Sustainable Development and Utilization of Groundwater Resources in Arid Areas of Karst Plain —— A Case Study of Litang Demonstration Zone in Guangxi

Tang Jiansheng 1, 2, Xia Riyuan 1, 2, Li 1, 2, Xu Yuanguang 2, Lan Funing 2.

China Geo University (Beijing), Beijing100083; 2. Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 54 1004)

Supported projects: National Science and Technology Key Project 2002Ba901A13; China Geological Survey Project 200310400043; Guangxi science and technology research 0 13300 1- 1.

About the author: Tang Jiansheng, male (1957—), Yao nationality, researcher of Institute of Karst Geology, Chinese Academy of Geological Sciences; Ph.D., School of Water Resources and Environmental Engineering, China Geo University. Mainly engaged in the research of karst resources and environment.

Based on the investigation and data analysis of water and soil resources distribution, the basic elements, allocation characteristics, occurrence and development conditions of karst water resources in karst plain area are analyzed. The characteristics of surface and underground double-layer hydrological network and strong karst development in karst plain are revealed, that is, groundwater hydrological network and surface hydrological network have high consistency in basin boundary and water flow direction, plain area is flat, hydraulic gradient is gentle, and the thickness of underground karst layer is small. Under the influence of old and new tectonic activities, a network of karst media system with water conduction and water storage has been formed. Karst aquifer has powerful water resources storage and regulation functions. In dry season, when the surface water is exhausted or cut off, the groundwater level drops by 5 ~ 8m, and the water resources stored in shallow karst aquifer (within 20m) can reach 300× 104m3/km2. By pumping groundwater and increasing the water level 1m, 253× 1.04 m3 more water can be pumped in the karst plain area of Litang groundwater system, which is equivalent to adding two small (one) reservoirs. On the basis of demonstration tests, the measures for sustainable development and utilization of water resources in karst areas are put forward, such as joint development of surface water and groundwater, full development and utilization of the storage function of karst underground aquifers, use of dispersed shallow wells to avoid geological disasters, popularization of water-saving technology and improvement of water conservancy utilization rate.

Central Guangxi is characterized by bare shallow karst plains, which mainly include peak forest plains, solitary peak plains and peak forest canyons. The surface of karst system is extremely karst, and various karst voids constitute a complex medium structure system with rapid leakage and transfer of surface materials and energy. As a result, the topsoil is shallow and barren, there are few plants, the soil structure is loose, the water-holding and water-retaining ability is poor, and the regulation function is weak. As a result, under the conditions of humid monsoon climate zone in the south subtropical zone and abundant rainfall (average rainfall 145 1mm), there are still serious difficulties in using water for farmland and drinking water for people and livestock, and droughts occur frequently. According to the statistics over the years, there are droughts in different degrees in central Guangxi almost every year [1]. Among them, the drought-affected area in the largest drought year (1963) reached 29.3×104hm2; ; For five consecutive years (1988 ~ 1992), a large area of drought occurred, with an average annual drought-affected area of 2 1.9× 104hm2, accounting for 48% of the total cultivated land area. Drought has a very serious impact on agricultural production, which is rare in history. The average annual grain yield reduction is18.34×, which shows that frequent droughts have seriously restricted the economic development of this area. Groundwater in karst plain area is widely distributed, with large storage capacity, strong water storage capacity and low investment in water supply [2]. Aiming at the comprehensive management of karst arid areas, this paper puts forward some technical approaches to comprehensively manage the agricultural ecological environment in karst arid areas, starting from the effective utilization of karst water resources and the rational utilization of land resources, implementing the joint deployment and development of surface water and groundwater, adjusting the allocation of water resources and land resources and the agricultural production structure. Typical demonstrations were carried out by using comprehensive management technologies such as water diversion and water lifting, water source regulation and storage, water-saving irrigation, soil improvement, high-efficiency agriculture and ecological reconstruction, and projects such as "comprehensive management demonstration of dry patches in central Guangxi" and "typical hydrogeological investigation and demonstration of groundwater resources in karst peak forest plain in central Guangxi" were organized. This paper summarizes some empirical research results.

