Water resources strategy and agricultural development in China
Water resources strategy and agricultural development in China
In the past 60 years, China's agriculture has developed rapidly. Food supply to most people in China has been guaranteed. Other agricultural products have greatly improved people's lives. Along with other technologies such as varieties, cultivation, fertilizers, and pest and disease control, the development of irrigated agriculture has played a very important role in delivering food security. With urbanization and the improvement in people's living standards, the amount of water used for domestic and industrial purposes has also increased. Environmental considerations suggest that irrigation water supply for Chinese agriculture should be maintained at around 320–340 billion m3 a year. Expansion of the irrigated area should be limited if water resources are to be conserved and the natural environment protected. In order to ensure the sustainable development of agriculture, especially food security, the future water-resources strategy must focus on changes in agricultural water-saving technology to increase the efficiency of use of precipitation and irrigation water.
China feeds 22% of the world's population from only 10% of the world's arable area and 6.5% of the water. Per capita grain production has been maintained above 350 kg since 1983. Other agricultural products are also in abundant supply and these ensure the improvement of living standards and nutrition. Grain output has exceeded 500 million tons five times since 1996 with an irrigation water supply of 320–340 billion m3. However, the state has suggested that the country must produce another 50 million tons of grain per annum by the year 2020. This suggests an increased water requirement for agriculture. With economic development, population growth and improved living standards, many industries will demand extra water resources. It is important that Chinese agriculture works to achieve increased grain yield while water use is stabilized, i.e. the production of more ‘crop per drop’.
Water resources and the agro-environment
China has a typical continental monsoon climate. The country covers three major climates with temperate, subtropical, and tropical zones distributed from north to south. The average precipitation is about 620 mm per year (560 billion m3 of available water resources). The precipitation is unevenly distributed across the country. The east has abundant rainfall with more than 800–1200 mm per annum. The North West has less rainfall with 250–600 mm per annum. China's topography gradually rises from the south east to the North West. The western part of the country is mostly hills and mountains except for the oases. On balance, there is less land with more water resource in the southern part of the country. There is more land with less water resource in the Northern provinces.
In 2007, cereals covered close to 70% of the arable land in the country. Other crops that are commonly grown are horticultural and cash crops such as cotton, sugar cane, and oil crops. In order to promote food production, different plant systems are targeted to climates of the different regions of China. The north east is a main center of spring corn growth, with one crop per year. In the North West, the main crop is spring corn and winter wheat is grown in upland areas. The fields are fallow from July to September after wheat is harvested. The central region of China is the main irrigated area, with a winter wheat and summer corn rotation, with two harvests per year. In south-east China, the climate allows production of a crop of rice twice a year, or annual rice–wheat production. Irrigation is essential for production on this scale, even though the region has abundant precipitation. In the hilly south west of the country, rice is grown in the river basins and the valleys, and corn and wheat are planted on the uplands.
About 70% of precipitation in China is concentrated in the summer and autumn months which is good for summer crop growth. In most parts of the country, there is a high frequency of drought because of a lack of rain in the winter and spring.
Agriculture has a deficit of soil moisture for around 8 months of a year and precipitation is not adequate for cropping during two or three seasons, particularly in the central and southern regions. The annual precipitation changes greatly from year to year with a 20–30% difference from wet years to dry years. It follows that irrigation is essential for high grain yield in most parts of China.
Water resource and grain production
The Chinese government has allocated substantial funding to developing water resources in many parts of the country, i.e. building reservoirs, wells, and irrigation channels to promote food/crop production. The irrigated area has expanded rapidly and reached 48.4 million ha in 1984, up from around 16 million ha in 1949 (an increase of around 14% per year). The gross grain yield reached over 400 million tons for the first time in 1985 by increasing at around 7.5% per annum for several years.
The irrigated area expanded over 27 years by 6% per year from 9.4 million ha. Grain production reached 500 million ton in 1996. The capacity of water supply for irrigation increased to 360 billion m3 in 1997 from 100 billion m3 in 1949.
Irrigation has supported about 70% of grain production. Another output of grain in China comes from rainfed farming. There is about 64 million ha of rainfed farming in China, accounting for 54% of arable land. Food production in rainfed farming areas relies on natural rainfall without irrigation, the output from which is about 30% to 32% of total gross grain production in the north of the country. There are 16 provinces with more than 50% of rainfed farming.
