AbstractThe discharge of agricultural chemicals (i.e., soil‐fertilizer nutrients and pesticides) in runoff waters is important from both agronomic and environmental standpoints. Presented here is an overview of our current concepts and approaches employed for describing this discharge, based on studies we have conducted over the past decade. Most of our field testing and validation of concepts regarding chemical discharge has focused on approximately 24 grassland and cropland watersheds across the Southern Plains. Chemicals considered include N, P, K, S, atrazine [2‐chloro‐4(ethylamino)‐6‐(isopropylamino)‐s‐triazine], alachlor [2‐chloro‐2′,6′‐diethyl‐N‐(methoxymethyl) acetanilide], and cyanazine [2‐[[4‐chloro‐6‐(ethylamino)‐s‐triazine‐2‐yl]amino]‐2‐methylpropionitrile]. Soluble chemical discharge has been described by kinetic desorption and uniform or nonuniform mixing approaches, incorporating parameters reflecting watershed management and the nature of the surface soil × precipitation interaction. Particulate chemical discharge has been described by the relationship between the discharge enrichment ratio (chemical content of eroded sediment/source soil) and soil loss. Special situations considered include type of tillage, computed water and sediment runoff, severe storms, bioavailability of P, cover crops, and manure applications. For the most part, predicted chemical discharge values compared favorably with their measured counterparts, r2 values often being >0.9. Further research needs include refinement and development of the prediction equations, data bases, runoff indices, and multidisciplinary systems.