1 general situation of geography and geological environment

1. 1 physical geography and hydrology

Litang Demonstration Zone is located in the east of Binyang County, Guangxi (Figure 1), east longitude10902 ′ ~10918 ′, and north latitude 23 04 ′ ~ 23 20 ′. There are railways in the area, with Litang as the hub, connecting Liuzhou in the north, Nanning in the west and Guigang and Zhanjiang in the southeast. Highway traffic network criss-crosses, Liunan Expressway runs through this area in the north-south direction, and Nanwu high-grade highway runs through this area in the east-west direction, with convenient transportation. The distribution area of karst area accounts for 66.86% of the land area, and the landform is karst butte plain and karst hilly area. Located in the south of the Tropic of Cancer, it belongs to the subtropical monsoon climate zone, with the annual average temperature of 265,438 0.0℃, the highest temperature of 40.0℃ in July, the highest temperature of 65,438 0.0℃ in Leng Yue and the lowest temperature of 0.5℃. The annual precipitation in Litang is 1, 200 ~ 000 mm, and the average precipitation for many years is 1, 584 mm The monthly precipitation in the year is unevenly distributed, mostly from May to September, accounting for more than 76% of the annual rainfall.

Figure 1 Traffic Location of Litang Demonstration Zone

Xinbujiang is the main river in this area, with a total length of 25 kilometers. Hongshui River in the Pearl River Basin is a secondary river of Qingshui River, the first tributary of Hongshui River Basin, with an average hydraulic gradient of 3‰, which turns from northeast to southwest and flows into Qingjiang River in the north. In addition to the main stream, there are tributaries of Xijiang River, which are intermittent, and the dry season is generally from June 165438+ 10 to March of the following year. In normal season, the river flow of the main stream is about 4 ~ 6 m3/s, and the main recharge sources are atmospheric precipitation and groundwater.

The terrain of the demonstration area is open and flat, the lithology of bedrock stratum is Devonian-Carboniferous carbonate rock, and underground karst is extremely developed. The buried depth of groundwater level is generally less than 10m, and the annual available resources are 25.15×104m3/a km2, which is mainly used for the industry and residents' life in Litang town. The water conservancy projects in this area are aging and most of them have been abandoned. Most of the farmland water is pumped by farmers themselves from Xinbu River for irrigation, and most of it is flooded, so the utilization efficiency of water resources is very low. The areas far away from rivers are "Wang Tiantian" and dry land, and the problem of drought and water shortage is serious.

1.2 geological environment

The karst aquifer in this area consists of carbonate rocks from the upper member of Yujiang Formation of Lower Devonian to Upper Carboniferous. Clastic rocks in the lower member of Yujiang Formation from Cambrian to Lower Devonian are mainly distributed in non-karst areas, Cretaceous clastic rocks are distributed in the shallow part near the surface of some sections, and Quaternary rocks are locally distributed, with the thickness of 1 ~ 10m. The main occurrence space of groundwater in this area is carbonate karst cave pipeline-fractured medium.

Limestone and dolomite are mainly distributed in this area, and structural cracks are developed. According to the data of lithology, structure, karst development degree and hydrogeological phenomenon, the water-bearing rock groups are divided into rich aquifer, medium aquifer, weak aquifer and relative aquifuge. The water-rich aquifer includes Maping Formation limestone, Datang Stage limestone mixed with dolomite, lower part of Rongxian Formation limestone and middle part of Donggangling limestone mixed with dolomite. The medium aquifer is limestone dolomite of Huanglong Formation, upper dolomite of Rongxian Formation, upper limestone dolomite of Donggangling and lower limestone dolomite. The weak aquifer is siltstone of Lianhuashan Formation and impure limestone with dolomite in the upper part of Liujiang Formation. Relative water-resisting layer includes Liujiang Formation siliceous rock, Yujiang Formation shale and Changling Formation mud shale. Underground karst is very developed. According to the data revealed by drilling, there are caves with different sizes in the underground 10 ~ 88m, and the largest one is 7 ~ 10~88m, which creates favorable conditions for the storage and enrichment of groundwater.

Structurally, this area is located at the top of Guangxi mountain arc and belongs to the north wing of anticlinorium in Zhenlongshan. The strike of strata is generally NE-SW, with NW dip angle of 20 ~ 40. Because the F5 fault in Central Lingcun-Guang Zhi is affected by a group of NE-SW trending reverse faults, the strata are missing and repeated. Along with the NW-SE horizontal inference layer generated by this group of faults, the central and western strata have been displaced in different degrees along the strike.

