Storage and Distribution of Water for Irrigation

The regime of most great rivers is irregular. Frequently, they carry their greatest volume of water in spring and in late autumn and winter they mау be reduced to mere trickles of water. То control these rivers and to regularize their regimes by storing water in the dry seasons, dams and reservoirs are constructed.

Dams, known also as barrages and weirs, are barriers built across rivers or streams to control the flow of water.

Today most dams have several functions, which mау include the storage and diversion of water for irrigation, the raising of water for generating hydroelectricity, and the provision of flood control. Dams have been constructed for thousands of years, at first of earth and later of stone.

Sometimes, the source of water may be lower than the area to be irrigated, especially if the river runs in а canyon, and the water itself often has to be transported considerable distances from the river to the fields. In such cases, complex systems of pumping stations and canals may be necessary to lift and move the water from the reservoir to the fields.

А large-scale system of irrigation requires а complex network of dams, pumping stations and canals. In addition to the main dam, whose reservoir is the main storage unit, sma11er diversion dams are needed to direct the water into an intricate canal system. The water is led from the dams into broad canals by gravity, and where these major canals, because of local physical conditions, are unable to receive their required water by gravity, pumping stations may be installed. These plants frequently receive their power from energy generated from power stations at the main storage dam.

From the main canals, water is diverted into а system which will distribute it throughout the farm. The most common means by which this is done is with open ditches or laterals, and the flow of water into them is controlled by headgates or regulators. They are frequently earth ditches, which may suffer from excessive losses owing to seepage and evaporation, especially in arid regions or in areas of porous, sandy or gravelly soils. То eliminate such wastage, the use of tubing in place of open ditches to carry water from the canal to the land has recently been introduced. Tubing also prevents the loss of land otherwise used for ditches.

Leading from the permanent open ditches are secondary or field ditches. They are ploughed in at the end of the growing season, as they wou1d otherwise act as obstacles during harvesting. Water is delivered from these ditches to the areas to be irrigated by means of check structures or turn-outs. Increasingly, however, water is being transferred from the ditch over the ditch bank into individual fields or furrows by means of siphons. These may be plastic, metal or rubber, and depending upon the size and volume of the water supply, may have flow capacities from as little as one gallon per minute to over 1,000 gallons per minute.

А very efficient way of conveying and distributing irrigation water is by means of pipe-lines. This method has many advantages: it practically eliminates losses due to evaporation and seepage, it reduces maintenance work, makes water control easier, eliminates the problem of weeds which grow along the banks of the open ditches, and makes it possible for water to be carried by gravity or under pressure. Such рipе1ines may be permanent installations or portable. The former usua11y consists of buried concrete supply and distribution lines, while the latter consist of metal or flexible surface pipes. Because of the very high cost of installation, however, pipe-1ines are still relatively uncommon, and they are generally found in areas where water is scarce and crops are valuable.