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Initial Evaluation of a Phosphorus Index on Pasture and Cropland Watersheds in Texas

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Paper number  022075,  2002 ASAE Annual Meeting . (doi: 10.13031/2013.9185) @2002
Authors:   R.D. Harmel, C.W. Richardson, P.B. DeLaune, P.A. Moore, Jr., B.E. Haggard, H.A. Torbert, K.W. King
Keywords:   P Index, nutrient loads, water quality, poultry litter

This paper presents results from the first 23 months of a field scale evaluation of the predictive ability of a phosphorus (P) Index for pastures and crop fields in the Texas Blackland Prairie. To perform this evaluation, water quality monitoring on pasture and cultivated watersheds was initiated in 2000 prior to poultry litter application and continues to the present under various poultry litter application rates. The P Index was able to accomplish its specified purpose of estimating the relative susceptibility of fields to excessive phosphorus loss. When results from both pasture and tilled fields were pooled, measured P loss tended to increase as P Index values increased. P Index values also correlated well with both average and maximum dissolved P concentrations in runoff. These initial results indicate that the Index might perform well for cultivated fields, with only slight modifications, even though the P Index is not currently set up to evaluate tilled systems. The P Index did not, however, perform as well when we attempted to use it to quantitatively predict P losses. For all fields in the fallow year and for control fields with no litter application, P Index values under-predicted P loss. Following litter application, P loss from the pasture watersheds was drastically over-predicted. For the tilled watersheds, P loss predictions did improve substantially following litter application, but relative errors were still quite large.

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