Irrigation has been the technology underlying many of the world's greatest civilizations. The ancient irrigation techniques depended mostly on the terrain, water supply, and the engineering skills of the civilization. Many different irrigation systems were developed very early in agriculture history.

Some of the first irrigation systems were developed in Egypt and Mesopotamia. The prosperity of the civilizations of the Nile River Valley has been recorded throughout history and their success depended on the efficiency with which the government organized the best use of the river water. Abundant crops could be stored for years and irrigation techniques could be improved.

At first, the Egyptians based their agriculture on growing winter crops after the annual floods were gone. The Egyptian irrigation systems depended on several factors. There was only one source of water, the Nile, which was too powerful to control. Therefore, irrigation systems had to be simple in terms of construction, and built high along the riverbank in order to deal with only the peak of the flood. Since the river valley is never more than 15 miles wide, irrigation systems could not carry water a great distance from the river (Ancient, 1999).

The Egyptians built large, flat basins for growing crops along the riverbanks. They developed simple sluices, or water channels with gates, that diverted water into the basins at the peak of the flood. It was simple in terms of both engineering and labor to arrange for good water flow through several basins controlled by simple gates. Water was allowed to sit in the fields for 40 to 60 days. It was then drained using ditches and canals, at the right time during the growing cycle, downstream back into the river. Salts never built up in the soil because there was plenty of water and the flow of water in the canals and ditches was strong enough to avoid silting. Any silt that did settle in the basins was good, because it kept the basin floors evenly flat and the silt brought in a large amount of nutrients with each flood (Ancient, 1999).

Irrigation systems were completely different in dry, hilly areas. Rivers and springs don't have the flow of the major rivers, so water storage systems needed to be developed. One of the earliest successful techniques was a diversion dam. One of the most impressive dams used for ancient irrigation is located near Marib, the ancient capital of Yemen. It was built around 600 BC. The dam is over 1,600 feet long and is built of carefully crafted masonry, strengthened by copper fastenings. The dam was not built in order to make a reservoir with usable water, but rather to hold back some of the annual floodwaters and divert some of the floodwater, under control, out of sluices into canal systems (Ancient, 1999).

Irrigation has been the base for agriculture in Mesopotamia for over 6,000 years. Mesopotamia contained the area that is now Iraq and part of Iran. The area has low rainfall, and is supplied with surface water by only two major rivers, the Tigris and Euphrates. Although the rivers are much smaller than the Nile, they have more severe floods and carry more silt. The area of Mesopotamia also has flat, poorly drained plains, so the region has always had a problem with poor soil, drought, flooding, silting and soil salinity (Ancient, 1999).

The engineers of the area had to worry about flood control and water storage, as well as irrigation. The canals were easily choked off due to the high amount of silt in the rivers. This problem was overcome by constant dredging, as long as the manpower was available. However, there was another, more difficult problem. It was difficult to drain water off of the fields, which meant salt could easily build up and sterilize the soil.

Although the plains of Mesopotamia are very flat, the bed of Euphrates is actually slightly higher than the Tigris. Engineers used this slight slope to their advantage and used the Euphrates as a water source and the Tigris as a drainage channel (Ancient, 1999).

As irrigation developed, civilizations learned that there are some things needed to have a successful irrigation system. A region that can be irrigated on a long-term basis has to have an abundant supply of good water, well-drained soil, good regional drainage, and a supply of fertilizer for the soil.

Water supplies must be safe and reliable. In each growing season, water of the right quality must be supplied at the right time and right place in the right amount. Even in one growing season, an irrigation system has to have continuous maintenance. Water is never pure because it has mineral salts dissolved in it. Evaporation will therefore make it saltier. Rivers flowing through dry or desert areas lose water by evaporation, and become salty. For example, the water in the lower Colorado River contains over a ton of salt per acre-foot of water (Ancient, 1999). An acre-foot is the amount of water it would take to cover an acre to the depth of one foot.

As water is applied to the crops, it spreads out into a thin sheet, which is exposed to the surface. Much of the water may evaporate, making it even saltier. The water may dry up altogether, leaving a thin layer of salts in and on the soil. Under normal circumstances, plants absorb nutrients from the soil, leaving behind excess salts. Eventually the salts build up in the surface soils until they become unfertile. This is why soils in dry irrigated areas tend to become salinized and sterile.

