MONITORING AND MODELING LONG-TERM CHANGES OF RAINFALL-RUNOFF RESPONSE AND WATER QUALITY IN A SMALL AGRICULTURAL WATERSHED RESTORED TO PRAIRIE

Although agricultural development generally leads to a step alteration in rainfall-runoff response and water quality, subsequent changes in land use create dynamic hydrological conditions. Furthermore, will restoration to pre-development land cover in a small watershed lead to full recovery of original hydrology? We analyzed 110 yr of land-use change for Judicial Ditch 66, which drains 3800 ha of recently restored prairie in NW Minnesota. The well-instrumented site lies on the margin of glacial Lake Agassiz, where strandlines parallel bands of wetland and prairie in the basin and comprise a component of the central North American flyway. Dominant land use can be divided into 4 phases: 1. nonintegrated drainage, groundwater and runoff lost to wetlands; 2. drainage and grazing; 3. cultivation, aggregate mining and drainage, and feedlot operation; 4. prairie reconstruction and closure of ditches. Curve number and Muskingum method routing were applied to estimate storm hydrographs. A USGS gage established in 2002 provides data for model calibration, enabling hydrological back- and forecasting. No runoff occurred during the pre-development stage, with an abrupt start and progressively increasing watershed runoff and flashiness through phases 2 and 3. For a 5 yr storm, model comparison to regional statistical methods suggests that 50% of the watershed was non-contributing during maximum phase 3 ditching and cultivation, increasing to 72% with diminishing stream flow following restoration. Active gravel leases and continued drainage of remaining agricultural land in the basin prevent a full recovery of the original conditions. Carbonate-rich soil and till substrate mitigated phosphorus "flush" following wetland restoration. A large store of P, however, remains near the feedlot. Although concentrations of NO₃ as high as 55 mg/L occurred in surficial aquifers, elevated concentrations of labile organic C in discharge zones and ditches greatly diminish NO₃ transport.