Abstract: Aimed at the specific characteristics of agricultural non-point source pollution being difficult to monitor and control in irrigation areas,and based on the viewpoint of unit analysis,a concept of load contribution rate was put forward.On such a basis,a model was established for assessment of agricultural non-point source pollution load in irrigation areas.A case study of the Qingtongxia irrigation area was carried out making use of the quality monitoring data of irrigation and drainage waters in the irrigation year of 2005-2006.Annual mineralization degree and ammonia nitrogen,total nitrogen and total phosphorus loads of agricultural non-point source pollution were estimated by using the model.Comparison with the mean concentration method showed that the new method was more reasonable.The results show that the annual agricultural non-point source pollution in the Qingtongxia irrigation area outputted 283×104 t salt,0.55×104 t ammonia nitrogen,4.11×104 t total nitrogen,and 123 t total phosphorus.Compared against the water quality data of the control section of the Yellow River,the output of salt accounted for about 70% of the increment of salt in the mainstream of the river;and the load of ammonia nitrogen and total nitrogen was 0.28 and 1.52 times as much as the point source pollution load of the control section,which causes great impact on water quality of the trunk course of the Yellow River;however,the output of total P was quite limited and so was its impact.

Key words: irrigation area, agricultural non-point source pollution, pollution load, unit analysis, estimation method