The basic design process
Surface irrigation design process is a procedure matching the most desirable frequency and depth of irrigation and the capacity and availability of the water supply. This process can be divided into a preliminary design stage and a detailed design stage.
The operation of the system should offer enough flexibility to supply water to the crop in variable amounts and schedules that allow the irrigator some scope to manage soil moisture for maximum yields as well as water, labour and energy conservation.
Water may be supplied on a continuous or a rotational basis in which the flow rate and duration may be relatively fixed. In those cases, the flexibility in scheduling irrigation is limited to what each farmer or group of farmers can mutually agree upon within their command areas. At the preliminary design stage, the limits of the water supply in satisfying an optimal irrigation schedule should be evaluated.
The next step in the design process involves collecting and analysing local climatological, soil and cropping patterns to estimate the crop water demands. From this analysis the amount of water the system should supply through the season can be estimated. A tentative schedule can be produced by comparing the net crop demands with the capability of the water delivery system to supply water according to a variable schedule. On-demand systems should have more flexibility than continuous or rotational water schedules which are often difficult to match to the crop demand. Whichever criterion (crop demand or water availability) governs the operating policy at the farm level, the information provided at this stage will define the limitations of the timing and depth of irrigations during the growing season.
The type of surface irrigation system selected for the farm should be carefully planned. Furrow systems are favoured in conditions of relatively high bi-directional slope, row crops, and small farm flows and applications. Border and basin systems are favoured in the flatter lands, large field discharges and larger depths of application during most irrigations. A great deal of management can be applied where flexibility in frequency and depth are possible.
The detailed design process involves determining the slope of the field, the furrow, border or basin discharge and duration, the location and sizing of headland structures and miscellaneous facilities; and the provision of surface drainage facilities either to collect tailwater for reuse or for disposal.
Land levelling can easily be the most expensive on-farm improvement made in preparation for irrigation. It is a prerequisite for the best performance of the surface system. Generally, the best land levelling strategy is to do as little as possible, i.e. to grade the field to a slope which involves minimum earth movement. Exceptions occur where other considerations dictate a change in the type of system, say, basin irrigation, and yield sufficient benefits to off-set the added cost of land levelling.
If the field has a general slope in two directions, land levelling for a furrow irrigation system is usually based on a best-fit plane through the field elevations. This minimizes earth movement over the entire field and unless the slopes in the direction normal to the expected water flow are very large, terracing and benching would not be necessary.
A border must have a zero slope normal to the field water flow which will require terracing in all cases of cross slope. Thus, the border slope is usually the best-fit subplane or strip. Basins, of course, are generally 'dead' level, i.e. no slope in either direction. Thus, terracing is required in both directions. To the extent the basin is rectangular, its largest dimension should run along the field's smallest natural slope in order to minimize land levelling costs.
The detailed design process starts with and ends with land levelling computations. At the start, the field topography is evaluated to determine the general land slopes in the direction of expected water flow. This need not be the extensive evaluation that is needed to actually move the earth. In fact, the analysis outlined earlier under the subject of evaluation is sufficient. Using this information along with target application depths derived from an analysis of crop water requirements, the detailed design process moves to the selection of flow rates and their duration that maximize application efficiency, tempered however by a continual review of the practical matters involved in farming the field later. Field length becomes a design variable at this stage and again there is a philosophy the designer must consider. In mechanized farming and possibly in animal power as well, long rectangular fields are preferable to short square ones in most cases except paddy rice. This notion is based on the time required for implement turning and realignment. In a long field, this time can be substantially less and therefore a more efficient use of cultivation and harvesting implements is achieved.
The next step in detailed design is to reconcile the flows and times with the total flow and its duration allocated to the field from the water supply. On small fields, the total supply may provide a satisfactory coverage when used to irrigate the whole field simultaneously. However, the general situation is that fields must be broken into 'sets' and irrigated part by part, i.e. basin by basin, border by border, etc. These subdivisions or 'sets' must match the field and its water supply. Thus, with the subdivisions established, the final land levelling is undertaken.
Once the field dimensions and flow parameters have been formulated, the surface irrigation system must be described structurally. To apply the water, pipes or ditches with associated control elements must be sized for the field. If tailwater is permitted, means for removing these flows must be provided. Also, the designer should give attention to the operation of the system. Automation will be a key element of some systems. The treatment of these topics is not detailed since there are other technical manuals and literature already available for this purpose.
The design methodology used in the guide relies on the kinematic-wave analysis for furrow and border advance and a fully hydrodynamic model for basin advance. These are transparent to the user of the guide, however, and further explanation for those interested can be found in Walker and Skogerboe. Simple algebraic equations are used for depletion and recession. This guide has reduced the role of these hydraulic techniques to the advance phase to allow the User to participate more in the design process. The interested reader can refer to several references in the bibliography for other graphical techniques which extend beyond those given here, but as one does so, it becomes more important to understand the nature of the hydraulic assumptions.
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