Surface water hydrology and quality in farmed prairie potholes of the Des Moines Lobe in Iowa
The prairie pothole region ranges from central Iowa to the northwest into Montana and south central Canada, totaling around 700,000 km2. This area contains millions of potholes, or enclosed topographical depressions, which often inundate with rainfall. Many are located in areas that have been converted to arable agricultural land through installation of artificial drainage. However, even with drainage, potholes will pond or have saturated soil conditions during and after significant rain events. There is little information about the amount of water and nutrients that travel through potholes and the impact that they have on crops and downstream environments. In this two year study, surface water depth and nutrient data (NH3, NO3-N, TN, DRP, TP, and TSS) were collected daily from eight prairie potholes on the Des Moines soil lobe in central Iowa to determine the surface water hydrology and quality. These potholes included surface and subsurface drained row crops and undrained retired pothole, allowing for drainage comparisons Inundation lasted five or more days at least once at six of the eight potholes, including four potholes with surface and subsurface drainage, which resulted in four of fourteen growing seasons not producing a yield in part of the pothole. Water balances of four different drainage intensities showed increased infiltration due to subsurface drainage and up to 78% of outflow due to surface drainage. Overall, drainage decreased the number of average inundation days, but heavy precipitation events still caused lengthy inundation periods that resulted in crop loss. For water quality, seasonal differences were investigated using samples from the first day of each event, with surface water concentrations in the early growing season being higher than late season for TN (p < .0001), NO3-N (p < .0001), NH3-N (p = .005), TP (p = .036), and TSS (p = .028). Average event concentration changes were determined, and TP and DRP had significant increases that averaged 0.51 mg/l and 0.46 mg/l per event, respectively, with an average event length of 6.9 days (range of 2 to 19). Lastly, potential nutrient loads to surface inlets were estimated for two of the potholes that saw significant inundation, with similar TP, DRP, and TSS unit-area loads (average of 0.21 mg/l, 0.18 mg/l, and 16 mg/l, respectively) to other watersheds in the area in 2016. Overall, this study is useful in understanding the water quality of potholes to inform policy regarding field management and conservation.