Scholarly article on topic 'China's food security is threatened by the unsustainable use of water resources in North and Northwest China'

China's food security is threatened by the unsustainable use of water resources in North and Northwest China Academic research paper on "Agriculture, forestry, and fisheries"

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Academic research paper on topic "China's food security is threatened by the unsustainable use of water resources in North and Northwest China"

REVIEW

China's food security is threatened by the unsustainable use of water resources in North and Northwest China

Taisheng Du1, Shaozhong Kang1, Xiying Zhang2 & Jianhua Zhang3

1Center for Agricultural Water Research in China, China Agricultural University, Beijing 100083, China

2Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Shijiazhuang 050021, China

3School of Life Sciences and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Hong Kong, China

Keywords

China, crop production, food security, sustainability, water resources, water-saving agriculture.

Correspondence

Jianhua Zhang, School of Life Sciences and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Hong Kong, China. Tel: +852 39436288; Fax: +852 26036382; E-mail: jhzhang@cuhk.edu.hk

Funding Information

We are grateful for research grants from the National Natural Science Foundation of China (51222905, 51079147, 51321001, 50939005), the National High-Tech 863 Project of China (2011AA100502), the Ministry of Water Resources of China (201001061), China-EU Int'l Collaboration Projects (S2010GR0692), National Basic Research Program of China (2012CB114300) and Shenzhen Overseas Talents Innovation & Entrepreneurship Funding Scheme (The Peacock Scheme).

Abstract

China has a growing population of 1.35 billion at the moment and has been largely self-sufficient in its food production. In consideration of the major agricultural resources (land, available fresh water, and ambient temperature), China is actually very vulnerable in maintaining its food security. Due to the unfavorable distributions of its water resources and temperature conditions, almost all the land that can be cropped has been utilized. Over cropping and over irrigation has led to some serious problems in two major areas of this country. In Northwest China where most of the land belongs to the inland river system, over expanding of irrigated area has resulted in some serious ecological problems, such as shrinking of oasis and desertification of grasslands. In North China Plain where about half of the country's wheat and maize are produced, over cropping has been supplemented with underground water for several decades, which has led to fast drop of underground water table, for example, 0.88 m per year in the recent 30 years at Luancheng County. China's food security is threatened by its diminishing and unsustainable use of water resources. Integrative agronomic, biological, engineering, and administrative practices that can sustainably use the water resources will be the key ways out to secure the country's future food production.

Received: 2 April 2013; Revised: 29 September 2013; Accepted: 30 September 2013

Food and Energy Security 2014; 3(1): 7-18

doi: 10.1002/fes3.40

Introduction

Globally, 2.6 billion people lack improved sanitation, 800 million people lack safe drinking water, 1 billion people

go to bed hungrily, 2 billion people are undernourished, and 60% of our ecosystem services are deteriorating according to the numbers released by the World Water Week in Stockholm, 26-31 August 2012. The theme of

© 2013 The Authors. Food and Energy Security published by John Wiley & Sons Ltd. and the Association of Applied Biologists. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

World Water Day this year is Water and Food Security, showing us that the whole world food security will not be guaranteed if we do not take integrated water resources management to make our water resources more sustainable (Watson 2004; Biswas 2008; Garcia 2008). It is expected that the world population will reach 8.3 billion by 2030 and food and fiber demand will be increased by 70%. However, we are already using 70% of our fresh water resources for crop production and have resulted in many serious environmental problems due to overexploitation of the water resources (Von Braun 2007; FAO 2012). Apparently any further increase in water consumption in agriculture is impossible. We are facing the inevitable changes in our focuses in agriculture: a greater emphasis on water productivity, defined as the ratio between crop yield and water consumption, rather than the yield per se is required.

