Following release of pesticides into the environment, they undergo a series ofphysical and (bio)chemical processes. Leaching in the vadose zone and further downto an underlying aquifer is one of the primary pathways of pesticides, which inducescontamination of the subsurface environment. A variety of mathematical models havebeen developed for simulating pesticide fate and transport in the vadose zone. Theyhave various limitations and thus they are suitable for specific conditions and researchpurposes. Their performance can be evaluated theoretically by analyzing the errorsintroduced from modeling assumptions and/or numerical approximations. In addition,they can be evaluated by comparing their simulations against the observed data. Acritical 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 specificspace/time point. A model, however, often provides an averaged level over a spacevolume during an interval of time. Sometimes, the representative volume can be toolarge to capture the real spatial variability in pesticide exposure levels. Worse of all, amodel may average over a much larger space domain far beyond the range pesticidescan 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 incorrectenvironmental assessment conclusions. In this study, a Windows-based, integratedpesticide transport model (IPTM-CS) is used for simulating three-phase pesticide fateand transport in the vadose zone. Various spatial discretization schemes are examinedand the corresponding simulations are compared. Particularly, effects of the spatialand temporal modeling scales on the model performance are discussed. It isconcluded that special caution should be taken in comparison and calibration ofpesticide transport models and different evaluation criteria should be used, dependingon their time and space scales.
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