Woody Plants and the Water Cycle on Karst Rangelands: Large Scale Rainfall Simulation Experiments

In the last 150 years, large tracts of grasslands and savannas have converted to woodlands as a result of changes in grazing and fire regimes. These changes have important socioeconomic, biological and potentially hydrological consequences. In this study we use large scale rainfall simulation experiments to evaluate the hydrological consequences of woody plant encroachment in Central Texas. The study was on the karst rangelands of the Edwards Plateau. Rainfall simulation experiments were conducted on 3 X 12 m plots for both canopy and inter-canopy sites. After an initial set of simulations, the woody plant canopy was removed for an additional set of simulations 1 and 2 years after the removal. For each set of simulations we monitored soil water, stem flow, canopy interception, surface runoff and interflow. A 2 m deep trench was constructed at the base of each plot for interflow monitoring. For the canopy plot we found that interflow was the dominant flow process. Over 90 percent of the water applied on the plot drained via interflow. The pattern of interflow was approximately the same after cutting the tree canopy for at least 2 years. In contrast, much more surface runoff was generated from the inter-canopy plots. Interflow was measured on the inter-canopy plots as well. Our results highlight runoff is generated via both surface and subsurface pathways in these karst landscapes and would suggest that runoff processes do not immediately shift following tree removal.