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

A modelling 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 Doña Ana County, New Mexico, USA. 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 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 and metolachlor 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 pesticide 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 pesticide 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 ~20 times less than the Health Advisory Level under the tensiometer-based irrigation scheduling practices, while the farmer's practices resulted in metolachlor concentrations ~625 times less than the Health Advisory Level. Similarly, the predicted cyanazine concentration in sandy soil class was 3125 times less than the Health Advisory Level under the tensiometer-based irrigation scheduling and ~416 000 times less than the Health Advisory Level under farmer's practices. Simulation results suggested that current farmer's practices do not pose a threat to the area's groundwater quality 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. The model's calculated leaching fractions using farmer's practices were similar to measured leaching fractions in Doña Ana County, giving credibility to the use of simulation models for assessing and comparing the potential impact of different irrigation scheduling practices on environmental quality at a regional level

Threads and Boundaries: Rethinking the Intellectual History of International Relations

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