Impacts of Precipitation Rate and Land Use on the Contribution of Groundwater Discharge to Streamflow Under Current and Past Conditions

Groundwater discharge to streams helps link terrestrial and aquatic habitats and determine surface water quality and quantity. Precipitation rates and land use both influence the contribution of groundwater discharge to streamflow, yet impacts of recent climate change on these relationships are not well known. To help fill this knowledge gap, this study uses the Kansas precipitation gradient as a natural laboratory to examine how precipitation rates and land use affect groundwater discharge to streams and whether those relationships are changing. We performed hydrograph separation calculations on stream discharge data from 27 watersheds across the precipitation gradient. The calculations evaluated runoff, baseflow, and baseflow index (BFI), which we used as an estimate of the proportion of groundwater discharge in streamflow. We assessed current conditions by calculating average values for the past 15 years for all sites and we assessed changes over time by examining data for the past 60 years from seven sites. Under current conditions, runoff and baseflow both increase with increasing average annual precipitation (p < 0.001), but the increase is greater for runoff than baseflow. As such, BFI tends to decrease with increasing precipitation (p < 0.02). This result suggests that a greater proportion of precipitation ends up as runoff rather than recharge as precipitation rates increase, possibly reflecting the limits of infiltration rate. In addition, variation in the permeability of watershed surfaces/subsurface units also appears to impact BFI. Consistent with this possibility, BFI decreases as the proportion of urban land use increases (p < 0.005). Results for the past 60 years show that the proportion of groundwater discharge (BFI) has significantly increased at the three sites receiving the least amount of precipitation (p < 0.05). Increases in irrigated acreage and groundwater pumping suggest that BFI is increasing in response to land use effects. Changes in climate may also contribute. However, average annual precipitation and temperature have not changed significantly for most of the watersheds. These results highlight the importance of understanding long term changes in groundwater discharge in response to climate as well as land use for better management of water resources in the future.