China still has a growing population. The latest census shows that the country has a population at 1.35 billion at the moment and will reach 1.5 billion at 2030. If the grain food per capita increases from today's 400 to 470 kg in 2030, which is realistic with the rapid increase in living standard, we will need at least 35% increase in grain production in less than 20 years. China's total food production has surpassed 500 million tons over the last several years. Technical advancements, such as extensive breeding programs and applications of large amount of chemical fertilizers and agrochemicals, have helped maintaining an increasing trend over the last several decades. More importantly, the agricultural policy changes over the last two decades, things like the "Household Responsibility System", which distributes land into individual households, the government subsidies to grain crops and the phasing-out of agricultural land taxes, are the major driving forces for the food production increases since 1980s (Zhang 2011).

In terms of the water consumption for crop production in China, we should note that the food production increase has resulted from an extensive increase in irrigated areas in this country (Fig. 1). There is a linear correlation between the total grain production in China and the effective irrigated area (National Bureau of Statistics of China 2010a,b). However, this correlation does not indicate that further increase in food production needs to be achieved with the expansion of irrigated area, because we have reached the limits of water resources in many parts of China (discussed in the following sections). We really have to find other ways to maintain the increase in food production with the same or even less water resources.

China is a big country but its water resources per capita are only about 25% of the world average (Jiang 2009). In addition, the distribution of the water resources is very

600.00 500.00 400.00 300.00 200.00 100.00 0.00

GP = 10.29 IA - 69.55

R2 = 0.918

- 0/ o

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 Irrigation area(106 ha)

Figure 1. Relationship between irrigation area and grain production in China during the last 60 years. Data showing selected years are from National Bureau of Statistics of China (2010a,b).

uneven by locations and by timing. The southeast part of China near the coastline has an annual rainfall over 1500 mm but the inner northwest part with a third of the country's land area only has less than 200 mm per year. China's rainfall is also affected by typhoons in the summer. From June to August, about 70% of the annual precipitation is delivered by typhoons. Northern China, with 40% of the country's population and 51% of the country's cropped land, has only 20% of its water resources. North China's water availability is only about 271 m3 of per capita value, which is only 1/25 of the world's average (Guan and Hubacek 2008). The water scarcity is as serious as in areas such as Israel which is 290 m3 water resources per capita. The average agricultural water consumption accounts for 68% in China, but the value varies in different region. For example, in North China Plain (NCP) which is the important political, cultural, and industrial center, the value is between 71.6% and 76.9%, while in Northwest China the value is above 90% (Fig. 2).

In recent decades, developing irrigation agriculture has changed the transformation and relationship among water cycles, crop water use, and ecology, especially in arid areas where the water resources are very limited and agriculture heavily depends on irrigation. The desert areas have expanded, but river, lake, pasture, and grassland acreage have shrunk. Therefore, competition for the limited water resources between agriculture and ecosystem is increasingly becoming a serious problem. Due to the lack of surface water, groundwater has become a major source of irrigation water in most parts of North and Northwest China. Groundwater levels are persistently declining in many parts of the areas. Overutilization of groundwater resources has led to some serious consequences, for example, gradually falling groundwater table, shrinking of vegetation areas, soil salinization, and desertification in oasis region (Kang et al. 2008). Increased pumping costs and

Figure 2. Agricultural water consumption ratio in different regions of China. Data are from Ministry of Water resource (2012).

saltwater intrusion have forced farmers to abandon thousands of wells in NCP in recent years (Kendy et al. 2004). It should be noted that this area accounts for about 50% of wheat and 33% of maize production in China. The wheat crop uses more than 70% of irrigation water resources in this area (Hu et al. 2010).

When compared to North and Northwest China, the other parts of China should have enough water resources for their crops but unique problems for their water resources still exist. For example, in Northeast China annual rainfall reaches 600 mm, which should be enough for the only one crop per year there. Periodic spring and summer droughts are still reported in recent years. They are largely attributed to abnormal atmospheric circulation, air-sea interaction (Kang et al. 2013) or local climatic fluctuations resulted from land use and land cover change mainly by rapid expansion of irrigated areas. It is believed that enhanced water engineering efforts and adoption of precise and water-saving irrigation technologies should help solve the issues. In fact, the Chinese Government has initiated a campaign to "Increase grain production with less water" there with an aim to boost the food production by 10% within 4 years. This area currently produces 22.0% of total grains with only 23.5% of the land in this country. It is believed that it has the potential to produce more if more and better irrigation can be provided.

