PLANNING OF AN IRRIGATION CANAL SYSTEM
During of irrigation canal projects include the determination of: (i) canal alignment, and of water demand. The first step in the planning of an irrigation canal project is to carry core liminary survey to establish the feasibility or otherwise of a proposal. Once the gravity of the proposal has been established, a detailed survey of the area is carried out and, thereafter, the alignment of the canal is fixed. The water demand of the canal is, ten, worn out.
To determine the feasibility of a proposal of extending canal irrigation to a new area, information on all such factors which influence irrigation development is collected during the preliminary (or reconnaissance) survey. During this survey all these factors are observed or enquired for the local people. Whenever necessary, some quick measurements are also made.
The information on the following features of the area are to be collected:
· Type of soil,
· Topography of the area,
· Crops of the area,
· Rainfall in the area,
· Water table elevations in the area,
· Existing irrigation facilities, and
· General outlook of the cultivators with respect to cultivation and irrigation.
The type of soil is judged by visual observations and by making enquiries from the lost people. The influence of the soil properties on the fertility and water holding capacity has already been discussed.
For a good layout of the canal system, the command area should be free from too many undulations. This requirement arises from the fact that a canal system is essentially a gravity flow system. However, the land must have sufficient longitudinal and cross slopes for the channels to be silt free. During the preliminary survey, the topography of the area is judged visual inspection only.
Water demand after the completion of an irrigation project would depend upon the cross being grown in the area. The cropping pattern would certainly change due to the introduction of irrigation, and the possible cropping patterns should be discussed with the farmers of the area.
The existing records of rain gauge stations of the area would enable the estimation of the normal rainfall in the area as well as the probability of less than normal rainfall in the area. This information is obviously, useful in determining the desirability of an irrigational projection in the area.
Water table elevation can be determined by measuring the depth of water surface in well from the ground with the help of a measuring tape. Water table elevation fluctuation considerably and information on this should be collected from the residents of the area and checked by measurements. Higher water table elevations in an generally indicate gone rainfall in the area as well as good soil moisture condition. Under such conditions, the demand for irrigation would be less and introduction of canal irrigation may cause the water table rise up to the root zone of the crops. T he land is then said to be water logged and the productivity of such land reduces considerably. Water logged land increases the incident of malaria in the affected area. Thus, areas with higher water table elevation are not suitable for canal irrigation.
Because of limited financial and hydrological resources, an irrigation project should be considered only for such areas where maximum need arises. Areas with an extensive network of ponds and well systems for irrigation should be given low priority for the introduction of canal irrigation.
The success or failure of an otherwise good irrigation system would depend upon the attitudes of the farmers of the area. Enlightened and hard-working cultivators would quickly adapt themselves to irrigated cultivation to derive maximum benefits by making use of improved varieties of seeds and cultivation practices. On the other hand, conservative farmers will have to be educated so that they can appreciate and adopt new irrigated cultivation practices.
The information collected during preliminary survey should be carefully examined to determine the feasibility or otherwise of introducing canal irrigation system in the area. If the result of the preliminary survey is favourable, more detailed surveys would be carried out and additional data collected.
The preparation of plans for a large canal project is simplified in a developed area because of the availability of settlement maps (also called shajra maps having scale of 16 inches to a mile (i.e., 1/3960 @1/4000) and revenue records in respect of each of the villages of the area. The settlement maps show the boundaries and assigned number of all the fields of the area, location of residential areas, culturable and barren land, wells, ponds and other features of the area. Usually for every village there is one settlement (or shajra) map which is prepared on a piece cloth. These maps and the revenue records together give information on total land area, cultivated area, and crop-wise cultivated area and the area irrigated by the existing ponds and wells.
With the help of settlement maps of all villages in a doab, a drawing indicating distinguishing features, such as courses of well-defined drainages of the area, is prepared. On this drawing are then marked the contours and other topographical details not available on the settlement maps but required for the planning of a canal irrigation project. Contours are parked after carrying outleveling‘’ survey of the area.
The details obtained from the settlement maps should also be updated in respect of developments such as new roads, additional cultivated area due to dried-up ponds, and so on. A an undeveloped (or unsettled) area, however, the settlement maps may not be available and the plans for the canal irrigation project will be prepared by carrying out engineering survey of the area.
One of the most important details from the point of view of canal irrigation is the watershed which must be marked on the above drawing. Watershed is the dividing line between the catchment areas of two drains and is obtained by joining the points of highest elevation on successive cross-sections taken between any two streams or drains. Just as there would be the chain watershed between two major streams of an area, there would be subsidiary watersheds between any tributary and the main stream or between any two adjacent tributaries.