Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2011
Publication Date: 11/4/2011
Citation: Seyfried, M.S., Marks, D.G., Chandler, D.G. 2011. Long-term soil water trends across a 1000 m elevation gradient. Vadose Zone Journal. 10:1275-1285.
Interpretive Summary: Considerable research on global climate change has indicated that global temperatures are warming. Local measurements we have made at the Reynolds Creek Experimental Watershed in southwestern Idaho over the past 45 years show that temperatures are rising at all elevations. Some of the concern about global warming involves the predicted reduction in plant growth in many areas do to draught stress brought on by a warmer climate. More specifically, the concern is that, in many area, soils will be drier longer because evaporation will be greater with warmer temperatures. This is reasonable, but there are many complications in soil-plant –atmosphere relations that make predictions uncertain. We evaluated trends in soil water content over a 30 year period to see if a general drying trend could be detected. Precipitation during that time period did not change. We found that no trend of soils either being drier or wetter over time. This was true at four different sites where the annual precipitation ranged from less than 250 mm/yr (<10 in) to about 800 mm/yr (>32 in). We think that the reason no change was detected is that, in this climate the combination of cold winters and dry summers means that the raised temperatures affect plant growth for only a very short period during the spring. At that time the climate is highly variable and soil drying is sensitive to soil moisture from the winter, the amount of spring rain and cloudiness, in addition to air temperature. This study show that the impacts of climate change on vegetation cannot be directly determined by climate data.
Technical Abstract: 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 year 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 four to five weeks per year. Second, the large degree of inter-annual variability of soil water during those critical weeks tends to obscure any subtle temporal trends in soil water dynamics that may be present.