|Torbert, Henry - Allen|
Submitted to: American Society of Agricultural Engineers Meetings Papers
Publication Type: Proceedings
Publication Acceptance Date: 7/28/2002
Publication Date: 7/28/2002
Citation: Harmel, R.D., DeLaune, P.B., Haggard, B.E., King, K.W., Richardson, C.W., Moore Jr., P.A., Torbert, H.A. Initial evaluation of a phosphorus index on pasture and cropland watersheds in Texas. American Society of Agricultural Engineers. 2002. Paper No. 02-2075. Interpretive Summary: With the expansion of the poultry industry and increased litter application to agricultural lands, management tools are needed to limit negative environmental impacts associated with excessive nutrients such as phosphorus (P) and nitrogen (N) in water. The focus of this study is the initial evaluation of one such tool called the Phosphorus Index or the P Index, which was developed to determine areas susceptible to excessive phosphorus loss. The P Index was developed for pastures in Arkansas, but we wanted to evaluate its effectiveness in Texas. To accomplish this goal, we determined P Index values for pasture and cropland fields and compared to them to measured phosphorus losses before and after application of poultry litter. To date in almost two years, the Arkansas P Index did not perform well in predicting the amount of annual P lost from pasture and tilled fields. However, the P Index did perform relatively well in ranking the fields based on P losses. In words, fields that had high P index values tended to have relatively high P losses.
Technical Abstract: 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.