According to the statistical data of various points near Litang Town, two groups of high-angle (above 50) tensile cracks and shear cracks are mainly developed, which are 310 ~ 340 and 50 ~ 90. Generally, the width is 0.0 1 ~ 0. 15m, the length is several meters, and the visible depth varies from 0. 1 ~ 1m, mostly filled with cuttings and clay. After long-term dissolution of groundwater, some fractures are interconnected to form water-rich karst channels.

Controlled by structure and stratum, the southeast is a low hill composed of clastic rock series of middle and lower Devonian, with an altitude of 250 ~ 650 m. The valley develops in a fan shape to the northwest and northeast, and seasonal water flows out of the supplementary plain area. The eastern part is a peak forest valley composed of carbonate rocks from Middle Devonian to Carboniferous, with an altitude of 2 10 ~ 320m, exposed bedrock, and depressions and funnels developed in the middle, which is the main area for atmospheric precipitation to replenish groundwater. Gu Feng plain is in the west and north, with an altitude of 90 ~ 105 m, and the cover is very thick. There is a local water-resisting layer composed of siliceous rocks of Liujiang Formation of Upper Devonian between the northern plains, which extends from northeast to southwest, blocking the hydraulic connection of groundwater runoff area in Litang Plain, forming a water-rich section from Linshan to Baishuitang in the south and from Wuya to Shilong in the north.

2. Accumulation conditions of karst groundwater

2. 1 characteristics of karst groundwater storage construction

Fig. 2 density distribution map of karst aquifer interpretation by electrical logging in Litang area

(According to the electrical prospecting data in Litang area of Guangxi Hydrogeological Engineering Team)

Litang area is located in the front arc of Guangxi mountain structure, and the main control structure is the Neocathaysian structural system. With the evolution of Mesozoic-Cenozoic regional structures, under the control of two groups of structures, namely, NNE or NE and NWW or NW, a carbonate monocline water storage formation was formed. The development of karst caves and dissolution fractures in karst aquifers is closely related to two groups of structures, and the development direction of dissolution fractures is similar to the distribution direction of structural fractures. Karst pipelines or underground rivers in this area are mostly developed along the NNE or NE fault zone or stratum contact zone, forming the main runoff zone where groundwater migrates from northeast to southwest, and NWW or NW-trending corrosion cracks often become the main way for regional groundwater to collect in the main runoff zone. Geophysical prospecting results show that the apparent resistivity of karst development zone is low. In the relatively low apparent resistivity area in Litang area, it is not only distributed along the main fault zone, but also closely related to the karstification of two groups of structural fractures in this area, forming a network pattern in the area (Figure 2), thus forming a network system of karst groundwater accumulation and migration.

In the vertical direction, the distribution of groundwater storage space depends on the karst development degree at different depths and is influenced by the regional erosion datum. According to the analysis of drilling data, the bedrock thickness in Gu Feng Plain is generally 4 ~ 9m, with local outcrops [3]. Underground karst can be divided into strong development zones with depth, and the development depth is less than 20m. The depth of strong development zone is 20 ~ 60m, the depth of moderate development zone is 60 ~ 90m, and the karst weak development zone is below 90m. It can be seen that the karst water in this area mainly exists in the karst water-bearing medium space within 60m underground.

2.2 Karst water dynamics

The observation results show that in the karst water system, from recharge area to runoff area and then to catchment area, the groundwater depth becomes shallow and the variation range of groundwater level becomes smaller (see the table below). The karst plain area has a gentle terrain, generally 3 ~ 5m. Even if it is superimposed with water level fluctuation, the buried depth of groundwater in dry season in this area is generally less than 10m, mostly 2 ~ 6m, and the time lag of water level change after rainfall is short. During the wet season, groundwater is exposed to the surface, forming dissolution pools, lakes or karst lakes and seasonal springs, which lays a material foundation for popularizing shallow well irrigation technology.

Table of Dynamic Observation Results of Karst Groundwater in Xinbujiang River Basin of Litang

Note: This is the observation result from the rainy season in 2004 to the dry season in 2005.

