Assessment on leading factors for dropping of shallow ground water level

The economy in the North China Plain is developed, with a dense population and a lot of ground water exploration. The shortage of water resources has become a bottleneck for sustainable development in the area. Since the 1960s, the shallow ground water level has been dropping in the North China Plain, the dropping amplitude in the sub-mountain plain area is larger than the amplitude in the central and eastern area, and more than 10 ground water level dropping funnels have been shaped. In accordance with monitoring in the year 2005, the water level buried depth at the center of the funnel was 40∼50m. The area with water level lower than “0”m is 19.8%. Increased exploration of ground water, less precipitation and reduced direct runoff are induced factors for dropping of the ground water level in the area. In the 1970s, the ground water exploration was 15.657×10 9 m 3 /a,and 18.524×10 9 m 3 /a in the 1990s. The exploration strength in the sub-mountain area is high, namely: Beijing 400.9×10 3 m 3 /(a·km 2 ), Shijiazhuang 258.3×10 3 m 3 /(a·km 2 ), Baoding 234.8×10 3 m 3 / (a·km 2 ); average precipitation in the North China Plain from 1956∼2003 was 558.6mm/a and from 1991 ∼ 2003 the average was 513.2mm/a, which has decreased by 8.1%. The decrement in the sub-mountain plain cities have decreased more, for example, since the 1950s, in Beijing and Shijiazhuang, the precipitations in the following 30 years have decreased by 16.1% and 9.1% than precipitations in the earlier 30 years; the surface water retention and water consumption out of the river course to reduce direct runoff, the surface water retention project retains 83.5% runoff in the mountain area, and average water inception in the river course in the plain area is more than 300 days per year. Leading factors for the dropping of the ground water level are assessed with a relative analysis method, statistics method and the water balance method of the Gray Theory, and the following was discovered: ➀ relevancies of the ground water level dropping to the ground water exploration quantity, precipitation and river leakage changes are 0.79, 0.69 and 0.61, and it can be seen that the exploration quantity is related in maximum and is a major influencing factor; ➁ relevancies of the ground water buried depth change to the accumulated ground water exploration quantity, accumulated precipitation and surface water accumulated leakage are 0.97, 0.92 and 0.88, which means that the leading factor for dropping of the ground water level is ground water exploration, the secondary factor is less precipitation and the last is that the surface water retention plays a certain role; ➂supply and drainage modules of 1956∼1984 series and 1991 ∼ 2003 series are −2.1×10 3 m 3 /(a·km 2 ) and - 17.1×10 3 m 3 /(a·km 2 ), with a contribution rate of the explored ground water to the supply and drainage for 1956∼1984 series of 52.1% and contribution rate of the precipitation to the supply and drainage of 47.2% while the contribution rates for 1991∼2003 series are 53.7% and 40.8%. Ground water exploration is the leading factor, contributing more to the dropping of the ground water than precipitation. Further, according to quantified assessments, ground water exploration is the leading factor for the dropping of the ground water level.