Assessing the impacts of hydrologic disturbances on urban water supply and demand in the western United States

Abstract

In the semi-arid western U.S., urban water systems are facing growing challenges to both supply and demand associated with growing populations, urban development, wildfires in headwaters basins, and climate change. Wildfire and climate change can alter the volume and timing of water delivery to downstream systems, and projected increases in temperature are expected to increase demand in urban systems. Along the Colorado Front Range, extensive redevelopment is changing the characteristics of the urban systems that drive water demand. To better understand the impacts of disturbance on regional water supply and demand, this dissertation assesses post-fire changes to water yield in a burned watershed in the Rockies and investigates trends in and drivers of urban irrigation, a consumptive use of water, in Denver, Colorado. After the Chippy Creek Fire in 2007, the Mill Creek Basin in Montana experienced abrupt shifts in vegetation, from evergreen forest to shrub/scrub and grasslands, resulting in significant changes in local hydrologic partitioning and altering downstream supplies. Evapotranspiration from the basin decreased by 46%, and water yield increased by 140% during the first decade after the fire with no clear recovery trends. In Denver, temperature and land cover influenced demand for outdoor water use between 1995 and 2018. Increasing temperatures drove significant increases in irrigation rates in 37% of Denver census block groups, and the percentage of water used outdoors increased significantly across the city during this period. Finally, examinations of irrigation rates at the parcel scale in Denver show significant differences between land uses that are associated with variation in impervious land cover. Modeled residential redevelopment scenarios show reductions of 141,000 m3 (114 AF) of residential outdoor use per 1% increase in single-family parcels redeveloped to multi-family units. This work contributes essential insights toward improving the resiliency of water systems and understanding key factors that influence sustainable urban development. Despite the destructive nature of wildfire, results indicate that increases in water yield following fire in headwaters basins can be utilized for downstream urban supply if managers appropriately plan for altered volume and quality. As temperatures rise and indoor water use becomes more efficient or is recycled, outdoor use comprises an increasingly large portion of total urban water demand, posing challenges to climate adaptation within water-limited cities. However, by integrating land use and water planning, the residential redevelopment of urban areas provides opportunities to reduce outdoor demand and design urban green spaces to achieve multiple benefits efficiently.