Long‐Term Soil Water Dynamics in the Shortgrass Steppe

Abstract

To assess the temporal and spatial dynamics of soil water at a shortgrass steppe site in northcentral Colorado, we evaluated the precipitation regime for a 33—yr period and ran a simulation model for this period. Small precipitation events accounted for a large fraction of the total number of events and represented a source of water with small interannual variability. The difference between wet and dry years was related to the occurrence of a few large events. Average daily precipitation was concentrated during the warmest months of the year with a maximum in late spring. Water in the surface soil layers had a short residence time and no seasonal pattern. Intermediate layers reflected the seasonal pattern of precipitation. Maximum soil water availability occurred in the late spring, but this was also the period with the highest interannual variability. The wettest layer was at 4—15 cm of depth. The frequency of wet conditions decreased above this layer because of the strong influence of evaporation and below because recharge was infrequent. No deep percolation events were recorded. During dry years distribution of soil water was very shallow and during wet years wet conditions reached to depths of 120—135 cm. This shallow distribution of soil water matches the distribution of processes and structural elements in the steppe suggesting that there is a cause—effect relationship between them. We speculate that the pattern of water availability interacts with biotic constraints and determines the rate of ecosystem processes. The depth distribution of water in dry and wet years is compared to the root distribution of grasses, shrubs, herbs, and succulents to suggest the response of each group to modal and extreme conditions. Comparison of long—term soil water patterns and traits of the major species allows us to suggest why Bouteloua gracilis is the dominant species in the shortgrass steppe.