Springs on rangelands: runoff dynamics and influence of woody plant cover.

To date, little effort has been devoted to understanding the nature of streamfiow from dryland catchments where springs are found, and little is known about how changes in vegetation may alter that streamfiow. But where they do occur in drylands, springs are important—ecologically as well as hydrologically. Areas that naturally support spring flow, such as those having an underlying karst geology, hold the most promise for increasing streamfiow through control of woody plants. For rangelands, this possibility is a particularly appealing one. However, few studies have documented that such a method is effective (except in Mediterranean climates). For this reason, we studied runoff generation for a site on the Edwards Plateau of Texas: a first-order (19-ha) rangeland catchment supplied with intermittent streamfiow from a spring at its base. Using hydrometric as well as isotopic characterization, we evaluated the extent to which runoff changed following removal of most of the woody plant cover (predominantly Ashe juniper). After monitoring streamfiow for 4 years (two before plant removal and two following removal), we found that (1) streamfiow increased following removal of juniper, by about 46 mm annually, except during the summer; (2) for the 4 years of observation, runoff made up about 22% of the water budget, with baseflow from the spring accounting for about half the total flow; and (3) hydrograph separation analysis conducted for two events following juniper removal indicated that pre-event water made up 25% and 50% of the runoff (these numbers represent lower bounds for the percentage of water derived from the spring during storm events). These findings are important, not only because they add to our understanding of runoff generation from spring-fed catchments, but also in particular because they demonstrate that where springs are present, decreasing woody plant cover may augment runoff. [ABSTRACT FROM AUTHOR]