saturated vapor pressure curve (mbar K-1) |
Value
A vector for sensible heat fluxes for crop
Calaulating sensible heat fluxes for soil
Description
Calaulating sensible heat fluxes for soil
Usage
cal_sensibleHeatFluxesForSoil( gamma = 0.659, As, r_s_s, r_s_a, rho_cp, D0, DELTA)Arguments
gamma | is the psychometric constant (0.658 mbar K-1) |
As | energy available to the soil (W m-2 ground) |
r_s_s | soil surface resistance, (s m-1) |
r_s_a | is the aerodynamic resistance between the soil and mean anopy flow (s m-1); |
rho_cp | the volumetric heat capacity for air (1221.09 J m-3 K-1) |
D0 | the vapor pressure deficit at the mean canopy flow |
DELTA | the slope of the saturated vapor pressure curve (mbar K-1) |
Value
A vector for soil sensible heat fluxes(W m-2 ground)
Calculating clear sky solar radiation with as and bs
Description
The calculation of the clear-sky radiation, Rso, when n = N,is required for computing net longwave radiation.
Usage
cal_skySolarRadiation_withas_bs(as, bs, Ra)Arguments
as,bs | as+bs fraction of extraterrestrial radiation reaching the earthon clear-sky days (n = N). |
Ra | extraterrestrial radiation [MJ m-2 day-1].From cal_extraterrestrialRadiation_for_daily() |
Value
A vector for clear-sky solar radiation [MJ m-2 day-1].
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating clear sky solar radiation with elevation
Description
The calculation of the clear-sky radiation, Rso, when n = N,is required for computing net longwave radiation.
Usage
cal_skySolarRadiation_withas_elevation(z, Ra)Arguments
z | station elevation above sea level [m]. |
Ra | extraterrestrial radiation [MJ m-2 day-1].From cal_extraterrestrialRadiation_for_daily() |
Value
A vector for clear-sky solar radiation [MJ m-2 day-1].
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating slope of saturation vapour pressure curve
Description
Calculating slope of saturation vapour pressure curve
Usage
cal_slopeOfSaturationVapourPressureCurve(Tem)Arguments
Tem | is air temperature (degrees Celsius). |
Details
In the FAO Penman-Monteith equation, where it occurs in the numeratorand denominator, the slope of the vapour pressure curve is calculatedusing mean air temperature.
Value
A vector for slope of saturation vapour pressure curve at air temperature T
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating Soil/ground heat flux
Description
Calculating Soil/ground heat flux
Usage
cal_soilHeatFlux(pRn, Rn)Arguments
pRn | the canopy penetration probility for net radiation |
Rn | the net radiation (W/m2 ground) |
Value
A vector for the soil heat flux (W/m2 ground)
Calculating soil heat flux(G) for day/ten-day periods
Description
As the magnitude of the day or ten-day soil heat flux beneaththe grass reference surface is relatively small, it may be ignored .
Usage
cal_soilHeatFlux_day()Value
A value for 0
References
FAO Irrigation and drainage paper 56 (P54)
Calculating soil heat flux (G) for general
Description
Complex models are available to describe soil heat flux. Becausesoil heat flux is small compared to Rn, particularly when the surface iscovered by vegetation and calculation time steps are 24 hours or longer,a simple calculation procedure is presented here for long time steps,based on the idea that the soil temperature follows air temperature.
Usage
cal_soilHeatFlux_general(cs, T1, T0, delta_t, delta_z)Arguments
cs | soil heat capacity [MJ m-3 degrees Celsius-1]. |
T1 | air temperature at time i [degrees Celsius]. |
T0 | air temperature at time i-1 [degrees Celsius]. |
delta_t | length of time interval [day]. |
delta_z | effective soil depth [m]. |
Value
A vector for soil heat flux [MJ m-2 day-1]
Note
Complex models are available to describe soil heat flux. Because soilheat flux is small compared to Rn, particularly when the surface iscovered by vegetation and calculation time steps are 24 hours or longer,a simple calculation procedure is presented here for long time steps,based on the idea that the soil temperature follows air temperature.
