Irrigation is artificially watering the soil to initiate or increase the growth of crops. The first large irrigation schemes developed in the eastern hemisphere about 4000 B.C. and made possible the advanced cultures of Egypt, Syria, Persia, India, Java, and Ceylon. In the western hemisphere, elaborate schemes developed much later, about 2000 years ago, for example in Mexico and Peru. Because the river flow or the amount and timing of the rainfall in these countries was capricious, success or failure often balanced on a razor edge. In fact irrigation was the success it was only because social systems were largely concerned with making sure that it worked. This called for a high degree of personal discipline in the management of irrigation schemes, and there were also strict rules that were enforced by inspectors. Careless irrigation ranked among the most serious of crimes, often punishable by death; and in China, the prestige of some of the earlier kings was measured by their wisdom in using and controlling water.
Some ancient irrigation dams and canals are still in use and testify to the care and skill with which they were made. But they are only the remains of much larger schemes that fell into ruin because it became impossible—especially in times of war—to keep up the constant vigilance and the enforcement of laws on which all successful irrigation depends. In general, the simple systems have lasted longest, and when, during the last two centuries, complex schemes became necessary to supply rapidly growing populations with more food, the art of successful irrigation usually had to be rediscovered.
Irrigation accounts for most of the water used throughout the world. All agricultural land in Egypt is irrigated, about half in China, Japan, and Pakistan, about 33 million acres in the U.S.A., and large parts of Europe. No country, in fact, is without its irrigation schemes. Yet only a very small percentage of the agricultural land is irrigated—about 400 million acres—often using only a small proportion of the available water. One reason for this is that irrigation requires enormous quantities of water. For instance, one ton of sugar-beet needs 1000 tons of water during its period of growth, wheat 1500, and rice 4000. Furthermore, most of the water used for irrigation cannot be re-used. At least half of it is lost by evapotranspiration; the rest is incorporated into the plants themselves , and drains down into the subsoil. At least it is possible to re-use a proportion of the water supplied to the domestic consumer and industry.
This is not to say that we cannot further increase the acreage of irrigated land. Most countries have ambitious plans to do just this. For instance, Russia plans to build an immense dam on the river Ob, creating a lake nearly the size of Italy, into which water will pour by canal from the river Yenesei. From the Ob, water would flow through a vast system of canals, rivers, and lakes to the Aral Sea, irrigating on the way 50 million acres of cropland, and vast stretches of pasture in arid western Siberia. As for the underdeveloped countries, irrigation will presumably increase when there is better education and enough money to finance large works. Some regions, however, such as the Mekong River basin , will also have to wait for a more settled political situation.
By far the greatest area of irrigated land exists in hot, humid climates. Here the commonest crop is rice, which provides the main calorific intake for about one third of the world’s population. Most rice is grown in paddy fields that are flooded with about six inches of water during the growing season. There is usually no shortage of water in humid climates, either because the annual rainfall is very high, or because there are large rivers with a reliable flow. The problem is rather one of controlling and distributing large quantities of water at just the right time. If the rains come too early or too late, or if water is applied at the wrong time, rice suffers. Another problem in some humid areas is draining away the large surpluses of water after the growing season.
In arid lands, on the other hand, the main problem of irrigating large areas is shortage of water. It is most unlikely that there will ever be enough, which is unfortunate because the soil in arid zones can be very fertile. Even when countries have a local abundance of water, as in the north of Israel and the north of California, there is still not enough to irrigate the entire arid section. Agriculture in arid lands is also beset by problems that do not occur in humid regions. Sudden heavy storms, causing flash floods, are typical of these parts in the brief wet season, and where there is sloping land that is devoid of vegetation, water flows so quickly that it has no time to sink into the soil. On the other hand, on flat plains where there is bad drainage, irrigation may eventually raise the water table so that the land becomes waterlogged.
Good drainage is just as important as irrigation. In the past, many peoples have neglected this principle, and so millions of acres now lie unused. Waterlogging prevents oxygen reaching plant roots, which then suffocate. Also, waterlogged soil does not encourage crops to develop deep roots, so that during drought they are unable to tap the deeper moist soil. Waterlogging occurs either because the water table reaches the surface or because water cannot percolate down beyond the root zone. The latter situation can arise in three ways. First, the openings and channels in the topsoil may be too small for water to infiltrate quickly enough; this can be avoided by proper cultivation of the topsoil . Second, the subsoil may be too compact to let water through; this is remedied by deep ploughing. Third, the subsoil may be naturally impermeable, as when it consists of clay, in which case the only solution is to lay underground drains that discharge surplus water into especially dug ditches.
