Contrasting watershed-scale trends in runoff and sediment yield complicate rangeland water resources planning

Matthew D Berg,Franco Marcantonio,William E Fox,Jason Mcalister,Mead A Allison,Bradford P Wilcox

doi:10.5194/hess-20-2295-2016

Abstract

Abstract. Rangelands cover a large portion of the earth's land surface and are undergoing dramatic landscape changes. At the same time, these ecosystems face increasing expectations to meet growing water supply needs. To address major gaps in our understanding of rangeland hydrologic function, we investigated historical watershed-scale runoff and sediment yield in a dynamic landscape in central Texas, USA. We quantified the relationship between precipitation and runoff and analyzed reservoir sediment cores dated using cesium-137 and lead-210 radioisotopes. Local rainfall and streamflow showed no directional trend over a period of 85 years, resulting in a rainfall–runoff ratio that has been resilient to watershed changes. Reservoir sedimentation rates generally were higher before 1963, but have been much lower and very stable since that time. Our findings suggest that (1) rangeland water yields may be stable over long periods despite dramatic landscape changes while (2) these same landscape changes influence sediment yields that impact downstream reservoir storage. Relying on rangelands to meet water needs demands an understanding of how these dynamic landscapes function and a quantification of the physical processes at work.

Highlights

Diverse rangeland ecosystems falling along a grassland– forest continuum cover roughly half of the earth’s land surface (Breshears, 2006)
As growing populations look to these dynamic landscapes to provide critical ecosystem services – including water supply and water storage – their ability to keep pace with these demands is uncertain (Havstad et al, 2007; Jackson et al, 2001)
Some of this uncertainty is due to the tremendous variability of runoff and erosion through time and space, which can vary by orders of magnitude even between portions of a single small field (Gaspar et al, 2013; Ritchie et al, 2005). Landscape changes affect these processes further still, and water and sediment yields depend on interactions between climate, vegetation, and local geology

Summary

Introduction

Diverse rangeland ecosystems falling along a grassland– forest continuum cover roughly half of the earth’s land surface (Breshears, 2006). As growing populations look to these dynamic landscapes to provide critical ecosystem services – including water supply and water storage – their ability to keep pace with these demands is uncertain (Havstad et al, 2007; Jackson et al, 2001) Some of this uncertainty is due to the tremendous variability of runoff and erosion through time and space, which can vary by orders of magnitude even between portions of a single small field (Gaspar et al, 2013; Ritchie et al, 2005). Landscape changes affect these processes further still, and water and sediment yields depend on interactions between climate, vegetation, and local geology. Further interdisciplinary study is imperative to develop a coherent picture of the link-

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Conclusion