Drought index assessment in the watersheds of affluents from the Rio Bravo/Rio Grande River

There are several methods and indices to characterize drought, but none of them is superior to the others in all circumstances. The most frequently used indices in North America are the Standardized Precipitation Index (SPI) and the Palmer Drought Severity Index (PDSI). Each one has characteristics which can be advantageously used to characterize drought and trigger actions established in Drought Mitigation Plans. The 1944 Mexico-US Treaty on conjunctive management of the Colorado, Tijuana and Bravo/Grande Rivers, for example, foresees ways to modify the mutual water allocations between the two countries in case of extreme drought. However, it does not define precisely such concept. Therefore, it is important to examine in detail the applicability and characteristics of both methods to characterize droughts. This paper shows the application of the SPI, which is based only on precipitation data and reflects the temporary rain efficiency, considered as the hydrological component that determines, to a great extent, the occurrence and characteristics of a drought. The PDSI method is also studied. This index is based on soil moisture balance for the soil layer where crops grow, and, therefore, not only precipitation and temperature have influence in water availability, but also soil characteristics are decisive. Both indices are applied to the Conchos and Pecos watersheds, in order to evaluate their behavior When time scales are appropriate for both methods, the results obtained are similar, and they show that droughts have been persistent and recurrent over the region during the last few years. Besides, a sensitivity analysis of PDSI to some parameters is presented.