evap (Granger, 1989; Granger and Pomeroy, 1997)
This module defined in Classevap, calculates the evapotranspiration using interval values from after snowmelt to fall snow cover.
It has been enhanced to partition calculated daily Granger evaporation over the day to intervals with positive net radiation.
Other evaporation algorithm available is Priestley-Taylor.
Observations
- none
Variables
- hru_evap (mm/int) - average depth of evaporation from an HRU in a time step, Dt.
- hru_cum_evap (mm) - cumulative evaporation – calculated as the sum of interval estimates of hru_evap.
- hru_evapD (mm/d) - daily evaporation – calculated as the daily sum of interval estimates of hru_evap. Should equal evap_GrangerD
- evap_GrangerD (mm/d) - daily evaporation calculated from daily means of t, u and vapour pressure.
- evap_G () - relative evaporation from Granger.
- evap_D () - relative drying power from Granger.
- hru_actet (mm/int) - actual evapotranspiration over the HRU. Evaporation and evapotranspiration are limited by the amount of soil moisture available.
- hru_cum_actet (mm) - cumulative actual evapotranspiration over the HRU.
Parameters
- evap_type (flag) - Evaporation method for HRU, 0 - Granger, 1 - Priestley Taylor.
- Ht (m) - crop height.
- hru_elev (m) - altitude.
- basin_area (km^2) - basin area.
- hru area (km^2) - HRU area.
- F_Qg () - fraction to ground flux. Qg = F_Qg*Rn.
- inhibit_evap (flag) - 0/1 enable/inhibit.
Variable Inputs
The source of the variable is given in brackets after the name. A full description of the variable may be found there.
- hru_t (obs) (°C)
- hru_tmean (obs) (°C)
- hru_umean (*) (m/s)
- hru_eamean (obs) (kPa)
- Rn (*) (mm/m^2*int)
- RnD (*) (mm/m^2*d)
- RnD_Pos (*) (mm/m^2*d)
Returns
Daily calculation of Granger.
- Ea = fdaily(u, Ht)*(e*-ea)
- D = Ea/(Ea+(Rn-Qg-Qs))
- G = 1/(0.793 + 0.2*exp(4.902*D) + 0.006*D
- E = (delta(t)*G*(Rn-Qg-Qs) + gamma(Pa, t)*G*Ea)/( delta(t)*G + gamma(Pa, t))
where t, u and ea are the mean of interval values and e* is calculated using the mean daily temperature t.
Priestley Taylor
- 1.26*delta(t)*(Rn-Qg-Qs)/(( delta(t) + gamma(Pa, t)) for positive incoming net radiation,
where values are interval. Evaporation is zero when incoming net radiation is <= 0.0.
Atmospheric Pressure
- P = P0*exp((293.0-0.0065*elevation)/293.0, 5.26),
where P0 = 101.3.
Granger Daily Evaporation Partitioned over daily intervals when Rn is positive.
- hru_evap = hru_evapD*Rn[tn] / RnD_POS,
where RnD_POS is the sum of daily intervals when Rn[Dt] is positive.