4000 60Or Daily fiow rate distribution (200 3000 ×≤ Figure 7 Annual flow in the main channel of the Weihe River from 1990 to 2002 and its daily flow rate distribution in 2001( data source: Department of Water Resources of Shaanxi Province, 2003) annual flow before 1993. The main reason for the declined annual flow is also an over Ithdrawal of water from the river channel for agricultural and industrial uses at the upper stream. In addition to this, through a detailed analysis of the daily flow rate distribution within one year(2001 as one example as shown by the inserted graph), it is understood that the daily flow rate fluctuates widely. Most of the runoff is concentrated in the period from September to October and there was apparently a dry period from March to July with very low or almost no daily flow With a very low flow for a long period of time within a year, the river almost loses its rated water quality in the main channel of the Yellow River and its tributaries, suce ability of self-purification and a serious problem of water pollution occurs. The deterio the Weihe River, is drawing public attention( Qian and Tang, 2004). In China, the quality of surface water has been classified into five categories according to its suitability for var- ious water use: Category I for headwater and national natural reserves; Category II for the protected area of first class source water for drinking water supply; Category Ill for the protected area of second class source water for drinking water supply; Category IV for industrial water supply; and Category V for agricultural water use( State Environmen- tal Protection Administration of China, 2002). In many sections of these rivers, the main water quality parameters, such as CODMn and NH3-N, fall into or even below Category V, and the water is unsuitable for any purpose of water use(Qian and Tang, 2004) Needs for water re-use In 2002, the total amount of wastewater discharge through the collection systems in China was 63. 1 billion m,, which was 11.5% of the total water withdrawal(549.7 billion m)or 35.8% of the non-agricultural water withdrawal (176.2 billion m). Of this amount, 38.8 billion m"(61.5%)was from industry and the remaining 24.3 billion m (38.5%) was from domestic wastewater discharge(Ministry of Water Resource, 2002) However, at present, only approximately 30%o of the domestic wastewater from cities and towns are treated. The government has set a goal of popularisation of domestic waste- water treatment of 45% by 2005. This will amount to approximately ll billion m per ear. If 1/ to In of the treated wastewater can be re-used, it can provide a supply of 3.5 to 5.5 billion m per year. Compared with the projected amount of water shortage in more than 400 cities at approximately 6 billion m per year(Zhang and Qian, 1997), the potential of treated domestic wastewater as a water resource is extremely largeannual flow before 1993. The main reason for the declined annual flow is also an over withdrawal of water from the river channel for agricultural and industrial uses at the upper stream. In addition to this, through a detailed analysis of the daily flow rate distribution within one year (2001 as one example as shown by the inserted graph), it is understood that the daily flow rate fluctuates widely. Most of the runoff is concentrated in the period from September to October and there was apparently a dry period from March to July with very low or almost no daily flow. With a very low flow for a long period of time within a year, the river almost loses its ability of self-purification and a serious problem of water pollution occurs. The deterio￾rated water quality in the main channel of the Yellow River and its tributaries, such as the Weihe River, is drawing public attention (Qian and Tang, 2004). In China, the quality of surface water has been classified into five categories according to its suitability for var￾ious water use: Category I for headwater and national natural reserves; Category II for the protected area of first class source water for drinking water supply; Category III for the protected area of second class source water for drinking water supply; Category IV for industrial water supply; and Category V for agricultural water use (State Environmen￾tal Protection Administration of China, 2002). In many sections of these rivers, the main water quality parameters, such as CODMn and NH3-N, fall into or even below Category V, and the water is unsuitable for any purpose of water use (Qian and Tang, 2004). Needs for water re-use Potential of wastewater as a water resource In 2002, the total amount of wastewater discharge through the collection systems in China was 63.1 billion m3 , which was 11.5% of the total water withdrawal (549.7 billion m3 ) or 35.8% of the non-agricultural water withdrawal (176.2 billion m3 ). Of this amount, 38.8 billion m3 (61.5%) was from industry and the remaining 24.3 billion m3 (38.5%) was from domestic wastewater discharge (Ministry of Water Resource, 2002). However, at present, only approximately 30% of the domestic wastewater from cities and towns are treated. The government has set a goal of popularisation of domestic waste￾water treatment of 45% by 2005. This will amount to approximately 11 billion m3 per year. If 1/3 to 1/2 of the treated wastewater can be re-used, it can provide a supply of 3.5 to 5.5 billion m3 per year. Compared with the projected amount of water shortage in more than 400 cities at approximately 6 billion m3 per year (Zhang and Qian, 1997), the potential of treated domestic wastewater as a water resource is extremely large. Figure 7 Annual flow in the main channel of the Weihe River from 1990 to 2002 and its daily flow rate distribution in 2001 (data source: Department of Water Resources of Shaanxi Province, 2003) X.C. Wang and P.K. Jin 41