AGRICULTURAL CHEMICAL DISCHARGE IN SURFACE-WATER RUNOFF

The discharge of agricultural chemicals (i.e., soil-fertilizer nutrien ts and pesticides) in runoff waters is important from both agronomic a nd environmental standpoints. Presented here is an overview of our cur rent concepts and approaches employed for describing this discharge, b ased on studies we have conducted over the past decade. Most of our fi eld testing and validation of concepts regarding chemical discharge ha s focused on approximately 24 grassland and cropland watersheds across the Southern Plains. Chemicals considered include N, P, K, S, atrazin e loro-4(ethylamino)-6-(isopropylamino)-s-triazine], alachlor [2-chlor o-2',6'-diethyl-N-(methoxymethyl) acetanilide], and cyanazine no)-s-tr iazine-2-yl]amino]-2-methylpropionitrile]. Soluble chemical discharge has been described by kinetic desorption and uniform or nonuniform mix ing approaches, incorporating parameters reflecting watershed manageme nt and the nature of the surface soil x precipitation interaction. Par ticulate chemical discharge has been described by the relationship bet ween the discharge enrichment ratio (chemical content of eroded sedime nt/source soil) and soil loss. Special situations considered include t ype of tillage, computed water and sediment runoff, severe storms, bio availability of P, cover crops, and manure applications. For the most part, predicted chemical discharge values compared favorably with thei r 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.