Tang Jiansheng^1，2, Xia Riyuan^1，2, Xu Yuanguang², Guan Bizhu², Chen Hongfeng²

1.China University of Geosciences, Beijing 100083;

2.Institute of Karst Geology, CAGS, Guilin 541004, China

Abstract: Despite the sufficient precipitation and ground water resources droughts nearly happen annually in karst areas in the centre of Guangxi, the area of tillable fields suffering drought amounts to 125 thousand hm², about 55.9% of that in Guangxi, which restricts the development of agricultural economy greatly. Comprehensive studies show that droughts are caused by such factors as climate, karst geology condition and construction project. 7 measures to prevent droughts were put forward in the paper. First of all, perfecting and reconstructing the irrigation works should improve the efficiency of synthetic utilization of which. Secondly, the combined development of surface and ground water can take full advantage of water resources with the uneven distribution characteristics in time and space. Thirdly, the restoration and reconstruction of water source ecosystem will reduce the waste of water resources and improve the function of water supply. Fourthly, adjusting structure of land use and constructing well ecosystem of farmlands can advance the benign circulation of resources utilization and environmental improvement. Fifthly, the planting structure modulation is adjusted by extending the techniques of water-saving, soil improvement and drought prevention of agriculture. Sixthly, agriculture structure should regulated to form disparate agricultural economy system. Finally, the system of drought defying must be founded and perfected. Thus, it will achieve sustainable development of rural economics and society.

Key words: karst areas, agriculture drought, status, cause, countermeasure, the centre of Guangxi

Introduction

Guangxi Zhuang Autonomous Region locates in the monsoon area of southern semitropics, where the annually average precipitation fluctuates from 1500 to 2000 mm, and is one of the areas with most abundant precipitation in China (ZHANG Zhongfu 2002). Although annual precipitation in the centre of Guangxi is a little lower than that in circumjacent areas, which ranges from 1300 mm to 1600 mm(LI Yaoxian 2001), whereas agricultural productivities undergo enormous loss because of frequent droughts in the area (Tab.1). From 1988, such measures being continuously conducted as project of drought-control in karst drought areas in the centre of Guangxi, study of synthesized techniques in order to advance the ability of resisting or decreasing calamities, to improve the conditions of agricultural productivity and life, to obtain the reasonable balance among resources, ecology, population and economy, and to realize the benign circulation of environment, society and economy. Main situation and causes of agricultural droughts were found out through studies and demonstrations of synthesized fathering performed in 3 typical areas with different types of agricultural resources based on regional investigation and analysis, with the result that countermeasures are established according to natural conditions and techniques or economic factors in the area.

1 Present situation of drought

Drought, a typical kind of natural disasters in karst areas in Guangxi, restricts development in economy greatly. As recorded in history, droughts with various scales had happened for 152 times from 1700 to 1949, 12 times of severer droughts had occurred covering the whole autonomous region of Guangxi. After liberation, drought emerged annually almost. From 1950 to 1990, the area of croplands stricken by droughts was 568.4 thousand hm2 in average per year. The area of croplands suffering droughts is between 54.8 and 1552.0 thousand hm2 every year during the 41 years, severely drought about 13.7~720.5 thousand hm2, and completely no harvests about 1.3~321.7 thousand hm2. There were 14 years when the area of croplands suffering droughts exceeded 666.7 thousand hm2 in each of that.

Tab.1 Survey of droughts in the latest 6 years in Guangxi (from 1999 to 2004)

Notes: from bulletin of water resources in Guangxi

Lands suffering karst drought with the area of each patch over ten thousand mu (a unit of area in China, 1/15 of a hectare) amounted to 83 patches in the centre of Guangxi, and the total area accounted for 3348.8 thousand mu (GUANG Yaohua 2000), where karst plain covered 55.9% of the total area, the peak-cluster karst plain 31.3%, the peak-forest karst in northeast 10.1%, and karst areas in east of Guangxi 2.7%. Where the average annual area suffering droughts is about 600 thousand hm2, the sum is even much higher up to 1000-odd thousand of hm2 in the years when more serious droughts happened, which the direct economic loss outstripped RMB 2 billion Yuan. The higher frequency of droughts happens in springs or autumns in one year; this situation becomes more and more serious because of abnormal climates in recent years. As shown in Tab. 1 during 6 years, 2 years when continuous drought happened from spring to autumn, 1 year of drought from spring to summer and spring drought in 2 years happened, only 1 year appeared with abundant precipitation and water resources and subsequently slight drought.

