Part 8, Surface Irrigation
By Ed Butts, PE, CPI
In this series on irrigation fundamentals and concepts we’ve discussed irrigation terms and definitions, soil-water-plant relationships, water quality considerations, application efficiency, required system flow rate and volume, distribution uniformity, and methods of sprinkler irrigation.
This month, we will undergo a slight change in direction and present an overview on the most common method of irrigation worldwide: surface (flood) irrigation techniques and design.
Introduction to Surface Irrigation
Irrigation is one of the major single uses of water throughout the United States as well as the world.
In 2015, total U.S. irrigation withdrawals were estimated at 118.1 million gallons per day or 42% of all freshwater withdrawals. Irrigation withdrawals from surface water sources were 60,900 million gallons per day, which accounted for 52% of the total irrigation withdrawals. Groundwater withdrawals were at 57,200 million gallons per day.
This amounts to 132,000 acre-feet per year or a national average around 2.09 acre-feet per acre with around 63,500 thousand acres under irrigation. Of this total acreage, about 23,300 thousand acres, or 36.6% of all irrigated acreage, were irrigated using surface flooding systems—making this method the second most common type of all irrigation methods behind only sprinkler irrigation.
Surface irrigation is the oldest method of irrigation known and is still in use in many developing nations and regions where there are ample water supplies and low levels of technology, as well as used in the United States.
Surface irrigation, often referred to as flood irrigation, implies that water distribution is uncontrolled, and therefore inherently inefficient. In reality, several of the irrigation practices grouped under this name—such as furrow, surge, and gated pipe irrigation—involve a significant degree of design and management to be successful and efficient.
The main problem with flood irrigation is that not all water delivered from the source will necessarily reach the plants. Up to 40% can be lost through soil leakage, waste, deep percolation loss, evaporation, or runoff—making it among the most inefficient irrigation methods.
Recapturing runoff can help to address this issue, as can the increasing use of advanced methods such as surge irrigation and precise field leveling, which encourages the water to be well distributed and absorbed rather than leaving shallow ponds of standing water in the field to evaporate.
Another common detrimental issue is flood irrigation can encourage the growth of certain kinds of undesirable crop pests, mold, and diseases. Standing water can also attract foraging animals looking for drinking water as well as mosquitoes and other insects—making it important to distribute water on properly managed fields with appropriate drainage.
While flood irrigation is primarily performed for agricultural use, it is sometimes used in residential landscaping as well. This is most common in areas where there is a considerable volume of available water for this use and it is managed carefully to avoid flooding streets and sidewalks as well as possibly causing some other problems related to erosion and mosquitos.
In some regions, entire neighborhoods use flood irrigation and the irrigation is controlled remotely and on a strict watering schedule by a water purveyor for people who want to sign up and pay for it.
The application process of all methods of surface irrigation can be described by using four distinct time phases illustrated in Figure 1.
As water is applied to the top end of the field, it will flow or advance over the length of the field. The advance phase refers to that length of time water is applied to the top end or entrance of the field and flows or advances over the entire length of the field.
After the water reaches the end of the field, it will either run off or start to pond. The time period between the end of the advance phase and the shutoff of the inflow is termed the wetting phase (ponding or storage). As the inflow ceases, the water will continue to run off by force of gravity and infiltrate into the soil until the entire field is drained.
The depletion phase is that short period of time after flow cutoff when the length of the field is still submerged and before runoff or infiltration occurs.
The final phase, the recession phase, describes the time period while the water is retreating toward the downstream end of the field. The depth of water applied to any point in the field is a function of the length of time in which water is present on the soil surface.
How surface irrigation works
Typically, the basic functions of surface or flood irrigation are as follows:
- At the start of irrigation, water flow is initiated and begins to advance down the field.
- At the same time, water begins to pond on the upper soil surface. The amount of ponded or stored water is substantial and can amount to at least 3 to 4 inches in depth, especially within downslope regions of the field.
- The ponded water infiltrates into the soil as water continues to flow across and down the field.
- At the time of flow cutoff, irrigation water is suspended but the ponded water continues to flow down the field by gravity. The ponded water continues to infiltrate the soil after cutoff, and depending on the slope and soil conditions, may provide the entire replenishment of soil moisture along the lower part of the field.
