Table 1 Water resources in the main hydrological zones in China (as of 2002) Basin Main river(s) Basin area Population Annual Water resources 1,2484512331 (billion m) Per capita 000 m per and l Heilongjiang 31113.4145.34 ver. Liao river 318.1612845 400 15.9 123.78 125.72 474 48.95 185. ver1,808.50440.591,1761,089.12471.899301 Zhu river 58064155431,708 525.1 146.5 VIll Southeast rivers 239.80 71.93 1,862 2314 133.03 Southwest rivers 851,41 20.55 564.127 3.374442698 187 145.7 17.04 Table 2 Water withdrawals in the main hydrological zones in China (as of 2002) tal(billion m) Domestic (96) Industry (96) Agriculture (96) I and Il 18.6 72.1 41.22 129 38.9 769 8207 612 73.1 168.2 28 555 15.44 645 16.21 X 10.3 1.83 75 93.9 3946 Low annual rainfall. As can be seen from Table l, all the north river basins(Zones IV and X) have very low annual rainfall. The inland rivers basin(Zone X), which covers the desert and Gobi area at the far northwest region, has the lowest annual rainfall at only 187 mm, but because of the vast basin area and relatively small population, the calculated per capita water resource is not low. Another parameter to affect the accessibility of water resource is water resource in unit area. From the last column of Table I it can be seen that the water resource per km- in the inland rivers basin is only approximately 17, 000 m, showing the difficulty in access for wate Uneven distribution of daily rainfall. Another important feature of rainfall in north China is the uneven distribution of daily rainfall within I year. Figure 3 is an example of distribution of recorded daily rainfall in 2002 at Xi'an, a central city in the Yellow River basin. The total annual rainfall in 2002 was 406 mm and the number of days with measurable rainfall(daily rainfall larger or equal to 0. I mm) was 76 days. Among them, the largest daily rainfall was recorded as 75.2 mm on the 9th of June, which was 18.5% of the annual rainfall. The days with rainfall larger than 20 mm were 5 days with a total of 183. 5 mm or 45.2% of the annual rainfall, those with rainfall larger than 10 mm were 12 days with a total of 280.5 mm or 69% of the total annual rainfall, and those with total annual rainfall. The other 43 rainy days were all with a rainfall of less than 2 mm. Such types of uneven distribution may result in intermittently concentrated runoff duringCharacteristics of annual rainfall in north China Low annual rainfall. As can be seen from Table 1, all the north river basins (Zones I– IV and X) have very low annual rainfall. The inland rivers basin (Zone X), which covers the desert and Gobi area at the far northwest region, has the lowest annual rainfall at only 187 mm, but because of the vast basin area and relatively small population, the calculated per capita water resource is not low. Another parameter to affect the accessibility of water resource is water resource in unit area. From the last column of Table 1 it can be seen that the water resource per km2 in the inland rivers basin is only approximately 17,000 m3 , showing the difficulty in access for water. Uneven distribution of daily rainfall. Another important feature of rainfall in north China is the uneven distribution of daily rainfall within 1 year. Figure 3 is an example of distribution of recorded daily rainfall in 2002 at Xi’an, a central city in the Yellow River basin. The total annual rainfall in 2002 was 406 mm and the number of days with measurable rainfall (daily rainfall larger or equal to 0.1 mm) was 76 days. Among them, the largest daily rainfall was recorded as 75.2 mm on the 9th of June, which was 18.5% of the annual rainfall. The days with rainfall larger than 20 mm were 5 days with a total of 183.5 mm or 45.2% of the annual rainfall, those with rainfall larger than 10 mm were 12 days with a total of 280.5 mm or 69% of the total annual rainfall, and those with rainfall larger or equal to 2 mm were 33 days with a total of 376 mm or 92.4% of the total annual rainfall. The other 43 rainy days were all with a rainfall of less than 2 mm. Such types of uneven distribution may result in intermittently concentrated runoff during Table 1 Water resources in the main hydrological zones in China (as of 2002) Basin Main river(s) Basin area (000 km2 ) Population (million) Annual rainfall (mm) Water resources Total (billion m3 ) Per capita (m3 /person) 000 m3 per km2 I and II Heilongjiang River, Liao River 1,248.45 123.31 460 137.3 1113.41 45.34 III Hai River 318.16 128.45 400 15.9 123.78 125.72 IV Yellow River 794.71 109.19 404 47.4 434.13 48.95 V Huai River 329.21 208.09 726 69.5 333.98 185.90 VI Yangtze River 1,808.50 440.59 1,176 1,089.1 2,471.89 93.01 VII Zhu River 580.64 155.43 1,708 525.1 3,378.41 146.56 VIII Southeast rivers 239.80 71.93 1,862 231.4 3,216.85 133.03 IX Southwest rivers 851.41 20.55 1,073 564.1 27,446.81 12.10 X Inland rivers 3,374.44 26.98 187 145.7 5,401.27 17.04 Table 2 Water withdrawals in the main hydrological zones in China (as of 2002) Basin Water withdrawals Total (billion m3 ) Domestic (%) Industry (%) Agriculture (%) as a % of water resource I and II 56.6 9.3 18.6 72.1 41.22 III 40 12.9 15.5 71.6 251.57 IV 38.9 9 14.1 76.9 82.07 V 61.2 11.7 15.2 73.1 88.06 VI 168.2 12.8 31.7 55.5 15.44 VII 85.1 14 21.5 64.5 16.21 VIII 31.9 14 28.2 57.8 13.79 IX 10.3 9 6.2 84.8 1.83 X 57.5 3.4 2.7 93.9 39.46 X.C. Wang and P.K. Jin 38