One very serious effect of waterlogging in arid regions is the accumulation of natural salts in the soil, called salinization. High salt concentrations affect plants in two ways: first, certain salts, such as boron, are toxic in excess; second, when the total concentration of various salts is too high, water is unable to enter plant roots, and may even leave them to enter the soil. In hot, dry weather, evaporation is extremely fast from the soil surface, and since all irrigation water contains salts , these are left behind in the soil. Say, for example, river water used for irrigation contains 400 parts of salt per million parts of water; then if 12 inches evaporates from one acre , 10 cwt. of salt per acre would be left behind. But it is difficult to avoid salinization in arid regions: if too much water is used, the level of high water tables rises until water reaches the surface by capillary action and then evaporates, leaving behind salts in the topsoil. If, on the other hand, too little water is used, accumulated salts are not flushed to below the root zone.
The right quantity of water to use depends on the concentration of its salts, the permeability of the soil, the level of the water table, and the type of crop. Poor quality water can be used for permeable soils with deep water tables; better quality water must be used for less permeable soils. Furthermore, crops differ in their tolerance to salts, and also require different quantities of water. And lastly it is important to apply water at just the right time, and this too varies from one crop to another. Really efficient irrigation is thus not simply a matter of applying water to the soil and waiting for crops to flourish. It requires a knowledge of soil-water conditions in relation to the particular crop to be grown.
Waterlogging and salinization exist in many parts of the world, especially in arid regions. They are particularly serious in countries that rely on their own land to provide food for a large population, rather than on heavy imports. West Pakistan, for example, with a population of 43 million , is the largest single irrigated region in the world: out of 39 million acres of fertile soil, 23 million are irrigated by an extensive system of boreholes and feeder canals. Yet the population lives in hunger and poverty; for even the vast, fertile Indus plain cannot provide enough food because of inefficient irrigation and farming practices. The flat plain of West Pakistan has very poor natural drainage, and irrigation has produced 11 million acres of waterlogged land. The main cause of this is that a third of the water in irrigation canals has seeped through their beds and has raised the water table. Waterlogging, coupled with the high rate of evaporation, has, in turn, ruined 5 million acres by salinization; in other parts salinization has been caused by farmers using too little irrigation water during the dry season. West Pakistan is at present engaged in one of the largest engineering projects in the world in an attempt to reclaim these lost acres, and to increase the quantity of water available for irrigation. Thousands of boreholes are being sunk to transfer ground water to the canals and hence lower the water table; the extra water in the canals is then used to flush accumulated salts from the topsoil. At the same time, efforts are being made to increase the crop yield by using fertilizers and better seed strains. The pattern of the new irrigation schemes is largely a result of the Indus Water Treaty of 1960, which settled a long dispute with India. The treaty states that by 1970 the three eastern tributaries will be used by India alone. Pakistan is therefore diverting the three western tributaries into canals at present dependent on those in the east. This is being achieved by a system of canals, barrages, and dams, including two immense earth dams, Mangla and Tarbela, that will conserve some of the floodwater of the Indus. However, surface storage is not a final solution; the Mangla and Tarbela reservoirs may be silted up in about 50 years and there are few sites for new reservoirs. Greater use will then have to be made of the extensive aquifer in the north, which is already tapped by half a million wells. With its enormous capacity, this aquifer would be better able to store floodwater than surface reservoirs.
With the help of foreign aid and knowledge, at least the old mistakes of waterlogging and salinization are not being repeated. Where possible, the beds of new canals lie below the water table so that they act as large drains , as well as serving for irrigation; canals with beds above the water table are sealed to prevent water leaking out and raising the water table. But the most difficult task is persuading farmers to adopt modern methods; this involves abandoning many ancient social customs, as well as introducing a new system of land tenure that gives some incentive to improve the land.
There are several methods of irrigation. The most effective for a particular area depends on the hilliness of the land, the nature of the soil, the type of crop, and the amount of available capital. One of the oldest systems is basin flooding, and was the sole method used in the flat Nile valley until the middle of the 19th century. Land near the river is divided into lots ranging from 1 to 40 thousand acres, surrounded by artificial banks. When the Nile rises in summer, water enters the basins through short canals and sluices, flooding the land to a depth of three to six feet. The water remains in the basins from 40 to 60 days, and deposits a fine layer of silt. When the river falls, water drains out of the basins, leaving behind moist land of renewed fertility. This system is simple and effective, except when the river level is abnormally low or high, and it is the only type of irrigation that automatically fertilizes the land. But with a growing population, Egypt has been forced to develop elaborate systems to irrigate fields beyond the reach of the annual flood.
