IMPACTS OF HISTORICAL CHANGES IN LAND-USE AND DAIRY HERDS ON WATER-QUALITY IN THE CATSKILLS-MOUNTAINS

Surface water eutrophication has been linked to nonpoint sources of ag ricultural nutrients. Based on 1938 and 1968 aerial photos and 1993 La ndsat imagery of a 31.7 km(2) watershed, corn (Zea mays L.)/hay rotati on, pasture, forest, and impervious/water land uses were digitized In a raster GLS. Monthly runoff volumes resulting from each year's land u se were predicted using a variable source area hydrologic model. Histo rical 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 functi on of manure application intensity and soil test P was derived from fi eld data and published sources. The maximum predicted event-mean SP co ncentration in runoff was 15.8 mg L-1. Watershed average potential SP loading to perennial streams remained essentially constant st 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, counteracti ng the hypothesized trend that SP loading would drop. The analyses ind icate that, for 1993, <6% of the agricultural (and accounted for 50% o f the agricultural SP Lend) suggesting that with improved manure handl ing practices over a relatively small area, it may be feasible to redu ce SP loads to streams.