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Furrow irrigation design examples




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The Problem. Furrow irrigation designs are often needed either for new irrigation schemes or on existing projects where improvements are needed. Land consolidation has been carried out in a number of irrigation projects where implementation has included land reform policies and has resulted in field units amenable to furrow irrigation. Consider one such case where the new farm units have been organized around a 2 hectare block 200 m by 100 m. Flows of 30 litres per second are allocated to each block for 48 hours every 10 days. Initial field surveys showed that the fields needing first attention were comprised of a loam soil, sloped 0.8 percent over the 100 m direction and 0.1 percent over the 200 m direction. The furrows were placed on 0.5 m intervals across the 100 m direction (and running in the 200 m direction). The furrows were assumed to have a hydraulic section where p1 = 0.57 and p2 = 1.367.

 

The frequency and duration of each irrigation needs to be checked and then the headland facilities selected and designed. During the first irrigation, the field will require just more than 35 hours to complete the irrigation (the sum of rreq + tL times the number of sets). The later watering will require 25 hours. If evapotranspiration rates were as high as .8 cm/day, the irrigation interval of 10 days waters the field well within these limits (Zreq divided by the crop use rate approximates the irrigation interval). Since the water supply is presumably controlled by an irrigation department, the design can be substantially hindered if the delivered flows are not as planned.

It may be useful to examine briefly the performance of this design. If the actual irrigations evolve as these design computations indicate, the farmer's irrigation pattern will waste about 44 percent of his water during first irrigations and about 40 percent during later irrigations. By today's standards, these losses are large and it may be cost-effective to add cutback or reuse to the system to reduce these losses.



Field operations. The question that arises at this point in the design is how to implement and operate the system on the field. How will the irrigator know what flow rates are actually running into the furrows, what the actual soil moisture depletion is, or when to terminate the flow into one set of furrows and shift the field supply to another set.

There are several types of furrow irrigation systems but probably the most common are those that either use open watercourses at the head of the field and divert into furrows using spires or siphon tubes, or those that utilize aluminium or plastic gated pipe. The task of sizing these headland facilities will be noted in a later section. The problem at this point in the design is the means of accurate flow measurement and management.

If the design is to be carried forward to an actual operation, the inlet must be equipped with a flow measuring device like those noted in Section 3. Then the irrigator with some simple instructions from the designer can 'share' this flow among the appropriate number of furrows and achieve a reasonably good approximation of the optimal discharge. In some cases, the outlets to each furrow can be individually calibrated and regulated. For instance, the size of the siphon tubes or spires might be selected by the designer. The irrigator can then adjust the flow by regulating the heads and/or the openings.

In short, this phase of irrigation engineering is highly dependent on the experience and practicality of the engineer. There is no single 'best' way to do things. What works well in one locale, may not in another. The computational procedures and methods of field evaluation provide the best values of the parameters. The good design can only give the irrigator the opportunity to operate the system at or near optimal conditions.

There is another point which is hidden by the hydraulics of surface irrigation (which have been largely omitted from this guide). The movement of the water over the soil surface is very sensitive to the relative magnitude of the furrow discharge and the cumulative infiltration rates. Irrigation practices which modify the field inflow, such as cutback, may actually reduce the performance of the system. In more practical terms, if the advance rate is slowed to accommodate a cutback regime, the gains in efficiency derived from reduced tailwater may be more than offset by increases in deep percolation losses. The user of this guide might repeat the following cutback design example using data and field conditions for a lighter soil to illustrate this problem. As described earlier, the inherent limitation of the cutback design is that the advance phase and the wetting phase must have the same duration..

Field layout. Once the advance phase inflows are established, the field design or layout commences with an estimate of the cutback flow. The one important constraint on the cutback flow is that it should not be less than the intake along the furrow and cause dewatering at the downstream end. Equation 71 was given to assist the designer in avoiding this problem, but it is only a guideline. Thus, for the first irrigation the cutback flow must be at least:




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