Irrigation and Drainage

High-altitude aerial view of irrigation in the Heart of the Sahara

Irrigation (in agriculture) is the replacement or supplementation of rainfall with water from another source in order to grow crops. In contrast, agriculture that relies only on direct rainfall is sometimes referred to as dryland farming.

Overview

The water source for irrigation may be a nearby or distant body of liquid or frozen water such as a river, spring, lake, aquifer, well, or snowpack. Depending on the distance of the source and the seasonality of rainfall, the water may be channelled directly to the agricultural fields or stored in reservoirs or cisterns for later use. In addition, the "harvesting" of local rain that falls on the roofs of buildings or on nearby unfarmed hills and its use to supplement the rain that falls directly on farmed fields also involves irrigation.

Various types of irrigation techniques differ in how the water obtained from the source is distributed within the field. In general, the goal is to supply the entire field uniformly with water, so that each plant has the amount of water it needs, neither too much nor too little.

Types of irrigation

Flood (furrow) irrigation

Ditches can be dug with hand tools, turned with a plow pulled by an animal or tractor, or precisely fashioned using laser-guided instruments depending on economic and physical factors such as the size of the field, the types of technology available, and the cost of manpower. Plants are grown in raised beds or listed rows. Water is distributed throughout the field via canals, unlined ditches, or furrows, between the rows or beds by use of rigid gated plastic or aluminum pipe, layflat plastic with holes punched at each furrow, concrete or plastic lined ditches, or unlined ditches. Where ditches are used, siphon tubes move water from the main ditch to the furrow. When pipes are used, water flow can be controlled by turning it on or off at the local source or by using automatic or manually controlled gates to transfer it from one set of ditches to another. Unless the field is small or very level, parts of it may suffer from water-logging while other parts may be too dry. Depending on heat, wind, and soil permeability, much water may be lost before it can benefit the plants. Automatic valves, also known as surge valves, can increase the efficiency of furrow irrigation because they alternately wet the furrows and allow the soil infiltration rate to slow prior to using the furrow for actual irrigation.

Once common in the United States, many ditch irrigation systems have been replaced because of high labor costs and increasing demands on water resources. Furrow irrigation also has a tendency to raise the water table in some areas and cause soil salination, requiring drainage. These types of systems are still common in other parts of the world.

Terracing

Terracing is a form of irrigation in which large steps are cut into hillsides and supported by stone or concrete walls. The level parts are used as garden plots or small fields. As water flows down the hillside it is channelled to each plot (probably most often by ditch irrigation). Terracing is usually very labor-intensive, since fields are small and access to them may be steep and narrow making it difficult to mechanize the work. In addition, the walls need constant maintenance, especially in rainy climates. However, terracing does allow steep mountainsides to be used to grow plants (although it may be more cost-effective to use them only for animal pasturage).

Overhead (sprinkler) irrigation

Overhead irrigation of blueberries in Plainville, New York

In overhead or sprinkler irrigation, water is piped to one or more central locations within the field and distributed by overhead high-pressure sprinklers or guns or by lower-pressure sprays. A system utilizing sprinklers, sprays, or guns mounted overhead on permanently installed risers is often referred to as a solid-set irrigation system. Some sprinklers can also be hidden below ground level, if aesthetics is a concern, and pop up in response to increased water pressure. This type of system is commonly used in lawns, golf courses, cemeteries, parks, and other turf areas.

Sprinklers that spray in a fixed pattern are generally called sprays or spray heads. Sprays are not usually designed to operate at pressures above 30 lbf/in² (200 kPa), due to misting problems that may develop. Higher pressure sprinklers that rotate are called rotors and are driven by a ball drive, gear drive, or impact mechanism. Rotors can be designed to rotate in a full or partial circle. Guns are similar to rotors, except that they generally operate at very high pressures of 40 to 130 lbf/in² (275 to 900 kPa) and flows of 50 to 1200 US gal/min (3 to 76 L/s), usually with nozzle diameters in the range of 0.5 to 1.9 inches (10 to 50 mm). Guns are used not only for irrigation, but also for industrial applications such as dust suppression and logging.

