The ability to evaluate shrubland ecosystem dynamics in a changing environment requires a historical perspective and quantitative analysis of one of the primary ecosystem inputs, precipitation. Historical time series of precipitation data collected from 25 raingages on the USDA Jornada Experimental Range since as early as 1915 are examined. Time series analyses are conducted to test for trends, autocorrelation, and periodicities in the data and establish an association between mean annual precipitation and time. These results are compared with precipitation data from the Walnut Gulch Experimental Watershed and records from 25 raingages on the Santa Rita Experimental Range in Southeastern Arizona. Because segments of the historic record analyzed can reveal increasing annual precipitation, decreasing annual precipitation, or no trend in annual precipitation, caution must be exercised in attempting to assign either natural variations in weather and climate or rangeland use and management practices as the cause of changes in vegetation over time.