Nutrient runoff change in a rural catchment in South Estonia.
Järvet, Arvo; Mander, Ülo; Kull, Ain; Kuusemets, Valdo
The purpose of this study was to (i) analyse trends in the runoff of organic matter (by BOD5) and nutrients from the medium-size (256 km2) rural watershed of Ohne River in southern Estonia, and (ii) model variations in nitrogen and phosphorous runoff. Chemical and hydrological monitoring data between 1980 and 2000 were used in this study. Due to a large and unprecedented decrease in agricultural production and land-use in the Ohne River catchment losses of nutrients and organic matter have decreased significantly. For example arable land decreased from 27 % to 16 % during the period 1980-2000. The results from this study showed that the runoff of NH4-N, NO3-N, P04-P and organic matter (after BOD5), calculated by linear trend, decreased from 1.50 to 1.05, 2.89 to 2.08, 0.111 to 0.053, and 7.73 to 5.29 kg ha-1 yr-1, respectively. To analyse temporal changes of the studied trends, the whole research period was divided into two equal parts: (1) 1981-1990 and (2) 1991-2000. The results of calculations show that the mean annual runoff of NH4-N, NO3-N, PO4-P and BOD5 from the Ohne River catchment in 1991-2000 was 28.4, 27.5, 35.7 and 21.7 % smaller than in the period 1981-1990. In the same time, a significant trend in water discharge was not observed - annual mean specific runoff decreased from 1981 to 2000 only by 0.05 l s-1 km-2. A simple empirical model, which incorporates land-use pattern, fertilization intensity, soil parameters and water discharge index, was used to describe variations in total-N and total-P runoff. This model is recommended to simulate annual nutrient flow in small and medium-size catchments with an area approximately between 50-500 km2, and medium diffuse pollution level (5-50 kg N ha-1 yr-1 and 0.1-2 kg P ha-1 yr-1) without remarkable point pollution sources.