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

Information about how current and proposed management practices impact environmental quality is required to develop best management practices. A modeling approach was used to evaluate the scheduling practices of local farmers and two other irrigation scheduling practices for their potential impact on groundwater pollution in Dona Ana County, New Mexico. Data about farmers' practices came from historical information about the timing and quantity of water delivered to the farms. The irrigation scheduling practices were: tensiometer-based with the tensiometers placed at 50% or 75% of the root-zone depth and irrigations started when tensiometer's readings reached 6 kPa for 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 scheduling model (IRRSCHM), a volume balance, mixing-cell, type irrigation scheduling and pesticide transport model, to assess and compare the impact of different irrigation scheduling practices on cyanazine (Bladex) and metolachlor (Dual) concentrations at 180 cm below the soil surface during a 30-year cropping sequence. The region was divided into different soil textural classes 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 practice. The tensiometer based irrigation scheduling resulted in the highest Bladex and Dual concentrations. The lowest concentrations were predicted under 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 5.25 x 10(-1) mg l(-1) Health Advisory Level under the tensiometer-based irrigation scheduling practices, while, the farmer's practices resulted in Dual concentration about 625 times less than the Health Advisory Level. Similarly, the predicted Bladex concentration in sandy soil class was 3125 times less than the 1.30 x 10(-2) mg l(-1) Bladex Health Advisory Level under the tensiometer-bassed irrigation scheduling and about 416 000 times less than the Health Advisory Level under farmer's practices. Simulation results suggested that current farmer's practices do not post: a threat to the area's groundwater duality and result in a 15-40% Leaching fraction depending on soil type. Tensiometer-based irrigation scheduling was similar to scheduling irrigations at 50% plant available water depletion and resulted in a 35-50% leaching fraction depending on soil type as long as the tensiometer was placed in the proper root zone depth. Thr model's calculated leaching fractions using farmer's practices were similar to measured leaching fractions in Dona Ana County, giving credibility to the use or simulation models for assessing 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.