Estimating Wetland Connectivity to Streams in the Prairie Pothole Region: An Isotopic and Remote Sensing Approach

Understanding hydrologic connectivity between wetlands and perennial streams is critical to understanding the reliance of stream flow on inputs from wetlands. We used the isotopic evaporation signal in water and remote sensing to examine wetland-stream hydrologic connectivity within the Pipestem Creek watershed, North Dakota, a watershed dominated by prairie-pothole wetlands. Pipestem Creek exhibited an evaporated-water signal that had approximately half the isotopic-enrichment signal found in most evaporatively enriched prairie-pothole wetlands. Groundwater adjacent to Pipestem Creek had isotopic values that indicated recharge from winter precipitation and had no significant evaporative enrichment, indicating that enriched surface water did not contribute significantly to groundwater discharging into Pipestem Creek. The estimated surface water area necessary to generate the evaporation signal within Pipestem Creek was highly dynamic, varied primarily with the amount of discharge, and was typically greater than the immediate Pipestem Creek surface water area, indicating that surficial flow from wetlands contributed to stream flow throughout the summer. We propose a dynamic range of spilling thresholds for prairie-pothole wetlands across the watershed allowing for wetland inputs even during low-flow periods. Combining Landsat estimates with the isotopic approach allowed determination of potential (Landsat) and actual (isotope) contributing areas in wetland-dominated systems. This combined approach can give insights into the changes in location and magnitude of surface water and groundwater pathways over time. This approach can be used in other areas where evaporation from wetlands results in a sufficient evaporative isotopic signal. Plain Language Summary Connectivity of wetlands within watersheds is critical to wetland function at local-to-landscape scales. However, actual measures of connectivity between wetlands and streams at the watershed scale are difficult to make, but critical for quantifying these functions, and for supporting policy and decision making. In this manuscript, we combine and compare direct field measurements of connectivity using stable isotopes of water, and remotely sensed measurements of connectivity using Landsat data. We found that prairie-pothole wetlands contributed significant amounts of water to the perennial stream from high to low-flow conditions. This wetland water flowed through surface water connections and not through groundwater flow paths. Hydrologic connectivity between prairie-pothole wetlands and the stream was dynamic, fluctuating with streamflow. Combining the isotopic and Landsat approaches to estimating contributing area appears to be a promising technique for determining potential and actual contributing areas in these flat wetland-dominated systems.