As a whole, droughts in Guangxi mainly happen in springs and autumns, while fewer appear in summers; despite of less precipitation in winters, the influences of droughts decrease due to lower temperature and evaporation. Central Guangxi easily suffers from droughts in autumn, which even last till winter. The main causes of which lie in atmosphere circumfluence being out of gear when monsoon advances and retreats, rainfall weathers could not or less impose on Guangxi, grave drought happening from spring to summer or from summer till autumn mostly owing to less precipitation in autumn and winter in the last year, a typical case is the drought in the year of 2004, which was affected by the drought of 2003, the drought not likely to happen in half a century brought tremendous loss.

2 Main causes of agricultural droughts

2.1 Climate

Influenced alternately by the winter and summer monsoon, seasonal change of precipitation is great uneven in Guangxi resulting in obvious rainy season and dry season. The time from Apr. to Sept. is the rainy season, when the precipitation occupies 70~85% of that in the whole year, and floods often arise; dry season with less precipitation (Tu Fangxu 1998) covers the period of time from Oct. to Mar. of next year, when the precipitation only accounts for the rest of which. Droughts caused by climate change contain two factors.

The first is monsoon circumfluence. In normal years, summer monsoon begins to enter Guangxi in the end of Apr. or early of May. Monthly precipitation multiples from May to the middle of Jun. The front strip hovers in South China and the first peak of precipitation forms in all over the area. Summer monsoon begins to retreat towards south in the middle of Sept., and the cold air mass invades from the north, the precipitation decreases sharply. The large difference that the intension of high atmospheric pressure of west Pacific semitropics changes between years brings about much diversification and uneven distribution of precipitation in various years; in the next place, typhoons or tropic storms arrive during the period from May to Nov. annually, tendency of droughts in summer or fall come into being if less the times of tropic cyclone invasion in the year. At the same time, Nanhai high-pressure and India low-pressure affect greatly the weather in winters or springs in Guangxi, influenced by northwest air current droughts appear in winters or springs. And besides, El-Nino and La-Nino are important factors, less precipitation be in springs in central Guangxi impacted by El-Nina, while which in falls or winters by La-Nino.

Next, Higher temperature causes more evaporation, and then droughts aggravate. Comparison analysis shows that precipitation in central Guangxi is less than that in Guiping (a city in Guangxi) and Guangzhou, the neighbor cities with the same in latitude, while evaporation is more, and the less precipitation, the more evaporation. Statistics data from 1990 to 1999 indicates that there are 6 years evaporation went beyond precipitation in which by Liuzhou weather station, while 8 years by Nanning station. The evaporation is 1.1 to 1.76 times that the precipitation in Liuzhou station, while 1.12 to 1.33 times in Nanning station. Droughts exacerbate inducing water in reservoirs reducing sharply, drying up in most water bodies, and ground water declining rapidly, which result in insufficient water using in producing and in croplands, then reduction of output of crops. For instance, from Jul. to Sept. in 2000 in the city of Guigang, monthly evaporation was above 210 mm, which is 20 to 40 mm more than that in the same time of past years. Because of continuous drought the areas of such crops as late rice, late corn and sugarcane suffering added up to 123 thousand hm², accounting for 70% of the gross area.

Fig.1 Precipitation in Nanning’s and Liuzhou’s weathe r station from Jan-90 to Jan-99

Tab.2 precipitation characteristics in two weather stations in central Guangxi

2.2 Karst geology

Karst leakage. The area of karst distributing in the counties of Shanglin, Binyang, Guigang, Heshan, Xingbin, Xiangzhou, Wuxuan and Xincheng, sum up 14069 km², which occupies 75.4% the total area (Xia Riyuan 1997). Complicated systems formed by exposed terranes, karst grooves, apertures, ponds or depressions, pits, sinkholes, shafts, funnels, caves, conduits and subterranean streams which act as approach to transfer surface materials and energy. Rainfall and surface water infiltrate into ground readily, as the proverb says that once storm can flood all depressions, however, the rice would wither if no rain in continuous ten days.

