Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Plants, Soils, and Climate

Committee Chair(s)

R. J. Hanks


R. J. Hanks


D. W. James


H. H. Wiebe


This research project was undertaken to determine if an evaporation pan would accurately predict evapotranspiration when placed in an actual irrigated field situation. Two potato fields in Southern Idaho with different micro-climates and soil types were used in this study. The in-field evaporation pan method was compared with the gravimetric method and the Jensen-Raise and modified-Penman climatic methods. Yield and quality responses were evaluated by varying the amount of sprinkler-applied water so that three distinct moisture regimes were evaluated. It was also necessary to evaluate recent crop coefficient (Kc) curves on potatoes to see if the Kc values predicted evapotranspiration (Et) accurately when related to the evaporation pan or climatic methods.

The results showed that the in-field evaporation pan method predicted Et as well as or better than the climatic methods. During July and August, the evaporation pan reading times a Kc of 0.95 predicted Et extremely well when compared with measured actual Et. Crop coefficient curves were developed for both fields by dividing measured actual Et by the evaporative pan reading. The two Kc curves were very similar for the entire growing season. Established Kccurves did not predict actual Et with accuracy or consistency.

Yield and quality was definitely correlated with the amount of applied water. The dry moisture regime for both fields received 37 percent less water than the normal plots (which were watered to gravimetric and evaporation pan depletion levels) and resulted in a yield reduction of 34 percent. The quality (percent number one potatoes) was decreased by about 50 percent with reduced water.