In Southwest China where rainfall usually exceeds 1000 mm per year, agriculture should not lack of water if the water resources are properly managed. In recent years, however, severe spring droughts are almost always the headline news in China. Lack of proper water

conservation measures or projects plus the abnormal weathers, arguably the result of climate change are the causes (Cao et al. 2012; Patterson et al. 2013; Zhang et al. 2013a). Traditional or indigenous water storage ponds for the households have largely been abandoned by converting them to crop land and relying on large reservoirs for irrigation during the last several decades. This has actually accelerated the drought damage and made the agriculture more vulnerable in this largely mountainous area in abnormally dry years when the reservoirs quickly dries up due to the ever increasing demand of water in all the sectors of society. Agricultural irrigation becomes a low priority in the dry years.

This review focuses on the North and Northwest China (Table 1) where water scarcity is most serious and simple

Table 1. Comparison of key data between Northwest China and North China Plain.

Northwest China1 North China Plain2

Area (km2) 3.43 x 106 3.20 x 105

Average annual 10-15 10-16

temperature (oC)

Annual rainfall (mm) ~50-400 ~400-800

Frost-free days per year 15-256 175-220

Principal crops Wheat, cotton, Wheat,

potato maize, cotton

Total land cultivated (ha) 1.21 x 107 2.65 x 107

Total land irrigated (ha) 6.20 x 106 1.62 x 107

Population 8.58 x 107 2.14 x 108

1Data from Zhang and Lu (2002).

2Data from Kendy et al. (2004) and Song et al. (2011).

water engineering approaches will not solve it. We believe the water-saving agriculture is the suitable option left to make these two areas sustainable in terms of water use.

Northwest China: Deteriorating Ecology and Environment

Northwest China belongs to the typical inland river system which means rivers end up within the land in lakes or wetland. The size of the area is about 29% of the country's land area, largely classified as arid or semiarid climate. Most of this area has an annual rainfall below 200 mm. In recent 30 years, 25 million hectares of farmland have suffered from drought disasters, which account for 20% of cropping acreage, with grain yield reduction at 57 billion kg (National Bureau of Statistics of China 2010a). The drought as a stress to crops exists almost all the time, regardless the seasons and accounts for more than 10% yield loss in most years, for example in drought-prone Gansu Province, an arid and semiarid area (Fig. 3). In recent 50 years, temperature has increased at 0.325, 0.339, and 0.360°C per decade in the mountain, oasis, and the desert areas, respectively. Precipitation has increased at 10.15, 6.29, and 0.87 mm per decade. It should be noted that only the increasing trend of temperature and precipitation in desert area was not significant (Li et al. 2013). In other areas of the watershed, such increases are significant. For example, in Wuwei, the average temperature only increased 1.2°C during last 50 years (Fig. 4).

The major water resources in this area come from glaciers in high mountains. Irrigation is therefore the lifeline for agriculture. The total area of this region is an estimated 2.78 million km2, about 29% of the national total,

1979 Year

Figure 3. Total grain yield and estimated yield loss due to drought in Gansu province of Northwest China during the past 58 years. Data during 1950-1990 are from Editorial Committee of Flood and Drought Disasters in Northwest Inland River Basin (1999) and data during 1991-2008 are from Water Resource Bureau of Gansu Province (2010).

1980 1990 Year

Figure 4. Annually average temperature change in Wuwei city during last 50 years. Data are from China Meteorological Administration National Meteorological Information Center (2013).

but almost 90% of people concentrate in the oasis which accounts for only 4% of the land surface there. Its water resources are estimated at only 3.8% of the country's total. Agriculture is in fact conducted in a very vulnerable ecological condition and almost totally relies on irrigation. As a consequence, agricultural water use is over 90% of the total water usage.

