IRRIGATION EFFICIENCY AND WATER USE DURING DROUGHT: A MODIFICATION OF THE PALMER DROUGHT INDEX (COLORADO)

Improving efficiency in irrigated agriculture is generally recognized as a major water management goal. This study addresses the role of normally surplus irrigation water as an in-system water reserve which may be drawn upon in times of drought.;Running the model for the period of analysis (1970-1980) showed that only the drought of 1976-1978 was severe enough to affect irrigated agriculture in the Boulder County study area, while over the same time period raindfed agriculture experienced two drought events. The drought index was seen to be a good indicator of moisture conditions, but the departure-from-normal value, d, was demonstrated to be more useful for analysis of irrigation agriculture.;The average monthly unit irrigation efficiency was seen to increase from sixty-seven percent in June to eighty-two percent in August. The overall average unit efficiency was seventy-four percent. This average efficiency level was related to an estimated average excess water availability of 1.1 inches per month, as measured by the departure (d) value.;Crop production and yield data showed that farmers in the study area apparently were able to minimize the impact of the drought. Production indices plateaued during the drought and crop yields fell below their trend line estimates, but in general the impact was modest.;The method was to modify the Palmer Drought Index technique for use in irrigated agriculture. Specific modifications included changing the empirical method for calculating potential evapotranspiration and correcting this value for seasonal crop transpiration characteristics; differentiating effective precipitation from total precipitation; modifying gross irrigation diversions for conveyance and reservoir efficiency; and adding a salt leaching requirement as part of the crop water demand.;When a probabilistic framework for analyzing the departure values was used, it was seen that the probability of obtaining d (GREATERTHEQ) 0 is eighty-five percent. This means that the probability of having some excess water in a given month in eighty-five percent, since d = 0 is the point at which water supply perfectly matches demand given the existing weather conditions. Likewise, the probability of having less than enough water in a given month is fifteen percent. This eighty-five percent/fifteen percent hinge point for d = 1.1 inches was found to correspond closely to other measurements of drought occurrence.