- The stored water also causes surface runoff to occur at the end of the field. The longer the time of water flow suspension, the more runoff will occur from the stored water. This is often collected and reused in a tailwater recovery system.
- Surface irrigation is generally applied in three major types: border irrigation, basin irrigation, and furrow irrigation.
Border irrigation is an old method of irrigation that is still extensively used, mostly in the western part of the United States to irrigate alfalfa, wheat, small grains, and row crops.
Border irrigation uses a large volume of water that is applied over a flat wide surface in a short period of time (Figure 2a). The borders are usually no more in configuration than a firm, high soil bed or row. Depth of water flow is shallow, generally a few inches up to a half-foot, and uniform down and along a contained bordered area called a bay (Figure 2b).
It is a system where a large volume of water is discharged into a defined border or bay at the top of the field and guided by the borders down the slope in a uniform, slow wetting pattern to the end of the field where unused water is often drained, collected, and returned to a collection basin or storage pond for future repumping. (This is generally called a tailwater recovery system, which will be discussed later in this series.)
The primary advantage of this method is it is inexpensive in terms of initial capital (construction) costs and operating energy costs. The principal disadvantage is its application is inefficient and its performance is highly dependent on adequate water supplies and three inherent field and soil properties—the slope of the field, the soil’s maximum infiltration rate, and the soil’s surface roughness.
The downhill grade or slope controls the rate of flow that can be applied by force of gravity. The infiltration rate impacts the amount or rate of water that can enter the soil. The surface roughness affects the ability of water to effectively and uniformly flow overland, as rough or irregular soil provides a greater resistance to gravity water flow and creates uneven water flow and distribution.
Deep homogenous heavy loam or clay soils with medium infiltration rates are preferred, although heavier clay soils can be difficult to irrigate because of the time needed to infiltrate an adequate depth of water into the soil. In these cases, basin irrigation is generally preferred.
Border irrigation is the most difficult irrigation method to manage effectively because of its high dependence on field and soil properties and limited performance characteristics. Therefore, a trial-and-error approach is normally used in design, field construction and testing, and ongoing management.
Border irrigation systems have several common design features. They usually work well with downgradient slopes between 0.1% to 0.2% or 1.2 inches to 2.4 inches per hundred feet of distance. However, in all cases border slopes should be uniform with a minimum slope of 0.08% to provide adequate distribution and drainage with a maximum slope of 2% to limit possible problems from soil erosion.
For best results, water should slowly meander down the field rather than flow at rapid speeds to avoid scour and soil erosion.
Border irrigation includes the use of border checks (small levies) that range from 6 inches to 20 inches in height. Depending on the available source or pumping rate and field slope, these typically confine water to an area from 10 feet to 300 feet wide so that water moves down the field through gravity. Field length in the path of flow varies but is usually determined by field constraints and soil characteristics.
There are several different styles of border irrigation with varying degrees of efficiency. This type of irrigation has been criticized because it can be extremely wasteful when not effectively designed and carefully managed, even though high irrigation efficiencies of up to 75% to 80% are theoretically possible with this method of irrigation.
However, they are rarely obtained in practice due to the difficulty of balancing the advance (inlet) and recession (drainage) phases of water application to prevent overland runoff or water loss. Thus, the typical application efficiency used in design is around 70%.
As this irrigation method uses the soil surface itself as the conveyance path to allow the flow of water down the field, any impediment to this flow will greatly disrupt and compromise the system’s efficiency. The rate at which the water flows down the field depends on several factors such as the inflow rate of water into the check width area, slope, length of the border check, soil infiltration rate, and the soil’s surface roughness.
The flow of water across the field is characterized by the advance curve (refer again to Figure 1), which shows the time at which water arrives at any given distance along the field length. This is also referred to as the time of concentration.
Border irrigation is generally best suited to close-growing crops such as pasture or alfalfa and larger mechanized farms, as it is designed to produce long uninterrupted field lengths for ease of machine operations. Borders can be up to 2500 feet or more in length and between 10 feet to 300 feet wide depending on a variety of factors. It is generally less suited to small-scale farms involving intensive hand labor or else animal-powered cultivation methods.
Obviously, the primary factor in determining a border width depends on the source capacity since most sources cannot produce the high quantity of water required to cover the width of an entire field in a single set. Therefore, prudent judgement must be used to figure how to best use the available capacity over the most efficient arrangement of bays.
