Agricultural potential estimated from the ratio of actual to potential evapotranspiration

The R-index, defined as the ratio of actual evapotranspiration (Et) and potential evapotranspiration (Ep), is a useful measure of plant water supply in relation to plant water demand. It also can be used to indicate soil moisture conditions. The R-index has a distribution which follows closely to the Beta distribution function. The R-index and its probability distribution together can be used to assess land use feasibility and irrigation potential requirements for selected crops. Studies have shown that R-index in the neighborhood of 0.90 can be assumed as optimum water requirement and in the neighborhood of 0.60 can be considered as requiring irrigation for crop growth. For example, the R-index was at the optimum level in Ames, Iowa (during the growing season) as little as 25% of the days in a dry season, and as much as 50% in a wet season for corn. Irrigation is indicated for this area about 2–3 years out of 10. Comparison between Illinois—Iowa and Nebraska indicates that Illinois—Iowa has a higher probability of R-index ≥ 0.90 and lower probability of an R-index < 0.60 than in Nebraska. This type of analysis can be used to study land classification and utilization.