Water 222Rn for evaluating the variation in groundwater inflows to discharge of the Big Sioux River in different flow periods

Knowledge of seasonal water sources can help for improved management strategies to sustain agricultural productivity and ecosystem-services in a basin. This study defines the distribution of gaining reaches and evaluates groundwater inflows into the Big Sioux River located in eastern South Dakota using 222Rn mass balance approach during two distinctly different flow periods. The groundwater inflows estimated for the middle catchment were higher (up to 12.8 m3/m/day) compared to the values estimated for the upper catchment (up to 0.3 m3/m/day) in the Big Sioux River basin during both low- and high-flow periods. During low-flow period, the overall cumulative groundwater inflow ranged between 2529 and 211,166 m3/day that corresponded to 5 and 53% contribution to the total flow rate. The inflow during high-flow period ranged between 2467 and 332,740 m3/day corresponding to 2 and 55% of the total flow rate. Despite having elevated cumulative groundwater inflow during high-flow period, the relative contribution of groundwater inflow into the river was high during the low-flow period (a mean of 24% evaluated for the low-flow period vs. 18% for the high-flow period), potentially associated with high meltwater contribution derived from the spring snowmelt during that period. These estimations are expected to provide a better knowledge on seasonal water resource allocations that may be crucial for better water resource management strategies for the Big Sioux River basin.