Long-term soil water trends across a 1000-m elevation gradient

There is a general consensus among climate models that the direct influence of increasing temperature on evaporative demand will result in drier soils, reduced water supply, and more frequent drought conditions. The data available to evaluate long-term soil water trends that may result from climate change are sparse. We examine soil water data collected over a 32-yr period at four sites covering a 1000-m elevation gradient in a semiarid watershed. Meteorological data from these sites have previously confirmed a significant, increasing temperature trend with no significant precipitation trend during the period of record. There are, however, well-documented climatic trends with elevation that may influence vegetative response to a warming climate. We evaluated the soil water data to: (i) determine any significant temporal trends and (ii) compare soil water among sites to evaluate spatial trends. No significant temporal trends were observed at any of the sites. There were, however, strong spatial trends with elevation and season. Two linked explanations for the lack of temporal trends emerge from the data. First, these semiarid systems are relatively insensitive to the effects of temperature increase because transpiration is limited by low leaf area, soil water content, and solar radiation for all but 4 to 5 wk per year. Second, the large degree of interannual variability of soil water during those critical weeks tends to obscure any subtle temporal trends in soil water dynamics that may be present.