Sprinklers may also be mounted on movable platforms connected to the water source by a hose. At the high-tech end, computerized, automatically moving wheeled systems may irrigate large areas unattended. At the low end, such as in a small greenhouse or landscape, a person may be watering each plant individually with a hose end sprinkler or even a watering can.

One drawback of overhead irrigation is that much water can be lost because of high winds or evaporation, and irrigating the entire field uniformly can be difficult or tedious if the system is not properly designed. Water remaining on plants' leaves may promote fungal and other diseases. If fertilizers are included in the irrigation water, plant leaves can be burned, especially on hot, sunny days.

Overhead irrigation is generally the best solution for watering lawns and golf courses, although drip irrigation is gaining in popularity in some lawn applications. (See also center pivot irrigation.)

Manually assembled systems of piping that are broken down to permit tillage and harvesting are sometimes called "hand set" or "hand move pipe". These are also commonly used on athletic fields where permanently installed sprinklers or outlets are not desired or where low initial costs are a factor.

The hub of a center-pivot irrigation system.

Center pivot irrigation

Central pivot irrigation is a form of overhead irrigation consisting of several segments of pipe (usually galvanized steel or aluminum) joined together and supported by trusses, mounted on wheeled towers with sprinklers positioned along its length. The system moves in a circular pattern and is fed with water from the pivot point at the center of the arc. These systems are common in parts of the United States where terrain is flat. Most center pivot systems now have drops hanging from a u-shaped pipe called a gooseneck attached at the top of the pipe with sprinkler heads that are positioned a few feet (at most) above the crop, thus limiting evaporative losses. Drops can also be used with drag hoses or bubblers that deposit the water directly on the ground between crops. The crops are planted in a circle to conform to the center pivot. This type of system is known as LEPA (Low Energy Precision Application).

Pivot irrigation in progress

Originally, most center pivots were water powered. These were replaced by hydraulic systems (T-L) and electric motor driven systems (Lindsay, Reinke, Valley). Most systems today are driven by an electric motor mounted at each tower.

Center pivot equipment can also be configured to move in a straight line, where the water is pulled from a central ditch. In this scenario, the system is called a linear move irrigation system.

Lateral move (Side roll, Wheel line) irrigation

A series of pipes, each with a wheel of about 1.5 m diameter permanently affixed to its midpoint and sprinklers along its length, are coupled together at one edge of a field. Water is supplied at one end using a large hose. After sufficient water has been applied, the hose is removed and the remaining assembly rotated either by hand or with a purpose-built mechanism, so that the sprinklers move 10m across the field. The hose is reconnected. The process is repeated until the opposite edge of the field is reached.

This system is less expensive to install than a center pivot, but much more labor intensive to operate, and it is limited in the amount of water it can carry. Most systems utilize 4 or 5 inch diameter aluminum pipe. One feature of a lateral move system is that it consists of sections that can be easily disconnected. They are most often used for small or oddly-shaped fields, such as those found in hilly or mountainous regions, or in regions where labor is inexpensive.

Drip, or trickle irrigation

A dripper

See main article at drip irrigation

Water is delivered at or near the root zone of plants, drop by drop. This type of system can be the most water-efficient method of irrigation, if managed properly, since evaporation and runoff are minimized. In modern agriculture, drip irrigation is often combined with plastic mulch, further reducing evaporation, and is also the means of delivery of fertilizer. The process is known as fertigation.

Deep percolation, where water moves below the root zone, can occur if a drip system is operated for too long of a duration. Drip irrigation methods range from very high-tech and computerized to low-tech and relatively labor-intensive. Lower water pressures are usually needed than for most other types of systems, with the exception of low energy center pivot systems and surface irrigation systems, and the system can be designed for uniformity throughout a field or for precise water delivery to individual plants in a landscape containing a mix of plant species. Although it is difficult to regulate pressure on steep slopes, pressure compensating emitters are available, so the field does not have to be level. High-tech solutions involve precisely calibrated emitters located along lines of tubing that extend from a computerized set of valves. Both pressure regulation and filtration to remove particles are important. The tubes are usually black (or buried under soil or mulch) to prevent the growth of algae and to protect the polyethylene from degradation due to ultraviolet light. But drip irrigation can also be as low-tech as a porous clay vessel sunk into the soil and occasionally filled from a hose or bucket. Subsurface drip irrigation has been used successfully on lawns, but it is more expensive than a more traditional sprinkler system. Surface drip systems are not cost-effective (or esthetically pleasing) for lawns and golf courses.

