Lawn Irrigation Contributions to Semi‐Arid Urban Baseflow Based on Water‐Stable Isotopes

In semi‐arid cities, urbanization can lead to elevated baseflow during summer months. One potential source for additional water is lawn irrigation. We sought to quantify lawn irrigation contributions to summertime 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 service areas. The grassland streams flowed for 60% of summer 2019 while urban streams flowed for 90%–100% of the summer. An isotope mixing analysis using tap and precipitation end‐members over a two week antecedent period estimated that tap water contributed 65% ± 10%–93% ± 3% with 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% ± 10%–82% ± 21% of analyzed baseflow. Quantifying lawn irrigation contributions to urban baseflow provides a basis for understanding how changes to lawn irrigation efficiency would affect water yield in the Denver metropolitan area. Plain Language Summary Excess lawn irrigation may contribute to flow in urban streams when more water is applied than plants can use or when sidewalks or driveways receive water from mis‐aimed sprinklers. We have applied a new approach to estimating how much flow in streams originally came from excess lawn irrigation by using a tracer that is different between tap water in the Denver, Colorado USA area and local rain. We applied this approach to 13 urban streams and two grassland streams in the Denver area. We found that, on the dry weather days analyzed, over 65% of flow in urban streams was from tap water, and lawn irrigation was a larger source of water in the streams than leaking water pipes. This work gives a new way to analyze how much flow is coming from different sources in urban areas, which is important for understanding consequenc