A typical story can be told by the example of Shiyang River basin (Kang et al. 2008). It is located in the eastern part of the Hexi Corridor of Gansu Province in Northwest China and on the northern slope of the Qilian Mountain. The river originates from the northern part of the mountain and ends at the Minqin oasis that is sandwiched between the Badanjilin and Tenggeli deserts (Kang et al. 2004). The whole river basin has an area of 41,600 km2 with eight subcatchments from east to west. The annually average water resources in the basin are estimated to be 1.661 x 109 m3 between 1995 and 2000 (Fig. 5). The population there is 2.3 million with cropped land at 373,000 ha. The total river flow in the upper reach did not change much during the last six decades. The flow to the lower reach, the Minqin oasis, however, has been greatly reduced during the later 50 years of the last century, largely due to the intensive human activities, mainly the expanded irrigation, in the upper and middle reaches (Fig. 5). The oasis only received an annual flow at about 60 millions m3 of water in the early 2000s, a drastic reduction if it is compared to the annual flow over 500 million m3 of water in the 1950s. The Minqin oasis, once nurtured by lakes, is now largely an oasis mainly relying on the underground water. It is consistently under the invasions by the Badanjilin and Tenggeli deserts from the east, north, and west and there is the risk that it will disappear altogether. The Minqin oasis currently prevents fusion of the two large deserts and its disappearance will threaten the two neighboring cities of Jinchang and

0 !■■■■■■■■■■■■

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015

Figure 5. Annual river flows (ARF) of Shiyang River at the foot of Qilian Mountain (upper plot) and at the lower reach of Minqin Oasis (lower plot). Data are from Kang et al. (2009).

Liangzhou with desertification and block the Hexi Corridor, the ancient Silk Road, which connects Xinjiang Uyghur Autonomous Region to central China. The ecology and climate of the whole northern China may also be affected if such a mega desert is formed.

During the later 50 years of last century, irrigation in the Shiyang River region has increased steadily and rapidly. The area of irrigated land started from 185,900 hm2 in the 1950s to 293,600 hm2 in 2000, nearly 60% increase. Seventeen large irrigation complexes, each with an area of over 100 hm2 have been constructed. An extensive system of lined canals has been built and the water transport efficiency of the canal system (ratio of canal inflow at the headwork to outflow of the end canal in an irrigation district) has increased from 0.30-0.35 in the 1950s to 0.54-0.72 in 2000. The total annual water usage in this area has reached 2.854 x 109 m3. Agricultural water use was 2.455 x 109 m3 which accounted for 80% of the total water use. The gross water utilization ratio (ratio of gross water utilization to the total water resources of the basin) reached 172% with water repeatedly used and combined with excessive groundwater extraction. The net water consumption by irrigation, industry, and domestic utilization increased from 0.867 x 109 m3 in the early 1950s to 1.718 x 109 m3 in 2000, virtually it has doubled in 50 years. The net water consumption ratio (ratio of net water consumption to the total water resources of the basin) reached 103% in 2000 (Kang et al. 2008).

During the last decade, much effort has been made in the Shiyang River basin to reduce the water use. The central government implemented the layout of a comprehensive reparation scheme of Shiyang River Basin and more

than 4.7 billion RMB has been spent since 2003 in repairing the damaged environment and improving water use efficiency and water productivity at different scales from crops, farmland, irrigation-covered areas, and watershed. Integrative approaches including agronomic water-saving measures, cropping system adjustment, water resource allocation, water engineering projects, ecosystem, and water resources conservation projects, inter-watersheds water diverting projects, and water resources information system, have been implemented. Rational and optimal allocation of water resources based on crop production and ecological demand has been put forward (Kang et al. 2009). A water-saving experimental station has been set up to demonstrate all sorts of water-saving irrigation techniques and facilities. The local government has been advocating the adoption of water-saving practices, setting the policies of water ration, and regulating the irrigation water use and decommissioning the irrigation areas. For example, they have rationed the underground water exploitation by trimming down the outputs of many "luxury" wells using IC card (carrying the user's identity and the amount of water available) on the pumps, subsidized the building of sun-warming plastic greenhouses (much less water consumptions for the vegetable crops inside due to the closed environment and high humidity) and adopted crops that require less irrigation. These measures are complemented by the extensive use of plastic mulching and advanced irrigation techniques such as improved furrow irrigation or partial root zone irrigation. There are also many reports that partial root zone drip irrigation increases crop water use efficiency more than conventional deficit irrigation (Kang et al. 2004; Du

