Selecting a procedure for estimating free water evaporation for the state of Texas

Evaporation of water has great bearing on society. Inspite of this, the State of Texas has not kept up with the recommendations of professional societies like the American Society of Civil Engineers and the Natural Resource Conservation Service in adopting scientific and rational procedures to estimate losses of water through evaporation. This study was conducted to decide on the best procedure for estimating evaporation and conclusions were based on statistical analyses performed.

Five combinational methods were used to compare the estimates of evaporation against measured evaporation. These are the FAO-24 Penman Method, the Penman- Montieth method, the Kohler-Parmele method, the Kohler-Nordenson-Fox method, and the Borrelli-Sharif method. Three data sets, one from Lubbock, Texas and two from Lake Hefner, Oklahoma were considered. Each of the data sets had daily air temperature, dew point temperature or relative humidity, wind speed, percentage sunshine or incident solar radiation and measured evaporation. Evaporation was measured using either water budget or energy budget.

The statistical analyses included average, standard deviation, variance, covariance, standard error of estimate, levels of significance for the means, two sample í-tests, and time series analysis. Results from two of the three data sets show that the Penman- Montieth method gave a better standard error of estimate than all other methods studied, though only marginally.