PHOSPHORUS CRITERIA AND WATER QUALITY MANAGEMENT FOR AGRICULTURAL WATERSHEDS

ABSTRACT

The concentrations and amounts of soluble and total phosphorus in rainfall, runoff, impoundments, and stream flow from several unfertilized watersheds in the Southern Plains have been monitored for the past decade to assess the effects of varying land uses on water quality as indicated by phosphorus concentration. Mean annual soluble and total phosphorus concentrations of 12 and 25 μg L⁻¹ were observed in rainfall, 199 and 710 μg L⁻¹ in runoff from unfertilized grassland, and 165 and 1,875 μg L⁻¹ from unfertilized cropland. Soluble phosphorus from rainfall averaged 41 g ha⁻¹, while that exported in runoff averaged 119 and 82 g ha⁻¹ yr⁻¹ for grassland and cropland watersheds, respectively. The load enrichment of phosphorus in runoff compared to rainfall was related to the water soluble phosphorus content of surface soil for each watershed (r² = 0.96 to 0.99). Soluble and total phosphorus concentrations in rainfall, runoff, and stream flow were sufficient to stimulate accelerated eutrophication according to critical phosphorus concentration guidelines. These guidelines — 10 and 20 μg L⁻¹ for soluble and total phosphorus, respectively—have been slightly modified by states for stream flow criteria. Data on phosphorus concentrations in rainfall and runoff from unfertilized watersheds at other U.S. and overseas locations are similarly above critical values; therefore, best management practices may not lower phosphorus concentrations on cultivated or grazed land to the critical values in runoff or stream flow. Management practices aimed at reducing nonpoint source phosphorus loading should be focused for areas where soil phosphorus levels have been raised above natural levels by applying fertilizers, manure, or crop residues.