Impact of weather and climate scenarios on conservation assessment outcomes

Authors: Garbrecht, Jurgen; Nearing, Mark; SHIELDS, DORY - Consultant; Tomer, Mark; Sadler, Edward; Bonta, James - Jim; Baffaut, Claire

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2014
Publication Date: 9/1/2014
Publication URL: http://handle.nal.usda.gov/10113/60551
Citation: Garbrecht, J.D., Nearing, M.A., Shields, D., Tomer, M.D., Sadler, E.J., Bonta, J.V., Baffaut, C. 2014. Impact of weather and climate scenarios on conservation assessment outcomes. Journal of Soil and Water Conservation. 69(5):374.

Interpretive Summary: The Conservation Effects Assessment Project (CEAP) provides an accounting of environmental benefits and effectiveness of conservation practices on agricultural landscapes. The distinct possibility that climate change may already be occurring introduces another dimension and challenge to the assessment and interpretation of the effectiveness of conservation programs. Intensified precipitation and increased frequency of extreme events lead to more runoff, higher soil erosion rates, potentially severe gullying, and related off-site sedimentation problems. In this investigation, the results of selected CEAP watershed assessment studies that address runoff and soil erosion are reviewed and interpret for potential impacts of climate scenarios on conservation outcomes. It was shown that runoff, soil erosion, and sediment leaving the watershed are sensitive to climate change scenarios of increased rainfall intensity and frequency. Rainfall intensity had greater impact than rainfall frequency, and sediment transport responded more sensitively to changes in rainfall than did runoff and soil erosion. Increased frequency of channel peak flow did not affect the performance of flood and grade control structures, but did increase annual sediment yield. In environments where channel bed and bank material consist of non-cohesive fine textured soils, watershed sediment yield is controlled by channel discharge and flow velocity neither of which is influenced by traditional hillside conservation practices or channel bank stabilization structures. Natural variability of weather, diversity of sediment sources, cumulative and integrative effects of watershed drainage processes, anthropogenic activities, and legacy impacts of past watershed management contribute to making it inherently difficult to identify, extract, and attribute trends in observed soil erosion and runoff to climate change. This is further impeded by the uncertainty that is still associated with projections of future precipitation for many regions of the contiguous US.

Technical Abstract: This paper reviews selected watershed studies of the Conservation Effects Assessment Project (CEAP) and interprets findings from the perspective of potential climate change impacts on conservation outcomes. Primary foci are runoff, soil erosion, sediment transport, watershed sediment yield, and associated processes. The intended purpose of this review is to provide climate change related background information that contributes to planning and management of agricultural watersheds, assessment of conservation needs, and development, funding, and implementation of conservation programs. The selected conservation assessment studies include: a thought experiment on the sensitivity of soil erosion, runoff, and sediment yield to changes in rainfall; a computer-based investigation of possible climate change effects on runoff and soil erosion in a southeastern Arizona rangeland; the complex response of northern Mississippi watersheds to runoff variations and channel stabilization measures; the impact of conservation practices and a persistent pluvial period on watershed runoff and sediment yield in Oklahoma; and, stream bank erosion during major flooding in Iowa during summer 2008 and river-corridor management. A review of rainfall and runoff records of a North-central Missouri watershed; and a Curve Number analysis of rainfall and runoff in a Northern Appalachian experimental watershed are also included herein. The review and interpretation of the selected studies revealed that climate change scenarios of increased precipitation intensity lead to an exponential increase in soil erosion, runoff, and watershed sediment yield, thereby stressing current conservation practices or future practices designed with present day practice standards. This leads to diminishing conservation practice effectiveness, and increasing the sediment supply to the stream network. Climate change scenarios of increased precipitation frequency lead to a proportional increase in mean annual runoff and sediment yield. This sensitive response of the watershed hydrologic system can lead to renewed soil erosion on hillsides that are large enough to offset the reduction in soil loss achieved by current conservation practices. However, in alluvial environments with bed and bank material consisting of fine sandy, silt loams, watershed sediment yield is controlled by channel discharge and energy slope, neither of which is influenced by traditional hillside conservation practices or channel bank stabilization structures. Thus, control of sediment yield will gradually shift in downstream direction from sediment supply to sediment transport capacity and blur any existing relation between a climate change signal, upland conservation outcomes, and sediment yield at a watershed outlet. More effective conservation practices, more extensive conservation coverage, and adaptive agronomic practices may be necessary to prevent excessive soil erosion and downstream sedimentation under a climate of intensified precipitation.