4.2. Disadvantages

There is one disadvantage of surface irrigation that confronts every designer and irrigator. The soil which must be used to convey the water over the field has properties that are highly varied both spatially and temporally. They become almost undefinable except immediately preceding the watering or during it. This creates an engineering problem in which at least two of the primary design variables, discharge and time of application, must be estimated not only at the field layout stage but also judged by the irrigator prior to the initiation of every surface irrigation event. Thus while it is possible for the new generation of surface irrigation methods to be attractive alternatives to sprinkler and trickle systems, their associated design and management practices are much more difficult to define and implement.

Although they need not be, surface irrigation systems are typically less efficient in applying water than either sprinkler or trickle systems. Many are situated on lower lands with heavier soils and, therefore, tend to be more affected by waterlogging and soil salinity if adequate drainage is not provided. The need to use the field surface as a conveyance and distribution facility requires that fields be well graded if possible. Land levelling costs can be high so the surface irrigation practice tends to be limited to land already having small, even slopes.

Surface systems tend to be labour-intensive. This labour need not be overly skilled. But to achieve high efficiencies the irrigation practices imposed by the irrigator must be carefully implemented. The progress of the water over the field must be monitored in larger fields and good judgement is required to terminate the inflow at the appropriate time. A consequence of poor judgement or design is poor efficiency.

One sometimes important disadvantage of surface irrigation methods is the difficulty in applying light, frequent irrigations early and late in the growing season of several crops. For example, in heavy calcareous soils where crust formation after the first irrigation and prior to the germination of crops, a light irrigation to soften the crust would improve yields substantially. Under surface irrigation systems this may be unfeasible or impractical as either the supply to the field is not readily available or the minimum depths applied would be too great.

Text 5.

5.1.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, Z_req, 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.