ABSTRACT MONTHLY evapotranspiration (Et) of alfalfa (Medicago sativa L.), cotton (Gossypium hirsutum L.), corn (Zea mays L.), and sorghum (Sorghum bicolor (L) Moench) was measured in nonweighing lysimeters for various years and locations in New Mexico to evaluate the effects of various climatic conditions. Potential evapotranspiration (E0) was determined using Penman's equation, and a crop coefficient (the ratio of Et to E0) was determined for each month of the growing season for each crop. Growing-degree-days (G) were determined from temperature data. A third order polynomial crop curve, relating crop coefficient to cumulative G, was estimated for each crop. There was no statistical difference in the crop curve between locations in New Mexico. The coefficient of determination (r2) of the combined polynomial for alfalfa was 0.70. Accumulated G had less variability than calendar days in predicting the time duration to harvest for corn and sorghum, but not for cotton.