modules_lake (12/30/11)

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.