Identification and classification of critical soil and water conservation areas in the Muskingum River basin in Ohio

The Muskingum River basin (MRB) of Ohio is the fourth most polluted watershed in the United States. To address water quality issues in the MRB, it is important to identify critical source areas (CSAs) that contribute disproportionately to high amounts of nonpoint source pollution. The Soil and Water Assessment Tool (SWAT) was used to develop a hydrological model in the MRB to identify conditions that cause land to be classified as CSAs in the basin. The model was calibrated for 10 years (1995 to 2004) and validated for 6 years (2005 to 2010) for streamflow, total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP) on a monthly scale. The validated model was then used to locate CSAs and the contributing sources of high nutrient discharges at the subbasin level in the MRB. Performance of conservation practices including terraces, buffer strips, 10% reduction in commercial and manure fertilizer use, and conservation tillage practices in the cropland of the CSAs were examined for their effectiveness in reducing nutrient loads at subbasin and watershed scales. In the MRB, average annual (1995 to 2010) subbasin yield for TSS, TN, and TP ranged from 0.02 to 1.23 Mg ha−1 y−1, 0.82 to 17 kg ha−1 y−1, and 0.03 to 2.6 kg ha−1 y−1, respectively. The slope of the landscape combined with land use practices were found to influence the water quality in the region. The cropland that comprises 22% of the basin was found to generate 70%, 63%, and 71% of the TSS, TN, and TP, respectively. Two percent of the cropland that is in steep landscape (>10% slope) was found to contribute to 29%, 6%, and 22% of the TSS, TN, and TP loads. While pasture land was found to generate 18% TSS, 15% TN, and 15% TP loads in the entire basin, these loadings were found to be significantly higher (53% TSS, 32% TN, and 64% TP) in the subbasins with intensive grazing activities. Evaluation of five conservation practices in the MRB suggested that terraces, vegetative filter strips, and controlled manure application are the most effective conservation practices to mitigate water quality issues in steep agricultural landscapes with complex management practices. The use of terraces was found to reduce the TSS (4% to 44%), TN (4% to 26%), and TP (5% to 53%) loads. Similarly, TSS, TN, and TP loads were reduced by 4% to 45%, 5% to 28%, and 7% to 45%, respectively, with filter strips practice in the basin. Nevertheless, none of the conservation practices in the cropland helped reduce sediment and nutrient loads by more than 12% in the outlet of the basin, suggesting that the conservation practices should be expanded to other land uses of the basin.