Development of Crop Coefficients Using Lysimeters

A long term record of evapotranspiration by grass as measured with the large weighing lysimeter at the University of California at Davis provided an excellent background for the initiation of lysimeter-based crop coefficient studies in 1968. Over the next nine years the large floating lysimeter provided crop evapotranspiration data (ETc) for a number of crops for direct comparison with evapotranspiration of alta fescue grass (ETo) grown in the weighing lysimeter. Results are used to illustrate the development of seasonal curves for both crop coefficients Kc, as well as basal coefficients, Kcb. Smooth curves for Kc for most of the crops studied are then presented. Next, weekly Kc data for corn is presented showing some 10% errors in July of 1970 due to 40-50 cm height differences between lysimeter and field plants. A section is presented on estimating Kc values for early (or initial) crop periods using FAO-24 methodology. A graph is provided showing the expected length of initial periods for several crops as a function of planting date. Finally other problems experienced in our crop coefficient studies are discussed with a suggested review of an earlier paper by Pruitt and Lourence (1985).