et al. 2006, 2008a,b, 2010; Fereres and Soriano 2007; Dodd Ian 2009; Sadras 2009). Furthermore, classification of water prices has been carried out, that is, greenhouse crops or drip irrigation crops enjoy 20% discount within the water quota. In contrast, for wheat and maize under traditional flood irrigation, the water price will be increased by 20%, and the cost per cubic meter of water increased from 1.93 Yuan RMB in 2006 to 5.7 Yuan in 2012. As a result, in the downstream area, the planting structure was adjusted, reducing the high water use crops (wheat and maize) and increasing cash crops (cotton, sunflowers, and vegetables) that can bring more profit (Fig. 6). The net income per farmer has increased from 4665.5 Yuan in 2006 to 7035 Yuan in 2012. The total water use in the middle reaches of the river basin has been reduced by 15.7%, and exploitation of groundwater in the whole area has been reduced by 55.3% at 2006 when compared to 1 year earlier at 2005. The water flow to Minqin has been gradually increased since 2006 (Fig. 5). The trend of ecological deterioration has been gradually reversed and groundwater table at Qingtu Lake in Minqin oasis has shown some increase. For example, the down trend of groundwater level became slow in recent years and even increased by 0.17 m at 2012 when compared to 1 year earlier.

2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 Year

2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 Year

Figure 6. Planting acreage and crop yield change in Minqin county at the downstream of Shiyang River Basin during past 12 years. Data are from Kang et al. (2009) and local government statistic report.

Minqin oasis's story can be visually shown by the landscape changes of the Qingtu Lake, which used to receive the final flows of Shiyang River. In the history books, the lake of 2000 years ago was described as a water surface of several hundred square kilometers with numerous kinds of fishes and crowds of wild ducks. Before the 1958, the lake area was largely a wetland with an area of about 100 km2. In 1958, a reservoir was built at Minqin to divert the water for irrigation. After that, the lake area has gradually become a desert with moving sand dunes. Surprisingly, due to the last 10 years of water conservation efforts, for example, in Wuwei city which is mainly in Shiyang River basin, the water distribution ratio of domestic, industry, ecology, and agriculture increased from 2:4:2:91 in 2006 to 5:14:8:73 in 2012, the annual river flow through Caiqi section reached 3.48 x 108 m3 during 2012, the groundwater exploitation is controlled within 0.86 x 108 m3, and the Qingtu Lake has resumed a water surface of 3 km2! Especially in the Huangantan village, Jiahe Town, Minqin County near Tenggeli desert, seven wells which were closed in 2008 became usable in the spring of 2012. As a result of the above described integrative measures, the declining trend of water table in the whole river basin was suppressed, and a restoration of 2.05 x 104 ha man-made forest and 1.0 x 104 ha grassland expansion has been achieved. In addition, 1.1 x 104 ha sand dune has been controlled by wheat straw grid in the lower reach.

The NCP: Over Exploitation of Underground Water

NCP is located in the eastern part of China. It covers the area from Bohai Sea at its east to the Taihang Mountain at its west, and from Yellow River at its south to the Yanshan Mountain at its north. It includes the flat plain area of Beijing, Tianjin, and Hebei Province, and parts of the plain area of Henan and Shandong provinces. The total area is approximately estimated as 320,000 km2, mainly including the Yellow, Huai, and Hai river basins. In the NCP, the area of agricultural land is about 8.85 x 107 ha with an irrigated area of 6.53 x 107 ha, and consumes about 70% total water resources (Fang et al. 2010; Zhang et al. 2011). The mean annual accumulated temperature has increased significantly by 348.5°C-day due to global warming during last half century (Song et al. 2011).