The only way to deal with this problem is to supply enough water so that the salt is flushed off or flushed through the soil. The flushing must remove salts from the area altogether, whether it is through natural or artificial drainage. In well-drained areas with a dry season and a wet season, flushing takes place naturally. However, in poorly drained areas, supplying too much water simply "mobilizes the salt while the water table rises to ground level" (Ancient). This means the water is drawn to the surface where it dries out and the salt is left as a surface deposit, which makes the problem worse rather than better. Once the soil is saturated with salt, with water up to the surface, there is no way to flush the salts out of the soil, and the fertility of the region is destroyed unless major drainage channels are built to carry away the salts. Even this is not necessarily a good thing because the flushing simply delivers the salt somewhere else, either to downstream users or into underground water supplies. Severe flushing also depletes the soil of nutrients (Ancient, 1999).

Irrigation can only be maintained on a long-term basis if certain conditions apply. Water has to be applied in a way that salt is not allowed to build up in the soil. This usually means that a lot of good-quality water is applied, and the drainage system is rapid and efficient. Soils also need a large supply of fertilizer to balance the flushing that is required to keep them free of salt. If any one of these systems fail, it is only a matter of time until the whole irrigation system fails (Ancient, 1999).

It took several hundred years and much trial and error before researchers figured out the principals of irrigation and engineers developed mechanical water control systems that drastically changed irrigation techniques, and irrigation into a form of science.

In 1800, just before the beginning of the modern irrigation age, the total area of irrigated land in the world was 19,760,000 acres. From this small starting base, the scientific and technical foundation of irrigation that was developed in the next century, made possible large new irrigation practices. By 1995, due to the advancements in irrigation technology, the total area of irrigated land in the world was just over 555,750,000 acres (Postel, 1999).

Much of this growth was due to the introduction of new products and techniques. Major advances in groundwater irrigation techniques began in the late 1930's. These evolutions followed the technical development of rotary well-drilling, deep well turbine pumps, right angle gear drives, improved internal combustion engines and electric motors, the widespread availability of natural gas and electricity for pumping energy. Higher crop prices and a drought in the 1950's brought about the need for the expansion of irrigation systems (Kromm, 1992).

Furrow irrigation developed using unlined ditches and siphon tubes for water distribution. Underground concrete pipes were introduced in the 1950's. Plastic underground pipes were introduced in the 1960's. As unlined ditches were replaced with underground pipelines, aluminum gated pipe became common with furrow irrigation.

Furrow application caused substantial wastewater problems. Due to the wastewater problems, reuse systems, including pumps and pipelines, became common in the early 1960's (Kromm, 1992). Pipeline distribution systems and water reuse systems greatly increased the water usage efficiencies.

The availability of affordable equipment such as aluminum pipe and impact sprinkler heads caused the expansion of sprinkler irrigation after World War Two. Later, the development of center pivot sprinkler systems allowed for the irrigation of soils with rough terrain and sandy textures, which are not suited for surface irrigation methods (Kromm, 1992).

The introduction of these new methods brought about irrigation's modern age. Today, there are two main types of irrigation; surface irrigation and sprinkler irrigation. Surface irrigation ranges anywhere from wild flooding, where little if any soil preparation is needed, to basin systems where soil is leveled precisely. There are two main types of sprinkler systems: set systems and mobile systems.

Surface irrigation can be used on nearly all soils, and most crops. The systems can be changed to fit a wide range of stream sizes and usually still maintain high water application efficiency. Surface irrigation systems are also very inexpensive compared to other systems, such as sprinkler irrigation (Hagen, 1967).

Wild flooding is one of the oldest surface irrigation systems in the USA. The soil surface is not changed for flood irrigation. In order for this to work, irrigators, based on their experience, direct the water to the high points on the fields in ditches, where it is released to cover the field in a "wild" manner. If the water has to flow a great distance, it will run to the low spots, causing over-irrigation, and leave the ridges and high areas too dry (Stewart, 1990).

Graded furrow irrigation is another type of surface irrigation that is commonly used for row crops such as corn and beans, as well as some tree and vine crops such as apples and grapes. Before irrigation, furrows are made between each row of the crop. Water is let into each furrow at the upper end of the field by siphon tubes, cutout ditches, or gated pipes. The irrigator must be sure that the water does not break out of the furrows, causing poor water distribution and faster soil erosion. This is an especially severe problem with soils that crack between irrigations. Also, as with any furrow irrigation system, water is left over at the bottom of the field. Because of this, furrow irrigation is often considered inefficient. However, in most cases, this wastewater is not totally lost, but is able to be reused either on the same farm, by neighboring farms, or by downstream water users (Stewart, 1990).

An irrigation system similar to the Graded Furrow method is the corrugation system. This system is used mostly for close growing crops such as alfalfa where the soil surface was not carefully smoothed to prevent slope changes. Small, shallow furrows, or "corrugates," help carry the water over the soil in a uniform manner. The small rate of flow in the corrugates helps minimize soil erosion (Stewart, 1990). This is probably the most common irrigation system used in this area.

