modules_evapX (12/18/12)

CRHM Borland

evapX - Evaporation calculated using Granger and Pomeroy, 1997, Priestley-Taylor and Penman-Monteith methods.

This module defined in ClassevapX, calculates the evapotranspiration from after snowmelt to fall snow cover.

It calculates the daily evapotranspiration using Granger and partitions it over the day to interval values using intervals with positive net radiation.

The  interval values are calculated for the Priestley-Taylor and Penman-Monteith methods. 

Observations

  • RnObs (W/m^2) - all-wave net radiation used by Penmon_Monteith and Dalton Bulk transfer method (optional).
  • Qsi (W/m^2) - incident short-wave (optional).

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_Granger_D etc.
  • 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.
  • Granger_D (mm/d) - daily evaporation calculated from daily means of t, u and vapour pressure using Granger_D method.
  • Priestley_Taylor_D (mm/d) - daily evaporation calculated from daily means of t, u and vapour pressure using Priestley_Taylor method.
  • rc (s/m) - stomatal resistance (used by Penman-Monteith and Dalton).
  • Pa (kPa) - Atmospheric pressure.

Parameters

  • evap_type (flag) - Evaporation method for HRU, 0 = Granger, 1 = Priestley-Taylor, 2 = Penman-Monteith.
  • 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.
  • Zwind () - wind measurement height (used by Penman-Monteith).
  • rcs (s/m) - stomatal resistance (used by Penman-Monteith).
  • soil_type () - HRU soil type (used by Penman-Monteith) [1->11]: sand/loamsand/sandloam/loam/siltloam/sasclloam/clayloam/siclloam/sandclay/siltclay/clay.
  • soil_Depth (m) - depth of soil column (used by Penman-Monteith).
  • Htmax (m) - maximum vegetation height (used by Penman-Monteith).
  • LAImax (m^2/m^2) - maximum leaf area index (used by Penman-Monteith).
  • LAImin (m^2/m^2) - minimum leaf area index (used by Penman-Monteith).
  • s () - seasonal growth index (used by Penman-Monteith).
  • PM_method () - Penman-Monteith method (used by Penman-Monteith), 0 = RC min, 1 = LAI, 2 = bulk.

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_u (*) (m/s)
  • hru_umean (*) (m/s)
  • hru_ea (obs) (kPa)
  • hru_eamean (obs) (kPa)
  • Rn (*) (mm/m^2*int)
  • RnD (*) (mm/m^2*d)
  • RnD_Pos (*) (mm/m^2*d)

Evaporation calculations Daily/Interval.

    Interval observations or daily mean observations can be used in Priestley-Taylor and  Penman-Monteith methods. The selection is made by the parameters PT_Daily, PM_Daily and DB_Daily. When

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.

    The Granger daily evaporation is partitioned over the 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.

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.