Get reliable rangeland science

Annual and seasonal precipitation in the American southwest is highly variable and projected to become more so. Rangeland managers must cope with this variation in their stewardship efforts. The objective of this study was to quantify variation in precipitation at seasonal to annual intervals and relate these fluctuations to herbivore grazing management decisions. Standardized precipitation index (SPI) values were calculated from PRISM data for the Texas A&M Sonora Research Station in the Edwards Plateau region of southwest Texas from 1895 to present. Annual precipitation (55 � 5.0 cm yr-1) is typically bimodal with peaks in spring and fall. Annual precipitation extremes include a minimum of 20.8 cm in 1951 (SPI = �2.76) and maximum of 93.8 cm in 2007 (SPI = 2.02). Seasonal precipitation extremes range from a minimum of 0.4 cm in winter of 1974 (SPI = �2.60) and a maximum of 42.1 in spring of 1900 (SPI = 2.94). Decadal coefficient of variation (%) in annual SPI values average 28.4 and range from a minimum of 20.4 in the 1960�s to a maximum of 51.4 in the 1950�s. These values for the 1970�s (21.2), 80�s (24.0) and 90�s (21.4) were all below average and have increased in the 2000�s (34.0) and so far in the 2010�s (34.4). In the most recent 50 years, annual SPI was used to create the following categories: > 0.50 = �wet�, 0.49 to � 0.49 = �neutral� and < � 0.50 = �dry�. Using these categories as a proxy for forage production and then determining stocking rate (SR) for the coming year from the previous year�s SPI would have resulted in: 1) maintain SR, 42 %; 2) overestimate SR, 26 %; and 3) underestimate SR 32 %. Determining SR from previous precipitation is often a mismatch. If climate variability increases, probability of mismatch will increase.