| For a long time, the agricultural cultivation in the Yellow River Basin is mainly dependent on the irrigation with the water of the Yellow River. Ningxia irrigation district is the largest and also the oldest one of the irrigation areas along the Yellow River, which makes a historic contribution to and still plays an important role in feeding people and protecting ecological environment. However, with the increase of modern agricultural inputs, the influence of non-point source pollution in the environment caused by agricultural production and rural life is increasing. More serious agricultural pollution is created by a more intensive agricultural production and extensive farming format in Ningxia irrigation district. Agricultural drainage water in Ningxia irrigation district goes directly into the Yellow River, and the water quality is affected by agricultural non-point source pollution (ANPS) in irrigation area. It is of very important significance to calculate pollution load for the quantitative indicators of pollution and estimating the pollution.Based on the conclusion of domestic and abroad agricultural non-point source pollution research improvement, Johnes Export Coefficient Model with the feature of less-and-available data and simple calculation was selected. In this paper, we pointed out the importance of irrigation in the irrigated areas and added the irrigation factor into the model in order to make it more reasonable and suitable for irrigated areas. Then, the pollution loads of Nitrogen and Phosphorus in Ningxia irrigation district were estimated using the model. Through the analysis, the space-time distribution of loads of ANPS in Ningxia irrigation district since the 1990's was revealed; at the same time, the situation of ANPS in Ningxia in 2006 was evaluated using Equal Standard Pollution Load and Pollution Load Ratio. The main conclusions of this paper were as follows:1. The cultivated field, livestock breeding and living pollution were the main sources of Ningxia irrigation area. The TN and TP export coefficients of cultivated fields were 19.04 kg / (hm2 ? a) and 0.75 kg / (hm2 ? a); the TN and TP export coefficients of livestock excretion were 10.584 kg / (ca ? a) and 0.31 kg / (ca ? a), pigs were 0.568 kg / (ca ? a) and 0.142 kg / (ca ? a), sheep were 0.91 kg / (ca ? a) and 0.045 kg / (ca ? a), poultry were 0.03 kg / (ca ? a) and 0.005 kg / (ca ? a); the TN and TP export coefficients of human waste were 0.961 kg / (ca ? a) and 0.022 kg / (ca ? a), waste water were 1.325 kg / (ca ? a) and 0.16 kg / (ca ? a).2. The correlation function was established by analyzing the relationship between irrigation and the monitoring value of non-point pollution loads, and it showed an exponential positive correlation. In addition, the values of irrigation factor were confirmed: the irrigation factor of TN and TP loads in 1998 was respectively 1.07 and 1.22; 0.96 and 0.88 in 2006.3. According to the real situation of irrigation district, irrigation factor was added into the model, making that the results of export coefficient were closer to the calculated results of the actual ANPS situation in Ningxia. That was to say it was reasonable to modify the model. Using the modified model, the pollution load estimated results of TN and TP showed that: the relative error of the modified method respectively decreased 5% and 13% than the traditional method. The traditional method showed the pollution loads under annual average conditions so that annual variation was only influenced by land use. The improved model reflected not only the changes of the characteristics of land-use types in traditional model, but also the impact of irrigation amount changes on the TN and TP load from the point of view of irrigation.4. From the space-time distribution, since 1990's, the total amount of TN and TP load in Ningxia irrigation district showed a rising trend, which marked an visible upward trend in TN load; the pollution before 2000 was more serious than after 2000; both TN and TP loads were decreasing in 2003, which was resulted from that irrigation was less than other years. In the distribution around the city, the differences were obvious. In 2006, TN load showed Yinchuan > Wuzhong > Zhongwei > Shizuishan, and TP showed Yinchuan > Zhongwei > Wuzhong > Shizuishan; in addition, the pollution in Wuzhong showed an obvious rising trend, and Shizuishan and Zhongwei showed a flat.5. From the different pollution sources, since 1990's, in the three major sources of pollution, that was cultivation field, livestock and poultry farming and rural life, TN load of cultivation field was the largest, TP load of livestock and poultry was the largest, but TN load of livestock and poultry showed a rising trend in recent years; the pollution load ratio of cultivation filed, livestock farming and rural life were 36%, 43% and 20%, which can be seen that livestock and poultry was the most important pollution sources in Ningxia irrigation district in 2006, followed by cultivation filed, and the proportion of rural life was the lowest. |
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