Long-Term Soil Water Trends along a 1000 m Elevation Gradient.

One of the issues of great concern related to global climate change is how food and water supply will be impacted. There is a general consensus among climate models that, in general, increasing temperature will lead to increasing evaporative demand and, eventually, drier soils and increased drought occurrence.  However, there is considerable local variation in these model projections, and very little measured data that might be used to evaluate the accuracy of simulation results. We present soil water data collected at 4 sites over a 31 year period. A warming trend during the past 45 years has been documented for these sites. The sites are only kilometers apart, but elevation range 1000m, so that the regional climate is the same for all, but the local site climate varies considerably. We found no temporal trend in soil water dynamics over the period of record. It appears that the projected increase in evaporative demand is only fully expressed for a relatively short period of time when the vegetation is active and soil water content is high. This period of time also is the time of year that soil water is most variable. The result is that we are looking for a rather small signal within a noisy set of data. Given a continuation of current trends, the warmer temperatures will be expressed as drier soils, but it will require a greater temperature increase in this environment.