Level basin irrigation is another surface irrigation technique where the soil surface is leveled completely, with no slope in any direction. Close-growing crops such as rice most commonly use the level basin system.

Even though surface irrigation systems are cheaper to operate, they usually end up costing more in the long run. The lower efficiency of these systems, compared to others, results in the need for larger quantities of water. Therefore, there must be larger storage areas and more extensive drainage systems for the wastewater, all requiring a large investment of money (Goldberg, 1976).

Sprinkler irrigation is "human's attempt to duplicate natural rainfall" (Stewart, 1990, p.484). Sprinkler irrigation is a versatile way of applying water to the surface of any crop or soil. In other words, a sprinkler system can be used for most all soil and geographic conditions, and for those areas where surface irrigation methods may be expensive and inefficient (Hagen, 1967).

Sprinkler irrigation uses water sprayed form a pressurized pipeline to supply water to sprinkler heads, where water is sprayed into the air as the water breaks into drops, which fall to the soil like rain. The size of the drops, the consistency with which they fall, and the rate at which they fall are all affected by the design of the sprinkler system. Therefore, the design of the sprinkler system is very important of its overall success (Stewart, 1990).

A "set" sprinkler system is one in which the sprinklers are placed in a fixed area. There may or may not be enough sprinklers to cover the entire field. If there are enough, the sprinklers are not moved. If there are not enough, the sprinklers must be mobile.

A mobile system is usually made of lines of sprinklers, and an entire line is moved from one place to the next. Aluminum is the most common material used for portable system pipelines because it is light and fairly inexpensive (Stewart, 1990).

The mobile system is the other kind of sprinkler system. The center pivot is the most widely used portable system. A center pivot system includes a pipeline mounted on a series of wheeled towers. The entire pipeline rotates around a fixed end where the water is fed into the system (Stewart, 1990). Center pivot systems are available to irrigate fields ranging from 24 to 260 square acres (Hagen, 1967).

The side-roll system is the most commonly used sprinkler system in this area. The side-roll uses the pipe as an axle, with wheels attached to the pipe. A small motor at the center of the system rotates the pipe, causing the sprinkler to move across the field. These sprinkler systems can be as long as one quarter of a mile (Hagen, 1967).

Sprinkler systems can also be useful for purposes other than supplying water to crops. Two of the most important uses for sprinklers are crop cooling and frost protection. Frost protection will be explained later. Crop cooling occurs when water is sprayed on the crop during times of high temperatures. Some cooling results from heat transfer to the water, because the water is cooler than the crop. However, most of the cooling comes from evaporation of the water. It takes a lot of energy to change water from a liquid into a vapor. The energy needed must be removed from the surrounding environment. This causes a lowering of temperature of the crop. The crop cooling method is most commonly used on crops such as apples or onions (Stewart, 1990).

Frost protection by sprinklers is used for crops such a strawberries. Frost protection actually uses ice to protect the crops. Water is sprayed onto the crop when there is a chance of a frost. The heat released when water freezes into ice protects the sensitive parts of the crop (Stewart, 1990).

Even though sprinkler systems are more efficient than surface irrigation methods and they can be used for such a wide variety of things, there are some disadvantages of sprinklers. Irrigation can create environments favorable to diseases when the conditions were previously unfavorable. Sprinkler system can alter the plant environment, giving rise to diseases that would not commonly be present. Also, water must be delivered to the system under pressure. Without the proper pressure, the system will not work properly. Wind can also affect the distribution of water from a sprinkler system (Stewart, 1990).

Overall, sprinkler systems are more efficient than surface irrigation systems. A sprinkler system has virtually no runoff, therefore, there is no wastewater to be dealt with. In surface irrigation systems, there is usually always some wastewater that is lost. Sprinkler systems provide a more consistent application of water than surface irrigation techniques do. Even though surface irrigation is usually cheaper, it takes less work to operate a sprinkler system. Irrigators have to choose between money and labor when choosing an irrigation system.

Ancient irrigation. (1999). Retrieved March 18, 2002 from the World Wide Web:

http://www-geology.ucdavis.edu/~GEL115/115CH17oldirrigation.html

Goldberg, D. (1976). Drip irrigation. Drip Irrigation Scientific Publications.

Hagen, R.M. (Ed.). (1967). Irrigation of agricultural lands. Madison, Wisconsin.

American Society of Agronomy, Inc.

Kromm, D.E., & White, S.E. (Eds.). (1992). Groundwater exploitation in the highplains.

University Press of Kansas.

Postel, S. (1999). Pillar of sand: Can the irrigation miracle last? New York:

W.W. Norton & Company, Inc.

Stewart, B.A., & Nielsen, D.R. (Eds.). (1990). Irrigation of agricultural crops.

Madison, Wisconsin. American Society of Agronomy, Inc.

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