Rusteau-1D HelpTheory and equations
Hargreaves equation
When only air temperatures are available, the potential evapotranspiration can be estimated with the Hargreaves equation Hargreaves and Samani (1985); Hargreaves (1994):
ETp = 0.0023 Ra (Tm + 17.8) √(Tmax − Tmin)
| Symbol | Meaning |
|---|---|
| ETp | potential evapotranspiration [mm/d], converted to the project's units |
| Ra | extraterrestrial radiation, expressed as equivalent evaporation (Ra [MJ/m²/d] × 0.408) [mm/d] |
| Tm | mean daily air temperature, (Tmax + Tmin)/2 [°C] |
| Tmax, Tmin | maximum and minimum daily air temperature [°C] |
Ra follows from the latitude of the site and the day of the year Allen et al. (1998). The temperature range √(Tmax − Tmin) stands for the cloudiness: clear days have a large range and high radiation. ETp is partitioned into evaporation and transpiration with the soil cover fraction (partitioning).
The Hargreaves equation is recommended by FAO-56 when solar radiation, humidity and wind data are missing Allen et al. (1998); it tends to overestimate under humid and underestimate under windy, dry conditions.
Literature cited in this topic
- Allen, R. G., L. S. Pereira, D. Raes, and M. Smith (1998). Crop Evapotranspiration – Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56, Food and Agriculture Organization of the United Nations, Rome, 300 pp. https://www.fao.org/4/x0490e/x0490e00.htm
- Hargreaves, G. H. (1994). Defining and using reference evapotranspiration. Journal of Irrigation and Drainage Engineering 120(6): 1132–1139. doi:10.1061/(ASCE)0733-9437(1994)120:6(1132)
- Hargreaves, G. H., and Z. A. Samani (1985). Reference crop evapotranspiration from temperature. Applied Engineering in Agriculture 1(2): 96–99. doi:10.13031/2013.26773
All sources: References.