investigating the proportion of wheat planted area in iran with wheat yield and water demand by focusing on virtual water approach

introduction the average annual precipitation in iran is about 250 mm that is not regularly distributed spatially and temporally. more than 70 percent of total precipitation is not accessible due to evapotranspiration. total renewable water resources of the country are estimated to be around 130 billion cubic meters. in spite of limitation in water resources, they are not used properly. agriculture is considered as the biggest sector that uses around 90 percent of total accessible water. accordingly, any mismanagement of water usage in this section can make a remarkable damage to the country water resources. paying attention to virtual water content of products is one of the methods to reduce water usage in this section by selecting proper agricultural products for each area.   virtual water is a measure of the total water used in production of a good or service. the concept was initially used to illustrate the advantages to water scarce nations of trade with other nations, rather than attempting to produce all goods locally. in recent times the concept has been applied to argue against production of commodities with high embodied water content, or to argue against their export on the basis that these activities waste scarce water resources. virtual water estimates have also been used as an indicator of environmental damage of certain production activities. estimation of a product’s virtual water content contains more than just considering water directly applied to growth or to process. in the case of agriculture, it should also attribute, for example, the water contained in producing fertilizers and pesticides used on a farm, and the water used to grow and process grains fed to animals. moreover, it must do so over the full lifespan of a plant, and also include all the water used at postharvest stages of production, including any inputs to those stages. generally, water associated with transportation must also be included, but this usually turns out to be a small and negligible amount. regarding its remarkable demand all around the world, wheat is one of the most strategic agricultural products. accordingly any decision towards changing the crop production may have distinct local, regional and global effects. on the other hand, considering the dominance of conventional irrigation methods in iran which impose remarkably higher stress on water resources in comparison with developed countries, an effort should be made. to achieve a wheat farming template through which the optimized amount of water is needed to have the maximum potential crop, a survey should be run and the whole country should be classified in categories. the wheat farming priority then should be attributed to provinces where the most optimized conditions are observed. wheat is the main agricultural product in iran and considering its significance and remarkable consumption, its farming in areas with lower water consumption would cause a reduction of water usage in agriculture section. wheat virtual water is equal to the amount of net water required for farming divided by produced wheat per hectare in each province.    materials and methods in this paper wheat yield, net water required for farming, irrigation water and farming area in each province has been investigated and by comparing these data for each province, proper areas for farming this important and strategic agricultural product is recommended.  in order to evaluate the water demand in different areas cropwat software is used.  finalized data are shown through maps in gis environment.   results and discussion as it is seen in figure 1 net water demand for wheat crop in central parts of iran is higher than marginal areas. minimal values are seen in caspian sea coastline.   figure 1- net water demand for wheat crop in different provinces of iran as it is seen in figure 2 the highest values of irrigation need are observed in central and southeastern parts (qom, isfahan, yazd, kerman, sistan and baluchestan provinces).   figure 2- wheat irrigation need in different provinces of iran   on the other hand the lowest values are seen in north and northwestern areas (guilan, mazandaran, golestan, ardebil, east and west azerbaijans, ilam, kohkilooyeh and boyerahmad and kermanshah provinces). besides water demand and irrigation need, wheat yield should also be taken into consideration. wheat yield in different provinces of iran is shown in this paper. as it is seen the lowest yield is observed in eastern provinces and also in bushehr where remarkable water shortage and conventional irrigation methods exist. wheat yield in provinces like tehran and kordestan are estimated to be more than five tons per hectare, while provinces like qazvin, hamedan, zanjan and hormozgan have wheat yields greater than four tons per hectare. as a rule of thumb, the farming priority should be done to provinces where the highest wheat yields are observed.   the largest wheat farms are located in khuzestan province. this province holds the 13th rank in irrigation need and 18th rank in wheat yield. accordingly, any enhancement project should be stopped. a similar status is seen in provinces like fars, khorasan razavi and kerman.   conclusion according to the results there is no coincidence between the optimization of water demand, wheat yield and surface area of wheat farming in different provinces. this fact would be terminated in more water loss and stress. the largest wheat farming areas are located in provinces like khuzestan, fars and khorasan razavi where a remarkable irrigation demand is observed. accordingly a shift should be occurred towards northern and western provinces where less irrigation need is observed.