Because the gradient of the land is an essential element of water flow and control, the land must be formed so as to give a precise gradient, predetermined according to the situation (Figure 5.5).
Consequently, large efforts are made to accurately survey the existing land surface, and move the topsoil until the required gradient is achieved. High points are cut away and that earth is moved to nil low points, using earthmoving scrapers, often guided by laser control systems. Efforts are also made to create a smooth surface, using large land planes (Figure 5.6). Bays or furrows are then constructed on this surface.
The resulting gradient partly determines the speed with which water gravitates from the top to the bottom of the bay or furrow. Consequently, the chosen gradient must not only suit the natural site conditions (you want to shift as little earth as possible) but must also be matched to the soil infiltration rate and the length of the bay or furrows.
Figure 5.6 A land plane provides final smoothing to the land surface.
Apart from the average gradient between the ends of the bay or furrow, it is important that there be little variation of gradient along the full length. Even minor variations will create depressions mat collect water and interfere with uniform water flow. (The exception might be where the bay or furrow crosses different soil types.) Further, the gradient across the field must be considered, because excessive cross-slope will also interfere with uniform watering and could result in excessive earthmoving.
In developing a site for surface irrigation, extreme care must be taken not to cut too deeply, exposing subsoil during earthmoving operations. Not only will this require special treatment to establish crops, but the fill areas will settle below the cut areas, upsetting the gradient. It may be necessary to land plane the site for some seasons after initial construction.
Specialised services and machinery are required to survey, calculate depths of cut and fill, and perform the earthmoving operations.
Reticulation of water.Surface irrigation methods usually utilise open channels to distribute water about the farm. The main supply enters the property usually at the highest point, so that secondary channels reticulating from this point can do so using gravity. The ability of the channels to command higher ground is a major determinant of die final layout of the irrigation system. It is often necessary to build up the ground under proposed channels to provide this command. Channels are constructed using specialised earthmoving machinery and like die bays or furrows they supply, must be installed on a precise gradient. In the same way that valves and fittings control water flow in pipes, mere are various structures installed in channels:
• Gates to control the direction of flow.
• Checks and weirs to stop water flow and increase water level in the channel.
• Drop structures, where it is necessary to bring water to a channel lower in height.
• Culverts, where access roads are required to cross the channel.
Various means are used to deliver the water from the channel into die head of the bay or furrow.
• Syphons. The water level in the channel is higher than the water level on the bay or furrow, so it will easily syphon, using aluminium or plastic tubes. This system is common with furrow irrigation, where one siphon is used for each furrow. It is also used for bays. The siphon is started manually at die start of die irrigation, and die irrigation can be stopped by draining the channel or by removing the siphon. Using syphons has a high labour cost (except for the larger diameter syphons, shifted and started by machine).
• Gates. Used with bays, a gate is opened in the wall of the channel.
• Pipe outlets. Also used with bays, a pipe is installed in the wall of the channel at the time of construction. A cap or plug is removed to allow water from the channel.
Many versions of die above techniques are in use, in an attempt to maximise efficient use of labour.
Channels have some disadvantages:
• Construction cost.
• Seepage, leakage and evaporation of water, significantly reducing application efficiency.
• Large land areas must be devoted to diem.
In more intensive horticultural applications, alternative distribution methods are common using low pressure pipelines. In the "gated pipe" system a pipeline is installed at the top of die field, with gates allocated to correspond with each furrow. Distribution losses are eliminated, and minimum land area wasted, but at die cost of the pipelines and pumping equipment necessary. The number of furrows that can be watered at any one time is limited by die flow capacity of die pipeline.
Various types of rigid and "lay-flat" plastic pipes are available as alternatives to channels in furrow irrigation layouts. In some applications, buried pipes with risers at each furrow head are used.
Management.Surface irrigation methods apply a relatively large amount of water at relatively infrequent intervals. It is difficult to apply a small amount of water with any evenness. Further, in most areas where surface irrigation methods are used, water is supplied from regulated sources and ordering of water in advance is required.
This has a number of implications. Because large amounts of water are applied at long intervals, some crops may not be ideally suited, as moisture stress may start before die next irrigation. Also, since large amounts are applied at die time of irrigation and because of die method with which die water is applied, temporary waterlogging occurs for a short period during and after irrigation. When water is in short supply (when allocations are reduced during drought), it is difficult to give precise control over a small depth of watering because surface irrigation methods require a certain depth of water to provide die driving force causing die water to flow.
Figure 5.7 A number of factors influence e amount of water entering the profile under surface irrigation, and the uniformity of that application.
Efficiency of watering.Consider a single bay or furrow, of a fixed length and gradient and with soil with a particular infiltration rate. At die time irrigation commences, water first contacts the soil at die head of die bay or furrow and starts infiltrating at mat point. As more water is applied, die water spreads across die bay, builds up in depth and starts flowing down die bay or furrow. It will take some time before water reaches die end of die bay or furrow, depending on slope, length, flow rate and infiltration rate.
