lake evaporation (Meyer Formula with revisions by PFRA)
This module defined in Classlake, calculates the monthly evaporation and distributes it uniformly over every interval of the month.
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.
- lake_evap_month (mm) - monthly evaporation from rh.
- 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.
- Va (mm) - water vapour pressure, Vw*rh (Meyer).
- Vw (mm) - air vapour pressure (Meyer).
- hru_t_mean (°C) - avereage monthly temperature.
- hru_rh_mean ( ) - avereage monthly relative humidity.
- hru_u_mean (m/s) - avereage monthlywind speed.
- hru_t_acc (°C*N) - accumulated monthly temperature.
- hru_rh_acc ( N) - accumulated monthly relative humidity.
- hru_u_acc (m/s*N) - accumulated monthly wind speed.
Parameters
- Meyer_C () - Meyer coefficient.
- Meyer_B () - Meyer monthly intercept coefficient.
- basin_area (km^2) - basin area.
- hru area (km^2) - HRU area.
- start_open_water () - Julian date of start of lake evaporation.
- end_open_water () - Julian date of end of lake evaporation.
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_rh (obs) ()
- hru_u (obs) (m/s)
Equations.
hru_t_Mmean = hru_t_acc/N_intervals; // where N_intervals is the number of time steps
in the month.
hru_rh_Mmean = hru_rh_acc/N_intervals;
hru_u_Mmean = hru_u_acc/N_intervals;
float Tw = 0.6*hru_t_Mmean + Meyer_B[ThisMonth-1];
Vw = pow(10,(-7.903*(373.16/(Tw+273.16)-1)+(5.028*log10(373.16/(Tw+273.16)))
-(0.0000001382*(pow(10, (11.34*(1-(373.16/(Tw+273.16)))))-1))
+(0.008133*pow(10, (-3.491*((373.16/(Tw+273.16))-1)))))+3.006);
Va = Vw*hru_rh_Mmean/100.0;
lake_evap_month = 0.75002*Meyer_C*(Vw-Va)*(1+0.06214*hru_u_Mmean);
hru_evap = lake_evap_month/N_intervals;
hru_cum_evap += hru_evap;
Note.
The months containing the Julian date of start of evaporation and end of operation
must be complete months for the correct calculation of the monthly evaporation.