Study On Spatial And Temporal Distribution Of Water Use Efficiency In China’s Grain Production

The fact of water shortage and uneven distribution between water resource and soil is the main factor that limits stable and continual grain production in China. So in order to use water in high efficiency which is the goal of water resource management, it is very fundamental to assess water occupation by grain production in a scientific and reasonable way as well as assess water use efficiency based on it. Contraposing shortages of traditional ways used in agricultural water use efficiency, this paper used the theory of water footprint and established indexes of water use efficiency of grain production based on calculation of water footprint of crop production in 31 provinces of China from 1998 to 2010 to assess water use efficiency of grain production in China. Based on comprehensive analysis of spatial and temporal difference of it, we searched the reasonable indexes that can evaluate the difference of water use efficiency in grain production among regions. Mainly conclusions are as following:(1) This study revealed spatial and temporal distribution of water footprint and its composition. Water footprint is the volume of water that has been used to produce grain, including blue water footprint and green water footprint. The average water footprint of China was about 6900×108 m3, exhibiting significant spatial difference. Water footprint produced by irrigated farmland was more than two-thirds of the whole amount of water footprint and exhibited increasing trend. For the whole country and every province, the proportion of effective water footprint(the part of water footprint that has been consumed in farmland, EWF) showed increasing trend. In province and in irrigated farmland, the proportion was 76.9% and 65.9%, respectively. The increase of GWF and EWF meaning that improve of composition of water footprint. Distribution of grain production and water footprint was changing gradually, showing gathering trend to the following districts simultaneously: huang-huai-hai region, northeast region and the middle and lower reaches of the Yangtze River. What’s more, northeast region was becoming the mainly region that produced grain and water footprint.(2) The paper showed water footprint of crop production both in irrigated and rain-fed farmland. Meanwhile it revealed the spatial difference of its composition. The average WFP of China was 1.340 m3. In general, WFP and BWFP in each province showed significant decline trend, while GWFP didn’t show any regular fluctuation in different years; Comparing with WFP of rain-fed farmland, WFP of the whole farmland and irrigated farmland showed more significant decline trend. WFP of given crops in each province didn’t change greatly in different years; Except WFP in rain-fed farmland, in general, each index exhibited significant gathering trend in space: low value of WFP mainly distributed in the north of Yangtze River, Huang-Hai-Hai region and southwest region; high value of WFP mainly distributed in southeast region, South China coastal areas and northeast of China; The spatial distribution of WFP didn’t change greatly in different years. All these factors, regional climate, the amount of water resource, the type of agricultural production, crop varieties, level of irrigation facility and so on, had important influence both on the spatial distribution of WFP and on its temporal evolution.(3) Relationship between water productivity indexes and WFP wasr explored in the research. Using irrigation water as input indicator, this paper established different water productivity indexes based on different scale, such as regional water productivity(IWPg), canal water productivity(IWPc), and field water productivity(IWPn); using general water resource as input indicator, this paper established WPg, WPu, WPET to reflect water productivity. Each water productivity index showed increasing trend, suggesting that water productivity of grain production was improved in general. Each index of water productivity that used general water resource as input exhibited gathering trend in space, high value of water productivity mainly distributed in Huang-huai-hai region and regions around it; low value of water productivity mainly distributed in northeast region, southeast, South China coastal areas and part of northwest regions. Every water productivity index could be used to evaluate the relative size of input of water resource-output of grain between different regions; Water productivity based on input of general water resource was most suitable to evaluate the spatial difference among regions; Based on liner relationship between WFP and water productivity, it was feasible to derive WFP from water productivity or derive water productivity from WFP.(4) The report quantitatively explored the influence that irrigation could have on grain production and water utilization. Using increase of grain output that has been produced by irrigation as output indicator and using consumption of blue water(CWUb) and TWUb as input indicators, this paper established indexes of marginal benefit of irrigation in regional scale(MRR) as well as in field scale(MRF). Irrigation played dominated role in producing regional water productivity, increasing spatial difference of it and enhancing the stability of this difference. MRR and MRF were marginal efficiency of grain output created by CWUb and TWUb respectively. MRR and MRF in China were 1.108 kg/m3 and 0.542 kg/m3 respectively. Marginal benefit of irrigation of each province showed increasing trend; MRF and MRR showed decline trend along with the increase of CWUb and TWUb respectively; Marginal benefit of irrigation and consumption of blue water can be calculated by fitting Power Function. The index of marginal benefit of irrigation is supplement of water productivity in traditional way, offering new content to the assessment of agricultural water use efficiency.(5) The report also proposed comprehensive index of water use efficiency(CEI) of grain production and searched its characteristic of temporal and spatial distribution, then the paper analyzed the influence of virtual water flow having on water use efficiency. Water using in grain production was becoming more reasonable. For China, CEI has increased from 0.525 in 1998 to 0.604 in 2010. For province, CEI exhibited significant gathering trend in space, and it was similar to the trend of WFP. Due to the decrease of the use of green water and proportion of effective water footprint, virtual water flows among provinces increased water productivity without enhancing the reasonability of water use. Based on relative role of regional grain production and CEI in the whole country, the paper discussed grain production in each province and the policy used to manage water. Northeast region should be an important area to increase water use efficiency of grain production.This study used water footprint to assess agricultural water use efficiency and systematically quantified water usage status during the process of grain production in China. Then the paper evaluated function of water footprint of crop production and water productivity in measuring the spatial difference of relationship between water resource and grain output, meanwhile the paper analyzed their relationship. What’s more, this paper established marginal benefit index and comprehensive indicator of water use efficiency of grain production, meaning that this paper evaluated water use efficiency of grain production in China and its spatial difference in a more comprehensive way. It helps to enrich and improve theories and indicators that have been used to assess water use efficiency, and provides reference to policy making in water resource management.