Subirrigation

Used in commercial greenhouse production, usually for potted plants, water is delivered from below, absorbed upwards, and the excess collected for recycling. Typically, a solution of water and nutrients floods a container or flows through a trough for a short period of time, 10-20 minutes, and is then pumped back into a holding tank for reuse. Subirrigation requires fairly sophisticated, expensive equipment and management. Advantages are water and nutrient conservation, and labor-saving through lowered system maintenance and automation. It is similar in principle and action to subsurface drip irrigation. The same concept of subsurface flooding and drainage is also being experimented with as an outdoor subirrigation method.great system of irrigation is on indus river

How an irrigation system works

Most commercial and residential irrigation systems are "in ground" systems, which means that everything is buried in the ground. With the pipes, sprinklers, and irrigation valves being hidden, it makes for a cleaner, more presentable landscape without garden hoses or other items having to be moved around manually.

The beginning of a sprinkler system is the water source. This is usually a tap into an existing (city) water line or a pump that pulls water out of a well or a pond.

History of irrigation

Evidence exists of irrigation in Mesopotamia and Egypt as far back as the 6th millennium BC.

There is also evidence of ancient Egyptian pharaohs of the twelfth dynasty using the natural lake of the Fayûm as a reservoir to store surpluses of water for use during the dry seasons, as the lake swelled annually as caused by the annual flooding of the Nile. Ancient visitors reported the appearance of "an artificial excavation, as reported by classic geographers and travellers" (CATHOLIC ENCYCLOPEDIA: Egypt: I. GENERAL DESCRIPTION: Flora and Agriculture).

In the Zana Valley of the Andes Mountains in Peru, archaeologists found remains of 3 irrigation canals radiocarbon dated from the 4th millennium BC, the 3rd millennium BC and the 9th century. These canals are the earliest record of irrigation in the New World. Traces of a canal possibly dating from the 5th millennium BC were found under the 4th millenium canal.(Dillehay, et al., 2005)

By the middle of the 20th century, the advent of diesel and electric motors led for the first time to systems that could pump groundwater out of major aquifers faster than it was recharged. This can lead to permanent loss of aquifer capacity, decreased water quality, ground subsidence, and other problems. The future of food production in such areas as the North China Plain, the Punjab, and the Great Plains of the US is threatened.

Problems in irrigation

• Competition for surface water rights.
• Depletion of underground aquifers.
• Ground subsidence (e.g. New Orleans, Louisiana)
• Buildup of toxic salts on soil surface in areas of high evaporation.

See also

• Aquifer
• Evapotranspiration
• Geohydrology
• Gezira Scheme
• Groundwater
• Irrigation in Saudi Arabia
• Paddy Field
• Qanat

References

• Jess Stryker's Landscape Irrigation Tutorials. URL accessed on August 1, 2005.
• Green Media Online. URL accessed on August 1, 2005.
• Dillehay TD, Eling HH Jr, Rossen J (2005). Preceramic irrigation canals in the Peruvian Andes. Proceedings of the National Academy of Sciences 102 (47): 17241-4. PMID 16284247

External links

Wikimedia Commons has media related to:

Irrigation

• Irrigation techniques. USGS. URL accessed on December 8, 2005.
• "The Irrigation Association." Accessed December 8, 2005.
• "Irrigation Forum." Accessed December 8, 2005.

Drainage is the natural or artificial removal of surface and sub-surface water from a given area. Many agricultural soils need drainage to improve production or to manage water supplies.

Reasons for artificial drainage

Wetland soils may need drainage to be used for agriculture. In the northern USA and Europe, glaciation created numerous small lakes which gradually filled with humus to make marshes. Some of these were drained using open ditches and trenches to make mucklands, which are primarily used for high value crops such as vegetables.

