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Non-linear response of a mixed land use watershed to nitrogen loading

Nikolaidis Nikolaos, H Heng, R Semagin, J.C Clausen

Πλήρης Εγγραφή

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Περίληψη

Nonpoint source pollution comprises a significant portion of the nutrient loads that adversely affect the water quality of freshwater and marine ecosystems. Current research indicates that reductions in nonpoint source loading of nitrogen to a basin should be considered in management decisions if significant nitrogen export reduction is to be achieved. Managers need to know the percent reduction in nitrogen loading necessary to achieve a desired reduction in the export. What makes this task difficult to achieve is that watersheds respond non-linearly because of the nature of hydrogeochemical processes that operate in a spatially variable land use, soils, geology and vegetation environment. The non-linear response of a watershed to nitrogen loading has been addressed through field monitoring and mathematical modelling. This study included the monitoring of a small, multiland use watershed for two years, the development of a mathematical model that can simulate the movement of nitrogen and the evaluation of the non-linear response of the watershed to nitrogen loadings. The site was the Muddy Brook watershed located in north-central Connecticut, U.S.A. The drainage area is 360 ha. The watershed land use consists of 54% unmanaged forest, 23% residential areas, 12% grass and hay lands and 11% maize fields. The Nutrient Transport and Transformation (NTT) model was used to assess the fate and transport of nitrogen in the watershed. The model is a distributed code that can simulate both the hydrology and chemistry of watersheds. Two management scenarios were simulated to assess the non-linear response of the Muddy Brook watershed. The first scenario simulated the possibility of reducing the nitrogen loading generated from a residential area by eliminating the septic tank input. The results indicated that there is going to be a 25% reduction in nitrogen export from the watershed if that scenario was followed. This dramatic reduction in export primarily results from the residential development being very close to the stream and there is a preferential flow that short-circuits the groundwater to the stream. On the other hand, an 100% reduction in fertilizer and manure input to the agricultural land will only result in less than 1.5% reduction in nitrogen export after a three year of simulation period. These simulations illustrate the non-linear nature of the Muddy Brook watershed response to nitrogen loadings and can be used to guide managers in selecting appropriate best management practices to improve the water quality of the stream.

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