A rule of thumb for border design, or at least a reasonable place to start, is to apply 5 gallons per minute per foot of width for a 1320-foot (quarter-mile) run length and 10 GPM per foot of width for a 2640-foot (half-mile) run length with a 0.10% slope.
Therefore, a quarter-mile-long field with a 0.10% slope would require approximately 500 GPM for a 100-foot-wide bay.
These flow rates provide nearly a 12-hour set time depending on field width, soil type, well yield, starting soil moisture deficit, and the field’s surface roughness.
Designers have several ways to get water from the source to the borders, but it is much more desirable to pipe the inlet water into a border with multiple valved inlets across the top than to discharge all the water into a single location at the top of the border, as multiple inlets give more uniformity of coverage than a single inlet can. Where an open channel is preferred, use of flumes or gates to direct water into each area are acceptable.
Another form of surface irrigation is called basin irrigation (Figure 3a), in which water floods a smaller area or basin surrounded by raised berms, usually made from earth. It is closest to meeting the definition of “flood” irrigation as an artificial method of watering plants in which a level or slightly sloped field is surrounded by a ridge of earth so that a shallow body of water may accumulate before it soaks into the soil.
This technique can be extremely useful for crops which need to remain submerged such as rice, and for soil that absorbs water slowly such as clays and loams.
Level basin irrigation has historically been used in small areas having level surfaces that are surrounded by earth banks called bunds. Bunds are permanent or temporary small earth embankments containing irrigation water within each basin and are sometimes referred to as banks, ridges, dykes, or levees.
Bunds can be placed in a field as permanent water retention barriers on the perimeter of the field or as temporary structures within the field to allow crop changes and rotations as well as variable basin widths (refer again to Figure 3a).
The height of a bund is determined by the needed irrigation depth and required height above the applied water depth to be sure that water will not overtop the bund. The width of bunds should be such that they are reasonably impervious to lateral infiltration so leakage will not occur and sloped as necessary on both faces to remain stable.
Water is applied rapidly to the entire basin and is allowed the time to infiltrate the soil. In a traditional basin, water is not permitted to drain from the field once it is irrigated. Basin irrigation is often favored in soils with relatively low infiltration rates (Walker 1989). Fields are typically set up to follow the natural contours of the land, although the recent introduction of laser leveling and land grading procedures has permitted the construction of large rectangular basins that are more appropriate for mechanized broadacre cropping.
The floor of the basin may be flat, ridged, or shaped into small beds depending on cropping and cultural practices. Basins need not be rectangular or have straight sides, and the border bunds may or may not be made from locally available mounded soil, as imported soil is often used where onsite soil may be too pervious to flow or with a higher potential for erosion and slope instability.
Basin size is limited by the available water flow across the basin. This is also known as the stream size, topography, soil factors, and degree of leveling required. Individual basins may be quite small or as large as 40 acres.
Level basins simplify water management since the irrigator need only supply a specified volume of water to the field. With an adequate stream size, the water will spread quickly over the field, minimizing non-uniformities during the inundation time.
Basin irrigation is most effective on uniform soils, precisely leveled, when large stream sizes relative to basin area are available and high efficiencies are possible with low labor requirements. There are few crops and soils not amenable to basin irrigation, including most field and row crops. It is generally favored by applications with moderate to slow intake soils and deep-rooted and closely spaced crops.
An example of basin irrigation on a wheat crop is shown in Figure 3b. Crops sensitive to flooding and soils which form a hard surface crust following an irrigation can be basin irrigated by adding furrowing or by using raised bed planting. Reclamation of salt-affected soils is easily accomplished with basin irrigation and providing for drainage of surface runoff is usually unnecessary.
Of course, it is always possible to encounter a heavy rainfall or mistake the cutoff time, thereby resulting in too much water in the basin. Consequently, some means of an emergency drainage path to a safe area that can withstand and store overflow water is generally a good design practice.
Basins can be served with less command area and field watercourses than can most border and furrow systems because their level nature allows water applications from virtually anywhere along the basin’s perimeter.
In addition, automation is easily applied by using automated valves to control flow into individual basins.
