| As one of the three largest irrigation districts in China, the Hetao irrigation district is located in the alluvial plain of the Yellow River, western Inner Mongolia. The Hetao irrigation district is also one of the most important agricultural producing areas in the northern China. The water scarcity has become more serious due to the combination of increasing water demand caused by socio-economic development, and the decreasing water supply. The water withdrawn from the Yellow River has decreased from 5 billion per year to 4 billion per year since 1999, when the water management for the Yellow River was done by the central government. More than 90% of water used in the Hetao irrigation district is related to the crop production and large volume of water resources flow out to other areas in virtual form due to the crop trade. For the Hetao irrigation district, the analysis of variation of virtual water flows and its sustainability is of great significance for its water resources management and water resources planning.Based on the virtual water theory and the production-trade situation in the Hetao irrigation district, this study put forward the calculation method for virtual water flows related to crop trade at the scale of irrigation district firstly. Then the temporal variation of(total, blue and green) virtual water flows related to crop trade during 1960-2010 and that of inter-county virtual water flows during 2001-2010 were analyzed. After that, the main influencing factors of virtual water flows related to crop trade was studied. Lastly, the sustainability of regional water resources use were evaluated by using the following indicators: the rate of virtual water flows in total water withdrawn, the rate of net blue virtual water export in water withdrawn from the Yellow River, the water saving and the water saving efficiency of virtual water flows. The following conclusions could be drawn:(1) This study put forward the calculation method of virtual water flows related to crop trade at the scale of irrigation district.Based on virtual water theory, this study presented the limits and defects of calculation methods of virtual water flows at the large scales(such as the global scale and the national scale). Then the water consumption process and the trade situation were studied based on water resources cycle process, water balance theory, the crop production-consumption-trade system and the available meteorological, agricultural and social-economic data. Lastly, the calculation method of virtual water flows related to crop trade at the scale of irrigation district was put forward which is based on the virtual water content including the irrigation water loss and the trade volume based on the crop surpluses and deficits. The actual water consumption for the traded crop could be presented and the trade data could be assumed by using this method. At the same time, some suggestions could be provided for other irrigation districts.(2) This study analyzed the temporal variation of virtual water flows related to crop trade during 1960-2010 in the Hetao irrigation district and that of inter-county virtual water flows during 2001-2010.This study analyzed the temporal variation of(total, blue and green) virtual water flows related to different crops trade during 1960-2010 in the Hetao irrigation district, and that of inter-county virtual water flows during 2001-2010, from the perspectives of volume, direction and constitute, based on the available meteorological, agricultural and social-economic data and the calculated method mentioned above. Ten different kinds of crops were included in this study. Results show: the volume of virtual water export presented a fluctuating increasing trend during the study period and the value in 2010 was 3.28 billion m3, which was 1.56 times of that in 1960. The volume of virtual water import increased from 1960 to 1979, and then decreased from 1.58 billion m3 in 1979 to 0.32 billion m3 in 2010; During the study period, the temporal variation trends of virtual water flows related to different crops trade were different significantly and four types could be seen: the value increased firstly and then decreased, the value increased fluctuated, the value decreased fluctuated or the value changed slightly; Virtual water flows related to rice trade were mainly from other areas to the Hetao irrigation district and those related to other crops trade were mainly in the opposite direction; Most of the total virtual water flows related to crop trade were blue water and the green water fluctuated significantly during the study period, compared with blue water; The virtual water export related to total crop trade was mainly due to the trade of grain crops firstly, and then was mainly due to the trade of non-grain crops, and the virtual water import was mainly due to the grain crop trade during the whole study period; The inter-county virtual water flows, most of which were blue water, were caused by the trade of wheat, coarse cereals, sugar beet and vegetable, and the largest virtual water export and import areas were Dengkou and Qianqi.(3) This study analyzed the main influencing factors of virtual water flows related to crop trade in the Hetao irrigation district.Based on the virtual water theory and the existing studies, this study analyzed the main influencing factors of virtual water flows related to crop trade in the Hetao irrigation district qualitatively using the Pearson correction analysis. Then the contribution rates of different factors(meteorological, agricultural and social-economic factors) were calculated to show the influence on virtual water export or virtual water import quantitatively, based on the statistical methods and the regression analysis. Results show: the volume of virtual water export related to crop trade was mainly influenced by crop yield, irrigation efficiency, area sown and mean annual temperature, and the contribution rates were 84.22%,-14.68%, 9.21% and 4.35%, respectively; the volume of virtual water import related to crop trade was mainly influenced by total population(84.39%) and proportion of population in urban areas(9.18%) during 1960-1979, while the main influencing factors were retail-price index for crop(89.52%) and total population(-2.01%) during 1980-2010.(4) This study evaluated the sustainability of water use in the Hetao irrigation district.Based on the virtual water theory and the available date, several indexes(the contribution rate of virtual water flows on regional water stress, the water saving efficiency of virtual water flows and others) were defined in this study. And then the sustainability of water use in the Hetao irrigation district was evaluated using these indexes. Results show: contribution rate of virtual water export on regional water stress increased during 1960-2010 and the value in 2010 was 73.26%. In other words, about 3/4 of water resources in the Hetao irrigation district was used to produce the exported water-intensive crops, which makes the water shortage in this area more serious and the sustainability for water use weaker; it is helpful for alleviating water shortage in the Hetao irrigation district by importing virtual water while the contribution rate of virtual water import on regional water stress decreased since 1980 s. The value of the contribution rate of virtual water import on regional water stress was 7.10% in 2010, less than 1/10 of that for virtual water export; Rate of net blue virtual water export in water withdrawn from the Yellow River decreased at an average rate of 0.67% per year since 1980 s and the value in 2010 was 53.90%. The high value shows that most of blue water in this area is used by agriculture rather than other sectors and the water use in this area is unsustainable; a water loss of 1.96×106 m3 could be seen for inter-county virtual water flows in the Hetao irrigation district during 2001-2010 and the rate of water loss in virtual water flows may as high as 40% in certain years. In other words, the water distribution and the sustainability for water use should be improved.This study put forward the calculation method for virtual water flows related to crop trade at the scale of irrigation district. Then the temporal variation of virtual water flows related to crop trade during a long period was analyzed and the main influencing factors of virtual water flows related to crop trade was studied qualitatively and quantitatively. Lastly, the sustainability of regional water use was evaluated by using different indicators. This study could provide suggestions for the water resources management, the alleviation of regional water pressure and the water resources management in other irrigation district. |
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