Modeling nitrogen loading in a small watershed in southwest China using a DNDC model with hydrological enhancements

摘要

The degradation of water quality has been observed worldwide, and inputs ofnitrogen (N), along with other nutrients, play a key role in the process ofcontamination. The quantification of N loading from non-point sources at awatershed scale has long been a challenge. Process-based models have beendeveloped to address this problem. Because N loading from non-point sourcesresult from interactions between biogeochemical and hydrological processes,a model framework must include both types of processes if it is to beuseful. This paper reports the results of a study in which we integrated twofundamental hydrologic features, the SCS (Soil Conservation Service) curvefunction and the MUSLE (Modified Universal Soil Loss), into a biogeochemicalmodel, the DNDC. The SCS curve equation and the MUSLE are widely used inhydrological models for calculating surface runoff and soil erosion.Equipped with the new added hydrologic features, DNDC was substantiallyenhanced with the new capacity of simulating both vertical and horizontalmovements of water and N at a watershed scale. A long-term experimentalwatershed in Southwest China was selected to test the new version of theDNDC. The target watershed's 35.1 ha of territory encompass 19.3 ha ofcroplands, 11.0 ha of forest lands, 1.1 ha of grassplots, and 3.7 ha ofresidential areas. An input database containing topographic data,meteorological conditions, soil properties, vegetation information, andmanagement applications was established and linked to the enhanced DNDC.Driven by the input database, the DNDC simulated the surface runoff flow,the subsurface leaching flow, the soil erosion, and the N loadings from thetarget watershed. The modeled water flow, sediment yield, and N loading fromthe entire watershed were compared with observations from the watershed andyielded encouraging results. The sources of N loading were identified byusing the results of the model. In 2008, the modeled runoff-induced loss oftotal N from the watershed was 904 kg N yr?1, of which approximately67 % came from the croplands. The enhanced DNDC model also estimated thewatershed-scale N losses (1391 kg N yr?1) from the emissions of theN-containing gases (ammonia, nitrous oxide, nitric oxide, and dinitrogen).Ammonia volatilization (1299 kg N yr?1) dominated the gaseous Nlosses. The study indicated that process-based biogeochemical models such asthe DNDC could contribute more effectively to watershed N loading studies ifthe hydrological components of the models were appropriately enhanced.