2.3 Water storage function of karst aquifer

As mentioned above, the karst in the supergene zone of karst plain area is strong and relatively uniform, and the karst pipelines are mostly reticulate; The unfilled caves and pipelines in the central karst zone are well developed and the lateral water flow is smooth. After long-term development and evolution, the internal structural order of karst system in karst plain has reached the best state [4]. The main manifestations of this area are as follows: the runoff discharge area of karst water system has the characteristics of solitary peak plain, and the surface elevation difference of karst plain is small, generally 2 ~ 6 m, which is close to the horizontal two-dimensional plane; The basin area and water flow direction of surface and underground hydrological networks have tended to be consistent; The leveling degree of strata in plain area corresponds to its water-bearing performance and the ability to form underground rivers. The strong karst aquifer has been basically leveled on the surface, leaving only a few solitary peaks. Karst water storage building is composed of karst cave pipeline-fractured medium with strong karst, which has a relatively uniform groundwater guide and storage network and plays a macro role in groundwater storage.

According to borehole exposure, karst development has obvious vertical zonation (Figure 3). The shallow (supergene) karst zone, the middle karst zone and the deep karst zone have depth limits in the range of 20m, 20-60m and 60-90m below the ground, and the karst development is weak below 90m. ① The shallow karst zone (0 ~ 20m section) is mainly composed of karst ditches, stone teeth and funnels, with wide karst joints, which mostly overlap with shallow weathered cracks. Karst caves are developed, with high total karst rate, but strong filling. ② The average total karst rate in the central karst cave area (20 ~ 60m section) decreased, but the porosity increased to 4.5%, mainly water-filled karst caves and dissolved fissures. One of the characteristics of cave morphology is beaded development, which consists of relatively wide caves and relatively narrow Adam's apple. The bottleneck effect of Adam's apple restricts the downward movement of the debris in the supergene zone, thus forming the middle karst cave zone with low filling degree. Due to the high degree of karst development, good tunnel connectivity and smooth lateral water flow. (3) Deep karst cave zone (60 ~ 90m section) The karst research degree in this depth section is low, and the cave pipelines mainly develop along large near-vertical structural planes (lithology or structure), and the pipeline isolation performance is more and more obvious, and the uneven development of karst is strengthened.

Fig. 3 Vertical zoning of karst caves in Litang area

(Modified according to Gui Xian Biao120 Hydrogeological Investigation Report of Guangxi Geological Bureau)

According to the borehole pumping test, the water released from the aquifer actually represents the exchange of water and gas in the karst water-bearing space. For the karst medium space with shallow burial and open aquifer, when the water contained in it is analyzed and expanded, it has the function of replenishing water. It can be seen from the above that the thickness of karst aquifer in this area can reach 90m, and the development depth of strong karst zone can reach 50 ~ 60m, while the interannual water level fluctuation under natural conditions is generally less than 10m. It can be seen that under natural conditions, the karst aquifer is mainly filled with water, and the water-gas exchange in the water-bearing medium space only accounts for about 20% of the karst space, which still has great regulation potential.

According to the analysis of borehole pumping test results in this area, the average water supply in shallow karst zone and central karst zone is 0.022 and 0.0 15 respectively. For shallow karst zone, it is equivalent to the storage capacity of 22mm water column per meter thickness; The total storage capacity of 20m-thick shallow karst zone is 440mm (equivalent to 440000 m3/km2), which is equivalent to 27.8% of the local average rainfall for many years (1584mm), indicating that it has a strong storage capacity for groundwater, and the water level drops by 1m, resulting in a storage space of 2.2×1.

2.4 Karst groundwater resources and development potential

The groundwater system of Xinbujiang River in Litang is a relatively independent circulation system of recharge, runoff and drainage, with a complete and closed boundary. Karst water-bearing medium has the characteristics of dissolving pipeline-fracture, and karst water storage structure receives lateral recharge from atmospheric precipitation and groundwater in non-carbonate areas in the system.

According to the calculation of the average precipitation for many years, the precipitation resource of the groundwater system in the Xinbujiang River in Litang is 6.50× 108m3/a, the average annual runoff in this area is 3. 18× 108m3/a, and the reservoir capacity is 0.062×108m3/a. ..

