Assessing runoff and erosion on woodland-encroached sagebrush steppe using the Rangeland Hydrology and Erosion Model

Authors: Williams, Christopher - Jason; Pierson, Fred; AL-HAMDAN, O.Z. - West Texas A & M University; Nouwakpo, Sayjro; Johnson, Justin; Polyakov, Viktor; KORMOS, P.R. - National Weather Service; SHAFF, S.E. - Us Geological Survey (USGS); SPAETH, K.E. - Natural Resources Conservation Service (NRCS, USDA)

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2022
Publication Date: 6/19/2022
Citation: Williams, C.J., Pierson Jr., F.B., Al-Hamdan, O., Nouwakpo, S.K., Johnson, J.C., Polyakov, V.O., Kormos, P., Shaff, S., Spaeth, K. 2022. Assessing runoff and erosion on woodland-encroached sagebrush steppe using the Rangeland Hydrology and Erosion Model. Ecosphere. 13(6). Article e4145. https://doi.org/10.1002/ecs2.4145.
DOI: https://doi.org/10.1002/ecs2.4145

Interpretive Summary: Public and private land managers throughout the western US are challenged with selecting and implementing effective tree removal treatments to mitigate ecohydrologic impacts of woodland encroachment on sagebrush rangelands. The primary challenge is forecasting the hydrologic and erosion benefits of potential treatments without extensive data and without expensive and laborious hydrologic field studies. The Rangeland Hydrology and Erosion Model (RHEM) is a free web-based model for assessing rangeland hydrologic and erosional responses to conservation practices but has not been extensively evaluated for application on woodlands and in assessment of tree removal. This study utilized measured runoff, erosion, vegetation, ground cover, and soils data from rainfall simulation studies to evaluate RHEM performance in predicting hillslope runoff and erosion responses on intact woodland sites before and 9 yr after tree removal. RHEM performance across two woodland sites and multiple treatment applications was “good” to “very good’ based on model statistics. Further, RHEM effectively predicted measured hydrologic and erosion responses for the intact woodlands and for conditions following tree removal by fire, cutting, and shredding treatments. Results from the model testing yielded multiple possible model frameworks for applying RHEM to diverse woodlands conditions. This study is the first published direct assessment of RHEM application to woodlands, and the frameworks developed therein provide land managers valuable insight for applying RHEM in assessment of hydrologic vulnerability and erosion potential for diverse management scenarios.

Technical Abstract: The retention and restoration of sagebrush-steppe vegetation are paramount concerns in the western US. Sagebrush rangelands provide a host of ecosystem services, including critical wildlife habitat, cultural resources, forage for wild and domestic ungulates, and retention of water and soil. These services at a given site decline with reductions in density, productivity, and distribution of sagebrush and associated perennial bunchgrasses and forbs. Prolonged fire-free periods on sagebrush rangelands allow encroaching pinyon and juniper conifers to persist and outcompete sagebrush and understory herbaceous vegetation for limited water and soil resources. Overtime, competition for resources propagates woodland formation, degraded understory vegetation, extensive bare ground, and amplified runoff and long-term soil loss. Public and private land managers throughout the western US are challenged with selecting and implementing effective tree removal treatments to mitigate these ecohydrologic impacts of woodland encroachment on sagebrush rangelands. Further, land managers need tools and information to predict woodland responses to tree removal practices and to aid treatment selection. This study utilized existing runoff, erosion, vegetation, and soils datasets from rainfall simulation experiments in woodlands to develop and evaluate multiple model scenarios in predicting woodland ecohydrologic responses to tree removal treatments. We applied the Rangeland Hydrology and Erosion Model (RHEM) with the existing datasets to predict patch-scale (12 m2) and hillslope-scale (100 m2) runoff and erosion before and after tree removal (burning, cutting, and shredding) at two woodland sites. RHEM effectively predicted measured runoff (R2 = 0.55) and erosion (R2=0.85) from rainfall simulation plots in untreated and treated woodland conditions, and model performance was “good” to “very good” as assessed by PBIAS. Multiple hillslope-scale RHEM frameworks were developed to apply the model with varying types/sources of vegetation and ground cover data. All model frameworks effectively captured measured effects of tree removal on hydrologic vulnerability and erosion potential. The model scenarios found tree removal by burning, cutting, and mastication reduced runoff 1.3-fold and sediment yield by 2-fold, on average, as assessed 9 yr post-treatment. Hydrologic and erosion responses were associated with increases in vegetation and ground cover following tree removal treatments. Measured and modelled runoff and erosion responses were well correlated with percent bare ground and litter cover, the primary controls on runoff and erosion processes in woodlands. The study results provide a suite of RHEM modeling frameworks for assessing hydrologic vulnerability and erosion potential on woodland-encroached sites and predicting the effectiveness of tree removal to re-establish a water and soil resource-conserving vegetation structure on sagebrush rangelands. We anticipate the RHEM modelling approaches may be applicable to analogous water-limited landscapes elsewhere subject to woody plant encroachment.