HargreavesSamani Subroutine
private subroutine HargreavesSamani(time, Tavg, Tmax, Tmin, etRad, pet)
Compute potential evapotranspiration with Hargreaves Samani model at daily time scale
References: Hargreaves, G.H., and Z.A. Samani (1982). Estimating potential evapotranspiration. J. Irrig. Drain. Eng., ASCE, 108(3), 223-230
Allen, R.G.; Pereira, L.S.; Raes, D.; Smith, M. Crop Evapotranspiration-Guidelines for Computing Crop Water Requirements-FAO Irrigation and Drainage Paper 56, 9th ed.; Food and Agriculture Organization of the United Nations: Rome, Italy, 1998; ISBN 92-5-104219-5.
Arguments
| Type | Intent | Optional | Attributes | Name | ||
|---|---|---|---|---|---|---|
| type(DateTime), | intent(in) | :: | time |
simulation time |
||
| real(kind=float), | intent(in) | :: | Tavg |
average daily temperature [°C] |
||
| real(kind=float), | intent(in) | :: | Tmax |
maximum daily temperature [°C] |
||
| real(kind=float), | intent(in) | :: | Tmin |
minimum daily temperature [°C] |
||
| real(kind=float), | intent(in) | :: | etRad |
extra terrestrial solar radiation [mm/day] |
||
| real(kind=float), | intent(out) | :: | pet |
potential evapotranspiration [m/s] |
Variables
| Type | Visibility | Attributes | Name | Initial | |||
|---|---|---|---|---|---|---|---|
| real(kind=float), | public, | parameter | :: | HC | = | 0.0023 |
empirical coefficient |
| real(kind=float), | public, | parameter | :: | HE | = | 0.5 |
empirical exponent |
| real(kind=float), | public, | parameter | :: | HT | = | 17.8 |
empirical temperature coefficient |
Source Code
SUBROUTINE HargreavesSamani & ! (time, Tavg, Tmax, Tmin, etrad, pet) IMPLICIT NONE !Arguments with intent(in): TYPE(DateTime) , INTENT(in) :: time !!simulation time REAL (KIND = float), INTENT(in) :: Tavg !!average daily temperature [°C] REAL (KIND = float), INTENT(in) :: Tmax !!maximum daily temperature [°C] REAL (KIND = float), INTENT(in) :: Tmin !!minimum daily temperature [°C] REAL (KIND = float), INTENT(in) :: etRad !!extra terrestrial solar radiation [mm/day] !Arguments with intent(out): REAL (KIND = float), INTENT(out) :: pet !! potential evapotranspiration [m/s] !local declarations. REAL (KIND = float), PARAMETER :: HC = 0.0023 !!empirical coefficient REAL (KIND = float), PARAMETER :: HE = 0.5 !!empirical exponent REAL (KIND = float), PARAMETER :: HT = 17.8 !!empirical temperature coefficient !-----------------------------------------end of declarations------------------ !calculate evapotranspiration IF ( ( Tmax - Tmin) < 0.) THEN CALL Catch ('warning', 'Evapotranspiration', '(Tmax - Tmin) < 0.' ) pet = 0. RETURN ELSE pet = HC * etRad * (Tmax - Tmin) ** HE * (Tavg + HT) END IF !conversion from mm/day to m/s pet = pet * millimeter / day RETURN END SUBROUTINE HargreavesSamani