The Ogallala Agro-Climate Tool

Despite a generally semi-arid climate the Ogallala aquifer region is one of the United States most productive agricultural areas. The Ogallala states - Texas, New Mexico, Oklahoma, Kansas, Colorado, Nebraska, South Dakota, and Wyoming - include six of the top ten U.S. wheat producing states and five of the top ten sorghum producers. Texas is the nation's leading grower of upland cotton, and together these states account for ~40 % of total U.S. cattle inventory and ~70 % of cattle in feedlots. Much of this productivity is dependent on the aquifer's water, which lies under 45 million hectares of the Ogallala state's area. But over the latter half of the 20th and the first years of the 21st century pumping from irrigated agriculture has led to declines in saturated thickness that have not been compensated for by natural aquifer recharge. This drawdown of an essentially fossil water resource has led to questions about the long-term viability of the area's agricultural economy. Approaches to sustaining the region's economy might include changing agricultural practices to conserve the aquifer as a water resource for as long as possible, or preparing for the transition to predominantly dryland farming. Both responses require knowledge of the region's climatology and of the water requirements of the area's major crops. The Ogallala Agro-Climate Tool, a Visual Basic application that can be run on Windows 2000, XP, and Vista operating systems, was developed to provide that information. The application's meteorological database consists of daily precipitation and temperature data from 141 U.S. Historical Climatology Network stations during 1976-2005. From that daily data the program calculates climate and crop evapotranspiration (ETc) statistics over arbitrarily defined periods within summer or winter growing seasons at user-selected latitude-longitude coordinates. The statistics reported include: estimates of seasonal and sub-seasonal ETc derived from the FAO-56 single crop coefficient algorithm, probabilities of exceedance of cumulative rainfall, irrigation demand and growing degree days, the probability that minimum and maximum daily temperatures will exceed user-defined temperature thresholds, and the probability of heat stress, cold stress and dry periods of varying duration.