At present, groundwater exploitation in this area is mainly used for drinking, industrial production and farmland irrigation. Groundwater exploitation has formed a certain scale, mainly concentrated in densely populated areas and industrial areas in Litang Town. The mining method is mainly mechanical wells, and a small number of large wells are drilled or groundwater from natural karst water points is directly extracted. According to investigation and statistics, only mechanical pumping (in 2002) produced 0.082× 108m3/a of groundwater, of which drinking water accounted for 47.4%, industrial production water accounted for 32.8%, and farmland irrigation water accounted for 19.8%. By taking water from private wells, the groundwater used for water supply for people and livestock in rural areas reaches 0.062× 108m3/a, and the annual exploitation amount of groundwater is 0. 144× 108m3/a, accounting for 32.7% of the proven groundwater resources. The exploitation of surface water is 0.73× 108m3/a, of which domestic and industrial water accounts for 2 1.5% and agricultural water accounts for 78.5%.

To sum up, the annual average precipitation resources of groundwater system in Xinbujiang River Basin of Litang are 6.50× 108m3/a, and the annual average runoff is 3. 18× 108m3/a, of which the natural groundwater resources are (1.65 ~/kloc-) At present, the amount of surface and underground water mined is 0.908× 108m3/a, accounting for 28.55% of the annual runoff. Two-thirds of the proven groundwater resources have not been utilized, which shows that the surface water and groundwater resources in this area still have great potential for development and utilization.

3 Effective utilization measures of water resources in karst plain area

Litang Town covers an area of 5267hm2, including 3267hm2 in paddy field and 2000hm2 in dry land. The total storage capacity of existing reservoirs is 8.03 million m3, the total installed capacity of electric irrigation stations is 6.5438+200,000 kW, and the designed total irrigation area is 367hm2. Due to the aging and disrepair of reservoir engineering, the main canal and supporting facilities were damaged and leaked, and the irrigation efficiency could not be fully exerted, resulting in the conversion of 367 hectares of paddy fields into single fields and 647 hectares of paddy fields into dry fields. The annual water shortage is 3.4 million m3. In the study area, groundwater is closely related to surface water, which is an inseparable system, and the development and utilization of groundwater should be considered as a whole. Therefore, the rational allocation of surface water and groundwater can effectively and reasonably use water resources, improve drought resistance and ensure the sustainable utilization of water resources [5].

3. 1 joint development of surface water and groundwater to strengthen the drought-resistant function of groundwater.

The material circulation of water system, the most important thing is water circulation, the exchange and migration of water between atmospheric precipitation, surface water and groundwater in a certain scale space. Surface water and groundwater are transformed into each other through karst cracks and pipelines, which changes with the seasons. In dry season, groundwater supplies surface water and becomes the basic flow of rivers; During the flood, surface water flows back through the karst pipeline to replenish the underground aquifer, thus increasing the groundwater level [6]. However, the space of underground water storage medium in karst water system is limited. When precipitation recharge exceeds its storage capacity, groundwater overflows the surface and is discharged from the system through surface runoff. In the dry season, the atmospheric precipitation drops sharply, and the surface water of Xinbu River in the area is discharged and replenished by groundwater, and the upstream tributaries dry up and cut off with the decline of groundwater level. This period of time is the season of high drought in this area, and the surface water resources are insufficient, and groundwater resources are the only source of water supply for agricultural production. Under this condition, the joint development of surface water and groundwater should be carried out, surface water should be pumped by pumping stations, and groundwater should be built by supporting irrigation canals or shallow well networks of cultivated land, so as to establish paddy field-dry land compound irrigation area. During drought or dry season, shallow wells are used to extract groundwater to ensure the water supply of farmland and dried vegetables and fruits.

3.2 Fully develop and utilize the water storage function of karst underground aquifer.

Karst plain area has flat terrain, shallow groundwater depth and few natural outcrops. The basic feature of groundwater distribution is that the buried depth of groundwater in the northeast peak forest valley area is relatively large, mostly between 10 ~ 15m, and the natural outcrops are mainly caves and shafts. In the transition area from Hejian to LAM Raymond Plain, the natural outcrops of groundwater are mainly dissolved wells and ponds (Tang Ling). Litang area is dominated by Gu Feng Plain, with flat and open terrain. In addition to some karst pools, karst springs and underground river outlets can be seen on both sides of the river. The buried depth of groundwater is shallow, mostly in 2 ~ 8m. The basic movement law of groundwater is that the peak cluster mountain area is the recharge area, the peak forest plain is the runoff area, and the solitary residual peak plain is the overflow area. As mentioned above, controlled by the regional drainage datum, drilling reveals that the thickness of karst aquifer in this karst plain area can reach 90m, and the depth of strong karst zone can reach 50 ~ 60m. However, under natural conditions, the storage space formed by groundwater discharged from the aquifer with seasonal changes is only within 8 ~ 10m below the ground, which shows that the storage function of the aquifer only plays a role of about 20% in the whole karst aquifer.

