K.H. Tiedemann (Canada)
Time-series models, least squares regressions, maximum likelihood regressions, nitrogen levels, and fertilizer management
This study provides an analysis of the determinants of nitrogen concentration levels and nitrogen flux in the lower Yangtze River. The study has five main findings. First, the Government of China has subsidized the use of commercial and organic fertilizer and irrigation water to increase yields, but it has paid less attention to the impact of increased nitrogen loadings on water quality. Second, annual fertilizer use per hectare has increased from an average of 84.8 kg for 1963-1969, to 125.8 kg for 1970-1979, to 223.1 kg for 1980 to 1989 and to 341.9 kg for 1990 to 1997. Third, average nitrogen concentration levels and average annual nitrogen fluxes have risen respectively from 0.30 mg/L and 0.28 million tonnes in 1963-1969, to 0.31 mg/L and 0.27 million tonnes in 1970-1979, to 0.80 mg/L and 0.73 million tonnes in 1980-1989 and to 1.50 mg/L and 1.30 million tonnes in 1990-1997. Fourth, agriculture nitrogen concentrations and nitrogen fluxes in the lower Yangtze River are significantly driven by fertilizer loadings and by precipitation runoff. Fifth, initial ordinary least squares modeling suggested the presence of auto-correlated residuals, and this was successfully dealt with using a first-order auto-regressive scheme estimated by maximum likelihood methods.
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