The largest project of this type in the world has been in process for centuries in the Netherlands. The area between Amsterdam, Haarlem and Leiden was, in prehistoric times swampland and small lakes. Turf cutting (Peat mining), subsidence and shoreline erosion gradually caused the formation of one large lake, the Haarlemmermeer, or lake of Haarlem. The invention of wind powered pumping engines in the 15th century permitted drainage of some of the marginal land, but the final drainage of the lake had to await the design of large, steam powered pumps and agreements between regional authorities. The elimination of the lake occurred between 1849 and 1852, creating thousands of km² of new land.

Coastal plains and river deltas may have seasonally or permanently high water tables and must have drainage improvements if they are to be used for agricultural purposes. An example is the flatwoods citrus-growing region of Florida. After periods of high rainfall, drainage pumps are employed to prevent damage to the citrus groves from overly wet soils. Rice production requires complete control of water, as fields need to be flooded or drained at different stages of the crop cycle. The Netherlands has also led the way in this type of drainage, not only to drain lowland along the shore, but actually pushing back the sea until the original nation has been greatly enlarged.

In moist climates, soils may be adequate for cropping with the exception that they become waterlogged for brief periods each year, from snow melt or from heavy rains. Soils that are predominantly clay will pass water very slowly downward, meanwhile plant roots suffocate because the excessive water around the roots eliminates air movement through the soil.
Other soils may have an impervious layer of mineralized soil, called a hardpan or relatively impervious rock layers may underlie shallow soils. Drainage is especially important in tree fruit production. Soils that are otherwise excellent may be waterlogged for a week of the year, which is sufficient to kill fruit trees and cost the productivity of the land until replacements can be established. In each of these cases appropriate drainage is used to carry off temporary flushes of water to prevent damage to annual or perennial crops.

In farming drier areas, irrigation is often used, and one would not consider drainage necessary. However, irrigation water always contains minerals and salts, and these can be concentrated to toxic levels by evapotranspiration. Irrigated land may need periodic flushes with excessive irrigation water and drainage to remove these toxic minerals.

Drainage in the 19th century

From the 1881 Household Cyclopedia

This operation is always best performed in spring or summer, when the ground is dry. Main drains ought to be made in every part of the field where a cross-cut or open drain was formerly wanted; they ought to be cut four feet (1.2 m) deep, upon an average. This completely secures them from the possibility of being damaged by the treading of horses or cattle, and being so far below the small drains, clears the water finely out of them. In every situation, pipe-turfs for the main drains, if they can be had, are preferable. If good stiff clay, a single row of pipe-turf; if sandy, a double row. When pipe-turf cannot be got conveniently, a good wedge drain may answer well, when the subsoil is a strong, stiff clay; but if the subsoil be only moderately so, a thorn drain, with couples below, will do still better; and if the subsoil is very sandy, except pipes can be had, it is in vain to attempt under-draining the field by any other method. It may be necessary to mention here that the size of the main drains ought to be regulated according to the length and declivity of the run, and the quantity of water to be carried off by them. It is always safe, however, to have the main drains large, and plenty of them; for economy here seldom turns out well.

Having finished the main drains, proceed next to make a small drain in every furrow of the field if the ridges formerly have not been less than fifteen feet (5 m) wide. But if that should be the case, first level the ridges, and make the drains in the best direction, and at such a distance from each other as may be thought necessary. If the water rises well in the bottom of the drains, they ought to be cut three feet (1 m) deep, and in this ease would dry the field sufficiently well, although they were from twenty-five to thirty feet (8 to 10 m) asunder; but if the water does not draw well to the bottom of the drains, two feet (0.6 m) will be a sufficient deepness for the pipe-drain, and two and a half feet (1 m) for the wedge drain. In no case ought they to be shallower where the field has been previously leveled. In this instance, however, as the surface water is carried off chiefly by the water sinking immediately into the top of the drains, it will be necessary to have the drains much nearer each other--say from fifteen to twenty feet (5 to 6 m). If the ridges are more than fifteen feet (5 m) wide, however broad and irregular they may be, follow invariably the line of the old furrows, as the best direction for the drains; and, where they are high-gathered ridges, from twenty to twenty-four inches will be a sufficient depth for the pipe-drain, and from twenty-four to thirty inches for the wedge-drain. Particular care should be taken in connecting the small and main drains together, so that the water may have a gentle declivity, with free access into the main drains.