Although it is the most efficient surface irrigation method of the three mentioned here at an average of 75%, basin irrigation does have a number of limitations. Two already have been mentioned: soil crusting and crops that cannot accommodate complete submergence—making precision land leveling important to achieving high uniformities and efficiencies.
Many basins are so small that precision equipment cannot work effectively to create uniformly level fields. So perimeter dykes need to be accurately placed for height and well maintained to eliminate breaching and waste, and must generally be higher for basins than other surface irrigation methods.
To reach maximum levels of efficiency, the flow per unit width must be as high as possible without causing erosion of the soil. When an irrigation project has been designed for either small basins or furrows and borders, the capacity of control and outlet structures may not be large enough to improve basins.
Determining the size of basins
Basins should be small if the:
- Slope of the land is gentle or flat
- Soil is clay
- Stream size to the basin is large
- Required depth of the irrigation application is large
- Field preparation is mechanized.
Basins can be large if the:
- Slope of the land is steep
- Soil is sandy
- Stream size to the basin is small
- Required depth of the irrigation application is small
- Field preparation is done by hand or animal traction.
Furrows are sloping, parallel, and narrow pre-cut channels typically formed in and below the top of the soil where water infiltration occurs through the wetted perimeter of the furrow to the crop. In furrow irrigation the water runs down the furrows along the field or furrow length in the direction of the predominant slope that lies between rows of crops, reaching the roots as it is absorbed into the soil. Systems may be designed with a variety of furrow shapes, lengths, and spacings to accommodate field and cropping conditions.
As this process adheres to fundamental open channel flow, furrow irrigation requires the greatest degree of precise and careful engineering calculation and design for total effectiveness.
Unlike most other surface water methods, furrow irrigation can be applied across either an entire or partial field length. For instance, furrows can extend across a field’s entire length to a perimeter drain at the end of the field or over just a partial length where a drain diverts the unused water away from dry downfield land (refer to Figure 4a).
A crop is planted on the ridge (Figure 4b) between furrows which may contain a single row of plants or several rows in the case of a bed type system. The intake rates in furrows may be quite variable, even when soils appear to be reasonably uniform, due to cultural practices, crop demands, and soil and seeding inconsistencies.
The intake rate of a new furrow will generally be greater than an established furrow that has been irrigated, and wheelrow furrows generated by mechanical methods can have greatly reduced infiltration rates due to repeated soil compaction. Optimal furrow lengths are primarily controlled by the crop, intake rates, and stream size.
Furrow irrigation is suitable for many crops, especially broadacre row crops including cotton, maize, tomatoes, and sugar cane, but also suitable for growing many tree crops such as citrus and stone fruits.
In the early stages of tree planting, one furrow alongside the tree row may be sufficient, but as the trees develop, two or more furrows can be constructed on both sides to provide sufficient water.
Furrow irrigation is most useful for unsubmerged crops that use extensive feeder roots to extract water from the soil up into the plant. By retaining water in the furrow and below the plant, water can be pulled laterally and vertically into the plant’s roots through capillary action (see Figure 4c).
Thus, any crops that would be otherwise damaged if water covered their stem or crown should be irrigated by furrows. For example, many cotton fields are irrigated using quartermile- long furrows while many tomato and melon fields are irrigated using one-sixth-mile or one-eighth-mile furrows.
The shorter furrows allow farmers to achieve greater distribution uniformity, while reducing deep percolation and possible surface runoff loss. However, quarter-mile furrows require one additional head ditch and tailwater ditch; one-eighth-mile and one-sixth-mile furrows require two additional sets of ditches.
Because of the many design and management controllable parameters, furrow irrigation systems can be used in many diverse cropping situations, but within the limits of soil uniformity and topography. With proper furrow management and scheduling and the use of runoff return flow (tailwater) systems, furrow irrigation can be a highly uniform and efficient method of applying water, often exceeding an irrigation efficiency of 70%.
However, the required uniformity and efficiency are highly dependent on proper management and furrow maintenance, so mismanagement and percolation losses can severely degrade system performance and efficiency. Therefore, the typical efficiency used for design should be around 65%.
Water is normally applied to the top end of each furrow and flows down the field under the influence of gravity (see Figure 4d). Water may be supplied using gated pipe, siphon, and head ditch, or bankless systems.
