HargreavesSamani

Hargreaves-Samani (also written Hargreaves Samani) is an empirical method for estimating daily reference evapotranspiration (ET0) used in hydrology and agriculture. It was developed by Hargreaves and Samani in the 1980s as a simpler alternative to data-intensive approaches such as Penman-Monteith. The method is widely used in regions where meteorological data are limited because it requires fewer inputs.

The HS method estimates ET0 primarily from daily air temperature measurements and a radiation term. A common formulation expresses ET0 as a function of mean temperature and temperature range, scaled by a radiation component such as extraterrestrial radiation (Ra). Several variants exist, some using Tmax and Tmin alone, others incorporating Tmean and Ra or solar radiation data.

Advantages of the HS approach include its simplicity, low data requirements, and broad applicability, making it a practical option for rapid assessments and large-scale studies. Limitations include lower accuracy in very humid or very arid climates, sensitivity to the quality of radiation data, and the potential need for local calibration to improve performance under specific conditions.

Applications encompass irrigation scheduling, water resources planning, and drought monitoring. The HS method remains widely implemented in agricultural software and climate databases as a convenient alternative to the FAO Penman-Monteith method when complete meteorological data are not available.