The flat landscape of NCP means it is best suitable for agriculture. In fact this area today produces half the wheat, nearly half the maize and one-third of the cotton for the country, a remarkable achievement if we consider its area is only 3.3% of the country's total (Zhang 2011). There are not many rivers to deliver water from other areas to NCP. The Yellow River has an average runoff

only at 1175 m3 s_1 and almost dries up when it reaches the sea. In fact the Yellow River dried up seasonally for consecutive 10 years from later 1990s to early 2000s. From 2003 to 2012, the annual water withdrawal and water consumption from Yellow River increased linearly (Fig. 7), which is correlated with the expansion of irrigated area. The annual rainfall for the area ranges from 376 to 750 mm. The average water resources per capita in this area are only about 20% of the national average. It is estimated that the crop land in this area only has an average of 668 mm water per year, enough for one crop but short for two crops per year (Guan and Hubacek 2008).

Traditionally, NCP has two crops per year, mostly winter wheat plus summer maize or cotton. In the past, water requirement in this area is supplemented by rivers that come from the mountains at west and north and from the Yellow River at south. The agricultural development in the upper reaches of these rivers during the last several decades, however, has used much of the water resources. Approximately since 1980s, NCP has begun to extensively exploit underground water to supplement the water shortage for their two crops per year. The availability of electricity supply to the countryside and the cheap electric pumps have made this trend possible and much accelerated the process. Ironically, the initial rapid exploitation of underground water and quick decline of the water table has brought an unexpected benefit to the agriculture in this area. It solved the long-lasting problem of soil salinity (Li et al. 2012)! The salt is deposited down the soil profile and cannot come up to the surface because of the lack of the capillary movement of salt toward the top soil. As a consequence, major crops such as wheat now yield many folds more than before the 1980s, namely from about 1 or 2 tons per hectare to over six tons per hectare (Zhang et al. 1998). According to a long-term field study conducted from 1979 to 2012 at the Luancheng Agro-Eco-Experimental Station of the Chinese

Academy of Sciences, which is located in the northern part of the NCP, the winter wheat annual yield increase was ~193 kg/ha per year from 1992 to 2001 (stage 2). The average yield increased rapidly during the first stage (from 1979 to 1992) and still maintained increase in the third stage (2001-2012). Other measures, such as the improved culti-vars may also have helped (Zhang et al. 2013b), but salinity as a major problem existing in NCP for many centuries has largely disappeared (Yun and Wang 1997; Li et al. 2003; Chen et al. 2006; Ouyang et al. 2011).

The rapid and extensive exploitation of underground water indeed has helped the rapid increase in total wheat and maize production in NCP. For example, as the data released from Hebei Province, wheat and maize production increased by almost sixfolds when compared to 1970s. However, this fantastic achievement also accompanied a groundwater exploitation increase by the same six-folds (Fig. 8)!

During the recent three decades, it has been increasingly realized that the agriculture at the NCP is not sustainable in terms of water consumption. The over exploitation of underground water has led to the rapid expansion of irrigation to large areas. The Hebei Province has seen this expansion by threefolds over the last 50 years (Fig. 9). Apparently the consequence of such rapid expansion is the rapid declining of underground water table. As one of the experimental stations at Luancheng has reported, the groundwater table steadily dropped from about 10 m in 1970s to over 40 m today (Fig. 10).

The data shown in Figure 9 are typical for NCP but far from the worst ones. There are many reports in the newspapers and other public media that the land sinking is accelerating in many parts of NCP (Wang et al. 2009). Also the sea water infiltration near the coastline area has also been frequently reported. More and more serious water problems such as continuous decline of water table and worsening of water quality have also been reported

<8 400

SS £ 300

J § 200

£ 100

1997 1999 2001 2003 2005 2007 2009 2011 2013 Year

Figure 7. Annual water withdrawal and water consumption from Yellow River. Lines indicate recent significant increases in water consumption and water withdrawal. Data are from Yellow River Conservancy Commission, Ministry of Water Resources (2013).