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating soil heat flux(G) for hourly/shorter periods
Description
For hourly (or shorter) calculations, G beneath a dense coverof grass does not correlate well with air temperature.
Usage
cal_soilHeatFlux_hourly(Rn, periods)Arguments
Rn | net radiation.From cal_netRadiation(). |
periods | "daylight" or "nighttime". |
Value
A vector for soil heat flux [MJ m-2 day-1]
Note
Where the soil is warming, the soil heat flux G is positive. The amountof energy required for this process is subtracted from Rn when estimatingevapotranspiration.
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating soil heat flux(G) for monthly periods
Description
When assuming a constant soil heat capacity of 2.1 MJ m-3 °C-1and an appropriate soil depth, cal_soilHeatFlux_general can be used toderive G for monthly periods.
Usage
cal_soilHeatFlux_monthly(T1, T0, Tmonth2 = TRUE)Arguments
T1 | air temperature at time i [degrees Celsius]. |
T0 | air temperature at time i-1 [degrees Celsius]. |
Tmonth2 | Is the mean air temperature of next month know? |
Value
A vector for soil heat flux [MJ m-2 day-1]
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating soil surface resistance
Description
Calculating soil surface resistance
Usage
cal_soilSurfaceResistance( tau, l, PHI_p, Dm_v, lambda_p, THETA_v_l, THETA_v_sat_l)Arguments
tau | soil tortuosity (taken as 2) |
l | is the dry soil layer thickness (taken as the first soil layer thickness) (m) |
PHI_p | is soil porosity |
Dm_v | the vapor diffusion coefficient in air (24.7 10-6 m2 s-1) |
lambda_p | the soil pore-size distribution index from the Brooks-Corey equation. |
THETA_v_l | volumetric soil water content (m-3 m-3) of the first soil layer |
THETA_v_sat_l | saturated soil water content (m3 m-3) of the first soil layer |
Value
A vector for the soil surface resistance (s m-1)
Calculating soil surface to mean canopy flow
Description
the resistance between the soil surface and the mean canopy flow (s m-1)
Usage
cal_soilSurfaceToMeanCanopyFlow(h, nK, Kh, zs0, z0, d)Arguments
h | the plant height(m) |
nK | the eddy diffusivity extinction coefficient (taken as 2) |
Kh | eddy diffusivity at the canopy top(m2/s) |
zs0 | is the soil surface roughnesslength (m). Note: for flat, tilled land, zs0 can be taken as 0.004 m. |
z0 | the crop roughness length (m) |
d | zero plane displacement height (m) |
Value
A vector for aerodynamic resistancessoil surface to mean canopy flow (s m-1)
Calculating solar declination
Description
Calculating solar declination
Usage
cal_solarDeclination(td)Arguments
td | is the day of year. |
Value
A vector for solar declination (Radian)
Note
The solar declination actually varies throughout the day too butits variation is very small; thus, it is often ignored.Negative angles occur when the angle is below the equator plane,positive for above the equator.
References
Teh CBS.Introduction to mathematical modeling of crop growth:How the equations are derived and assembled into a computer model.Brown Walker Press, 2006.
Examples
cal_solarDeclination(34)Calculating solar declination with FAO56 method
Description
Calculating solar declination with FAO56 method
Usage
cal_solarDeclination_in_FAO(J)Arguments
J | is the number of the day in the year between 1 (1 January) and 365 or 366 (31 December) |
Value
A vector for solar declination(Radian)
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating solar inclination
Description
A parameter used to determine the position of the sun relativeto the observer (the other one is the angle from south).Conversionrelationship with solar altitude angle:solar inclination=pi/2-solar altitude.
Usage
cal_solarInclination(solar_declination, latitude, hour_anger)Arguments
solar_declination | is solar declination anger. It can be calculated from cal_solardeclination(). |
latitude | is the latitude data (Radian). |
hour_anger | is hour anger. It can be calculated from cal_hourangle(). |
Value
A vector for solar inclination (Radian)
References
Teh CBS.Introduction to mathematical modeling of crop growth:How the equations are derived and assembled into a computer model.Brown Walker Press, 2006.
