Using observations and a distributed hydrologic model to explore runoff thresholds linked with mesquite encroachment in the Sonoran Desert

Authors: PIERINI, N.A. - Arizona State University; VIVONI, E.R. - Arizona State University; ROBLES-MORUSA, A. - Sonora Institute Of Technology; Scott, Russell - Russ; Nearing, Mark

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/29/2014
Publication Date: 10/23/2014
Citation: Pierini, N., Vivoni, E., Robles-Morusa, A., Scott, R.L., Nearing, M.A. 2014. Using observations and a distributed hydrologic model to explore runoff thresholds linked with mesquite encroachment in the Sonoran Desert. Water Resources Research. 50:8191-8215.

Interpretive Summary: The proliferation of shrubs in grasslands has transformed semiarid landscapes worldwide over the past century. In this study, we use runoff observations from two paired watersheds in southern Arizona documented to have undergone the encroachment of mesquite shrubs, and where one watershed had mesquite removed in 1974, to understand how shrub proliferation affects the amount of runoff produced by a watershed. Forty years of runoff observations from the watersheds exhibit changes in runoff production over time, such that the watershed with more woody plants currently leads to less runoff for small rainfall events, more runoff for larger events, and a larger overall runoff coefficient. To explain this observation, we first test the hydrologic model against data from an environmental sensor network (e.g., soil temperatures and water content, landscape evaporation, etc.) and runoff data. We find good agreement between the model and observations and then use it to identify that the relative amounts of grass and bare soil covering the watershed determine the observed runoff response. These mechanisms help to explain how woody plants have different effects on watershed runoff depending on rainfall amounts.

Technical Abstract: Woody plant encroachment is a world wide phenomenon with implications on the hydrologic cycle at the catchment scale that are not well understood. In this study, we use observations from two small semiarid watersheds in southern Arizona that have been encroached by the velvet mesquite tree and apply a distributed hydrologic model to explore runoff threshold processes experienced during the North American monsoon. The paired watersheds have similar soil and meteorological conditions, but vary considerably in terms of vegetation cover (mesquite, grass, bare soil) and their proportions with one basin having undergone mesquite removal in 1974. Long-term observations from the watersheds exhibit changes in runoff production over time, such that the watershed with more woody plants currently has less runoff for small rainfall events, more runoff for larger events, and a larger runoff ratio during the study periods (summers 2011 and 2012). To explain this observation, we first test the distributed model, parameterized with high-resolution (1 m) terrain and vegetation distributions, against continuous data from an environmental sensor network, including an eddy covariance tower, soil moisture, and temperature profiles in different vegetation types, and runoff observations. We find good agreement between the model and observations for simultaneous water and energy states and fluxes over a range of measurement scales. We then identify that the areal fraction of grass (bare soil) cover determines the runoff response for small (large) rainfall events due to the dominant controls of antecedent wetness (hydraulic conductivity). These model-derived mechanisms explain how woody plants have differential effects on runoff in semiarid basins depending on precipitation event sizes.