MULTI-DECADAL SEDIMENT AND HYDROLOGY RESPONSE TO CROPLAND ABANDONMENT AND WOODY PLANT ENCROACHMENT IN DYNAMIC RANGELANDS

Landscape conversion to agriculture and woody plant encroachment are major issues in rangelands around the world. Many studies have examined the small-scale effects of these trends, but very little work has addressed the watershed-scale consequences on hydrology and sediment yield, and questions remain regarding even the timing and magnitude of change. Through an integrated approach involving lake sediment coring, remote sensing of historical imagery, and streamflow analysis, we present a holistic strategy for understanding multi-decadal dynamics for entire watersheds. In 230km² of central Texas rangelands, the picture of agriculture and shrub encroachment is complex. Approximately 85% of active cropland has been abandoned since the late 1930s, but the assumption of constant encroachment is incorrect. Individual watersheds have undergone a variety of shrub trends, from a 100% increase of woody plant cover to a 65% reduction over 75 years. However, the proportion of bare ground generally has increased in all areas. Analyses of Cesium-137 and Lead-210 in sediments collected from downstream impoundments present an ideal opportunity to relate erosion and deposition to these rangeland changes. Adjacent watersheds display very different trends, with watersheds experiencing anything from a 150% increase in sediment yield over the last 25 years in one area to a near cessation in downstream deposition since the mid-1970s in another. These responses largely can be related to changing watershed land cover and the construction of upstream retention ponds. This approach holds promise for widespread application toward rangeland management and water resource planning in numerous global contexts.