Examples
cal_solarInclination(solar_declination=-0.297,latitude=30,hour_anger=0)Calculating Solar radiation
Description
If the solar radiation, Rs, is not measured, it can be calculatedwith the Angstrom formula, which relates solar radiation to extraterrestrialradiation and relative sunshine duration. This is a shortwave radiation.
Usage
cal_solarRadiation(as = 0.25, bs = 0.5, n, N, Ra)Arguments
as | regression constant, expressing the fraction of extraterrestrialradiation reaching the earth on overcast days (n = 0).Default is 0.25. |
bs | as+bs is fraction of extraterrestrial radiation reaching the earthon clear days (n = N). Default is 0.50. |
n | actual duration of sunshine [hour]. |
N | maximum possible duration of sunshine or daylight hours [hour].from cal_daylightHours() |
Ra | extraterrestrial radiation [MJ m-2 day-1]. From cal_extraterrestrialRadiation_for_daily() |
Value
A vector for solar or shortwave radiation [MJ m-2 day-1]
Note
Rs is expressed in the above equation in MJ m-2 day-1. The correspondingequivalent evaporation in mm day-1 is obtained by multiplying Rs by 0.408(Equation 20). Depending on atmospheric conditions (humidity, dust) andsolar declination (latitude and month), the Angstrom values as and bs willvary. Where no actual solar radiation data are available and no calibrationhas been carried out for improved as and bs parameters, the valuesas = 0.25 and bs = 0.50 are recommended.
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating sunset hour angle
Description
Calculating sunset hour angle
Usage
cal_sunsetHourAngle(lat, solar_declination)Arguments
lat | latitude (Radian), positive for the northern hemisphere andnegative for the southern hemisphere. |
solar_declination | solar declination(Radina). |
Value
A vector for sunset Hour Angle(Radian)
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating the local solar time for sunset/sunrise
Description
Calculating the local solar time for sunset/sunrise.
Usage
cal_sunsetTime(solar_declination, latitude)Arguments
solar_declination | can be calculated by cal_solardeclination(). |
latitude | is latitude data(Radian). |
Details
Knowing the time of sunrise can calculate the time of sunset.sunrise_time=24-sunset_time. Day_length=2*(sunset_time-12).
Value
A vector for the local solar time for sunset/sunrise
References
Teh CBS.Introduction to mathematical modeling of crop growth:How the equations are derived and assembled into a computer model.Brown Walker Press, 2006.
Calculating total latent heat flux
Description
Calculating total latent heat flux
Usage
cal_totalLatentHeatFlux( DELTA, gamma, r_a_a, r_c_a, r_s_a, r_c_s, r_s_s, A, rho_cp = 1221.09, D, As, Ac)Arguments
DELTA | the slope of the saturated vapor pressure curve (mbar K-1) |
gamma | is the psychometric constant (0.658 mbar K-1) |
r_a_a | the aerodynamic resistance between the mean canopy flow and reference height (s m-1) |
r_c_a | the bulk boundary layer resistance (s m-1) |
r_s_a | is the aerodynamic resistance between the soil and mean canopy flow (s m-1) |
r_c_s | the canopy resistance(s m-1) |
r_s_s | soil surface resistance (s m-1) |
A | energy available to the system (total)(W m-2 ground) |
rho_cp | is the volumetric heat capacity for air (1221.09 J m-3 K-1) |
D | the vapor pressure deficit (mbar) |
As | energy available to soil (W m-2 ground) |
Ac | energy available to crop (W m-2 ground) |
Value
A vector for the total latent heat flux (W m-2 ground)
Calculating wind speed above and within the canopies.
Description
Calculating wind speed above and within the canopies.