According to the analysis results of water storage function of karst water-bearing medium system, if groundwater is pumped, the water level will drop by 1m and the water storage space will reach 253× 104m3/km2. The area of karst plain in this groundwater system is about 1 15 km2, and the depth of groundwater exploitation will increase by 1m, so more water can be taken. However, the amount of water to be replenished is only 8‰ of the runoff resources in this area, or 3.7% of the exploitable groundwater resources.

3.3 Scientifically arrange wells, rationally develop shallow karst water resources and avoid the risk of geological disasters.

According to the law of karst development in karst plain area and the characteristics of medium structure of underground water system, shallow well irrigation system is built according to the pipeline fracture network of underground aquifer, and irrigation water in dry area is solved by developing methods such as shallow well and large well. Decentralized well distribution and shallow groundwater development are characterized by small scale, low investment and quick results. At the same time, it can avoid the local deep drop of water source area and the strong change of groundwater level, which is beneficial to stimulate the water storage capacity of groundwater aquifer and groundwater recharge, and can avoid the harm caused by karst collapse.

3.4 Promote water-saving technology and improve water use efficiency.

Karst plain area has a large cultivated land area and high population density, and the contradiction between supply and demand of water resources is prominent. Agricultural water consumption accounts for 2/3 of the total water resources. Most of the existing irrigation and water conservancy projects were built in 1950s and 1960s, with low design standards, long operation time, disrepair of water conveyance channels, incomplete or damaged supporting buckets and branch canals in the field, serious leakage, low effective utilization rate of water conservancy, effective irrigation and tillage ratio below 30% [6], and effective utilization rate of irrigation level only around 40%. Therefore, we must adhere to the principle of paying equal attention to both open source and throttling, attach importance to the construction of new water source projects, strengthen the construction of throttling and water-saving water conservancy projects, popularize water-saving agricultural technologies, and improve drought resistance. It mainly includes ① supporting projects of water-saving irrigation canal network in irrigation area, perfecting the anti-seepage canal system of branch canals in irrigation area, improving the utilization rate of water supply and increasing the effective irrigation area. (2) Dry land water-saving irrigation project, using sprinkler irrigation, pipe irrigation and drip irrigation technology to solve the irrigation of sugarcane, vegetables and orchards. Dry land distribution area is often the tail area of surface water conservancy projects, and groundwater is the most important irrigation water source, and its water supply cost is high. Through the construction of dry land planting park, the introduction of high-efficiency and high-quality crop varieties and the use of water-saving irrigation, the irrigation area of single well and the value-added rate of water supply will be improved to obtain greater economic benefits. ③ Popularize agronomy water-saving technology, adopt plastic film planting technology to solve insufficient precipitation in spring and autumn, prevent strong evaporation, preserve water and fertilizer, and ensure crop growth; Introduce new drought-tolerant varieties, select or introduce high-quality crop varieties with early maturity or short growth period and strong adaptability according to the characteristics of high frequency and severe drought in autumn in this area, avoid the influence of drought and obtain good production benefits; Adjust the planting structure, change the paddy field planting in two seasons into alternating drought and flood, improve the contradiction between water demand and water supply resources in autumn, arrange rice planting in rainy season, and introduce high-yield and high-quality varieties to ensure food production; Planting vegetables, fruits and other crops in dry land that does not need a lot of water supply in a short time can make full use of the water storage and water supply function of groundwater aquifer, adjust the allocation of water and soil resources in time, and achieve the effect of drought prevention and drought relief.

