THE ROLE OF WETLAND DRAINAGE IN MAINTAINING LATE-SEASON BASEFLOW IN HIGH-ELEVATION HEADWATER STREAMS

Wetlands occur in many high-elevation headwater basins. Following snowmelt, wetland drainage can be a major source of baseflow which maintains late-season streamflow. Climate variability may influence the hydrology in headwater basins by modifying the timing, amount, and form precipitation. These changes can impact snowpack, water distribution, and timing of snowmelt and peak streamflow, influencing when baseflow contribution becomes meaningful. We investigated the importance of drainage from high-elevation wetlands to late-season baseflow in the Senator Beck Basin (SBB), at an elevation of 3359 masl, in the San Juan Mountains, SW Colorado, USA. The stream in the SBB, a tributary to the Colorado River, is gauged and has recorded discharge and specific conductance (SC) since 2005. In August 2020, the stream above the toe of the wetland had dried, yet the gauge still recorded flow. During a visit to the SBB in September 2021, the stream was dry above the wetland and seeps were observed coming from the toe of the wetland, generating streamflow. These observations led to two questions: (1) when does snowmelt end, and (2) when does baseflow discharge become important?

The time at which baseflow discharge becomes significant may vary between wet and dry years. To estimate the time, we took the second derivative (SD) of the stream discharge vs time data to determine the day of the year when the rate of change of discharge slows and approaches zero. The Conductivity Mass Balance method was used to estimate the baseflow contribution to streamflow using the flow and SC data. The baseflow index (BFI), the fraction of streamflow that is baseflow, was determined on a daily timestep. Comparing the SD results to the BFI results using several years of data, the timing when the SD approaches zero occurs around the time that BFI is 15%. Using data from 2018 (a dry year) and 2019 (a wet year), the day of year in 2018 was 174 (BFI; 171 SD) and for 2019, it was day 215 (BFI; 217 SD). Baseflow became significant approximately 30-40 days before the wet year. Future work will investigate how the time of peak snow water equivalent and snow all gone influences the timing when baseflow becomes important. These results may allow temporal variations to be used to estimate the potential effects of climate variability on wetland drainage and baseflow in headwater streams.