150 120 90 60 30

x Wheat yield X

- a Maize yield x X X

Aff / À

- & .-- x Vf .

x A J^ r A -S A x X i i i i

0 5 10 15 20 25 30 35 Exploitation of groundwater (108 m3/year)

Figure 8. Total wheat and maize production at Hebei Province, North China Plain, in relation to the total exploitation of groundwater from 1970s to 2000s. Data are from Zhang et al. (2012).

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 Irrigated area (106 ha)

Figure 9. Relationship between groundwater exploitation (GW) and irrigation area (IA) in Hebei plain of North China during last 50 years. Data are from Zhang et al. (2009).

1972 1977 1982 1987 1992 1997 2002 2007 2012 0

5 10 15 20 25 30 35 40 45

Figure 10. Ground water table (GWT) changes during the last 40 years at Luancheng, Hebei Province, North China Plain. Data are from Zhang et al. (2009) and Wu et al. (2012).

(Liu et al. 2001; Jia and Liu 2002). The impact of such deteriorating situation of water depletion on social and economic developments in this area is huge and immense (Zhang et al. 2004, 2009). We expect more serious consequences will appear before long. There is no sign that the trend is slowing at the moment.

The Way Out: Integrative Water-Saving Agriculture

Water-saving agriculture is a technology system that focuses on improving the efficient utilization of crop water, field water, channel water, and recycled water, and the benefits from agricultural production. It also includes key products and equipments characterized by high-efficiency, low-cost, eco- and environment-friendly advanta-

ges, and setting up a water-saving engineering and management pattern suited to the different needs of different areas. Water-saving irrigation should be the effective way to cope with the ever increasing food demand in China. Is it possible to make the water use sustainable for Northwest China and NCP where a crisis of water shortage is developing? The goal should be to reduce the agricultural water use to a level that can sustain itself. To achieve this goal, we need to set the target that we should reduce the water use by half if compared to the "luxury" water use during 1980s and still maintain the productivity of crop land in these areas. The way out for the sustainable use of agricultural water resources is to increase the water use efficiency and much more effort should be made to develop water-saving techniques for the field crops. In general, high efficient use of agricultural water resources can be achieved by water engineering, agronomic, and biological water-saving methods, and administrative policies.

The conventional engineering approaches for water saving in China are the rational exploitation of water resources such as temporal and spatial optimization and allocation of water resources and coordinative dispatch of multiple water resources including surface water and groundwater. Furthermore, in arid regions where crop production is heavily dependent on irrigation and water resources are scarce, unconventional water engineering approaches should be also encouraged and developed. For instances, rainwater harvesting and storage for agricultural use in arid mountainous areas of Gansu and Ningxia Provinces have been practiced. Some simple and new improvements have made this indigenous method more efficient and more widely accepted. Utilization of saline or brackish water with total dissolved solid up to 3-7 g/L or water electrical conductivity up to 3200-6500 is/cm for irrigation on salt-resistant crops such as cotton and sunflower in the ecologically fragile oasis areas in Xinjiang or Gansu has been successfully developed. Use of recycled sewage water for irrigation has been promoted in the suburbs of Beijing city. Desalination of sea water or sea ice for irrigation in the seaside agricultural areas near the Bohai Bay has also been tested with some successes. These approaches have been complemented with traditional water-saving irrigation and engineering technologies such as canal lining, irrigation with low-pressure pipe conveyance, improved surface irrigation by shortening or narrowing border or furrow, surge flow irrigation based on laser-controlled land leveling technology and so on. In recent decades, sprinkling irrigation, micro-irrigation, drip irrigation under plastic film mulch, and subsurface irrigation have also been rapidly popularized in arid areas of Northwest China. One good example is the drip irrigation under plastic film mulch on cotton in Xinjiang

Uygur Autonomous Region, where more than one-third of the country's cotton, or one-eighth of world's cotton, is grown. Nearly half of the irrigated water can be saved in comparison to the traditional surface irrigation with this new irrigation method.

