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item Hively, Wells - Dean

Submitted to: Hydrology and Earth System Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2005
Publication Date: 12/6/2005
Citation: Hively, W.D., Gerad-Marchant, P., Steenhuis, T. 2005. Distributed hydrological modeling of total dissolved phosphorus transport in an agricultural landscape, Part II: Dissolved phosphorus transport. Hydrology and Earth Systems Science. 2:S-1141-S1145.

Interpretive Summary: The majority of agricultural nutrients lost as non-point source pollution are produced from critical areas that occupy only a small portion of the landscape. These critical areas occur where nutrients are readily available (for example, in heavily manured areas or soils receiving long-term nutrient applications) and hydraulic transport pathways are present (for example, frequently saturated areas and impermeable roadways). This study used a combination of field sampling and mapping to estimate the concentrations of phosphorus produced in runoff from all areas of a small dairy farm watershed that is located in the headwaters of the New York City water supply reservoirs. The estimated concentrations, which included the effects of soil phosphorus, surface applied manure, and impermeable areas (barnyards, roadways), were combined with hydrological estimates of surface runoff production (calculated through the use of the Soil Moisture Distribution and Routing model, described in a companion paper) to estimate the daily loads of dissolved phosphorus lost from the watershed. Results matched fairly well with loads observed at the watershed outlet, and indicated that the bulk of phosphorus loss was attributable to field areas during the wintertime, and to near-barn areas during the summertime.

Technical Abstract: Reducing non-point source phosphorus (P) loss to drinking water reservoirs is a main concern for New York City watershed planners, and modeling of P transport can assist in the evaluation of agricultural effects on nutrient dynamics. A spatially distributed model of total dissolved phosphorus (TDP) loading was developed using raster maps covering a 164-ha dairy farm watershed. Transport of TDP was calculated separately for baseflow and for surface runoff from manure-covered and non-manure-covered areas. Soil test P, simulated rainfall application, and land use were used to predict concentrations of TDP in overland flow from non-manure covered areas. Concentrations in runoff for manure-covered areas were computed from predicted cumulative flow and elapsed time since manure application, using field-specific manure spreading data. Baseflow TDP was calibrated from observed concentrations using a temperature-dependent coefficient. An additional component estimated loading associated with manure deposition on impervious areas, such as barnyards and roadways. Daily baseflow and runoff volumes were predicted for each 10-m cell using the Soil Moisture Distribution and Routing Model (SMDR). For each cell, daily TDP loads were calculated as the product of predicted runoff and estimated TDP concentrations. Predicted loads agreed well with loads observed at the watershed outlet when hydrology was modeled accurately (R2 79% winter, 87% summer). Lack of fit in early spring was attributed to difficulty in predicting snowmelt. Overall, runoff from non-manured areas appeared to be the dominant TDP loading source factor.