Resolving the hydrologic signature of water spreader berms in the US Southwest

Abstract

In an attempt to restore degraded rangelands in the western United States, thousands of water and erosion control structures such as earthen water spreaders and contour berms were built in the mid 1900s to control runoff and sediment. Although many were installed by the newly formed USDA Soil Conservation Service, many others were designed without the benefit of local hydrologic data or technical design guidance. As a result, there is a wide range in the efficacy of these structures, and in many cases, the current status of hydrologic process interactions is unknown. In addition, structurally compromised, abandoned, and unmaintained structures are now interacting with runoff and sediment contrary to their intended purpose, in some cases exacerbating erosion. Because these structures are typically small relative to the resolution of available topographic data, they are not generally accounted for in runoff simulation models. Recent years have marked the increasing availability of LiDAR-based topographic data of sufficiently high resolution to incorporate water and erosion control structures in the digital elevation models underpinning hydrologic models. However, beyond the challenges of data acquisition, modeling tools capable of resolving berm topographies and characterizing their hydrologic impacts are needed. Here, the potential hydrologic impacts of water spreader berms are simulated in virtual experiments using a rainfall-runoff model (specifically, the Saint Venant Equations). The model simulations characterize the local and hillslope-scale effects of berms across a range of storm intensities, landscape attributes, and berm shapes while accounting for berm topography and elevated soil permeability upslope of berms in response to vegetation growth. We demonstrate how berms alter surface runoff and create spatially varied runoff patterns, and describe the impact of berm removal on re-establishing connectivity patterns.