Usage
cal_windSpeed_Canopy(z, h, u_, k = 0.4, d, z0, nu = 2)Arguments
z | the height (m) |
h | the plant height (m) |
u_ | the friction velocity (m/s) |
k | the von Karman constant (0.4) |
d | zero plane displacement height (m) |
z0 | the crop roughness length(m) |
nu | the wind speed extinction coefficient (taken as 2) |
Value
A vector for the wind speed (m/s) at height z(m)
Calculating zero plane displacement height
Description
Calculating zero plane displacement height
Usage
cal_zeroPlaneHeight(h)Arguments
h | the plant height (m) |
Value
A vector for zero plane displacement height (m)
Show the results of different models
Description
Show the results of different models
Usage
compare_model_plot(model_list, names)Arguments
model_list | List. Including output results of different models. |
names | Vector. Name of models. |
Value
A list for ggplot2 plot
Convert date to day of year
Description
Convert date to day of year
Usage
convert_Date_to_dayofyear(Date)Arguments
Date | is a date format data. |
Value
A vector for the day of year.
Convert Fahrenheit to degrees Celsius
Description
Convert Fahrenheit to degrees Celsius
Usage
convert_Fahrenheit_to_degreesCelsius(Fahrenheit)Arguments
Fahrenheit | temperature in Fahrenheit(°F). |
Value
A vector for temperature in degrees Celsius(°C)
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Convert radiation unit
Description
Type has the following types:MJ_m2_day_to_J_cm2_day;MJ_m2_day_to_cal_cm2_day;MJ_m2_day_to_W_m2;cal_cm2_day_to_MJ_m2_daycal_cm2_day_to_J_cm2_daycal_cm2_day_to_W_m2cal_cm2_day_to_mm_dayW_m2_to_MJ_m2_dayW_m2_to_J_cm2_dayW_m2_to_cal_cm2_dayW_m2_to_mm_daymm_day_to_MJ_m2_daymm_day_to_J_cm2_daymm_day_to_cal_cm2_daymm_day_to_W_m2
Usage
convert_Rad_unit(rad, type)Arguments
rad | Radiation data need to be converted from one unit to another unit. |
type | Used to specify how to convert. |
Value
A vector for radiation converted unit
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Converting angert to radian
Description
Converting the unit of angle in longitude and latitude intothe unit of radian.
Usage
convert_angert_to_radian(anger)Arguments
anger | Longitude or dimension in Angle. |
Value
A vector for longitude or dimension in radian.
Examples
convert_angert_to_radian(98.8)Convert degrees Celsius to Fahrenheit
Description
Convert degrees Celsius to Fahrenheit
Usage
convert_degreesCelsius_to_Fahrenheit(degrees_Celsius)Arguments
degrees_Celsius | temperature in degrees Celsius(°C). |
Value
A vector for atemperature in Fahrenheit(°F).
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Convert wind speed to the standard of 2m
Description
For the calculation of evapotranspiration, wind. Speed measuredat 2 m above the surface is required. To adjust wind speed data obtainedfrom instruments placed at elevations other than the standard height of 2 m,a logarithmic wind speed profile may be used for measurements above a short grassed surface.
Usage
convert_windSpeed_to_2m(uz, z)Arguments
uz | measured wind speed at z m above ground surface [m s-1]. |
z | height of measurement above ground surface [m]. |
Value
A vector for wind speed at 2 m above ground surface [m s-1].
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Create a csv file or a dataframe in R to store the model data
Description
#'@title Converting the every sheets in XLSX file to csv files#'@param xlsx_file the xlsx file path#'@export#'@importFrom utils write.csv#'@return No return value
Usage
create_modelDF(TreNum = 1, rowNum = 1)Arguments
TreNum | Number. Need to generate how many treatment columun |
rowNum | Number. The number of row. |
Details
convert_xlsx_to_csv<-function(xlsx_file)# library(readxl)# library(stringr)#sheetsnameSheetnames<-readxl::excel_sheets(xlsx_file)
#xlsx2csv
for (i in Sheetnames) data<-readxl::read_xlsx(xlsx_file,sheet=i)write.csv(data,file = stringr::str_c(i,".csv"),row.names = FALSE)
Latitude and Longitude use radians as units; Altitude use 'm' as units;Na use 'hour' as units;Tmax and Tmin use Celsius as units; Wind use m/s asunits;RHmean and RHmin use percent sign as uits;Rs use MJ M-2 day-1 as unitsHeight use cm as units;SoilWater and Irrigation use mm as units;GroundwaterDepth use cm as unit.
