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SIPNET (Simplified Photosynthesis and Evapotranspiration Model) is a lightweight ecosystem model for coupled carbon, water, nitrogen, and trace greenhouse-gas (N₂O, CH₄) dynamics. It was built for data-assimilation workflows with eddy-covariance observations and now supports applications from forest carbon inventories to managed croplands. Recent development has focused on agricultural management and nitrogen cycling, and it is coupled to thePEcAn Project modeling framework.

• Simulates photosynthesis, respiration, allocation, soil water, soil temperature, and nitrogen cycling on sub-daily time steps.
• Tracks a compact set of vegetation, litter, soil, and mineral-N pools, enabling fast single-site runs, ensembles, and Monte Carlo experiments.
• Assimilates high-frequency observations (eddy covariance, chamber fluxes, remote sensing) and works within PEcAn model calibration, assimilation, and analysis workflows.
• Event-based management handles planting, harvest, tillage, irrigation (including flooding), fertilization, and organic amendments.
• Nitrogen cycle includes plant, soil, and mineral N pools, with plant uptake, nitrogen limitation, mineralization, volatilization, and leaching.
• Model structure can be configured with a config file or command line options.
• Clean, modular, and thoroughly tested BSD-licensed codebase ready for both research workflows and production use.

• Browse the latest published documentation:https://pecanproject.github.io/sipnet/
• Getting started: Follow thequick-start guide to install prerequisites, compile, and run smoke tests.
• Running the model: Theuser guide explains configuration, climate/parameter files, and runtime options, with dedicated pages forinputs andoutputs.
• Understanding the code: Thedeveloper docs describe architecture, while sections ontesting andCLI extensions cover contributions.
• Project practices: Review theContributing guide,Code of Conduct, andChangelog before opening PRs or issues.

Need help? Open an issue onGitHub or join thePEcAn community Slack.

SIPNET is distributed under the BSD 3-Clause license—seeLICENSE for the full text.

Software

• Longfritz, M. J., W. J. Sacks, D. J. P. Moore, J. M. Zobitz, B. H. Braswell, D. S. Schimel, R. Kooper, M. C. Dietze, I. Fer, C. Black, and D. S. LeBauer. 2025.SIPNET: Simple Photosynthesis and Evapotranspiration Model (v2.0.0).https://doi.org/10.5281/zenodo.17148669

Model foundations

• Braswell, B. H., W. J. Sacks, E. Linder, and D. S. Schimel. 2005.Global Change Biology 11(2):335–355.https://doi.org/10.1111/j.1365-2486.2005.00897.x
• Moore, D. J. P., J. Hu, W. J. Sacks, D. S. Schimel, and R. K. Monson. 2008.Agricultural and Forest Meteorology 148(10):1467–1477.https://doi.org/10.1016/j.agrformet.2008.04.013
• Sacks, W. J., D. S. Schimel, and R. K. Monson. 2007.Oecologia 151(1):54–68.https://doi.org/10.1007/s00442-006-0565-2
• Sacks, W. J., D. S. Schimel, R. K. Monson, and B. H. Braswell. 2006.Global Change Biology 12(2):240–259.https://doi.org/10.1111/j.1365-2486.2005.01059.x
• Zobitz, J. M., D. J. P. Moore, T. Quaife, B. H. Braswell, A. Bergeson, J. A. Anthony, and R. K. Monson. 2014.Agricultural and Forest Meteorology 195–196:73–88.https://doi.org/10.1016/j.agrformet.2014.04.011
• Zobitz, J. M., D. J. P. Moore, W. J. Sacks, R. K. Monson, D. R. Bowling, and D. S. Schimel. 2008.Ecosystems 11(2):250–269.https://doi.org/10.1007/s10021-007-9120-1
• Zobitz, J., H. Aaltonen, X. Zhou, F. Berninger, J. Pumpanen, and K. Köster. 2021.Geoscientific Model Development 14(10):6605–6622.https://doi.org/10.5194/gmd-14-6605-2021

SIPNET + PEcAn applications

• Dokoohaki, H., B. D. Morrison, A. Raiho, S. P. Serbin, and M. Dietze. 2021.https://doi.org/10.5194/gmd-2021-236
• Dokoohaki, H., B. D. Morrison, A. Raiho, S. P. Serbin, K. Zarada, L. Dramko, and M. Dietze. 2022.Geoscientific Model Development 15:3233–3252.https://doi.org/10.5194/gmd-15-3233-2022
• Fer, I., R. Kelly, P. R. Moorcroft, A. D. Richardson, E. M. Cowdery, and M. C. Dietze. 2018.Biogeosciences 15:5801–5830.https://doi.org/10.5194/bg-15-5801-2018

Key Releases

• v2.0.0 “Add Events and Refactor”: major refactor to add event system for managed ecosystems, modularize code, and improve build/test infrastructure.
• v1.2.0 “fAPAR assimilation”: MODIS-derived fAPAR assimilation for joint NEE + remote sensing workflows (Zobitz 2014).
• v1.1.0 “Roots and Microbes”: microbial soil respiration, root pools, and joint CO₂/H₂O assimilation (Moore 2008; Zobitz 2008).
• v1.0.0 “First release”: baseline Braswell (2005) model with with improved hydrology, evergreen phenology (Sacks 2006), and respiration partitioning (Sacks 2007).