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By far the largest user of fresh water is agriculture, which now accounts for around three-quarters of global water consumption. If, as is almost certain, the population increases by 65 per cent over the next 50 years, around 70 per cent of this future world population will face water shortages and 16 per cent will have insufficient water to grow their basic food requirement. The necessary increases in food production cannot be achieved without higher productivity on existing land and with existing water resources. A central question for HELP is therefore "what is the scope for increasing the efficiency with which water is used in agriculture?" Although irrigation is often proposed as the way to achieve higher productivity per unit area of land, it increases the pressure on freshwater resources. Further, because irrigated agriculture only provides about a third of the world’s food, a relatively large increase in production would be required. On the other hand, increasing the efficiency of rain-fed agriculture would both increase food production and also reduce the demand on freshwater resources. Most global assessments of future food production ignore the possibility of increasing rain-fed food production per unit area. However, in regions such as sub-Saharan Africa where population growth is very high, most (>90 per cent) food comes from rain-fed agriculture, and this is likely to remain the case for the foreseeable future. It is therefore essential to increase water-use efficiency in rain-fed, as well as in irrigated, agriculture.
Food and water policy Five key policy issues are identified in respect of water and food:
Food and water management
We can only improve agricultural water management and at the same time increase food production by improving water-use efficiency. This is true for irrigated as well as rain-fed agriculture. It means producing the same or a larger amount of food with less water. While improving water-use efficiency relates mainly to water quantity, improving or maintaining water quality is also a central task for agricultural water management. While the increase of food production is the most pressing issue in many countries, the maintenance and improvement of water quality is of prime concern in others.We can improve water-use efficiency through:
The hydrological sciences contribution
The technical basis for improving agricultural water-use efficiency is to increase the total amount of the water made available to plants, and/or to increase the efficiency with which transpired water produces biomass. The main ways in which the former can be achieved is to improve infiltration(that is, reduce surface runoff), reduce direct evaporation of water from the soil (or irrigation water) and reduce drainage. We know some of the basic techniques for reducing surface runoff, soil evaporation and drainage, but the suitability and net effect of a particular approach in a given environment require further study. We can achieve another form of efficiency improvement by fixing more carbon per unit of water transpired. This "transpiration efficiency" varies with crop type and atmospheric humidity, with higher efficiencies in more humid environments. In principle, therefore, more biomass could be produced using the same amount of water by selecting species with high transpiration efficiencies or by growing plants in more humid air. The latter could be done on a macro scale, that is, by growing plants and/or using irrigation water at times, or in places, where air humidity is high. There is also some scope for microclimate manipulation in semi-arid regions where the relative humidity around crops can be increased using an over-storey of trees. Again the scope for improving transpiration efficiency has been identified, but there is a need for systematic study of which options are likely to work in particular environments. It should also be noted that too much water can lead to waterlogging and salinity problems. In brief, the main scientific question about food and water that requires research is:
There is much scope for improving water-use efficiency in agriculture, but appropriate solutions need to be developed for particular physical, social and economic conditions. We need to increase efforts to introduce technical innovations to the social, political and institutional structures that can encourage farmers to adopt the improvements. If this was achieved, then more areas of the world would be able to produce the food they need for their current and future populations. |
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