Water District Board Members

EVAP ESTM

About the Evaporation Estimation on the Home Page

The Evaporation Estimation on the WCTMWD home page is only an estimation and is intended to show the relationship between Hubbard Lake losses through pumping versus evaporation. While the estimation has been calculated using actual data and has been calculated using scientific formulas, it is only an estimation. Exact evaporation numbers are difficult to determine, but this estimation gives a relatively accurate representation of the kind of evaporation amounts that can be expected for the given weather conditions.

This page contains a brief description of the assumptions and calculations used to determine the Evaporation Calculations on the WCTMWD home page. The equations used to estimate the Evaporation in MGD have been supplied by Dr. Jia Zhenwen, Texas Water Development Board, and have been modified slightly to accomodate our field readings. The field readings have been taken from processed National Weather Service data for Abilene, TX. In the future, the readings will be taken from Hubbard Creek Lake. Each sample provides a reading for wind, temp, and relative humidity every hour which is later averaged for a daily reading. Solar radiation readings were not available, but may be available soon.

TWDB Equations Form Basis for Estimation

The equations and sub-equations below are necessary to determine evaporation per day in inches using the following field input data: solar radiation(SRAD), air temperature(TAIR), average wind speed(WSPD), and relative humidity (RELH),

The TWDB supplied equations have the following variables and units:

SRAD == CAL / SQUARE CENTIMETER
WSPD == KILOMETERS PER DAY
TAIR == DEGREES CELSIUS
RELH == PERCENT

These are the readings that WCTMWD records from the field:

Temperature == degrees F
Humidity == relative humidity in %
Wind == wind speed in mph

The following conversions are required to turn WCTMWD field readings into the TWDB variable format:

TAIR_avg = ("Temperature"-32)*5/9
SRAD_avg = 100 constant
RELH = "Humidity"
WSPD_avg = "Wind"*38.4

Here are the variables and accompanying equations and sub-equations:

xcompl = 1 - (RELH_avg/100)

TdC = TAIR_avg - ((14.55 + (0.114*TAIR_avg))*xcompl +pow((2.5 + 0.007*TAIR_avg)*xcompl, 3) + (15.9 +0.117*TAIR_avg)*pow(xcompl, 14))

del_lambda1 = pow(1+(0.66/pow(0.00815*TAIR_avg + 0.8912, 7)), -1)

del_lambda2 = 1 - del_lambda1

Qn = 0.00714*SRAD_avg + 0.00000526*SRAD_avg*pow(TAIR_avg+17.8, 1.87) +0.00000394*pow(SRAD_avg, 2) - 0.00000000239*pow(SRAD_avg, 2)*pow(TAIR_avg-7.2, 2) - 1.02

es_ea = 33.86 * (pow(0.00738*TAIR_avg + 0.8072, 8) -pow(0.00738*TdC+0.8072, 8))

Ea = pow(es_ea, 0.88) * (0.42 + 0.0029*WSPD_avg)

Daily Evap in inches = (del_lambda1*Qn + del_lambda2*Ea) / 25.4

References:

- The primary equation is the Penman Equation from "Hydrology For Engineers", third edition, by Linsley/Kohler/Paulhus, Copyright 1982, and published by McGraw-Hill.