Impacts of Historical Changes in Land Use and Dairy Herds on Water Quality in the Catskills Mountains
1998
Surface water eutrophication has been linked to nonpoint sources of agricultural nutrients. Based on 1938 and 1968 aerial photos and 1993 Landsat imagery of a 31.7 km² watershed, corn (Zea mays L.)/hay rotation, pasture, forest, and impervious/water land uses were digitized in a raster GIS. Monthly runoff volumes resulting from each year's land use were predicted using a variable source area hydrologic model. Historical data on dairy animal populations and the spatial distribution of current soil test phosphorus (STP) levels were used to derive manure loading intensities. Event-mean SP concentration in runoff as a function of manure application intensity and soil test P was derived from field data and published sources. The maximum predicted event-mean SP concentration in runoff was 15.8 mg L⁻¹. Watershed average potential SP loading to perennial streams remained essentially constant at 0.14 kg ha⁻¹ yr⁻¹ over the period 1938 to 1993. Herd size and total manure load dropped significantly over the period 1938 to 1993; however, the P content in manure and confinement of animals increased, counteracting the hypothesized trend that SP loading would drop. The analyses indicate that, for 1993, <6% of the agricultural land accounted for 50% of the agricultural SP load, suggesting that with improved manure handling practices over a relatively small area, it may be feasible to reduce SP loads to streams.
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