During mat time, water has been infiltrating at the head of die bay or furrow, whilst nothing has reached die bottom. Consequently, surface irrigation methods have an inherent unevenness in the amount of water applied. If die water is applied too long at die top, water in excess of die crop's requirements will percolate to die watertable and be "wasted", during die time it takes to adequately water die lower end. If only enough is applied at die top, then die bottom end will not receive its requirement. Either way, water application efficiency, and potentially die yield, are affected.
This situation is complicated further by die fact that minor variations in gradient along or across die bay or furrow may result in localised ponding, and soil infiltration rates may vary across die field and with die amount of water received.
Various strategies can be employed to minimise these problems:
• Attention to accuracy during landscaping.
• Varying die gradient along die bay or furrow to control velocity of water flow (not easy to construct).
• Change die flow rate during irrigation. Start with a high flow rate to get die water to die end quickly, then cut back die flow rate so it just matches intake rate along die full length. This is also not easy to manage.
• Construct bay or furrow length to suit soil intake rate.
Where gated pipe is used, "surging" techniques are worth considering (although the cost of special valves must be considered). The area being irrigated is divided into two parts (left and right). A special valve supplying die gated pipe sends water one way initially at a higher than normal flow rate to send a pulse of water down die furrow. After a shorter than normal period of time, the valve switches the other way, to send a pulse down those furrows. This switching is repeated frequently during die irrigation.
Because of their characteristic inefficient application of water, surface irrigation methods often contribute to groundwater accession and rising watertables.
On-farm efficiency can be improved in another way. In order to ensure adequate water is applied at die downstream end of die bay or furrow, it is frequently die case that water runs off die end of die bay or furrow during irrigation. This water will be wasted and contributes to reduced application efficiency (except where it runs onto pasture for improved grazing production but this is not usually "planned").
Most new installations construct drains to collect this tailwater and deliver it to storage dams at die lowest point of die system. In some designs, and with careful attention to levels and gradients, die tailwater drain can be terminated adjacent to die main storage reservoirs with tailwater pumps used to transfer it to die main storage.
Such systems can also be used to capture storm runoff to reduce dependency on delivered water.
Figure 5.8 Tailwater is collected in a sump, and pumped into the adjacent storage dam.
Surface irrigation methods are traditionally labour intensive, particularly associated with getting die water on to die field and checking when to terminate watering. Syphons are particularly time consuming.
Various devices are available, but not common, to minimise labour:
• "Giant" syphons, mechanically activated, to service multiple furrows from a head bay.
• Gates and other channel outlets, in preference to syphons.
• Various devices to automatically close channel checks, to start irrigation in a new part of the field.
• Sensors and transmitters to notify when irrigation water has progressed a certain distance down the bay or furrow.
Figure 5.9 In this layout, a single large diameter outlet pipe services many furrows.
The hinged gate on the pipe can be opened and dosed from a vehicle, saving time.
Figure 5.10 One type of sensor and its receiver to detect progress of the wetting front during irrigation. (Courtesy: Electronic Irrigation Systems P/L)
Layout can also contribute to efficient labour. Long runs and wide bays mean fewer outlets per hectare, but going too long and wide may contribute to non-uniform application.
Summary of design procedures.An estimate is required of die expected water requirement (seasonal and peak) for the area proposed, and die water supply is investigated to ensure supply is matched to demand. Initial feasibility planning should identify die approximate boundary of die irrigated area, based on limitations created by soil characteristics or topography.
A detailed topographic survey is conducted to enable accurate measurement of die slope of die ground surface. Rarely will this be satisfactory without some earthmoving. The preferred slope along proposed bays or furrows is estimated, based on soil characteristics and desirable length of run. The difference between actual and proposed slopes enables a prediction of the amount of cut and fill required at any point of die area, and therefore of die total earthmoving requirements.
This can be calculated by special software directly from die survey data.
Adjustments can be made to account for various management factors, provided application uniformity is acceptable:
• Long runs, of equal length in each block, make it easier to manage during irrigation, and for most crop husbandry operations.
• Minimise the depth of cut, to reduce risk of topsoil removal.
• Large individual paddocks assist mechanization.
• Layout should maximise die use of die irrigated area, with minimal "waste" areas in channels, drains, roads, etc.
• The number of structures and earthworks should be minimised.
Earthworks for final layout can then be costed, and pumping stations designed. The highest point to be irrigated is located. Because water is reticulated by gravity, usually through channels, this point will influence much of the proposed layout. The main supply channel is located to deliver water to irrigation blocks within the area, whilst being constructed at the correct gradient. In a similar way, tailwater drains are positioned to collect water from each block, and gravitate it to a sump at die lowest point of the layout.
Survey information is pegged onto the site, and earthmoving is conducted usually under laser control. The quality of die earthmoving job should be checked by comparing levels of the finished work with the levels proposed in me irrigation plan. Channels are constructed and structures installed.
Rubber-tyred elevating or carry scrapers are used where earth must be hauled from one point to another. Land planes are used to smooth the surface following scraping. This is performed in a number of compass directions to ensure die final surface has exactly the correct down-slope and cross-slope.
Bay checks, or furrows, are then constructed on the finished surface. Graded land will settle after irrigation, and re-grading may be required several times initially, and occasionally in subsequent years.