4. Demonstration of effective utilization of water resources

In Wujiang village, the core demonstration area, the joint development of surface water and groundwater and the demonstration of shallow groundwater aquifer storage were carried out. The village is located in the karst plain area, with seasonal surface rivers and streams, and the riverbed is 2.5~3.5m higher than the ground, which is the main water source for farmland water supply in this area. According to the interview, the water flow is cut off for about 2 months in general years, and the flow drops sharply in September in dry years, and the water flow is cut off from the middle of165438/kloc-0 to the early March of the following year. Therefore, agricultural production in autumn and winter depends on groundwater supply. The karst aquifer is middle-thick dolomitic limestone, dolomite and limestone mixed with thin marl in the Lower Carboniferous, with moderate water abundance, which is karst pipeline fissure water. The buried depth of groundwater is 0.5 ~ 6m. The joint development of surface water and groundwater is designed. Surface water is pumped by pumping station, and groundwater is constructed by shallow well network, so as to establish paddy field-dry land compound irrigation area.

The population of the demonstration area is 3,370, and the cultivated land area is 354hm2, including paddy field 1.37 hm2 and dry land 2 1.7 hm2. Since the implementation of the project, 3.2 million yuan has been invested to build a surface water pumping station and a 6000m-m light seepage-proof diversion canal. Six shallow groundwater wells were dug, and a reservoir 100m3 was built, with a pipe irrigation network of about 27hm2 to ensure normal irrigation of farmland. The effective irrigation area is 233hm2, which completely changed the situation of "depending on the weather for food" in the village. On this basis, an economic forest and fruit forest demonstration base of 20hm2 and a high-quality vegetable base of 1.67 hm2 were developed, and a diversified structure of rice, vegetables, corn, sugarcane and fruit was formed in agricultural planting. Through the implementation of demonstration projects, new varieties and technologies have been introduced continuously, and the brand of "Qiao Mei Radish" has been registered with special varieties. Due to the improvement of water sources and irrigation conditions, more than 200 hectares of white radish and carrot were planted in 2004, with an average yield of 52,500 kg/hm2, an increase of 40% over 200 1 year, and every family built a "radish house". The average yield of maize is 6750kg/hm2, which is 50% higher than that of 200 1 year. By demonstrating the effective utilization and development of water resources, the agricultural economy of the village Committee has developed rapidly. In 2004, the per capita net income of farmers was 2,302 yuan, an increase of 682 yuan over 200 1 year, with an average annual increase of 14%.

5 conclusion

A double-layer hydrological network with both surface water network and underground water network has been formed in karst plain area. Due to the uplift of the basin in the drought period and the downward movement of the drainage basement in the later period, karst water cuts down to form underground river channels and forms a double-layer drainage system with the valley surface river network. In rainy season, the river water level is propped up, and the groundwater level in the basin rises, with a buried depth of only 0.5 ~ 1m, and even overflows the ground to connect with the surface rivers. In the dry season, groundwater drops, and there is still some rainfall runoff on the surface, forming a double-layer discharge with an interval of 5 ~ 8m. In the driest season, the surface rivers dry up. Groundwater migrates and discharges along karst pipelines and cracks. In recent years, the NE-trending fault structure and NW-trending extension in this area are very developed, and the karstification is strong, which provides a good space for groundwater occurrence. Controlled by the flow datum, the hydraulic gradient is small and the runoff is slow. The groundwater is shallow, quick to recharge, high in water quality and superior in development conditions. In recent years, the exploitation amount in the coming year is less than 5% of the total, so the joint development of surface water and groundwater should be strengthened to give full play to the storage function of karst aquifers.

The use of shallow wells, large wells and other development methods belongs to micro-water conservancy, and ordinary people have the ability to build it. Groundwater development has the characteristics of small scale, low investment and quick effect. According to the pipeline-fissure network of underground aquifer, the water supply system and shallow well irrigation system in arid area can solve the irrigation water problem in arid area, and the water supply guarantee rate and utilization rate are high. We can implement the preferential policy of "who builds, who owns, who manages and who benefits" through effective incentive measures such as "replacing compensation with awards", mobilize the masses to build, reduce investment risks and management risks, and form economies of scale. However, groundwater development should be carried out moderately, and in principle, it should be controlled within 75% of the recoverable resources, that is, the annual mining scale should be within 0.52× 108m3, which can greatly alleviate the contradiction between agricultural irrigation, production water for small and medium-sized enterprises and rural domestic water. Judging from the hydrogeological environment conditions in this area, it is not appropriate to excessively concentrate on the exploitation of groundwater to avoid geological disasters such as funnel decline and ground collapse.

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