The Iowa P Index
The Iowa P Index uses a multiplicative approach to combine source and transport factors to estimate P delivered within three components: erosion (sediment-bound P loss), runoff (dissolved-P loss), and subsurface drainage (dissolved-P loss through tiles or coarse subsoil). The outputs from these components are summed to get an overall approximation of biologically available P delivered. The resulting number (expressed in lb P/acre/year) is placed into one of five risk classes (very low to very high). The guidelines establish that the P index ratings can be calculated for an entire field or for different conservation management units or zones within a field based on soil type, soil P level or landscape differences that justify different nutrient management plans. (The Iowa P Index Excel file.)
The Iowa P Index uses RUSLE2 to estimate sediment loss through sheet and rill erosion. Ephemeral gully erosion is calculated by the user. The P index requires a recent soil test P value and the distance from the center of the field to the nearest perennial or intermittent stream. The Iowa P Index uses a computer spreadsheet to access and integrate information on soil units, landscape forms, runoff curve numbers (RCNs), and county historical precipitation data to estimate and integrate volume of water runoff, sediment delivery ratio (SDR) and sediment trap efficiency of soil conservation practices. The spreadsheet processes this information to estimate P delivered within each of the three transport components.
The Iowa P Index reflects the concept that erosion from cropland is a major source of P loads to surface waters and that a large proportion of the particulate P can be made bio-available to aquatic vegetation over time through chemical, biological and hydrological processes in conditions such as shallow glacial-derived lakes or artificial reservoirs predominant in Iowa. Therefore, the index weighs particulate P loss heavily when the erosion risk is high. Partly due to this long-term approach and emphasis on cropland, the index does not differentiate based on solubility of commonly used P sources, even though these may be of short-term significance in a water body.