An understanding of the relationship between irrigation pumpage and change in ground-water storage was needed to quantify the amount of water returning to the High Plains aquifer as a result of intensive irrigation in Castro and Parmer Counties, Texas. Irrigation pumpage for the 9-year period, 1975-83, was estimated by using the Blaney-Criddle consumptive-use formula adjusted by a factor to account for irrigation demand and field-measured crop applications. Total estimated pumpage for the 9-year period was 11,269,000 acre-feet and 8,914,000 acre-feet. The estimated pumpage was based upon reported crop acreage data and LANDSAT acreage data, respectively.
Aquifer storage for the same period was estimated as the product of specific yield, net water-level change, and area. Change in storage was 5,168,000 acre-feet. Many of the areas of the largest change in storage also were the areas of the largest saturated thickness. The only locations that did not experience substantial water-level declines were the northwest and northeast parts of the study area.
A comparison was made of water returning to the aquifer by calculating the difference between irrigation pumpage and the change in aquifer storage. Two estimates of this comparison, expressed as a percentage of irrigation pumpage, were obtained on the basis of two different sources of acreage data. This comparison was 54 percent of pumpage based on reported crop acreage data and 42 percent of pumpage based on LANDSAT interpreted acreage data.
