Water scarcity: A major threat to agriculture

Alternate sources of irrigation can help overcome water shortage to some extent

By Dr. S.M. Alam, NIA, Tandojam
August 02 - 08, 1999

The increasing scarcity of water resources is a major threat to the country's agriculture. A dependable supply of good quantity water is essential for the success of any agricultural project. Both the quality and quantity of the water required will depend on the activity involved. For example, sprinkler irrigation requires large amounts of water of high quality. Approximately, 100,000 liters of water is needed to apply 20 mm of water to over one hectare. If the water is high in calcium (hardness) or contains particles then the sprinkler may become blocked or eroded and affects the irrigation systems. It is essential that the delicate balance between too little and much water in the plant root environment is carefully controlled. Increasing demands on marginal land for crop growth means increasing demands on spares irrigation water resources, and in cultivated land not under irrigation the problem may be too much, rather than too little water available for crop growth. Water management should, therefore, be viewed as a priority in agricultural production and development; water wastage through evaporation and drainage must be reduced to a minimum. The underlying principle of irrigation scheduling is to add only sufficient water to the soil to meet the daily requirements of a growing crop, with adding excess water which becomes wasted through evaporation from the soil surface or through drainage. Some of the sources of irrigation are given as:

River waters: In many areas, rivers are often the most easy and convenient source of water for irrigation purposes. However, the flow of river water is dependent on the rainfall in the catchment area. Consequently, during dry seasons, when maximum quantity of water is required, the flow will be the lowest. If too many users abstract water from the river, the flow may be reduced to such an extent that it silts up. The quality of irrigation is often poor in city areas. Since domestic and industrial sewage is frequently discharged directly into the water course.

Lakes and reservoirs: Natural lakes generally store water, when it rains and so provide a source which can be used at other times of the seasons. Similarly, dam or reservoir projects store water from the wet season for use in the dry season. This overcomes the problem of naturally fluctuating flow. Because a lake or reservoir is a closed system, nutrients and pollutants can accumulate and sometimes reach toxic levels. If there is a build-up of nitrates and phosphates, then entrophication may occur which will further lower the quality of the water.

Ground water: Ground water is obtained from boreholes sunk into water bearing rocks or aquifers. This water is often of great age and so providing it is not saline, it is often of very good quality.

Piped supply water: The capacity of many piped supplies is often insufficient for irrigation purposes, although it may be adequate for general farm or plantation use. Due to this problem, storage tanks are often used. These are filled up overnight so that pumped supplies can be delivered during the day. Piped water is often treated to a high standard to make it suitable for human consumption. Where water resources are limited, low quality supplies can be used for irrigation.

Rain water: Rain water is also good source of water for agricultural crops and for other uses. It is a natural water and its purity is considered to be of maximum extent. As rain falls, more water is added to the soil and the soil moisture content increases. The soil can not hold all the water received and, after period of time (usually 24 hours), the soil will have lost excess water through drainage and is then in a saturated equilibrium condition known as field capacity (FC). Any further addition of water will not be retained in the soil and will thus be unavailable to plants. For convenience, a value of 0.05 bars (suction) is taken to correspond to field capacity.

Water Quality: No natural water is actually pure since it contains a variety of materials, either dissolved or in suspension, as well as microorganisms (many of these are harmless to human being, plants and animals). However, it is normally difficult to determine visually whether a water sample is suitable for drinking purpose or for irrigation. During drought conditions it becomes acceptable to use water of lower quality, particularly for non-essential uses.

Chemical salts: The chemical salts such as sulphates, carbonates, calcium, magnesium and iron are present in most water supplies, but are not normally found in concentrations high enough to affect crop growth. However, if used for irrigation, water containing these salts can increase the alkalinity of the soil. Excessive hardness of water can lead to pipe and sprinkler blockages. Iron salts can also cause blockages, as well as leading to pipe corrosion through the growth of iron bacteria.

Metals: Natural water sometimes contains raised levels of metals, but they are more commonly associated with sewage sludge or the pollution of other water sources with industrial effluent. These metals sometimes cause direct damage to crop plants, but are also often accumulated in the leaves. In this way humans or animals may be subjected to a higher than normal intake of these when the crops are eaten.

Saline water: Few crops (and even fewer animals) are able to tolerate saline water. Many commercial crops are extremely sensitive to saline water and will either be damaged or produce reduced yield. Irrigation with saline water is also likely to damage the soil.

Micro-organism: Plant, animal and human diseases can be spread by water contaminated by bacteria, viruses and other micro-organisms. Contamination of water supplies with domestic sewage is one of the main problems. Water which is used to irrigate crops which are likely to be eaten raw should be checked for biological contamination before it is used.