Value
A dataframe and a csv file (if to_CSV_file==TRUE)
Note
The column of soilwater refers to the measured soil water (mm) in themaximum root layer, which is used to compare the difference between thesimulated value and the measured value, which is an optional variable.The columns of Stage includes four stages: Ini, Development, Mid, End,which are mainly determined by LAI and growth status and can referto Allen et al., (1998).
Estimating ET0 with Tmax and Tmin
Description
When solar radiation data, relative humidity data and/or wind speeddata are missing, they should be estimated using the procedures presentedin this section. As an alternative, ETo can be estimated using theHargreaves ETo equation.
Usage
estimate_ET0_with_TmaxAndTmin(Tmean, Tmax, Tmin, Ra)Arguments
Tmean | mean temperature. |
Tmax | max temperature. |
Tmin | min temperature. |
Ra | extraterrestrial radiation [mm day-1]. |
Value
A vector for reference evapotranspiration (mm day-1).
Note
Units for both ETo and Ra in Equation 52 are mm day-1. Equation 52should be verified in each new region by comparing with estimates bythe FAO Penman-Monteith equation (Equation 6) at weather stations wheresolar radiation, air temperature, humidity, and wind speed are measured.If necessary, Equation 52 can be calibrated on a monthly or annual basisby determining empirical coefficients where ETo = a + b ETo Eq.52, wherethe Eq. 52 subscript refers to ETo predicted using Equation 52. Thecoefficients a and b can be determined by regression analyses or by visualfitting. In general, estimating solar radiation, vapor pressure and windspeed as described in Equations 48 to 51 and Table 4 and then utilizingthese estimates in Equation 6 (the FAO Penman-Monteith equation) willprovide somewhat more accurate estimates as compared to estimating ETodirectly using Equation 52. This is due to the ability of the estimationequations to incorporate general climatic characteristics such as high orlow wind speed or high or low relative humidity into the ETo estimate madeusing Equation 6. Equation 52 has a tendency to underpredict under highwind conditions (u2 > 3 m/s) and to overpredict under conditions of highrelative humidity.
Estimate LAI for alfalfa
Description
Estimate LAI for alfalfa
Usage
estimate_LAI_for_alfalfa(hc)Arguments
hc | is the vegetation height in meter. (in meter) |
Value
A vector for leaf are index of alfalfa
References
Zhao C , Feng Z , Chen G . Soil water balance simulation of alfalfa (Medicago sativa L.)in the semiarid Chinese Loess Plateau[J]. Agricultural Water Management, 2004, 69(2):0-114.
Estimating solar radiation for island locations
Description
For island locations, where the land mass has a widthperpendicular to the coastline of 20 km or less, the air masses influencingthe atmospheric conditions are dominated by the adjacent water body in alldirections. The temperature method is not appropriate for this situation.Where radiation data from another location on the island are not available,a first estimate of the monthly solar average can be obtained from theempirical relation.
Usage
estimate_Rs_for_islandLocations(Ra, b = 4)Arguments
Ra | extraterrestrial radiation [MJ m-2 day-1]. |
b | empirical constant, equal to 4 MJ m-2 day-1. |
Value
A vector for solar radiation
Note
This relationship is only applicable for low altitudes (from 0 to 100 m).The empirical constant represents the fact that in island locations someclouds are usually present, thus making the mean solar radiation 4 MJ m-2 day-1below the nearly clear sky envelope (0.7 Ra). Local adjustment of theempirical constant may improve the estimation. The method is only appropriatefor monthly calculations. The constant relation between Rs and Ra does notyield accurate daily estimates.
