Impact of irrigation scheduling practices on pesticide leaching at a regional level

Information about how current and proposed management practices impact envi ronmental quality is required to develop best management practices. A model ing approach was used to evaluate the scheduling practices of local farmers and two other irrigation scheduling practices for their potential impact o n groundwater pollution in Dona Ana County, New Mexico. Data about farmers' practices came from historical information about the timing and quantity o f water delivered to the farms. The irrigation scheduling practices were: t ensiometer-based with the tensiometers placed at 50% or 75% of the root-zon e depth and irrigations started when tensiometer's readings reached 6 kPa f or sandy soils, 23 kPa for sandy loam soils, 44 kPa for loamy soils, and 74 kPa for clay loam soils; and at 50% plant available water depletion (PAWD) level regardless of soil type. The objective was to use irrigation schedul ing model (IRRSCHM), a volume balance, mixing-cell, type irrigation schedul ing and pesticide transport model, to assess and compare the impact of diff erent irrigation scheduling practices on cyanazine (Bladex) and metolachlor (Dual) concentrations at 180 cm below the soil surface during a 30-year cr opping sequence. The region was divided into different soil textural classe s to facilitate rapid estimation of soil parameters needed for the model. Very low Bladex and Dual concentrations were predicted at 180 cm below the soil surface. However, the predicted pesticide concentrations increased as soil sand fractions increased, regardless of the irrigation scheduling prac tice. The tensiometer based irrigation scheduling resulted in the highest B ladex and Dual concentrations. The lowest concentrations were predicted und er the farmer's practices due to deficit irrigation. Dual concentrations at 180 cm depth of the sandy soil class were about 20 times less than the 5.2 5 x 10(-1) mg l(-1) Health Advisory Level under the tensiometer-based irrig ation scheduling practices, while, the farmer's practices resulted in Dual concentration about 625 times less than the Health Advisory Level. Similarl y, the predicted Bladex concentration in sandy soil class was 3125 times le ss than the 1.30 x 10(-2) mg l(-1) Bladex Health Advisory Level under the t ensiometer-bassed irrigation scheduling and about 416 000 times less than t he Health Advisory Level under farmer's practices. Simulation results sugge sted that current farmer's practices do not post: a threat to the area's gr oundwater duality and result in a 15-40% Leaching fraction depending on soi l type. Tensiometer-based irrigation scheduling was similar to scheduling i rrigations at 50% plant available water depletion and resulted in a 35-50% leaching fraction depending on soil type as long as the tensiometer was pla ced in the proper root zone depth. Thr model's calculated leaching fraction s using farmer's practices were similar to measured leaching fractions in D ona Ana County, giving credibility to the use or simulation models for asse ssing and comparing the potential impact of different irrigation scheduling practices on environmental quality at a regional level, (C) 2000 Published by Elsevier Science B.V. All rights reserved.