An Evaluation of GIS Methodologies for Improving P Index Measurement in the Illinois Watershed.

In Arkansas a phosphorus index (P-Index) is used in nutrient management planning to assess the risk of offsite transport of phosphorus from fields.  The Arkansas P-Index has three components:  P source, P transport, and a best management practice multiplier.  Although the use of the P Index is a requirement of nutrient management planning, as a result of the assumptions involved and diversity of measurement techniques used, there are inconsistencies within P-Index calculations.  First, the transport component of the index for a specific field is calculated using one dominant soil series regardless of the number of soils occurring in a field.  Additionally slope, which is an important input to the transport component, can be calculated using several different methodologies.  The focus of this study is to evaluate the effect of these techniques and methods on the variability of P-Index calculations and to assess if Geographic Information System (GIS) modeling could be used to obtain these values in a more consistent manner.  The study is occurring in the Upper Illinois River Watershed in Northwest Arkansas. Slope information calculated from field work, from NRCS soil series, and Digital Elevation Model’s (DEM’s) are being used to calculate transport factor values and will be subsequently compared.  Also, the P-index is being computed using an area-weighted value comprised of individual risk values computed from each soil series in a field.  This area-weighted value will then be compared to the value calculated from using only the dominant soil in a field.  Performing such calculations and comparisons are important to understanding the variability associated with current P-Index calculations, as well as testing the overall sensitivity of the Index to slope measurement.