The program ONLINE_HARGREAVES calculates potential evapotranspiration (ETo) by the Hargreaves method.

  1. The input is:   (a) net radiation Qn (cal cm-2 d-1), (b) minimum air temperature Tmin (oC), (c) mean air temperature Tmean (oC), and (d) maximum air temperature Tmax (oC).

  2. The output is potential evapotranspiration ETo (mm d-1).

  3. Potential evapotranspiration is a function of:   (a) extraterrestrial radiation Ra (mm d-1), (b) temperature range Tr = Tmax - Tmin, and (c) mean air temperature Tmean.

  4. The extraterrestrial radiation Ra is a function of:   (a) net radiation Qn (cal cm-2 d-1), (b) density ρ (g cm-3), and (c) heat of vaporization H (cal g-1).

  5. The units of Ra are mm d-1.

  6. All temperature values are in oC.

  7. Input allowable ranges: (1) air temperature, 0-50 oC; (2) net radiation, 100-1000 cal cm2 d-1. Input data outside of these ranges will trigger an error message and stop execution.

  8. The calculator would stop execution and trigger an error message if:
    (1) Tmin > Tmax , (2) Tmax < Tmin , (3) Tmean > Tmax , and (4) Tmean < Tmin.