An Analysis of the Pollutant Loads and Hydrological Condition for Water Quality Improvement for the Weihe River For implementing water resources management in the Weihe River basin, water resources assessment is firstly necessary and important. The accuracy of water resources assessment relies on predictability of the hydrological cycle. Different land use, topography, geology and soil conditions, and artificial water uses (mainly agricultural irrigation) determine the complexity of hydrological characteristics in this basin. This research attempts to incorporate all available spatial information into the hydrological modeling by a distributed approach. Introduction Results & Discussion Figure 2 summarizes the results of water quality monitoring records of 2001 and 2002 along the main river channel. The dashed line in the figure indicates the guideline values of COD Mn and NH 3 -N for surface water which is considered to be tolerable with its quality as a public water body. Regarding CODMn, except for the first 4 locations in the upper rural area, the annual average values are much higher than the tolerable level (15 mg/L), showing serious contamination by organic pollutants especially in the most densely populated area (location number 7 and 8). Regarding NH3-N, even in the upper rural area, the annual average values (location number 2 and 3) are 4 to 8 times the tolerable level (2.0 mg/L), indicating an intensive source of agricultural pollution in that area. Conclusions Weihe River is the biggest tributary of the Yellow River - the second longest river in China. Figure 1 shows the map of the Weihe River basin and its location. With a total length over 810 km, the Weihe River covers a basin area of 134,000 km 2. The middle and lower basin area is called the Guanzhong Basin, one of the richest agricultural areas in China from the ancient time where the famous ancient capital city Xi’an is located. Nowadays with the fast urbanization of this area, population has increased quickly, and human activities have greatly influenced the water environment. Fig. 2 Water Quality Monitoring Results of COD Mn and NH 3 -N at 13 Locations Along the Weihe River (Annual averages of 2001 and 2002) Fig. 3 Variation of Annual Flow of Weihe River from 1990 to 2002 Constructing a system on efficient water utilization is an urgent need in this region. Furthermore, to apply this integrated model for water resources assessment and management in reference to the land use change and/or climate variation. The available water in 2050, which is set drought season, is estimated about 39 billion tons. This value is less than the water demand in the Weihe basin, so it is urgent need to introduce saving-water systems in this region. Fig. 1 Location Map of the Weihe River Basin Weihe River Basin Water Quality of Weihe River Variation of Annual Flow of Weihe River The maximum capacity of the Weihe River to receive pollutants but without causing a water quality lower than the tolerable level was estimated. The ability of the river water to dilute the incoming pollutants and its capability to perform self-purification were taken into account. If the low flow rate of 2001 is used for water quality control, the estimated capacities of the river to receive COD Mn and NH 3 -N are 4.1 x 10 4 and 5.5 x 10 3 kg/day, respectively, which are only about 14% and 8% of the present loads. This means that a further decrease of 86% of COD Mn and 92% of NH 3 -N loading would have to be considered. However, it is almost impossible to fulfil such a task from both the economic and technical standpoints, because some of the non-point sources, such as those from agricultural activities, would be very difficult to control. Even if all the possible measures were taken to reduce the pollutant loads from all the point sources to the attainable extent, the river water quality could only be improved to the tolerable level in accordance with the mean daily flow rate, i.e. a guarantee of water quality for merely half of the time within one year. The sudden decrease of the annual total flow from 1994 was partially caused by the accomplishment of a large scale irrigation system in the upper stream agricultural area, where an amount of 400 to 600 MCM/yr water has been regularly diverted from the river channel. The diverted amount is even larger in the dry years as the demand of irrigation water becomes bigger. This results in a great decrease of the total annual flow to the downstream, and also a much greater decrease in the low daily flow rate and, because the rate of water withdrawal from the river channel is often greater in the dry season. Hydrological analysis result shows that if measures can be taken to regulate the manner of water diverting and especially to restrict water withdrawal in the low water season, the uneven daily flow rate distribution as shown in Figure 3 can be much improved. The mean daily flow rate can be doubled and the low daily flow rate can be increased by 3 to 4 times. With a larger base flow in the river channel to increase its ability to dilute the pollutants, the task of pollutant load reduction, as discussed above, can be much lessened. The total annual load of COD Mn is estimated as about 1.57 x 10 8 kg, and that of NH3-N as 3.7 x 10 7 kg. In addition to the high pollutant loads, the river is suffering from a decline of its annual flow as is shown in Figure 3. Before 1993 the annual total flow had been around 3,000 million cubic meters per year (MCM/yr), while after 1994 it dropped to less than 1,000 MCM/yr. Within each year, the daily flow rate fluctuates widely. Taking the 2001 record as example (Figure 3), most of the runoff was concentrated in a period from September to October and there was apparently a dry period from March to July with very low daily flow. A hydrological analysis was conducted regarding the annual variation of the daily flow in The maximum, minimum, and mean (corresponding to a probability of 50%) daily flow rates are 501, 1.8 and 14.6 m 3 /s, respectively, showing a very uneven distribution. If the low flow rate corresponding to a probability of 75% is considered, it is only 6.41 m 3 /s.