Soil water is the most frequent limiting resource in grassland ecosystems. The most recent report by the IPCC contains predictions that the central grassland region of North America will warm substantially during the twenty first century. Associated predictions of precipitation changes are less certain and range from small increases to small decreases. Regardless of whether precipitation increases or decreases, the magnitude of the predicted increases in temperature will create drier conditions. A decrease in soil water, in turn, will affect important ecosystem processes. The objective of our research is to understand how reduced soil water will influence ecosystem processes in the driest portions of the central grassland region. We constructed rainfall manipulation shelters and control plots at two study sites on the shortgrass steppe and mixed grass prairie to examine the effects of altered soil water dynamics. Rainfall shelters were designed to decrease ambient rainfall by 50%. We are monitoring soil water at each site using soil moisture probes that measured the volumetric water content (VWC) of the top 10 cm of soil. We combine these data with a daily time step soil water simulation model to evaluate the effects of drier conditions on soil water dynamics throughout the profile as well as ecosystem water balance.

Results/Conclusions

We found that although the VWC of the manipulated plots was lower than the VWC of control plots, a designed 50% decrease in precipitation did not correspond to a 50% decrease in soil water content. The average difference in soil water content was 24% throughout the growing season, but ranged from 5 to 49% depending on the size of the precipitation event and the antecedent soil water content. The largest differences occurred under the driest conditions and the smallest rainfall events.