A Phosphorus Index transport factor based on variable source area hydrology for New York State

Abstract

Abstract:The Phosphorus (P) Index concept is used in many states to help develop nutrient management plans for livestock agriculture to protect water quality. Although many P indices conceptually incorporate variable source area (VSA) runoff processes, they generally assume proximity to a water course is an adequate proxy of runoff risk. Here we propose a VSA-based transport factor that uses the topographic index concept to indicate runoff risk. We compared both transport factors based on the current New York State P Index and our VSA-based P Index to field measures of runoff probability across an abandoned agricultural site in upstate New York. We also compared transport factors and P indices using the current and VSAbased approaches on a farm in the Catskill Mountains of New York to evaluate differences at whole-field and farm scales. Field runoff-risk measurements were better correlated with VSA-based transport factor (r 2 = 0.63, α = 0.05) than with the current dissolved-P transport factor (r 2 = 0.40, α = 0.05). Although these point-scale differences in transport factor values translated into field-scale differences in transport factor, the net differences at the farm scale and in P Index were not very large. On a field-by-field basis, 12 out of 21 fields had different transport factor categories with the VSA method. However, the total land area classified as high risk changed very little between the two methods. There was more land classified as moderate risk using the VSA-based approach than using the current methods, due to some low risk areas being classified as higher risk and some high-risk areas being classified as lower risk. The VSA approach allows managers and producers to more easily manage farm units (e.g., fields) at finer resolutions both spatially and temporally, which will increase the options for managing nutrients on fields. These types of more rigorous links between management tools and physical hydrology provide valuable, more scientifically defensible information for improving our ability to control nonpoint source pollution. Key words: nutrient management-Phosphorus Index-variable source areasThis paper proposes a variable source area (VSA) based transport factor for incorporation into an existing Phosphorus (P) Index to give farmers and nutrient management planners improved information about the relative risks of runoff from different parts of a farm. Phosphorus in agricultural runoff often contributes to eutrophication of streams and other fresh water systems (Carpenter et al. 1998). The P Index is a widely used tool for managing phosphorus (P) from nonpoint sources (NPS), which helps identify fields or parts of the landscape that are particularly susceptible to P loss, including P-enriched runoff (Sharpley et al. 2003). The information contained in the P Index is used in farm nutrient management plans to strategize manure spreading, fertilization, and other management decisions to reduce risks of NPS P transport to streams.The New York State (NYS) P runoff index was developed by a m...

Evaluating topographic wetness indices across central New York agricultural landscapes

“…Computed as ln(α/tan β), where α is the upslope contributing area per unit contour length and tanβ is the local slope, the index provides a relative, not absolute, measure of the moisture status of a particular area or pixel. Since its introduction, the TWI concept has been integrated into many popular hydrologic models (e.g., TOPMODEL, Beven and Kirby, 1979;VSLF, Schneiderman et al, 2007;SWAT-VSA, Easton et al, 2008) and pollution risk indices (Agnew et al, 2006;Reaney et al, 2011;Marjerison et al, 2011;Buchanan et al, 2013). Despite its wide application, large-scale corroboration of TWI-based predictions of landscape-scale soil moisture patterns using actual field observations are the exception rather than the rule.…”

Real-Time Forecast of Hydrologically Sensitive Areas in the Salmon Creek Watershed, New York State, Using an Online Prediction Tool

“…Soils are generally 200 cm deep, with well to somewhat poorly drained silt loams and gravelly silt loams [64]. Elevations range from 320 to 378 m. The watershed exhibits typical HSA type hydrology due to the shallow highly permeable soils overlaying a dense fragipan at a shallow depth [29]. …”

“…Specifically, it can provide general geospatial data sets used to calculate the P-Index transport factor (e.g., soil drainage, flood frequency) and, more importantly, provide better and more precise information about the coincidence of fields and HSAs. Recall that the current NYS P-Index identifies high-risk runoff areas based largely on proximity to a watercourse, which is not a reliable proxy of runoff risk [29,35]. Additionally, the current NYS P-Index transport factor is more or less static, restricting the application of manure within a fixed-width 30 m buffer to the stream [33], which has been shown to not effectively abate dissolved phosphorus transport to streams [4,8,9,13].…”

“…The spatial definition of HSAs used in management tools, like the NYS P-Index, is often too static to adequately capture HSA dynamics, which are, rather, better described by topography and hydrometeorological processes. Several researchers have, for instance, proposed approaches for identifying HSA-locations more precisely in space and time on a month-to-month [22,29,35] or even storm-size basis [28,30,36,37]. However, these approaches have not been widely adopted, probably because they require GIS and sometimes, hydrological modeling expertise, which is not widely available to nutrient managers and conservation planners.…”

“…Previous efforts to identify HSAs have largely used distributed hydrological models such as SWAT-VSA (Soil and Water Assessment Tool re-conceptualized for VSA) [18,19] and the VSA water balance model [19,[24][25][26][27]. Managers could potentially use the HSA concept to identify areas that are more likely to saturate or that are more hydrologically sensitive so that these areas could be protected from potentially polluting activities, such as animal manure applications to agricultural land (e.g., [9,22,28,29]). …”