Hydrologic responses to urbanization in Denver watersheds and investigation of precipitation thresholds for streamflow generation in pre-development semi-arid rangeland
Date
2021
Authors
Wilson, Stacy, author
Bhaskar, Aditi, advisor
Kampf, Stephanie, committee member
Green, Tim, committee member
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Abstract
Urbanization alters stream hydrographs and has been shown to have detrimental effects on water quality, stream morphology, and riparian ecosystem function. A thorough understanding of this alteration is crucial for effective and sustainable water management as communities in semi-arid areas continue to grow at an accelerated pace. However, the hydrologic response to urbanization in semi-arid rangeland environments has not been well documented. Using eight years of instantaneous flow data for twenty-one watersheds ranging in size from 1 to 90 km2 with impervious areas ranging from 1 to 47%, this study provides a comprehensive analysis of hydrologic alteration occurring with urbanization in the semi-arid area of Denver, Colorado, USA. Using a semi-automated method to identify 2,877 streamflow events, we analyzed event-based metrics of peak flow, runoff depth, runoff ratio, time to peak, and duration, in addition to precipitation threshold and number of streamflow events occurring in response to precipitation events and zero flow. We found that number of events and peak flow increased significantly with the fraction of impervious area (imperviousness), while duration, precipitation threshold, and zero flow decreased significantly with imperviousness. Runoff depth, runoff ratio, and time to peak either gave mixed results or did not vary significantly with imperviousness. Our results suggest that urban watersheds in semi-arid environments are more hydraulically efficient than their undeveloped counterparts, resulting in an increased number of streamflow events generated by smaller precipitation events, with a quicker delivery of runoff to receiving streams. This research also characterized the flow in West Stroh Gulch rangeland in Parker, Colorado through time-lapse photography in conjunction with climatological data. Our monitoring period was limited to one year in duration, while no streamflow events were observed throughout our study, suggesting the precipitation threshold to generate runoff in this undeveloped rangeland exceeds the largest rainfall events observed (30 mm depth and a 60-minute maximum intensity of 5 mm/hour). Our data provides important baseline information for future comparison as development in semi-arid areas rapidly progresses, contributing physical data useful for model calibration. Overall, this research makes an important contribution to understanding the streamflow response of grasslands and urban watersheds to precipitation in semi-arid environments.
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Subject
precipitation threshold
semi-arid
urbanization
rangeland
hydrologic responses
urban hydrology