Irrigation for crop production in the semi-arid Texas High Plains is dependent on groundwater withdrawals from the Ogallala Aquifer, which is declining because withdrawals exceed natural recharge. Irrigation development in the region accelerated during the 1950s. Both irrigated area and volume pumped peaked in 1974 and steadily declined during [1974][1975][1976][1977][1978][1979][1980][1981][1982][1983][1984][1985][1986][1987][1988][1989]. By 2004, however, irrigated area was nearly the same as it was in 1958, and volume pumped had increased slightly. Several strategies to reduce groundwater withdrawals were reviewed without any reductions in irrigated land area or crop productivity. The most promising evaluated were: (1) increasing weather-based irrigation scheduling using the Texas High Plains Evapotranspiration Network (TXHPET); (2) converting gravity-irrigated land (27% of total) to centre pivot irrigation; and (3) replacing high-water to lower-water demand crops (i.e., corn to cotton). If the land area using the TXHPET network was doubled, and if gravity-irrigated lands were reduced to 10%, groundwater withdrawals could be reduced by 14%. An additional reduction of 8% may be possible by converting half of the irrigated corn area to cotton. Copyright # 2008 John Wiley & Sons, Ltd. Plusieurs stratégies pour réduire les prélèvements d'eaux souterraines sans réduire la surface irriguée ou diminuer la productivité ont été analysées. Les plus intéressantes sont: (1) augmenter le pilotage de l'irrigation par le réseau de mesure de l'évapotranspiration des hautes plaines du Texas (TXHPET); (2) convertir l'irrigation gravitaire (27% du total) en irrigation par pivot; et (3) remplacer les cultures à forte demande en eau par des cultures à demande plus faible (soit le maïs par le coton). Si la surface utilisant le réseau TXHPET doublait, et si l'irrigation gravitaire était réduite de 10%, les prélèvements d'eaux souterraines pourraient être réduits de 14%. Une réduction complémentaire de 8% serait possible en convertissant la moitié de la surface irriguée de maïs en coton.

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Drought is a complex and highly destructive natural phenomenon that affects portions of the United States almost every year, and severe water deficiencies can often become catastrophic for agricultural production. Evapotranspiration (ET) by crops is an important component in the agricultural water budget; thus, it is advantageous to include ET in agricultural drought monitoring. The main objectives of this study were to (1) conduct a literature review of drought indices with a focus to identify a simple but simultaneously adequate drought index for monitoring agricultural drought in a semiarid region and (2) using the identified drought index method, develop and evaluate time series of that drought index for the Texas High Plains. Based on the literature review, the Standardized Precipitation-Evapotranspiration Index (SPEI) was found to satisfy identified constraints for assessing agricultural drought. However, the SPEI was revised by replacing reference ET with potential crop ET to better represent actual water demand. Data from the Texas High Plains Evapotranspiration network was used to calculate SPEIs for the major irrigated crops. Trends and magnitudes of cropspecific, time-series SPEIs followed crop water demand patterns for summer crops. Such an observation suggests that a modified SPEI is an appropriate index to monitor agricultural drought for summer crops, but it was found to not account for soil water stored during the summer fallow period for winter wheat.

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