Comparing soil pesticide movement for a finite-element model and field measurements under drip chemigation

Soil pesticide samples were collected from a field experiment where drip chemigation of the systemic insecticide, imidacloprid, was used to protect muskmelon (cantaloupe) plants from cucumber beetles. After the soil samples were analyzed and efficacy in controlling the insect pest was determined, the finite element model LEWASTE was configured to simulate: (1) drip chemigation of a pesticide in raised beds under plastic mulch, (2) advective/dispersive movement of pesticide from rainwater infiltration and drip irrigation, and (3) root extraction of pesticide with soil water. Only available parameters were input to the model domain without measuring specific soil parameters at the field site and then the model was executed to simulate soil distribution of pesticide during the period of efficacy. A linear regression of measured versus predicted pesticide concentration in the soil indicated a strong relationship between the model and the field study with a slope of 0.89, an intercept of -7.05 ppb and an r2 of 0.97. The model and soil samples also showed that the pesticide, imidacloprid, did not leach during the time frame of evaluation and that 70% of the pesticide was still located in the root zone when efficacy ceased. In addition, the model calculated that only 11% of the applied pesticide was absorbed by the muskmelon plants during the period of efficacy. These results suggest that as the plants developed from 9 to 800 g, the muskmelons outgrew the source of the pesticide instead of the pesticide source being depleted for use by the plant via leaching, uptake, and degradation. Numerical modeling of drip chemigation can help researchers understand crop production factors such as the efficacy of a pesticide in addition to the fate and transport of pesticide.