Space and Time Issues in Comparison and Evaluation of Different Pesticide Transport Models in the Vadose Zone

Following release of pesticides into the environment, they undergo a series of physical and (bio)chemical processes. Leaching in the vadose zone and further down to an underlying aquifer is one of the primary pathways of pesticides, which induces contamination of the subsurface environment. A variety of mathematical models have been developed for simulating pesticide fate and transport in the vadose zone. They have various limitations and thus they are suitable for specific conditions and research purposes. Their performance can be evaluated theoretically by analyzing the errors introduced from modeling assumptions and/or numerical approximations. In addition, they can be evaluated by comparing their simulations against the observed data. A critical issue is how to compare them. Field samples are often space and time specific. In other words, a "grab" sample in the field represents the pesticide level at a specific space/time point. A model, however, often provides an averaged level over a space volume during an interval of time. Sometimes, the representative volume can be too large to capture the real spatial variability in pesticide exposure levels. Worse of all, a model may average over a much larger space domain far beyond the range pesticides can reach. In such a case, the model actually "speeds up" the transport of pesticides. Thus, an improper selection of the spatial and temporal scales may lead to incorrect environmental assessment conclusions. In this study, a Windows-based, integrated pesticide transport model (IPTM-CS) is used for simulating three-phase pesticide fate and transport in the vadose zone. Various spatial discretization schemes are examined and the corresponding simulations are compared. Particularly, effects of the spatial and temporal modeling scales on the model performance are discussed. It is concluded that special caution should be taken in comparison and calibration of pesticide transport models and different evaluation criteria should be used, depending on their time and space scales.