Double-layer hydrology nets exist on surface and under ground. Movement and circulation of current in karst areas differ much from those in non-karst areas; and density of network of rivers is distinctly lower while well development occurs in the nets of ground water flow, a typical case is that the average density of network of rivers is only 0.15 km per km2 in Xingbin, merely one sixth to a quarter that in non-karst areas in South China with the value of 0.6 to 1.0 km per km2 (Zhang Zhigan 2005). Rain water, surface water and ground water high associate in karst areas, transformation among them occurs frequently. Karst aquifer belongs of a certain kind of duplex mediums with high conductivity and weak storage of water, where hydrological regime changes acutely，and flow distributes unequably, regenerative resources feebly modulate under natural conditions. Surface flux production is 30% to 50% lower than that in non-karst areas. Such sluice establishments as reservoirs or pools in karst areas generally possess grave leakage, water reserves of about two 3rds medium-sized or pint-sized reservoirs could not reach the designed standard owing to leakage.

Low capability of water maintenance of mantlerocks. Mantlerock of carbonate rock in the area basically inclines to laterite from preliminary investigation (area with grit soil accounting for 45.5% in Xingbin, Tab.3), which possesses 3 aspects of characteristics. Firstly, thin soil distributes in karst grooves carbonate-based just like vesicle, generally, the average width of the soil falls short of 1-2 m; secondly, high contents of gravels enhance the leakage; thirdly, a layer of clay with several decimeters in width usually cover the surface of bedrock, chaps emerge when being dehydrated. Water use in the croplands with grit soil covering, accounting for 46.5% of the total in Xingbin, is about 1 to 3 times more than that covered by powdery soil or clay (Tab.2), and water use by unit yield amounts to 12.5 cubic meters per kilogram.

Tab.3 characteristics and distribution of grit-based soil in Xingbin

（From the second general investigation of plowlands in Xingbin, by TAN Hongzhi, etc.）

Entironment worsening. Forest, an important modulation system, is scarce. The rate of forest covering is nothing more than 5.4% by 1973’s data, which drop to 3.1% by 1989’s survey; the decreasing forests conserve water with difficulty, so soil erosion accelerates. Deadwoods and fallen leaves in virgin forests can transform precipitation by 40-80% to underground flux discharging slowly. If forest covering rate be increased to 35% through ecology restoration, the capability to reserve water would be equivalent to 20 medium-sized reservoirs with storage capacity of 10000 m³.

2.3  Projects

Low function of irrigation systems. 80% of reservoirs in karst areas lock into leakage, 35% of those lose efficacy for grave leakage. Low efficiency of utilization exists in establishment and technique of water-saving irrigation, the average using coefficient of water by ditches is only 0.4, which is less than 60% of that in developed areas. Gross water use of rice production is about 4.1 m3 per 1 kg; the value would be even more in the year of drought to 6.5 m3 per 1 kg, which are 4 to 6 times that in developed areas respectively. Irrigation works in central Guangxi, without united programming, and bad quality of construction, mostly were constructed during the period of end 1950’s and early 1960’s, water use in agriculture fall short of 6% that of total water resources, which could not suffice irrigation in the years of droughts(Zhou Youyou 2003).

Deficiencies in united control of projects for water resources development. About 0.98 billion m3 of underground flux, accounting for 30% of the total flux with the sum of 3.29 billion m3 by branches inflowing. No matter what natural or manpower subsystems, surface water inclines to flow into ground because of double-layer of hydrology nets existing in karst areas. Upper constructions mainly constitute the whole irrigation works, which control surface flux, development of ground water is only 4% of the total.

Grave karst leakages. Present projects for water reserve or drawing mainly were constructed with small area of rain collection，little water source and grave leakage in Fenglin or Fengcong karst areas. A good quantity of water loses during the course of transportation. For instance, present irrigation works in Litang, the total designed area by irrigation is 2250 hm2, but 370and 650 hm2 of paddy fields had to be turned into single-harvest fields and glebes respectively for aging and disrepair of reservoirs and seepage in ditches.

Colligated measures in water resources deficiency and high expense for irrigation. In karst plains, it is much difficult in construction for sluice works, agricultural water supply depends on water elevating at a certain extent. But high lift of water pumping works causing much expense of electricity became heavy burdens to farmers. Such as irrigation station by electric power in Xingbin, cost per year amounts to RMB 1320 yuan per hm2.

3 Countermeasures for Karst Droughts Control

3.1 Reconstruction and improving efficiency of irrigation works

New techniques and materials should be employed to improve capability of leakage prevention and lessen loss of water in renovation of ditches systems; such factors should be emphasized as hydropower, irrigation, flood prevention and shipping in water power development; wellhead construction for lives and industrial producing, and aquiculture be performed according to the conditions of water in ground water development; sluice, elevation and drawing water could be adopted based on local conditions where surface water is absent, while karst ground water suffice with good quality for irrigation and drinking (Bao Haiying 2000).