In recent years, more widely used approach of water saving in agriculture is the agronomic approach by ways such as crop rotation, water conserving through minimum tillage or no tillage, plastic film mulching or straw mulching, watering coupled with fertilization, breeding for drought tolerant varieties, chemical methods such as using superabsorbent polymers, soil amendments and antitranspirants. There are many reports for the successful use of these methods (Akhtar and Malik 2000; Puoci et al. 2008; He et al. 2009).

The most challenging and promising approach is arguably the biological water saving. By definition, the meaning of biological water saving is that water is saved directly from the reduced water consumption by the plants. This may be achieved through ways such as crop genetic improvement and physiological regulations through reduced stomatal opening or reduced light interception. One method that has been practiced and reported is the alternate partial root zone irrigation (Kang and Zhang 2004; Tang et al. 2005; Du et al. 2006, 2008a, b; Fereres and Soriano 2007; Davies et al. 2011). The practice in Northwest China showed that partial root zone irrigation can reduce irrigation, up to 30% reduction, and still maintain economic yield in crops such as cotton and maize (Kang et al. 1998; Tang et al. 2005, 2010). An additional benefit of partial root zone irrigation is that the plant growth pattern can be changed by continuously exposing some roots in drying soil. It has been reported that grapevine can have reduced vegetative growth and therefore reduced demand for pruning (Loveys et al. 2004). However, partial root zone irrigation can have some contrasting effects on vegetative vigor which may be agronomically advantageous according to total soil water availability (Romero et al. 2012).

Based on the widely used agronomic and promising biological water-saving approach, it should be emphasized that government policies on water allocation and government subsidies for water-saving practices should never be ignored in areas where the allocation of water resources is the key decision to allow balanced developments in different social sectors and maintain sustainable environment. As mentioned earlier in Northwest China, governments at different levels have set up special bureaus for specific watershed areas to allocate water resources. They are responsible to ration the water resources, set water-saving targets for different industries, monitor and charge the water consumptions at canals as well as at pumping wells. Water engineering projects and water-saving equipment

are subsidized from central government or provincial governments. All of these administrative approaches should be the effective approach to reverse the deteriorating trend of ecological environment in the Northwest China. After more than a decade of effort, we have already seen some improving effects.

Concluding Remark

It can be foreseen that water will be the most limiting factor for food production in the world in the near future. As many have suggested, we shall need a "Blue Revolution", to increase the agricultural productivity per unit of water, or "more crop per drop" (UNIS 2000). Norman Borlaug said in 2000, "how can we continue to expand food production for a growing world population within the parameters of likely water availability? The inevitable conclusion is that humankind in the 21st century will need to bring about a "Blue Revolution -more crop for every drop" to complement the so(sic)-called "Green Revolution" of the 20th Century. Water use productivity must be wedded to land use productivity. Science and technology will be called upon to show the way".

In China, with a huge population and only limited agricultural resources, such a Blue Revolution is more urgent than ever. Specifically we should first of all identify our unique problems in water scarcity in different areas, analyze the possible consequences if the business-as-usual way of water use in food security and other social and economical developments, design-specific and strategic plans to cope with the challenges, and finally set the specific targets in water-saving and water usage. As discussed earlier, a 50% reduction in irrigated water use in North and Northwest China is the target that must be achieved in the near future to make our water resources sustainable. Water-saving agriculture is the only way to achieve the target. As we have discussed, the irrigated agriculture should be controlled in a suitable scope in the arid and ecologically fragile areas of NCP and Northwest China, and the allocation of water resources must include the ecological water demand. Optimized and integrated technologies for increasing water use efficiency, yield and quality are available. However, it needs cooperation from all aspects, such as scientific research, effective technology, optimum organization, and strong market and government applications.

Acknowledgments

We are grateful for research grants from the National Natural Science Foundation of China (51222905, 51079147, 51321001, 50939005), the National High-Tech 863 Project of China (2011AA100502), the Ministry of Water Resources of China (201001061), China-EU Int'l

Collaboration Projects (S2010GR0692), National Basic

Research Program of China (2012CB114300) and Shenzhen Overseas Talents Innovation & Entrepreneurship

Funding Scheme (The Peacock Scheme).

Conflict of Interest

None declared. References

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