The design of a surface irrigation system first involves evaluating the general topographic conditions, soils, crops, farming practices anticipated and farm operator's desires and finances for the field or farm in question. Information collected during the preliminary analysis should be sufficient to permit selecting one or more surface methods that will be most suitable. Then the basic information that will be needed to design the selected systems must be secured.
The basic data needed to design a system can be grouped into five general categories:
Water: Annual allotment, method of delivery (continuous flow, rotation or demand system, pumped etc.) stream size available at any time and during peak water use period, quality of irrigation water expected amount and distribution of rainfall, and irrigation water requirement including leaching requirement.
Topography: Major land slopes, field sizes and shapes, uniformity of grades, minor topographic undulations, point of water delivery and surface drainage characteristics.
Soils: Feasibility of constructing canals and ditches without excessive seepage losses, structural stability for canals and ditches, maximum root zone depth, available water holding capacity, effects of surface flooding such as crusting and cracking cumulative intake as a function of time and expected variability between irrigations, erodibility, salt content and internal drainage capacity.
Crops: Types and proportions of each crop to be grown, rooting depths and allowable soil water deficits at various stages of growth, anticipated germination problems, relative sensitivity to inundation, harvesting procedures required, crop rotation systems, and grazing needs.
Other: Availability and cost of labour, financial resources available, local customs, degree of maintenance anticipated and maintenance equipment available and construction equipment available to the operator or through the local contractors.
All of the above items have some bearing on the system selected and its final design. Overlooking or neglecting to consider any one of them can impair the effectiveness of the surface method selected.
At least ten principle criteria are important in the design of a surface irrigation system. All ten are in turn governed by the overall economics of the entire farming operation. Thus the design of an irrigation system is complex and not readily subject to quantitative analysis.
- • Store the required water in the root zone of the soil.
- • Obtain reasonably uniform application of water.
- • Minimize soil erosion.
- • Minimize runoff of irrigation water from the field. Provide for beneficial use of runoff water.
- • Minimize labour requirement for irrigation.
Minimize land used for ditches and other control to distribute the water.
- • Fit irrigation system to field boundaries.
- • Adapt system to soil and topographic changes.
- • Facilitate use of machinery for land preparation, cultivating, furrowing, harvesting etc.
The irrigation system designer must consider many soil and topographic factors in selecting the method of irrigation. The suitability of each method (surface, sprinkler, and trickle) must be considered for specific site conditions. Soil and topographic factors that the designer considers in selecting the systems include:
- • Topographic features such as surface irregularities, steepness of slope, changes in slope direction.
- • Soil features such as water holding capacity, intake rate and depth.
- • Geographic features such as field shape, natural drains, buildings, utilities or obstructions. Related factors that may influence selecting a surface system and land shaping include:
Water supply, quantity, delivery flow rate, delivery schedule and location.
Labour requirement and availability of labour
- • Energy requirement, cost and availability
- • Cost of system installation, operation and maintenance.
- • Available farm equipment.
- • The farmer's preference.
If surface irrigation method is selected, the field surface must be given detailed consideration. Rarely is it possible to establish a satisfactory surface irrigation system without some land grading to change the surface topography of a field to a planned grade.