The transfer of water from a river through canals has been used since Babylonian times, and it is still one of the commonest ways of bringing water to crops. If the river has a steep gradient, water is diverted into a canal some distance upstream, and led along a contour so that it can flow to fields by gravity. But if the riverhas only a slight gradient, like the Nile, low dams with sluices, called barrages, are built at intervals to raise the river level; from these barrages water flows by canal to land that lies above the river level below each barrage. Canals are expensive to build, especially if watertight; they also need occasional dredging to free them from deposited silt, although this can be reduced by constructing sedimentation pools near the canal intake. Water-weeds also are a menace.
Another method of irrigation used since ancient times involves lifting water from wells into channels that cross a field. Wells are the only source of water in many arid regions, but are seldom used to irrigate large areas. In poor arid areas many ingenious devices to lift water are still used, such as the shadoof , the Archimedean screw lift, the windlass, water-wheels, and so on. Gradually, though, these methods are being replaced by power-driven pumps, but with these great care has to be taken that the ground water is not permanently depleted.
An interesting method of collecting water to irrigate arid regions is the kanat. This consists of a tunnel that collects underground water from an aquifer at the foot of a mountain. Water flows along the gently sloping tunnel to the irrigated region by gravity, maybe for 30 miles. Every so often, vertical shafts extend from the tunnel to the surface, and give workmen access for repairs and for removing earth. Several thousand miles of kanats exist from North Africa to Asia, and are still used, the most famous being the vast network in Iran .
The most recent method of irrigation involves overhead sprinklers, from which water falls like rain. Sprinklers consist either of perforated pipes through which water flows under pressure, or of a rotating nozzle at the end of a pressure pipe that flings water over a large area. The great advantage of sprinklers is their lightness; they can be carried to different parts of a field as needed. They are also the only economic means of irrigating hilly terrain. Water from sprinklers is easily and accurately applied and the method is especially useful where labour is short.
Perforated pipes can also be laid beneath the surface of the soil—a method called subsurface irrigation. With sprinklers, about 10 per cent of the water evaporates before reaching the ground and about 20 per cent from the soil. Such losses do not occur with careful subsurface irrigation, which is invaluable in arid regions. Unfortunately, there are few soils where this method is practicable, because the openings in the pipes become blocked; gravel or sandy soil is the best for this purpose.
So far we have dealt with methods of irrigating existing soils. But all that plants require from soil is anchorage, and a supply of water, oxygen, and inorganic salts. This leads to an interesting method of cultivation in which soil is made artificially with small stones, or even plastic pellets, to provide anchorage; nutrients are supplied in the irrigation water. This method of cultivation, called hydroponics, is particularly useful in isolated, barren places where vegetables cannot be grown, such as the Cape Verde Islands in the Atlantic. It is also being increasingly used in growing intensive crops under glass in Europe and America.
A typical hydroponics plot consists of a level enclosure with waterproof sides and bottom. Seeds are sown in the gravel, and then the nutrient solution is pumped into the enclosure at regular intervals so as to flood the gravel without wetting the surface. After each flooding, the solution drains away and is used again and again. As the solution drains away, air is drawn into the interstices between the gravel to provide roots with oxygen.
Hydroponics has several advantages, such as high yields, less root disease, and less requirement for labour. The concentration of nutrients can be exactly controlled, and there is better aeration than with normal soils. Also, since the surface of the bed remains dry, there is less loss by evaporation than there is with conventional irrigation of fields. For instance, hydroponically grown tomatoes require half as much irrigation water as do tomatoes grown conventionally. Experiments in the Negev desert in Israel have also shown that it is possible to use water with a high concentration of dissolved salts— water that cannot be used for irrigation by any other method. In this process, certain compounds are added in exact amounts to the irrigation »jr • — V water to ‘neutralize’ the salts’ harmful effects.
These, then, are the methods of irrigation, all of which require a large source of water. But if there are no large rivers or lakes within a reasonable distance, water must be stored during the wet season. This is not as easy as it sounds. What we call ‘arid regions’ do have a short wet season, but when the rain comes it is usually torrential and very sudden, producing flash floods that are difficult and costly to impound. And to make matters worse, there are excessive losses by evaporation from stored water in arid climates. A great deal of thought has been given to ways of reducing evaporation, but the only remedy so far has been to apply a small quantity of an organic substance, such as cetyl alcohol, on the surface of a reservoir. In calm weather, this spreads out into a film only one molecule thick and reduces evaporation. This has been tried with some success in Australia, but with moderate winds the film breaks just when evaporation is fastest.
