Summary and Discussion

This paper examined the statistical relationship between subsurface tile drainage and nitrate-N concentrations in watersheds across Indiana. The results indicate that a strong linear relation exists between the flow-weighted nitrate-N concentration from nonpoint sources and tile drained area percentage. The relation is strong for the annual model and for monthly models from December to July. It is weaker but still statistically significant from September to October when the drain flow is low. Based on the relationship determined in this study, the watershed-scale nonpoint source nitrate-N load can be estimated based only on the percent of drained land in the watershed.

There are many uncertainties in the model, including the fact that only one factor was included in the regression, that the models were based on monthly data, and that other nonpoint sources such as atmospheric deposition also contribute to nitrate-N loads but were not included. Other studies have used annual flow or other additional variables in the regression models, which resulted in a stronger correlation coefficient but required more inputs that may not be available. In addition, the model is also limited by the uncertainty in the nitrate-N load estimates, due both to the infrequent (mostly once per month) sampling, and to the uncertainty of the measurements processes, as described by Harmel et al. (2006). Birgand et al. (2011b) found that the RMSE in calculating annual nitrate-N load using the linear-interpolation method and based on monthly sampling was 30% to 36%. Harmel et al. (2006) found that estimated uncertainty from the cumulative effects of measurement error is likely ranging from 8% to 69% for small watersheds. The point source estimates were limited by the fact that effluent concentrations for nitrate-N

were based on literature values since measured information is rarely available from permitted facilities.

These predictions could be used as a simple yet effective tool to estimate nitrate-N concentrations in unmonitored watersheds in Indiana and similar areas. If the estimated concentrations are converted to loads, they can be compared with target nitrate-N loads to provide a basis for estimating the amount and type of conservation activities needed to achieve water quality goals.

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