Objective and scope of design
The surface irrigation system should replenish the root zone reservoir efficiently and uniformly so that crop stress is avoided, and resources like energy, water, nutrient, and labour are conserved. The irrigation system might also be used to cool the atmosphere around sensitive fruit and vegetable crops, or to heat the atmosphere to prevent their damage by frost. An irrigation system must always be capable of leaching salts accumulating in the root zone. It may also be used to soften the soil for better cultivation or even to fertilize the field and spread insecticides.
The design procedures outlined in the following sections are based on a target application, Zreq, which equals the soil moisture extracted by the crop. It is in the final analysis a trial and error procedure by which a selection of lengths, slopes, field inflow rates and cutoff times can be made that will maximize application efficiency. Considerations such as erosion and water supply limitations will act as constraints on the design procedures. Many fields will require a subdivision to utilize optimally the total flow available. This remains a judgement that the designer is left to make after weighing all other factors that he feels are relevant to the successful operation of the system. Maximum application efficiencies, the implicit goal of design, will occur when the least watered areas of the field are just refilled. Deep percolation will be minimized by minimizing differences in intake opportunity time, and then terminating the inflow on time. Surface runoff is controlled or reused.
An engineer may have an opportunity to design a surface irrigation system as part of a new irrigation project where surface methods have been selected or when the performance of an existing irrigation system requires improvement by redesign. In a new irrigation project, it is to be hoped that the surface irrigation system design is initiated after a great deal of irrigation engineering has already occurred. The selection of system configurations for the project is in fact an integral part of the project planning process. If a new or modified surface system is planned on lands already irrigated, the decision has presumably been based, at least partially, on the results of an evaluation at the existing site. In this case, the design is more easily accomplished because of the higher level of experience and data available.
In either case, the data required fall into six general categories :
a) the nature of irrigation water supply in terms of the annual allotment, method of delivery and charge, discharge and duration, frequency of use and the quality of the water;
b) the topography of the land with particular emphasis on major slopes, undulations, locations of water delivery and surface drainage outlets;
c) the physical and chemical characteristics of the soil, especially the infiltration characteristics, moisture-holding capacities, salinity and internal drainage;
d) the cropping pattern, its water requirements, and special considerations given to assure that the irrigation system is workable within the harvesting and cultivation schedule, germination period and the critical growth periods;
e) the marketing conditions in the area as well as the availability and skill of labour, maintenance and replacement services, funding for construction and operation, and energy, fertilizers, seeds, pesticides, etc.; and
f) the cultural practices employed in the farming region especially where they may prohibit a specific element of the design or operation of the system.
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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