An alternative way of reducing evaporation is to build deeper reservoirs so as to reduce the ratio of surface to volume. But these are costly to build, and it is interesting to know that Australia has a scheme to create a large, deep reservoir below the land surface in a matter of seconds by exploding an atomic device.
In recent years, there have been some attempts to transport water from far-away lakes and reservoirs. Only well-developed countries, however, have been able to overcome the cost and difficulty of such a task, for, as we have seen, effective irrigation requires enormous quantities of water. One land that is being transformed in this way is Israel. The ancient Israelites had no large river to provide water for irrigation, unlike the more fortunate Mesopotamians and Egyptians. Agriculture depended entirely on direct rainfall, which varied from about 40 inches a year in the north to only 1 inch a year in the southern part of the Negev desert. Early settlers in the Negev, however, developed ingenious methods for irrigating small patches of land, such as by trapping dew, and sinking wells in wadi beds. They also developed a system of diverting rain that flowed over the hard-baked soil into channels and hollows. Some of these ancient systems still work effectively.
Ezekiel’s vision told of a ‘land traversed by a great river, fringed with many trees, whose waters go down into the desert and where everything shall live whither the river cometh.’ Centuries later this dream began to be realized. At the Paris Peace Conference in 1919, it was agreed to include Lake Tiberias and part of the river Jordan within the borders of Palestine so that irrigation might be possible. After the State of Israel was formed in 1948, irrigation became more intensive, and it was decided to concentrate on the driest part, the Negev, where irrigation would yield the most benefit. Much of this desert is potentially very fertile, and flat enough for mechanized cultivation, pipe laying, and road building. And since it has a healthy climate and small population, its development would relieve congestion in other parts of the country.
During the summer, only a little dew moistens the land; rain falls only in the four winter months and is very variable. Much of the rain rushes in sudden torrents into streams and wadis and hence into the Mediterranean and the river Jordan, and attempts to trap these sudden spates in reservoirs have proved unsuccessful. The only solution was to transport large quantities of water from Lake Tiberias 150 miles away. The immense 108-inch-diameter concrete pipe-line leading from the lake now also links up previous schemes into one integrated system .
By 1970, the population of Israel will have reached about three million and Lake Tiberias cannot supply all the extra water needed. Experiments are therefore in progress to intercept flash floods and recharge the water underground —a very difficult task. In addition, more wells will be dug, and efforts are to be made to purify municipal sewage. A desalination plant is also to be built at Tel-Aviv.
Large storage reservoirs for irrigation are usually economical only when they also serve another purpose. One such scheme is the Snowy Mountains project in Australia, which has been called one of the ‘seven future engineering wonders of the world.’ It is partly a hydroelectric project, and the total cost of the scheme will be repaid by the sale of electricity. Australia is largely desert with a rainfall of less than 10 inches a year, and has one of the highest evaporation rates in the world. The small population of 12 million lives mostly in the well-watered strip along the east and southeast coasts, and along the northern edge. Much of the crop production depends on irrigation, but this needs to increase to support a rapidly expanding population.
The Snowy Mountains—the highest land in Australia—are snow-bound for five months of the year, and give rise to three large rivers: the Murray and the Murrumbidgee, which flow westward across dry and fertile plains, and the Snowy, which flows southward through the well-watered coastal belt. The Snowy Scheme is designed to increase the irrigated land along the Murray and Murrumbidgee rivers by storing the surplus snow-melt in large reservoirs, which will then augment the flow of the two rivers during the dry summer. It also involves diverting the Snowy River, via tunnels and reservoirs, to the Murray and Murrumbidgee. The scheme, due for completion in 1970, covers 2500 square miles, and involves 17 large dams, 9 power stations, and 80 miles of aqueducts high in the ranges to collect mountain streams that would otherwise miss the reservoirs. Water from various rivers passes to the reservoirs through 100 miles of tunnels.
Irrigation schemes that enable enough crops to be grown to feed a large part of the population, as in Israel and Australia, are possible only because four conditions prevail: the population is relatively small, there is a large quantity of water that can be transported from at least one part of the country, there is sufficient capital to build long-term, multipurpose schemes, and farmers are well enough educated to accept the techniques of modern agriculture. Where one or more of these conditions is absent, much can still be done to improve agriculture in arid regions by making the best use of what water there is, and by properly cultivating the soil. In parts of Tunisia, for example, where there is not enough ground or surface water, a million acres of fruit trees have been planted on soil that catches and holds the night-time dew.