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Estimating solar radiation data derived from air temperature differences
Description
The difference between the maximum and minimum air temperatureis related to the degree of cloud cover in a location. Clear-sky conditionsresult in high temperatures during the day (Tmax) because the atmosphereis transparent to the incoming solar radiation and in low temperaturesduring the night (Tmin) because less outgoing longwave radiation is absorbedby the atmosphere. On the other hand, in overcast conditions, Tmax isrelatively smaller because a significant part of the incoming solar radiationnever reaches the earth's surface and is absorbed and reflected by the clouds.Similarly, Tmin will be relatively higher as the cloud cover acts as ablanket and decreases the net outgoing longwave radiation. Therefore, thedifference between the maximum and minimum air temperature (Tmax - Tmin)can be used as an indicator of the fraction of extraterrestrial radiationthat reaches the earth's surface. This principle has been utilized byHargreaves and Samani to develop estimates of ETo using only air temperaturedata.
Usage
estimate_Rs_from_airTemDiff(Ra, Tmax, Tmin, locations)Arguments
Ra | extraterrestrial radiation [MJ m-2 d-1]. |
Tmax | maximum air temperature. |
Tmin | minimum air temperature. |
locations | The adjustment coefficient kRs is empirical and differs forinterior' or 'coastal' regions. |
Value
A vector for solar radiation
Note
The temperature difference method is recommended for locations whereit is not appropriate to import radiation data from a regional station,either because homogeneous climate conditions do not occur, or becausedata for the region are lacking. For island conditions, the methodologyof Equation 50 is not appropriate due to moderating effects of thesurrounding water body. Caution is required when daily computations ofETo are needed. The advice given for Equation 49 fully applies. It isrecommended that daily estimates of ETo that are based on estimated Rs besummed or averaged over a several-day period, such as a week, decade ormonth to reduce prediction error.
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Estimating missing humidity data
Description
Where humidity data are lacking or are of questionable quality,an estimate of actual vapour pressure, ea, can be obtained by assumingthat dewpoint temperature (Tdew) is near the daily minimum temperature (Tmin).This statement implicitly assumes that at sunrise, when the air temperatureis close to Tmin, that the air is nearly saturated with water vapour andthe relative humidity is nearly 100 %.
Usage
estimate_ea(Tmin)Arguments
Tmin | the minimum tem daily. |
Value
A vector for humidity
Note
The relationship Tdew near Tmin holds for locations where the covercrop of the station is well watered. However, particularly for arid regions,the air might not be saturated when its temperature is at its minimum.Hence, Tmin might be greater than Tdew and a further calibration may berequired to estimate dewpoint temperatures. In these situations, "Tmin" inthe above equation may be better approximated by subtracting 2-3 degreesCelsius from Tmin. Appropriate correction procedures are given in Annex 6.In humid and subhumid climates, Tmin and Tdew measured in early morningmay be less than Tdew measured during the daytime because of condensationof dew during the night. After sunrise, evaporation of the dew will onceagain humidify the air and will increase the value measured for Tdewduring the daytime. This phenomenon is demonstrated in Figure 5.4 of Annex 5.However, it is standard practice in 24-hour calculations of ETo to useTdew measured or calculated durin early morning. The estimate for ea fromTmin should be checked. When the prediction by Equation 48 is validatedfor a region, it can be used for daily estimates of ea.
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.FAO Irrigation and drainage paper No. 56. Rome: Food and AgricultureOrganization of the United Nations, 1998.
Calculating the goodness-of-fit indicators between measured and simulated values
Description
Calculating the goodness-of-fit indicators between measured and simulated values
Usage
estimate_goodnessOfFit(Sim, Obs)Arguments
Sim | The simualtion value of model. |
Obs | The observed value. |
Value
A vector for the goodness-of-fit indicators
linear interpolation for vector
Description
Linear interpolation is performed by using the values on bothsides of the missing values.
Usage
linear_interpolation(DataVector)Arguments
DataVector | data vector.Note that the starting value of vector needsto be no missing value. |
Value
A interpolated vector