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

Authors: Williams, Christopher; Pierson Jr, Frederick; 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 transition of sagebrush-dominated (Artemisia spp.) shrublands to pinyon (Pinus spp.) and juniper (Juniperus spp.) woodlands markedly alters resource-conserving vegetation structure typical of these landscapes. Land managers and scientists in the western US need knowledge and predictive tools for assessment and effective targeting of tree-removal treatments to conserve or restore sagebrush vegetation and associated hydrologic function. This study developed modeling approaches to quantify the hydrologic vulnerability and erosion potential of sagebrush rangelands in the later stages of woodland encroachment and in response to commonly applied tree-removal treatments. Using experimental data from multiple sites in the Great Basin Region, USA, and process-based knowledge from decade-long vegetation and rainfall simulation studies at those sites, we (1) assessed the capability of the Rangeland Hydrology and Erosion Model (RHEM) to accurately predict patch-scale (12 m2) measured runoff and erosion from tree canopy and intercanopy hydrologic functional units in untreated and burned woodlands 9 yr post-fire, and (2) developed and evaluated multiple RHEM approaches/frameworks to model aggregated effects of tree canopy and intercanopy areas on patch- and hillslope-scale (50 m length) runoff and erosion processes in untreated and treated (burned, cut, and masticated) woodlands. RHEM accurately predicted measured runoff and sediment yield from patch-scale rainfall simulations as partitioned on untreated and treated tree canopy and intercanopy areas and effectively parameterized the dominant controls on runoff and erosion process in woodlands. With few exceptions, evaluated hillslope-scale RHEM frameworks similarly predicted reduced hydrologic vulnerability and erosion potential for conditions 9 yr following tree removal by burning, cutting, and mastication treatments. Regressions of RHEM-predicted hillslope runoff, sediment, and hydraulic/erosion parameters with bare ground and ground cover attributes indicate all RHEM frameworks effectively represented the dominant controls on hydrologic and erosion processes for rangelands and woodlands. The results provide RHEM frameworks and recommendations 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 our RHEM or similar modeling approaches may be applicable to analogous water-limited landscapes elsewhere subject to woody plant encroachment.