3.2 United development for surface and ground water taking full advantage of the time-space distribution of water resources

Surface water systems develops weakly in karst areas, surface branches usually dry in seasons of droughts, surface water exchanges with groundwater through karst fractures and conduits. United development for surface and ground water as water elevation before surface water drying to increase aquifer recharge from irrigation water; droughts lasting for several years could be solved by drilling and well digging to construct underground modulation reservoirs (Chen Weihai 1999). Well development of karst aquifers with good storage and infiltration for water possesses functions for water modulation, excessive exploitation in deep ground water in dry seasons when crops need more water were conducted to form modulation storage capacity, when rain seasons begin, ground water would be restored by natural infiltration of rains.

3.3 Restoration and reconstruction of entironment

Such method as forest ecology systems building to better water entironment, plant structure modulation to reserve water should be performed to mitigate the inhomogeneity of rainfall temporal distribution, delay the process of which transforming into rapid flow, and then decrease the loss of water resources(Jia Rong 2000). Agriculture, forestry and stockbreeding adapted to local climate with seasonal drought be constructed, vegetation of forest and grass can meliorate ecology, and accordingly improve the harvest of crops and stockbreeding. Increasing organic materials in soils can conserve water better.

3.4  Structural adjustment in land use

A majority of flats and gentle sloping fields were cultivated to croplands in karst plains, where crops and sugar canes act as the main plants, infields account for 50.57 to 55.03% of the total land in Xingbin andBinyang County through remote sensing analysis, orchards and tea gardens are less than 0.5% of which. Woodland with fruit trees can improve agricultural ecology, microclimate and sub-circulation of water, and at the same time, can decrease the water loss by evaporation.

3.5 Planting structure regulation and water saving agriculture extension

Regulation in planting time of crops should be adopted to avoid drought seasons, at the same time, water in atmosphere should be fully utilized (rain, fog, and dew, etc.) (Shou Shengnian 2001; Ren Yangjun 2000). Cultivation models for droughts prevention are introduced in Litang, such as rotation of rice and vegetables, of corns, melon and fruits, and vegetable, and of fruits, peanut and bean. Production value totalize to RMB 3000 to 5000 yuan per mu by green sword bean or balsam pear planted during winter, techniques of water saving can control the loss of water, improve the capability of water conservation by soil; straw and earth membrane covering could effectively control evaporation; shallow planting with deep furrow, much application of organic fertilizer, and stalk returning to fields, such measures as mentioned could improve water and fertilizer absorbed and capability to drought prevention; ditches along contour be constructed in sloping land cultivation can lessen surface flow, and increase more infiltration.

3.6 Agricultural property structure modulation to form agricultural systems with many entities

Many methods put forward aim at the seasonal, frequent, and territorial droughts and its restriction to water use by crops. ① Quick development in stockbreeding and augmentation in the proportion of herbivore be conducted. Sugarcane, corns and peanut are the main local agricultural species, plentiful byproduct such as straw, leaf of sugarcane and residue of peanuts can be taken as feedingstuff with high quality, furthermore, picked seeds of pasturage be introduced, and improve the proportion of stockbreeding in agricultural economy; ② intensive process of farm produce as foodstuff, fruits and vegetable to improve agricultural efficiency, income for farmers and economy development; ③ Alteration of the third industry and development in service for countries, such as circulation network and establishments, should be performed. Firstly, interior economic information centre be set up to induct producing and communicate between produce and sale; secondly, service system should be perfected. Folk information network systematic service constantly is reinforced; thirdly, information superhighway be constructed. Dependence of Agricultural economy on planting with high water consuming could be disassembled by modulation of industry structure.

3.7 Service systems for droughts prevention establishment

Service network for drought prevention use county’s team as leader, country’s team as link, and villages’ as main body, which supply specialized and standardized service for peasantry, such as mobile irrigation, maintenance and tenancy for irrigation machines, new techniques for water-saving irrigation, etc..

4  Conclusions

The large area of land suffering droughts presents in central Guangxi, with high rate of cultivation, only partial of which has guarantee for water supply under natural conditions. This study supply several methods to improve the capability for droughts prevention of agricultural production, and the efficiency of water and soil resources utilization and output, which ensure that agricultural production and economy could not be affected obviously by disasters. It is put forward that such factors should be considered to solve water deficiency caused by droughts effectively as development and utilization of water resources, configuration of water and soil resources, entironment administration, water saving, protection and management for agricultural resources, and improvement in agricultural production, to achieve sustainable development in agricultural economy and society.

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