Thirty Year Soil Water Trends Along a 1000 Meter Elevation Gradient

Many of the issues associated with ongoing global climate change hinge on the impacts of the documented physical changes (e.g., rising temperature) on the ecological systems that sustain life. A primary interface between these two is the soil, where water and nutrients are stored for plant consumption. Water is of particular interest in arid and semiarid regions because it limits plant growth. Most General Circulation Model forecasts indicate that increasing temperatures will result in increasing evaporation rates and an increase in drought occurrence. Positive trends in air temperature consistent with earlier snowmelt and spring streamflow have recently been documented during the 45 year period of record at the Reynolds Creek Experimental Watershed. Measurements were made over a 1000 m elevation gradient and four-fold precipitation gradient. In this presentation we examine soil water data collected over a 30 year period of record at the same sites in the watershed. Measurements were made by neutron probe at roughly two week intervals during the period. These data are expressed in terms of soil water storage to a depth of one meter and analyzed with respect to impacts on the duration of plant water stress as indicated by low plant available water contents. We found large differences in terms of year-to-year variability at different sites, and in the degree of correlation between water stress and annual precipitation, but our analyses yielded no temporal trend in soil water dynamics. It appears that below the snow line in semiarid mountains, the variability in the amount and timing of precipitation, which is large relative to soil water storage, has a much larger impact than the roughly one degree temperature increase has on evaporative demand.