Evaluating hydrologic change in a semi-arid city: the impacts of land cover, water consumption, and policy directives on local water fluxes in Los Angeles, CA

Abstract

California experienced one of its worst droughts in recorded history from 2013-2015, resulting in the first statewide mandatory water restriction. The stated restrictions specifically targeted urban water use, with a major focus on outdoor irrigation. With over 80% of the U.S. population living in urban centers, many of which are water-scarce cities, it is becoming increasingly important to understand the numerous impacts of urbanization and water management policies on local hydrologic fluxes. Two of the most common land cover types associated with urban regions are impervious surfaces and irrigated landscapes, however, they have opposite impacts on hydrologic fluxes. In most studies, the impact of urbanization is primarily characterized by impervious surfaces; however, in semi-arid regions, high rates of irrigation puts stress on water supplies by increasing demands and leading to major alterations to local hydrologic behavior. The objective of this dissertation is to improve understanding of how and to what degree urban land surfaces, water consumption practices, and conservation policies impact hydrologic fluxes in a semi-arid city, using Los Angeles, California as a case study. This was analyzed through three studies, each examining alterations to a component of the local water budget. The first study evaluated the response of streamflow to outdoor irrigation practices and water conservation policies by comparing pre- vs during-water conservation streamflow data. The second study applied a Bayesian Hierarchical Model to fill missing groundwater level data allowing for examination of land cover and water management impacts on recharge rates. The third study utilized empirical equations derived from in-situ measurements to analyze the effect of land cover composition and vegetation type on evapotranspiration rates. Overall, results indicate that irrigated landscapes play a large role in altering all evaluated hydrologic fluxes, often having a greater impact than impervious surfaces, making irrigation a crucial component to consider when studying urban regions.