Economic Importance of Agricultural Irrigation
Water provides goods (e.g., drinking water, irrigation water) and services (e.g., hydroelectric production, recreation, and freshwater) used by agriculture, industry, and households. The provision of most of these goods and services is related to the quantity and quality of available water. Water management and distribution require considering its unique characteristics as a resource.
Water used in irrigation can be pumped from groundwater reserves or provided from rivers, lakes, and dams. Water brought to the land is applied to crops through flooding, channels, by spraying, or by dripping. The applied water infiltrates the soil, evaporates, or flows as surface water. Part of the water infiltrating the soil is taken up by plants (and later lost through transpiration) and part of it recharges groundwater. Water infiltrating underground can pollute groundwater with chemicals (fertilizers, herbicides, and pesticides), salts leached from the soil, and waste from animal waste. In cases where river levels are low and groundwater levels are high, groundwater can replenish surface water levels, which creates a two-way link between surface and groundwater resources.
Limiting Water Use
It is not easy to control or prevent water use. Many water uses involve the withdrawal of water from the hydrological system (known as 'extractive' or 'off-stream'). Generally, only a small portion of the withdrawn water is consumed. Consumed water is used in plants, animals, or industrial products. However, most of the withdrawn water is not consumed and returns to the water system to be reused at a later time and in a different place. Water in the return flow re-enters the surface flow system, can infiltrate aquifers, or can return to the gaseous hydrological system by evaporating. Water can also be used in-stream without removing it from the hydrological system (for example, hydroelectric power generation or dilution by boat). Such uses generally require little or no water consumption, but they affect where and when water can be consumed for other uses (Young, 1996).
Water is a 'bulk' resource. This means that its economic value per unit weight or volume is relatively low. Therefore, it requires high costs per unit of transport volume and is not economically feasible over long distances unless a high marginal value can be achieved. Abstraction, storage, and any transport costs tend to be high compared to the low economic value placed on the use of an additional unit of water. This can create site-specific values for water (Young, 1996). Another characteristic of water is that the supply quantity cannot be easily determined. This is primarily determined by the following processes: water flow; evaporation from the surface; and supply as spreading onto the earth. The supply of surface water is largely determined by climate. Consequently, the quantity provided is variable and unreliable. This affects specific water uses (e.g., the development of water-dependent industries) and the value of water in some uses (e.g., irrigation). The quality of water (the nature and concentrations of pollutants) can exclude certain uses (e.g., drinking water for domestic use), but has no effect on others (e.g., hydroelectric power generation).
Water is an Important Resource
The nature of irrigation water demand relates to quantity, location, timing, and quality. Irrigation is generally carried out with large amounts of water. The large volumes of water required for irrigation must usually be transported over a distance to the field. For surface water, canals and pipes can provide conveyance, and for groundwater, it is provided through withdrawal pipes. In terms of timing, irrigation water demand can extend throughout the growing season and, when sufficient supply is available, can extend into the dry season. The highest demand for irrigation water generally occurs in surface waters. Surface waters can be the need for storage capacity, naturally occurring water bodies (lakes, wetlands, and aquifers), or specially constructed dams. High salinity levels occurring when the quality of water required for irrigation is low prevent irrigation use, and contaminated supplies can reduce crop quality (e.g., contamination of horticultural products by pathogens in polluted water sources).
Irrigation is a vital component of agricultural production in many developing countries. In 1997-99, while irrigable crop production occurred in two-fifths of developing countries on irrigated lands, irrigated lands accounted for about one-fifth of the total cultivated area. Developing countries are particularly dependent on irrigation: in 1997-99, 59% of cereal production in developing countries was irrigated (Bruinsma, 2003). Food production in developing countries is increasing in response to the demands of expanding populations and increasing prosperity. Irrigated agriculture will make a significant contribution to this demand.
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