Recently, there has been renewed interest in the effects of dew and fog on vegetation. In some parts of the world, dew or fog is the sole source of water for plants, as along the rainless coast of Peru. Fog is responsible for the distribution of plants along the west-central coast of North America, while along the south coast of Hokkaido in Japan, special strips of forest are planted to catch the fog moving inland. When there is a large amount of moisture, drops of water condense on leaves, fall to the ground, and are then absorbed by plant roots. With certain plants condensed moisture is absorbed directly by the leaves, although how common or important this is no one knows.
As far as agriculture is concerned, great advances in utilizing dew have been made by the Israelis in the Negev. Here the total annual rainfall varies from one to eight inches, all of which may fall within 10 days. The amount of dew, on the other hand, is very great, especially in the driest and hottest part of the country. Measurements showed that some dew occurs during 250 nights of the year and heavy dew on 140 of them.
Some authorities claim that the Israelites collected dew thousands of years ago with mounds of stones. When these cooled at night by radiation, water vapour condensed on their surfaces and then ran down to water the base of a vine plant. Be that as it may, today the Israelites use sloping polythene sheets with a gutter at the bottom edge. Dew runs down the sheet into the gutter, and from here gravitates along a duct to a pit around each seedling. The 0-8 gallons that collect each month on about 10 square feet of sheet is enough to keep seedlings alive until the winter rains. The same device, of course, can be used to collect rain: a sheet 10 feet square in area supplying a pit 1 foot square provides 10 times as much water as when rain falls directly on the pit.
As a result of dew collecting, there now stand in the Negcv desert avenues and woods of eucalyptus, Aleppo pine, and other trees. The method is simple, and avoids the cost of irrigation canals and pipe-lines; and because dew is almost pure distilled water, it is effective in flushing out accumulated salts from the soil. There is no reason why dew collecting should not be effective in some other deserts of the world, such as in Peru and Chile,
Agriculture in areas where there is little rainfall and no possibility of irrigation is called dry farming. It is especially effective for crops like barley, wheat, and sorghums. Although widely practised, really efficient dry farming is not easy, and many countries have learnt the art only during the last 50 years. The first condition for dry farming is to plant crops that are drought resistant and whose main growth period lies in the wet season. Just as important is to remove weeds and unnecessary plant growth, otherwise the little water present may be soon lost by transpiration. In any case, there is often too little water to cope with the continuous transpiration of the crop alone, in which case the land is ploughed and left fallow for a year or two so as to allow it to build up a store of water.
Another technique used in dry farming is to prepare a tilth, which is a surface layer of small, loose, soil crumbs. The large openings and channels in such a layer permit rain to seep immediately into the soil, in contrast to hard-baked surfaces, over which water flows away or stands as puddles until it evaporates. To a certain extent, tilth also reduces evaporation from the underlying moist layer. Evaporation from the surface of damp soil is proportional to the wind speed and heat energy falling on to it, and continues as long as water at the surface is replenished by the upward movement of water by capillarity. Tilth reduces evaporation because the small capillary tubes are broken, and it protects the soil below from the sun and wind. The main difficulty with dry farming is to prevent the correct tilth structure from being destroyed too quickly by wind and rain. If the crumbs are too fine, they are blown away, and if the land is not flat, muddy water flows over the surface and blocks the openings with sediment. Driving rain also blocks soil openings by knocking particles into them. The commonest way of preserving the correct tilth structure is by contour ploughing, in which theTurrows follow the contour of the land and trap rain. Another method sometimes used is mulching—that is, covering the soil with dead plant matter. This breaks the impact of rain, which reaches the soil as a slow trickle without plugging the pores. Mulch also prevents the sun and wind from drying out the soil surface. Many successful experiments are also being done with plastic mulching, whereby the soil is first irrigated and then covered with plastic sheets.
Irrigation is usually of no avail unless the soil is well looked after. Proper cultivation of the soil, however, takes so much time and skill that it is not surprising that vast tracts of land now lie barren. But most countries are gradually learning from past mistakes. The dangers of waterlogging and salinization are now appreciated; drainage is considered as important as irrigation. The greatest difficulty in improving agriculture at present is undoubtedly educating and persuading farmers in developing countries to adopt modern methods, such as using the correct amount of water, fertilizers, and better seed strains. Primitive methods of farming have developed along with rigid social and religious customs, while in parts of the East, politics are also involved. All this will doubtless change, but persuasion is difficult when farmers can neither read nor write. One also meets a lack of interest, due to disease and malnutrition, and the conviction that famine will result if the new suggested methods fail. We can see, therefore, that there is a lot more to successful, world-wide irrigation than just providing enough water.