Estimating lawn irrigation contributions to semi-arid, urban baseflow using water-stable isotopes

Abstract:

Excess lawn irrigation may return to streams and lead to widespread baseflow increases in semi-arid cities. The contributions of excess lawn irrigation to semi-arid streams are of particular interest because of the potential for concentrated nutrient inputs. We sought to quantify lawn irrigation contributions to summer baseflow in Denver, Colorado (USA) using water-stable isotope (δ18O and δ2H) analysis of surface water, tap water, and precipitation. If lawn irrigation contributed significantly to baseflow, we predicted the isotopic composition of Denver’s urban streams would more closely resemble local tap water than precipitation or streamflow from nearby grassland watersheds. We expected tap water to be distinctive due to local water providers importing source water from high elevations. Thirteen urban streams and two grassland streams were selected for sampling. None of the streams had high-elevation headwaters or wastewater effluent, and the grassland streams did not receive irrigation. Tap water was sampled from five water provider service areas. A two end-member isotope mixing analysis using tap and precipitation end-members estimated that tap water contributed more than 65% of urban baseflow on specific days in late summer. After taking contributions from infrastructure leakage into account, we estimated lawn irrigation return flows made up 32% - 82% of analyzed baseflow. Quantifying lawn irrigation contributions to urban baseflow will provide a basis for understanding how changes to lawn irrigation efficiency would affect water quality in semi-arid cities.