FAO-56 soil water balance
Simulate how water moves through a cropped field over one season: rain and irrigation fill the root zone, while crop transpiration, soil evaporation, runoff and deep percolation empty it. The app uses the FAO-56 dual crop coefficient method as implemented in pyfao56.
How the model works
Crop evapotranspiration is reference evapotranspiration (ETo, the water use of a well-watered grass surface) scaled by crop coefficients: ETc,adj = (Ks × Kcb + Ke) × ETo (FAO-56 1998, Eq. 80). Kcb describes transpiration by an unstressed crop, Ke adds evaporation from wet soil, and the stress coefficient Ks (0 to 1) reduces transpiration when the root zone gets too dry.
Each day the model updates the root-zone depletion: Dr,i = Dr,i-1 − (P − RO) − I − CR + ETc,adj + DP (FAO-56 1998, Eq. 85), where P is precipitation, RO runoff, I net irrigation, CR capillary rise and DP deep percolation. Water stress begins once Dr exceeds the readily available water, RAW = p × TAW.
Six steps
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Location
Pick the field on the map or type its coordinates. The location decides which weather and soil data the app can fetch (gridMET covers the contiguous United States; SSURGO soils cover the United States), and latitude and elevation enter the reference ET calculation.
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Crop & Season
Choose the crop and define its season. Crop coefficients, heights, rooting depths and the depletion fraction come from the project's crop table; blank values are flagged for you to enter. The crop curve can follow calendar days or accumulated growing degree days (GDD).
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Weather
Daily weather drives reference evapotranspiration (the FAO-56 Penman-Monteith equation; FAO-56 1998, Eq. 6) and supplies rainfall. Fetch gridMET data for the season, or upload your own file; the app checks the data for gaps and implausible values and shows the resolved season before you continue.
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Soil
Describe the soil profile: layer depths, field capacity and wilting point. Together with root depth they set how much water the crop can use. For US sites the values can be filled from SSURGO and then edited.
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Irrigation
Decide how water is added: no irrigation (rainfed), your own irrigation events, an uploaded schedule, or automatic scheduling that irrigates when the soil or crop reaches a threshold you choose.
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Results
Daily and seasonal results of the soil water balance: evapotranspiration, water stress, irrigation, runoff and deep percolation, with charts and downloads.
References
- Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration: Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56. FAO, Rome.
- Pereira, L. S., Allen, R. G., Paredes, P., López-Urrea, R., Raes, D., Smith, M., Kilic, A., & Salman, M. (2025). Crop evapotranspiration – Guidelines for computing crop water requirements. Second edition, revised 2025. FAO Irrigation and Drainage Paper No. 56 Rev. 1. FAO, Rome.
- Thorp, K. R., Gulati, D., Kukal, M., Ames, R., Pokoski, T., & DeJonge, K. C. (2025). Version 1.4.0 - pyfao56: FAO-56 evapotranspiration in Python. SoftwareX, 30, 102109. https://doi.org/10.1016/j.softx.2025.102109
- Abatzoglou, J. T. (2013). Development of gridded surface meteorological data for ecological applications and modelling. International Journal of Climatology, 33(1), 121–131.
- Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. Soil Survey Geographic (SSURGO) Database, accessed through Soil Data Access.
- USDA Soil Conservation Service (1986). Urban Hydrology for Small Watersheds. Technical Release 55 (TR-55), 2nd ed. Washington, DC.
- Thorp, K. R., DeJonge, K. C., & Kukal, M. S. (2025). The pyfao56 automatic irrigation scheduling algorithm. Agricultural Water Management, 322, 110013. https://doi.org/10.1016/j.agwat.2025.110013
- ASCE-EWRI Task Committee on Standardization of Reference Evapotranspiration (Walter, I. A., Allen, R. G., Elliott, R., Itenfisu, D., Brown, P., et al.) (2005). The ASCE Standardized Reference Evapotranspiration Equation. American Society of Civil Engineers, Reston, Virginia.
- ASCE Task Committee on Revision of Manual 70 (2016). Evaporation, Evapotranspiration, and Irrigation Water Requirements, 2nd ed. Jensen, M. E., & Allen, R. G. (eds.). ASCE Manuals and Reports on Engineering Practice No. 70. American Society of Civil Engineers, Reston, Virginia.