Characterization of streamflow response to storms in semi-arid rangeland and urban watersheds in the Denver, Colorado, USA area

Abstract:

Urbanization leads to increases in peak streamflow, frequency of high flows, and has detrimental effects on water quality, stream morphology, and riparian ecosystem function. Recent research has highlighted the variability in the hydrologic response of watersheds in differing climate and geologic settings, drawing attention to the underrepresentation of semi-arid systems in many studies of the urban stream syndrome. Our research aims to help fill this gap, with monitoring in a pre-development, rangeland watershed and analysis of streamflow in watersheds across a gradient of urbanization. West Stroh watershed in Parker, CO, consists of an ephemeral network of ungauged stream channels and is scheduled for residential development within the next two years. Developers have partnered with Mile High Flood District and Wright Water Engineers to incorporate development strategies designed to maintain crucial characteristics of the area’s natural hydrology. We are characterizing the pre-development, rangeland hydrology of this watershed using time-lapse photography in conjunction with rainfall data to evaluate threshold precipitation events for streamflow generation. The grassland watersheds in Rocky Flats, managed by the Department of Energy Office of Legacy Management, function as reference watersheds in mostly undeveloped condition. Initial analysis of four reference watersheds from 1.35 - 12.67 km² and 9 - 23 years of record showed frequency of zero flow days from 1.3 - 40.1%. Early monitoring of West Stroh (1.42 km²) from July 2020 captured no flow with small rainfall events (2.0 mm total rainfall, or less), with monitoring ongoing. This study seeks to provide the physical data necessary for modeling the proposed stormwater management strategies for West Stroh and important baseline information for future comparison. Furthermore, monitoring throughout the development process and post-development in West Stroh will provide the best indicators of the effectiveness of the strategies implemented. More widely, this research will make an important contribution to understanding the precipitation response of pre-development grasslands. In addition, analysis of streamflow metrics across a gradient of urbanization in Denver area watersheds will produce valuable knowledge characterizing the response to urbanization of grasslands in semi-arid climates.