In 2021 seminars will be held mostly virtually.

Dr. Aditi Bhaskar

This talk presents an analysis of the changes to streamflow in the semi-arid area of Denver, Colorado, USA with a focus on (1) contributors to stream baseflow and (2) an analysis of changes in streamflow response to rainfall events with urbanization.  To estimate contributions to stream baseflow, we used water-stable isotope (δ¹⁸O and δ²H) analysis of surface water, tap water, and precipitation. Thirteen urban streams and two grassland streams were selected for sampling. An isotope mixing analysis using tap and precipitation end-members over a two week antecedent period estimated that tap water contributed a mean of 80% of urban baseflow on specific days in late summer. After taking contributions from infrastructure leakage into account, we estimated that lawn irrigation return flows made up 32% to 82% of analyzed baseflow. For changes to streamflow response to rainfall events, we used eight years of instantaneous streamflow data in 21 watersheds ranging in size from 1 to 90 km² with impervious areas ranging from 1% to 47%.  Using a semi-automated method to identify a total of 2,877 streamflow events, we analyzed event-based metrics of peak flow, runoff depth, runoff ratio, time to peak, duration, and number of streamflow events occurring in response to rainfall events, in addition to precipitation threshold and zero flow.  We found that more impervious watersheds had perennial or nearly perennial flow, unlike the least impervious watersheds which usually were dry.  Streamflow events were shorter in duration and had higher peak flow in watersheds with more impervious surface cover.  This work points to the need for local adaptation of stormwater management that seeks to mitigate the effects of streamflow changes with urbanization.