Converting agricultural landscapes to native ecosystems has long been known to improve water quality, especially in US Midwest basins struggling with nutrient and sediment pollution. While the benefits of restoration have been well documented at field scales, few restoration efforts have been substantial enough to be detected at a larger watershed scale. Walnut Creek in central Iowa is a notable exception, undergoing conversion of 45% of cropland in the downstream half of the basin to tallgrass prairie at the Neal Smith National Wildlife Refuge. This study examined the influence of this large-scale prairie reconstruction on streamflow, nitrate, and sediment in Walnut Creek (1995-2024) using monitoring records upstream and downstream of the restoration. Mean annual streamflow yield and baseflow index were found to be lower in the prairie portion of the basin (264 mm/year and 0.58) than in its unaltered upstream counterpart (292 mm/year and 0.66). Likewise, nitrate and suspended sediment concentration (SSC) levels exhibited greater reductions within the prairie portion of the basin. Nitrate yields and flow-weighted concentrations decreased at rates of -0.370 kg/ha/year and -0.131 mg/L/year, respectively, while SSC decreases were -48.8 kg/ha/year and -17.0 mg/L/year. These results align with the hypothesized effects of converting cropland to tallgrass prairie and suggest that restoration efforts have had an appreciable impact on Walnut Creek's streamflow and water quality. The study highlights the benefits of long-term monitoring for quantifying changes in flow and water quality driven by land cover change, as improvements can be difficult to detect due to interannual rainfall variability.
Anderson et al. (Sun,) studied this question.