|
|
|
|
The CIMIS Equation used is a version of the Modified Penman Equation by Pruitt and Doorenbos (Proceeding of the International Round Table Conference on "Evapotranspiration", Budapest, Hungary. 1977). It also employs a wind function developed at the University of California Davis. The input variables used in the CIMIS equation and the steps required to calculate ETo are described below.
|
|
|
Variables Required
|
|
|
-
ea = Mean hourly vapor pressure (kPa)
-
RH = Mean hourly relative humidity (%)
-
Rn = Mean hourly net radiation (Wm-2)
-
T = Mean hourly air temperature (Celsius)
-
U = Mean hourly wind speed at 2 meters (ms-1)
-
Z = Elevation of the station above mean sea level (m)
|
|
|
Steps
|
|
|
-
Convert temperature from Celsius to Kelvin
Tk = T + 273.16
-
Saturation vapor pressure
es = 0.6108 * exp(T * 17.27/ (T + 237.3))
-
VPD - Vapor pressure deficit
VPD = es - ea (kPa)
-
DEL - Slope of the saturation vapor pressure vs. air temperature curve at the average hourly air temperature
DEL = (4099 * es)/(T + 237.3)2
-
Barometric pressure
P = 101.3 - 0.0115 * Z + 5.44 * 10-7 * Z2
-
GAM - Psychrometer constant (kPa C-1)
GAM = 0.000646 (1 + 0.000946*T) P
-
W - Weighting function
W = DEL/(DEL + GAM)
-
FU2 - Wind function
For Rn<=0 (nighttime)
FU2 = 0.125 + 0.0439U
For Rn>0 (daytime)
FU2 = 0.030 + 0.0576U
-
NR - Convert Rn from Wm-2 to mm
NR = Rn/(694.5 (1-0.000946*T))
-
Hourly ETo is approximately equal to RET
RET = W*NR + (1-W)VPD * FU2
-
Daily ETo equals the sum of 24 hours RET (mm)
|
|
