Surface Irrigation

Surface irrigation stands for a large group of irrigation methods in which water is distributed by gravity over the surface of the field (note: surface irrigation does not include spate irrigation). The three most common methods are basin irrigation, border irrigation and furrow irrigation. Water is typically introduced at the highest point or along the edge of a field, which allows covering the field by overland flow. Historically, surface irrigation has been the most common method of irrigating agricultural land. The defining feature of surface irrigation methods is that the soil is used as the transport medium (as opposed to pipelines (see subsurface irrigation, drip irrigation or through the air, as with sprinklers).

Surface irrigation methods contain two basic categories: ponding (surface water pooled in a puddle) and moving water. The moving water methods require some runoff or ponding to guarantee adequate infiltration at the lower end of the field. The soil type controls the length of the run and the depth of infiltrated over time. The better the quality of the soil is the less is the unnecessary runoff and the better the infiltration into the soil and therefore the use for the crops (BURT 2000). Due to ponding however, it is important not to irrigate the crops during the day but in the early morning or at night in order to avoid water loss due to evaporation.

Each surface system has its own unique advantages and disadvantages depending on such factors as initial development costs, size and shape of individual fields, soil characteristics, nature and availability of the water supply, climate, cropping pattern, social preferences and structures, and historical experience (WALKER 2003).

Basin Irrigation

Basin irrigation is the most common form of surface irrigation, particularly in regions with layouts of small fields. If a field is level in all directions, is encompassed by a dyke to prevent runoff, and provides an undirected flow of water onto the field, it is herein called a basin.

If the basins are small or if the discharge rate available is relatively large, there are few soils not amenable to basin irrigation. Generally, basin irrigation is favoured by moderate to slow intake soils and deep-rooted, closely spaced crops. Crops, which do not tolerate flooding and soils subject to crusting can be basin irrigated by furrowing or using raised bed planting. Basin irrigation is an effective method of leaching salts from the soil profile into the deeper groundwater.

Basin irrigation systems can be automated with relatively simple and inexpensive flow controls at the basin inlet. However, basin irrigation has a number of limitations in association with agriculture in the less developed countries: Accurate land levelling is prerequisite to high uniformities and efficiencies, but this is difficult to accomplish in small areas;the perimeter dikes must be well maintained to eliminate breaching and waste; and it is difficult and often infeasible to incorporate the use of modem farm machinery in small basins, thereby limiting small-scale basin irrigation to hand and animal powered cultivation (WALKER 2003).

Border Irrigation System

In many circumstances, border irrigation can be viewed as an expansion of basin irrigation to include long rectangular or contoured field shapes, longitudinal but no lateral slope, and free draining or blocked conditions at the lower end.

In border irrigation, a field is divided into strips separated by border ridges running down the slope of the field. The width of the stripes is usually from 20 to 100 feet (6 to 30 metres). The area between the ridges is flooded during irrigation. Border irrigation is used for tree crops and for crops as alfalfa (Medicago sativa) and small grains (UCCE 2003).

Furrow Irrigation

An alternative to flooding the entire field surface is to construct small channels along the primary direction of water movement. Water introduced in these furrows infiltrates through the wetted perimeter and moves vertically and laterally thereafter to refill the soil. Furrows can be used in conjunction with basins and borders to overcome topographical variation and crusting (WALKER 2003). Furrows are well adapted to row crops and orchards or vineyards (BURT 2000).

Furrow systems require more labour than border or basin systems. Some disadvantages are salinity hazards between the furrows, limited machinery mobility across the lateral field direction and an increased erosion potential. On the other hand, topographical conditions can be more severe and variable, and the smaller wetted area can reduce evaporation loss (WALKER 2003).

The three most important hardware items for an efficient furrow irrigation are (BURT 2000):

• A tailwater return flow system, which incorporates a reservoir
• Short furrows for an acceptable advanced ratio
• A large variable water supply stream

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 indefinable 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 (WALKER 1989).

Levelling the fields and building the water ditches and reservoirs might be expensive, but once this is done, costs are low and the self-help capacity is very high. Thus, the expected life of the system, fixed costs, and annual operation costs (energy, water depreciation, land preparation, maintenance, labour, taxes, etc.) should be included in the analysis when selecting an irrigation system (WALKER 2003). The water allocation is done by gravity, therefore no energy is required. On the other hand, a surface irrigation system is labour intensive, which should be considered.

Good operation of any irrigation system includes matching the irrigation duration with the rate of application and the intake rate of the soil to maximise the fraction of water stored in the root zone. Operation of surface irrigation requires being there to “tend” the water, i.e. to move the water to successive application points as it reaches the end of the run. (HILL et al. 2008).

Ditches should be cleaned out at least annually and more often if needed. A shovel can be used to clean smaller ditches. A mechanical ditcher and tractor is very helpful on larger ditches. Often ditch cleaning is an early spring “rite” to be completed prior to the first delivery of water. Many irrigation and canal companies require that shareholders maintain their own head gates and keep them in good operating condition. In areas where rodent damage is a problem, “tromping gopher holes” or otherwise fixing leaks in ditches may be a daily chore. Periodic re-levelling of surface irrigated fields may be needed to compensate for soil settlement or consolidation over time (HILL et al. 2008).