The evaporation data is used estimate the evaporation from the depression storage and the evapotranspiration from the active soil zone. It could be time-series data of pan evaporation and calculated potential evapotranspiration (PET) or the monthly evaporation.

Monthly Evaporation

Monthly evaporation data is usually available from local agencies. The default monthly evaporation data is shown in Table 7.

Table 7 Monthly Evaporation Data

Month Evaporation (mm/month) Month Evaporation (mm/month)
Jan 0 July 150
Feb 1.1 Aug 99.6
Mar 2.5 Sep 77.2
Apr 19.1 Oct 55.4
May 66.2 Nov 22.7
Jun 106.9 Dec 4.3

Calculated PET

Empirical equations are available to calculate PET based on other climate data, e.g. Penman-Monteith method, Priestley-Taylor method and Hargreaves method. The calculated PET time-series could be read into the model as the upper limit of evapotranspiration.

Pan Evaporation

When pan evaporation time series data is used, PET is calculated using a modified version of the U.S. Agricultural Research Service (ARS) equations (Warren Viessman, 1977). It accounts for the vegetation characteristics and amount of soil water available in the active soil zone. It’s given as:

where:
= potential evapotranspiration (mm),
GI = the growth index of crop in % of maturity,
k = the ratio of GI to pan evaporation,
= pan evaporation (mm),
S = total porosity,
SA = the available soil porosity (unfilled by water),
AWC = porosity drainable only by evapotranspiration,
x = the exponent and could be calculated as:

x = AWC / G

where G is the moisture freely drained by gravity.

Monthly growth index GI is used and the default growth index is given in Table 8.

TABLE 8 DEFAULT MONTHLY GROWTH INDEX

Month Growth Index Month Growth Index
Jan 0 July 0.98
Feb 0.08 Aug 0.83
Mar 0.29 Sep 0.57
Apr 0.57 Oct 0.29
May 0.83 Nov 0.08
Jun 0.98 Dec 0

The ratio of GI to pan evaporation, k, is usually 1.0-1.2 for short grasses, 1.2-1.6 for crops up to shoulder height, and 1.6-2.0 for forest.

The pan evaporation is calculated with lake evaporation with
= / ELKPAN

where ELKPAN is the lake evaporation to pan evaporation ratio with value between 0.6 and 0.8 (Bruce Withers, 1974) and the lake evaporation could be using the monthly lake evaporation as shown in Error! Reference source not found. or the measured data.

S and G is the physical soil parameter. Representative values are given in Table 9.

TABLE 9 HYDROLOGIC CAPACITIES OF SOIL TEXTURE CLASSES (WARREN VIESSMAN, 1977)

 Texture Class S (%) G (%) AWC (%) x (AWC/G) Coarse sand 24.4 17.7 6.7 0.38 Coarse sandy loam 24.5 15.8 8.7 0.55 Sand 32.3 19.0 13.3 0.70 Loamy sand 37.0 26.9 10.1 0.38 Loamy find sand 32.3 27.2 5.4 0.20 Sandy loam 30.9 18.6 12.3 0.66 Fine sandy loam 36.6 23.5 13.1 0.56 Very fine sandy loam 32.7 21.0 11.7 0.56 Loam 30.0 14.4 15.6 1.08 Silt loam 31.3 11.4 19.9 1.74 Sandy clay loam 25.3 13.4 11.9 0.89 Clay loam 25.7 13.0 12.7 0.98 Silty clay loam 23.3 8.4 14.9 1.77 Sandy clay 19.4 11.6 7.8 0.67 Silty clay 21.4 9.1 12.3 1.34 Clay 18.8 7.3 11.5 1.58

S and G could be calculated using soil total porosity, field capacity and wilting point, where S equals to porosity minus wilting point and G equals to porosity minus field capacity as shown in the figure below Relation between Soil Moisture Limits and Soil Texture Class (Rossman & Huber, 2016). Their values are given for different soil textures in Table 9.

RELATION BETWEEN SOIL MOISTURE LIMITS AND SOIL TEXTURE CLASS (ROSSMAN & HUBER, 2016)