Impact Of Agricultural Activities On Regional Groundwater Variation

Shijiazhuang is a typical area that exploits groundwater to irrigate agricultural crops and gets high agricultural yield in North China Plain. At the same time, shallow groundwater has been over exploited in Shijiazhuang area. Therefore, this study selected Shijiazhuang Plain as the key research area to analyze the relationship between groundwater and agriculture. Groundwater exploitation increasing for agricultural irrigation is the dominant factor that induces shallow groundwater changing rapidly in Shijiazhuang Plain. So it is significant to identify and diagnose the impact of agricultural activities on variation of groundwater. On one hand, it can relieve the contradiction between agricultural producing and groundwater deterioration condition. On the other hand, it can promote reasonable utilization and conservation of regional groundwater. In order to identify and diagnose the impact of agricultural activities on groundwater, selecting the appropriate indexes of agriculture that can reflect groundwater variation is an important work.The objectives of this study is first to identify the critical elements of agricultural activities that can effect regional groundwater variation. It is a basic work to study the interacted relationship among precipitation and groundwater exploitation and groundwater table under the condition that precipitation changes and scale of agricultural production expands too. Then it is to reveal the influence characteristics and mechanism and effected present condition of agricultural elements variation on regional groundwater. Furthermore, based on the regional groundwater capacity and agricultural development condition, the goal of the study is to diagnose the impact of agricultural activities on regional groundwater and put forwards up the countermeasures.Based on the above objectives, this study makes Shijiazhuang Plain as the key research area, and does works of three aspects as following. First, it is to make clear of the impact characteristics and mechanism of precipitation and groundwater exploitation variation on groundwater buried depth in agricultural area. Second, it is to study the impact characteristics and mechanism of agricultural elements variation on groundwater exploitation and water table in agricultural area. Third, it is to diagnose the impact of agricultural activities on regional groundwater variation.Based on system theory, this study takes geological statistic methods as the main methods, and combines qualitative diagnosis (analysis) and quantitative identification (assessment) together. Firs t, this thesis studies the precipitation characteristics and analyses the adaptability between precipitation and agricultural producing in Shijiazhuang Plain. Second, according to the variation characteristics and evolutive process of precipitation and groundwater exploitation, this thesis studies the impact and mechanism of precipitation and exploitation changing on groundwater table. T hen this study chooses crops planting structure, planting area and crop yield as the main agricultural indexes, and study the relationship between them and groundwater. At last, this thesis diagnoses and analyses the impact of agricultural activities on regional groundwater variation.(1) Precipitation is concentrate in the months from June to September in the study area. During the main growing period of winter wheat which is the main water-consumption crop, the amount of precipitation is less than 20% of the total precipitation that year. As a result, precipitation here cannot meet the crop water demand, so precipitation is unsuitable for agricultural water demand. It determines that a lot of groundwater needs to be pumped every year for agricultural irrigation, for surface water is scarce in Shijiazhuang Plain. So it can be concluded that agricultural irrigation aggravates the condition of groundwater over-exploitation. However, the value of total precipitation from March to May every decade increases continuously during recent 50 years, and it slows down the aggravating condition of groundwater over-exploitation to some extent.(2) Precipitation does something on groundwater from two aspects. For on thing, it affects the amount of groundwater recharge. For the other, it drives groundwater exploitation for agricultural irrigation changing. That is to say, precipitation doses a double impact on groundwater variation. In wet years, groundwater recharge is rich, and it makes water table rising or extent of declining obviously slowing down. At the same time, with effective precipitation increasing, agricultural groundwater exploitation is reducing. So extent of groundwater table declining is slowed down. However, in dry years, the condition is obviously changed. Precipitation is small and water supplied for crop demand is less than wet years. So it drives agricultural groundwater exploitation increasing. As a result, the extent of groundwater declining is aggravated. The impact intensity of agricultural activities on groundwater is stronger from January to May each year, and it is relatively weak from June to December. Generally, the impact intensity is the strongest in March, the mean value of groundwater declining in March is 1.005m during the period from 1990 to 2006. And it is the weakest in October, the mean value of groundwater declining is 0.876m.(3) It is the agricultural groundwater exploitation that make regional groundwater table continuously drawing down. Besides, it is an important area of high grain producing. Agricultural water consumption accounts for about 80% of the total water used, and groundwater supply is over 80% of the total water supply. For recent 30 years, from 1975 to 2005, irrigated area in this study area has increased 121 thousand acres every year. The amount of water-drawing well increased to 192.9 thousand in 2005 from 67.5 thousand in 1975.(4) The impact of groundwater exploitation for agricultural use on regional groundwater is becoming stronger and stronger. But the impact of farmland factors variation on regional groundwater is becoming weaker and weaker. Before 1982, there is a slow-type linear correlation relationship between groundwater table and agricultural groundwater exploitation. After 1982, the declining rate of groundwater table with exploitation increasing is obviously bigger than the rater before 1982. In the period from 1956 to 1981, with changing of planting area and yield of water-consumption crops like wheat and maize, regional groundwater exploitation changes too. That is to say, planting area and yield of water-consumption crops increasing makes agricultural exploitation obviously increase. But after 1982, the linear relationship between agricultural exploitation and planting area and crop yield is becoming weaker. It means that increasing rate of agricultural exploitation with increasing of planting area and yield of water-consumption crops becomes smaller. It is closely relates to application of water saving technology and improving of its level.(5) First time using the method of combining projection pursuit model and MapGIS technology, this study diagnoses the impact of farmland factors variation on groundwater. The results can be used to guide the adjustment of agricultural planting structure.