Planning water management.   The water-management system on a farm includes farm inlet and outlet structures, conveyance lines, the laying out of irrigation and drainage ditches, the erecting of water-control structures, such as water gates, diversion boxes and drop structures, and the grading or the levelling of the land. It may also include the construction of storage tanks, the installation of pumps, etc.

After a proper assessment of these factors and topographic survey, a farm is divided into a convenient number of blocks and plots. Each block should have an independent irrigation and drainage channel and an approach road. There should be the minimum wastage of land under the field bunds and channels. The water flow in the channel should be non-silting and non-erosive.

Water can be conveyed from the storage point or the farm inlet to the point of use through earthen or lined water-courses or through metallic, canvas, cement or plastic pipes, hoses or tubes. Several lining materials of varying cost and durability are now available. These are mud, cement, bitumen, asphalt, bricks, plastic membranes, sodium compounds, etc. Conveyance losses are extremely high, especially in light a href=soils1.html>soils, and lining is essential for using water economically. The water-courses should be at a higher elevation than the field level.

Land drainage.   The provision of drains is a must on every irrigated farm for sustained farming. Damage caused by excess water and the consequent development of the salt problem is enormous and unnoticeable in early stages. Except rice, almost all crops receive a severe setback during their growth when excess water stagnates even for a period as short as 3 to 4 days.

In heavy-rainfall areas, there should be adequate arrangements for surface drainage for the speedy disposal of water. In high-water-table areas, there should be subsoil drainage. Spacing between the drains depends upon the permeability characteristics of the soil, excess water-tolerance capacity of the crop grown, the amount of water to be disposed of and its speed. The drains may be open or closed. The open drains are easy to construct and maintain, but they occupy considerable land. The closed drains are costly, but save land and, hence, are desirable where the land is scarce. The closed drains may be of mole type or tile type. A mole drain is an unlined channel in the subsoil made with a tractor-drawn mole plough. This type of drainage is suitable only on heavy clay soils. These drains last for two to three years. The installation of the mole system is cheaper than the tile system.

Tile drainage is rather an expensive system but it lasts several decades. The investment varies with the area to be drained, the depth of installation, the quantity of water to be handled and the type of soil, and the cost may range from Rs 1,500 to 3,000 per hectare. A tile is made of clay or concrete and it is about 30 to 50 cm in length and 7 to 12 cm in diameter. These are placed end to end, with a gap of two to three mm between them. Excess water enters the system through this space and is conveyed along the gradient.

The preperation of land for irrigation also includes grading or levelling. Several implements, such as scrapers, floats and land planes are now available for moving the earth and smoothening the fields. These can be operated with animal power or tractors.

The term water requirement includes the amount of water for meeting the needs of evaporation, transpiration and metabolic activities (all together known as consumptive use), losses during the application of irrigation water and water needed for special operations, such as land preparation, transplanting, the leaching of excess water, etc.

The entity A, viz. the consumptive use depends upon the factors of weather, the type of crop canopy, the soil moisture status and the stage of the crop. The entity B, i.e. the application losses, depends upon the type of the irrigation system, the soil texture and structure, and management practices. The entity C, viz., special needs, depends upon such factors as the soil moisture status of the soil as well as water, and the nature of the crop species grown.

Recent researches have revealed that meteorological factors play a very prominent role in deciding the magnitude of the consumptive use of water which forms a major part of the value of water requirements of crop plants. Evapo-transpiration involves the conversion of soil moisture into water vapour and requires about 540 calories per gramme of water. The primary source of energy is the sun. The role of wind is two-fold. It carries water vapour away from the leaves, steepening the gradient (sink strength) or it may bring additional energy known as 'advective energy'. The advective energy can increase or decrease the consumptive use of water in a given locality. If there is a mass of active green vegetation, covering the ground completely and growing extensively under a non-limiting water-supply, the quantity of water evapo-transpiration by that mass of vegetation per day is independent of that species. Water loss by crop plants is, thus, primarily a physical phenomenon governed by the available solar and advective energy and the sink strength of the wind.

The consumptive needs of water by a crop cannot be reduced and no moisture economy is possible in the case of this item on a field scale, if the aim is to obtain the maximum yields.

The entity B in the expression of water requirement comprises application losses during the conveyance of water through seepage and deep percolation. The irrigation efficiency depends upon the soil texture. The finer is the soil texture, the lower are the application losses and the greater is the irrigation efficiency than surface irrigation. The management practices also influence the irrigation efficiency. The levelling of the land properly, the selection of the optimum plot size and the stream size and the proper scheduling of irrigation depth increase the efficiency. The application losses can also be minimized by using water-control structures, such as water gates, diversion boxes and by lining the canals.

The entity C, i.e. special needs, depends upon the nature of the crop grown, the salinity status of the soil or water or of the both, the soil moisture status at the time of land preparation. More water is required forcrop production if the transplanting of the seedlings is involved, if the soil or irrigation water is saline or if the soil is dry and hard before preparing the land for sowing.

The best, most rapid and most reliable method of finding out the consumptive use of water is with an open-pan evaporimeter which assesses all meteorological factors in an integrated way. The consumptive use of water is 0.7 to 8 times the value of the US class A open-pan evaporation when the soil moisture is adequate and the crop canopy is full and in an active stage of growth. A sunken screened open-pan evaporimeter furnishes directly the values of consumptive use of water by crop plants during their active growth.

The effeciency of field irrigation can be determined by measuring the quantity of irrigation water applied and stored in the root-zone. If it is not possible to measure the efficiency, the following broad values are suggested as a guide for surface-irrigation methods.

The quantity of water needed for special operations, such as land preparation, leaching down salts and transplanting, can be calculated by actual determinations in each case. The following values are indicated as a guideline.