The soil and water assessment tool (SWAT), a large-scale hydrologic model, is used to estimate phosphate (P) loading in streams and water bodies. The labile, active, and stable P pools are currently used to represent P cycling in SWAT; however, these pools are conceptual without any chemical basis. The current structure allows SWAT to reasonably represent P cycling; however, restructuring and incorporation of recent research results should produce more accurate P simulations. This paper presents a redefined SWAT structure using four inorganic soil P pools (water soluble, labile, active, and stable) combined with soil extraction methods to determine initial values for each pool. The redefined structure was selected by examining the chemical characteristics of laboratory tests relative to soil P pools and analyzing the relationships between test results. Water soluble P, labile P and active P are determined using water, H3A, Mehlich 3 extraction, respectively. Stable P is determined with an acid digestion. Redefining SWAT’s inorganic soil P pools based on soil and extractant chemistry will improve the defensibility, credibility, and the accuracy of SWAT P routines in water resource planning, management, and decision making.
SWAT, Soil and Water Assessment Tool; STP, soil test phosphorus; PAI, phosphorus availability index; NAPT, North American proficiency testing.
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