Developing new irrigation area is a major component of government policy for China's agriculture. The statistics for the last 10 years showed that the ratio of substantially irrigated land to total cultivated land has declined when compared with data collected in 1998. This illustrates that some of the water resources used for agriculture are not sustainable, despite enhanced engineering capability. Consequently, prospects for expanding the area of irrigated land in China are not good. In the future, China's agriculture may not be able to rely on large-scale irrigation and high water use. At the same time, over-exploitation of water resources will cause serious ecological problems. Agriculture must find new ways to increase food production, i.e. increasing rainwater use, wastewater use or restructuring agriculture technologies to allow a saving of water.
Contribution of irrigation water and precipitation
Crop water consumption includes not only irrigation from a variety of water sources, but also precipitation. Precipitation is assessed as ‘the rainfall onto a given area of arable land’. This will be a fundamental assessment of available rainwater resource supplied to a crop in the field. As an example, in 2007, the rainfall in China was about 430 billion m3. That year, the state made available about 320 billion m3 water for irrigation. So the total water resources was about 750 billion m3 water for agriculture production, about 43% of which was from irrigation water; about 57% of it was from the precipitation utilized with existing technologies. Except in the ‘special regions’ in China,
A higher proportion of the water used in irrigation comes directly from precipitation. About 750 billion m3 water is now used as a base for planning the future water strategy for agriculture in China. The different regions in China now make decisions on local water use and agricultural development; these are based on the cropping systems employed, agronomic technology, and field engineering available.
Channel seepage is a major factor causing a low utility rate of the 320 billion m3 water supplied for irrigation. A Ministry of Water Resources report shows that the utilization rate of irrigation water is about 45%. This means that 55% of irrigation water leaks from canals or in the field etc. (losses in delivery). To reduce water loss in delivery is an urgent need in China. For example, channels with a cement lining, micro or spray irrigation (rather than flooding), and pipe irrigation are necessary for efficient water delivery.
Over the past 10 years of grain production in the whole of China, the contribution of rainwater is always greater than the contribution of irrigation water.
The contribution of irrigation water is slightly higher than that of rainwater in the south east and twice that of rainwater in the North West. The contribution of rainwater is 72% in the north east and 58% in the central region. The variation in contribution from region to region shows that different kinds of irrigation/management technology will be required in different regions in the future.
In the north of China, over 44% of rainfall runs off from the fields and is lost (Xue, 2002).
Run-off can be intercepted through small-scale engineering facilities and provide additional water for the fight against drought. If 10% of run-off is intercepted, then the rainwater for agriculture in this region will increase by 43 billion m3.
Grain produce and water use efficiency (WUE)
The efficiency of water use (WUE) in the production of a grain crop (grain production per unit of water used) is a reflection of the effectiveness with which grain yield has been enhanced by crop improvement and also shows how the water used in producing the crop has been rationalized by modern methods of crop management. In the past 60 years, high yield technology has been dedicated to improving the production of crops for China's agricultural conditions and this has resulted in an almost continuous increase in yield.
This has occurred year on year for the last 30 years (Fig. 3). The WUE of grain production in China (both rainfed and irrigated crops) has been 0.85 kg m−3 for the period 1998–2007 (Li and Peng, 2009). WUE is very different between regions. The highest efficiency is in the central region and the lowest is in the North West.
The difference is likely to be due to climate variability and also to other factors.
(i) Varieties and crop planting structure. The characteristics of varieties determine how much water is needed for grain production. Each variety has a certain adaptability to the prevailing environmental conditions and to management conditions, i.e. cold, drought, and waterlogging. Rice consumes around 1177 m3 of water in the production of 1 kg of grain, around 200 m3 kg−1 more than is required to produce 1 kg of wheat, and 500 m3 kg−1 more than is required to produce 1 kg of corn (Li and Peng, 2009). In the last 15 years, the proportion of arable agriculture devoted to corn has increased from 25.2% to 34%, with a consequent increase in output ratio from 24% to 33%. The area devoted to winter wheat in the North West has been reduced in the last 15 years. Adjusting planting time and structure to maximize the efficient use of precipitation and to increase water efficiency, requires the careful selection of crops, avoidance of the dry season, and maximum use of rainfall.
