Predicting Solute Transport Using Mapping‐Unit Data: Model Simulations versus Observed Data at Four Field Sites

Solute transport models may be used in conjunction with soils and climatic databases to predict the sensitivity of soil mapping units to leaching of agrichemicals. The primary objective of this study was to compare predicted solute transport obtained from several modeling approaches to observed data collected at four irrigated field sites throughout Montana. Observed solute distribution data as a function of soil depth was collected using Br− and a moderately sorbing herbicide, bromacil (5‐bromo‐3‐sec‐butyl‐6‐methyluracil), for three sampling dates over a period of 1 yr and used for evaluating different approaches for generating model predictions. The observed data were compared to predictions generated by two process‐based solute transport models, Chemical Movement in Layered Soils (CMLS) and Leaching Estimation and Chemistry Models (LEACHM), which differ in level of sophistication. Model predictions were obtained using inputs based on either detailed site characteristics (i.e., soil profile characterization) or the 1:24 000 scale USDA‐NRCS Soil Survey (SSURGO) database coupled with parameter estimation routines. Comparisons of observed and simulated transport showed that although LEACHM and CMLS frequently underestimated solute transport, both models performed similarly using either site‐specific inputs or SSURGO derived inputs. Model simulations using SSURGO soils data and 23 yr of historical climate data resulted in only slight variations in predictions of solute transport, suggesting that SSURGO inputs coupled with historic climate data provide a fairly robust approach for evaluating the sensitivity of mapping units to pesticide leaching out of the root zone.