When the drains are finished, the ridges are cleaved down upon the drains by the plough; and where they had been very high formerly, a second clearing may be given; but it is better not to level the ridges too much, for by allowing them to retain a little of their former shape, the ground being lowest immediately where the drains are, the surface water collects upon the top of the drains; and, by shrinking into them, gets freely away. After the field is thus finished, run the new ridges across the small drains, making them about ten feet (3 m) broad, and continue afterwards to plough the field in the same manner as dry land.

It is evident from the above method of draining that the expense will vary very much, according to the quantity of main drains necessary for the field, the distance of the small drains from each other, and the distance the turf is to be carried.

The advantage resulting from under-draining, is very great, for besides a considerable saving annually of water furrowing, cross cutting, etc., the land can often be ploughed and sown to advantage, both in the spring and in the fall of the year, when otherwise it would be found quite impracticable; every species of drilled crops, such as beans, potatoes, turnips, etc., can be cultivated successfully; and every species, both of green and white crops, is less apt to fail in wet and untoward seasons.

Wherever a burst of water appears in any particular spot, the sure and certain way of getting quit of such an evil is to dig hollow drains to such a depth below the surface as is required by the fall or level that can be gained, and by the quantity of water expected to proceed from the burst or spring. Having ascertained the extent of water to be carried off, taken the necessary levels, and cleared a mouth or loading passage for the water, begin the drain at the extremity next to that leader, and go on with the work till the top of the spring is touched, which probably will accomplish the intended object. But if it should not be completely accomplished, run off from the main drain with such a number of branches as may be required to intercept the water, and in this way disappointment will hardly be experienced. Drains, to be substantially useful, should seldom be less than three feet (1 m) in depth, twenty or twenty four inches thereof to be close packed with stones or wood, according to circumstances. The former are the best materials, but in many places are not to be got in sufficient quantities; recourse therefore, must often be made to the latter, though not so effectual or durable.

It is of vast importance to fill up drains as fast as they are dug out; because, if left open for any length of time, the earth is not only apt to fall in but the sides get into a broken, irregular state, which cannot afterwards be completely rectified. A proper covering of straw or sod should be put upon the top of the materials, to keep the surface earth from mixing with them; and where wood is the material used for filling up, a double degree of attention is necessary, otherwise the proposed improvement may be effectually frustrated.

The pit method of draining is a very effectual one, if executed with judgment. When it is sufficiently ascertained where the bed of water is deposited, which can easily be done by boring with an auger, sink a pit into the place of a size which will allow a man freely to work within its bounds. Dig this pit of such a depth as to reach the bed of the water meant to be carried off; and when this depth is attained, which is easily discerned by the rising of the water, fill up the pit with great land-stones and carry off the water by a stout drain to some adjoining ditch or mouth, whence it may proceed to the nearest river.

Current practices

Modern drainage systems incorporate geotextile filters that retain and prevent fine particles of soil from passing into and clogging the drain. Geotextiles are synthetic textile fabrics specially manufactured for civil and environmental engineering applications. Geotextiles are designed to retain fine soil particles while allowing water to pass through. In a typical drainage system they would be laid along a trench which would then be filled with coarse material: gravel, sea shells, stone or rock. The geotextile is then folded over the top of the stone and the trench is then covered by soil. Groundwater seeps through the geotextile and flows within the stone to an outfall. In high groundwater conditions a perforated plastic (PVC or PE) pipe is laid along the base of the drain to increase the volume of water transported in the drain.

Alternatively prefabricated plastic drainage systems, often incorporating geotextile, coco fiber or rag filters can be considered. The use of these materials has become increasingly more common due to their ease of use which eliminates the need for transporting and laying stone drainage aggregate which is invariably more expensive than a synthetic drain.

Over the past 30 years geotextile and PVC filters have become the most commonly used soil filter media. They are cheap to produce and easy to lay, with factory controlled properties that ensure long term filtration performance even in fine silty soil conditions.

See also

• John Johnston, who introduced land drainage to the United States.
• Building construction
• Watershed

External link

• International Geosynthetics Society, information on geotextiles and geosynthetics in general.