(ii) Irrigation methods and scheduling. Many successful examples show that natural precipitation is very important even in irrigated areas. Precipitation can be used directly to reduce the amount of irrigation water applied. The necessity for this will depend on the irrigation method. The efficiency of use of water in flood irrigation or micro irrigation will be quite different. In the north west, flood irrigation is needed to flush the soil due to saline and alkaline soils. Alternate furrow irrigation and border irrigation, appear to have positive effects on WUE, but are only used in some regions of China. Another irrigation opportunity to increase WUE is to construct an appropriate irrigation scheme according to precipitation characteristics, soil moisture and crop growth period. A suitable scheme should be variable according to the amount of the precipitation and achieve high yield, especially in the regions where rainfall is high. For example, in the central region, winter wheat grows, from October to June, in the dry season. There is about 107 mm precipitation in a ‘normal’ year. Conventional irrigation methods take irrigation water of 4500 m3.
A revised irrigation scheme will reduce the number of irrigation events by one or two thereby saving 60–120 mm of water.
The proposal is to delay the irrigation period according to the precipitation during the period between the sowing and jointing stages of winter wheat. The more precipitation, the more the irrigation may be delayed.
(iii) Capacity for drought resistance. Drought is the largest abiotic stress for agricultural production in China, especially in the North West and north east where there is limited irrigation infrastructure and water. Corn production in wet years, average years, and drought years, is very different with yields of only 20–50% of potential yield. Due to drought, the gross grain yield in China can be reduced by 150 million kg per year. Technologies to raise yield include covering soil in the field (mulching with crop residues or plastic).
This strategy is popular when precipitation is 350–500 mm or less. Mulching can effectively inhibit soil evaporation and sustain soil moisture content during the early growth of corn. Rainwater use efficiency can be increased. Soil water content in covered fields is much higher than in open fields, especially in the seeding season and before the rainy season (Fig. 8) (Liu, 2008). The resulting yield of corn increased by 15% and water productivity as a 15 year average was 16.93 kg mm.
Future strategies for the use of water resources for agriculture development
Policy-makers in China have a new proposal to increase annual production capacity by 50 million tons (grain) by the year 2020. Research priorities aim at the improvement of water use efficiency and drought resistance of new varieties, new irrigation methods and equipment, water-harvesting techniques, integrated agriculture technologies, modelling of water requirements, and information services. To exploit new water resources and expand the irrigated area will be very difficult in China. Agriculture development must look for pathways other that increased water use. Some opportunities include the following.
(i) To save water and obtain more productivity from existing water. China has around 320 billion m3 of water but the utilization rate of water is only 45%. If the utilization of irrigation water reaches 55%, it would make available an extra 32 billion m3 of water resource. This may be used to expand the irrigated area or to increase assurance of water supply to the existing irrigated area. To achieve this strategic target, it is necessary to improve the efficiency of established irrigation equipment, i.e. sprinklers, drip systems and so on,
And to improve scheduling and water distribution, according to the particular crop, soil, and water availability.
(ii) Complementary use of groundwater and precipitation. The above data show that precipitation constitutes between 33% and 66% in the water used for agriculture. Most regions achieve above 50% of water use from irrigation, except for the oasis agricultural areas. Therefore, if irrigators can study and predict the patterns of the precipitation distribution and the water requirements of the crop, irrigation systems can more efficiently utilize water resources, minimizing its use, while still maintaining high crop yields. This approach requires precise technical services to achieve a water saving. In the rainfed agriculture areas, it is necessary to employ the technologies of rainwater harvesting for micro-water allocation and a higher utilization of rainwater.
(iii) The development of water-saving agriculture. Water-saving technology such as mulching of the soil surface, use of drip irrigation, deficit scheduling etc. will help to deliver a more rational use of a variety of water resources. The aim is to maximize crop growth and yield while minimizing water use, i.e. to achieve more ‘crop per drop’.
If WUE (water use efficiency) increases by just 0.1 kg of grain m−3 of water, from around 0.85 kg m−3 to 0.95 kg m−3, then China's annual grain yield can increase by 32 million tons. As a result, grain yield can be increased without accessing new water supplies. Efficient water-use technologies will interact in influence with all aspects of the crop and
The cropping system, such as the variety, density, soil management, fertilization management, and planting systems in the agricultural production process.
(IV) Early-warning disaster reduction. Drought disaster reduction needs an awareness of water resource availability and the impending weather. Engineering construction should then be appropriate to deliver water where and when it is needed and in the quantities required.