Siphon tube systems (see Figure 4e) are a popular and widespread method for transitioning water from a supply or head ditch or channel to a furrow. They are available with single, double, and triple bends to fit the needed channel design, field length, and direction of the slope (i.e., the required furrow flow rate). These tubes are typically available in diameters ranging from 1 inch to 3 inches and can be cut to length in both pipe and tube shapes using materials made from polyethylene plastic or metal.
The speed of water movement is determined by many factors such as slope, surface roughness, and furrow shape, but most importantly, by the inflow rate and soil infiltration rate. The spacing between adjacent furrows is governed by the crop species, with common spacings typically ranging from 2 feet to 8 feet.
Furrows may range anywhere from less than 300 feet to more than 6000 feet in length depending on the soil type, field location, and crop type. Shorter furrows are commonly associated with higher uniformity of application, but often result in increasing potential for runoff losses.
Surge irrigation is a variant of furrow irrigation where the water supply is pulsed on and off in planned time periods (on for one hour and then off for one and a half hours). The alternating wetting and drying cycles reduce infiltration rates, resulting in faster advance rates and higher uniformity than by using a continuous flow. The reduction in infiltration is a result of surface consolidation, filling of cracks and micropores, and the disintegration of soil particles during rapid wetting and consequent surface sealing during each drying phase.
The effectiveness of surge irrigation is dependent on the soil type. For example, many clay soils experience a rapid sealing under continuous flow, and therefore surge irrigation offers little overall benefit. Surge irrigation should almost always be performed using automated methods, such as control valves and timers, to ensure accurate application of water and cycles occur.
Another variation of furrow irrigation is a gated pipe system. Depending on the type of soil and depth, gated pipe irrigation systems may reduce the seepage losses that often occur in earthen head ditches.
Water is delivered to furrows through sliding gates in aluminum or PVC pipes that transport water from a turnout or canal. Gated pipe systems can be designed to deliver water in half-mile, quarter-mile, or one-sixth-mile furrows by placing additional lines of gated pipe in each field.
Some farmers prefer gated pipe systems when using shorter furrows, because there is no loss of crop area caused by the second and third sets of ditches that are required when using a siphon tube system. However, additional labor is required to place the gated pipes in each field before irrigations begin and to remove the pipe following the final irrigation event.
An example of a gated pipe system with erosion socks irrigating a tomato crop is shown in Figure 4f.
Reasons for and against furrow irrigation
As with all irrigation methods, furrow irrigation has several features recommending it as well as not recommending it.
Advantages of furrow irrigation:
- Lower initial investment of equipment.
- Simple and easy to understand and operate, eliminating extensive cost and time for training and orientation.
- Minimizing water loss inefficiencies in gravity irrigation systems allows irrigators to save labor costs.
- Lower pumping costs per each acre-inch of water pumped due to the lack of system pressurization.
- Furrow irrigation can minimize irrigation costs and chemical leaching, and result in higher crop yields.
Disadvantages of furrow irrigation:
- An accumulation of salinity can build up between furrows, reducing crop uptake and damaging soil.
- Increased level of tailwater or runoff losses. A solution is to build a retention pond along the edges of fields that helps capture this runoff, allowing it to be pumped back to the upslope side of the field for use in future irrigation cycles.
- The difficulty of transporting farm equipment across the open furrows.
- Added expense and time required for the extra tillage practices (the furrow construction).
- An increase in the erosive potential of the soil.
- Precise and accurate field layout, slope, and furrow construction is needed to ensure uniformity of flow.
- Needing to remove any small hills that would have allowed bypassing by the gravity water flow because a primary difficulty of furrow irrigation is ensuring uniform dispersion of water occurs over a given field.
- Furrow systems are generally more difficult to automate, particularly with regulating flow to provide an equal discharge in each furrow. This can require complex and expensive flow-balancing systems.
This wraps up this month’s discussion on irrigation fundamentals. Next month, we will kick off the new year with an overview on one of the most popular methods in current and widespread use: drip and trickle irrigation.
Until then, happy holidays to all, and as always, work safe and smart.
Many of the drawings and background information for this article were derived from: Walker, W.R. 1989. Guidelines for designing and evaluating surface irrigation systems. FAO Irrigation and Drainage Paper 45. Food and Agricultural Organization of the United Nations.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at firstname.lastname@example.org.