Storage Irrigation

Storage Irrigation Points : Storage Irrigation, Methods of Assessment of run off from a catchment area, Basin Recharge, Direct Run-off, In storage irrigation system, a solid barrier, such as a darn or a storage weir is constructed across the river and water is stored in the reservoir or lake so formed. Depending upon the water requirements of crops, or the hydroelectric power generation, and upon the flow of water in the river, the volume of storage required is decided. From the contour plan of the basin at the site of construction, the elevation storage curve for the reservoir is known. The height of the dam is then decided from this curve, corresponding to the storage-volume required. In Pakistan, most of the irrigation schemes fall under this category. Fig. shows the schematic plan for such a scheme.

Storage it irrigation scheme comparatively of a bigger magnitude, and involves much more expenditure than a direct irrigation scheme. One or two main canals take off from the reservoir. Due to the formation of reservoir, some land property may be submerged to the upstream of the dam. A network of canal system convey water to the agriculture fields, through various regulatory works. Cross- Drainage works such as aqueducts siphons aqueducts passages and cannal are constructed natural drains cross the canals.

Methods of Assessment of run off from a catchment area: Methods of Assessment of run off from a catchment area: The run off of a catchment area in any specified period is the total quantity of water draining into a stream or into a reservoir in that period. This can be expressed as centimeters of water over a catchment, or (ii) the total water in cubic-metre or hectare meters for given catchment.

The rainfall is disposed off in the following manner:
1. Basin Recharge.
2. Direct run-off (or simply run-cm.
3. Percolation down to ground water.
4. Evaporation. 1. Basin Recharge (i) Rain intercepted by leaves and stems of vegetation.
(ii) Water held up in surface depressions, commonly known as the depression storage.
(iii) Soil moisture held as capillary water in pore spaces of soil or as hygroscopic water absorbed on the surface of soil particles. 2. Direct Run-off Direct run-off is that water which reaches the stream shortly after it falls as rain. Direct run-off consists of:
(i) Overland flow (or surface run-off).
(ii) Interflow (influent stream). Overland flow is that portion of water which travel across the ground surface to the nearest stream. However, if the soil is permeable, water percolates into it, and when it become saturated, flows laterally in the surface soil to a stream channel. The essential condition for interflow is that the surface soil is permeable, but the sub-soil is relatively impermeable so that water does not percolate deep to meet the ground water. 3. Percolation Down to Ground Water (Base Flow) If the sub-soil is also permeable, water percolates deep downwards to meet the ground water. Much of the lo water flog of rivers is derived from the ground water. Stream channels which are below the ground water table are called effluent streams. Earth Dams: Earth dams are made of locally available soils and gravels and therefore, are most common types of dams used upto moderate heights. Their construction involves utilization of materials in the natural state requiring a minimum of processing. With the advancing knowledge of soil mechanics and with the advent of more sophisticated earth moving eq4ipment, earth dams are now becoming more common, even for higher heights. The foundation requirements of earth dams are less stringent than for other types.