By Darrel Dunn, Ph.D., PG, Hydrogeologist
A scientific paper published by United States Geological Survey hydrologists Ward Sanford and David Selnick titled Estimation of Evapotranspiration Across the Conterminous United States Using a Regression with Climate and Land-Cover Data uses a regression equation to estimate fraction of precipitation lost to evapotranspiration. They present a map that shows an estimated value for every county in the conterminous United States. This map can be used with local precipitation data to estimate groundwater recharge due to seepage to the water table of precipitation that is not returned to the atmosphere as by evaporation and plant transpiration. The map is reproduced below.
Recharge estimates may be made by various other methods. One method is Blaney-Criddle. The Blaney-Criddle method uses the following equation:
u is monthly evapotranspiration (consumptive use),
k is a crop coefficient, and
f = (t X p)/100
where t is mean monthly temperature in degrees Fahrenheit, and
p is monthly percentage of daytime hours of the year.
The coefficient k has been empirically determined for many "crops" in many geographic locations. Crop coefficients have been derived for natural vegetation as well as agricultural crops. The literature on Blaney-Criddle crop coefficients is extensive.