Irrigation

Flooding Irrigation

Perhaps the simplest method of irrigation, flooding (or basin) irrigation was most likely first practiced by the Egyptians along the Nile River. In this system, earth banks were built on the flat floodplains in order to divide the plains into a series of basins. During the river's annual flood, the basins filled up with water. This water was left in the basins for as long as six weeks, after which time temporary gaps were cut in the earthen ridges of the basins to allow the water to drain away, leaving behind a layer of rich silt. Seeds were then sown in the waterlogged soil. Among the flaws with this method of irrigation were the dependence upon the size of the flood and the limitation of being able to grow only one crop per year.

Flooding irrigation was also developed by the Chinese for growing paddy rice. Flooding is still effective for growing this crop due to the fact that paddy rice grows best when its roots are submerged in water.

 

 

Furrow Irrigation A furrow is a long shallow trench in the ground, usually dug by a plow. In furrow irrigation, crops are planted on the elevated ridges between the furrows. Water is admitted into the furrows along one of the sides, called the lateral. When the level of the water in the furrows has risen to the desired height, the inflow of water is ceased. The water then seeps into the soil and is absorbed by the roots of the plants. Although this method of irrigation is more expensive and more difficult to build and maintain compared to the flooding method, it enables one to control exactly how much water the plants receive, which increases its flexibility.

 

Subirrigation

When the groundwater table is near the surface of the field, subirrigation can be employed. In this method of irrigation, water is brought to the field via ditches. The water is then permitted to seep into the ground, thereby augmenting the water table. With the increase in water, the water table rises. After sufficient water is absorbed into the ground, the water table rises high enough to directly feed the roots of the crops. However, for subirrigation to be effective, the source water must have a low salinity. Subirrigation requires considerably less water than flooding. It also has the benefit of keeping the tops of the crops dry. This helps to prevent the growth of rot and mildew, both of which can cause spoilage.

 

 

Sprinkling Irrigation The sprinkling method is perhaps the most convenient irrigation method. It uses pipes and pumps to simulate natural rainfall. This system enables one to control very accurately the areas that receive water, as well as exactly how much water those areas receive. Additionally, sprinkling systems can be used to irrigate uneven and hilly ground that would be impossible to irrigate using other methods without first regrading and smoothing the land. Sprinkling systems can even be designed to be portable if such flexibility is desired. In the United States, rotary sprinkling systems are commonly used. In these systems, a long pipeline with sprinklers is connected to a central pivot. The pipeline is mounted on wheels, so it is able to rotate about the pivot as it sprinkles water onto the field. Thus, the irrigated sections define circles in the fields.

 

Drip Irrigation

Drip (or trickle) irrigation was developed in Israel, where water supplies are often scarce. In this method of irrigation, a perforated pipe is laid on the ground, with the perforations positioned very close to the roots of the plants. The perforations permit a controlled amount of water to flow out of the pipe and into the roots. Because water losses due to both evaporation and seepage below the root level are minimized in drip irrigation, it is ideal for regions having very limited sources of water.

 

 

Drainage

It is just as important to ensure that the fields do not receive too much water as it is to ensure that they do not receive too little water. If drainage is inadequate, salts may accumulate, a process called salination. Salination occurs because the irrigation water carries salts to the fields, but the plants absorb only the water and leave the salt behind. Due to the lack of drainage, these excess salts remain on the fields, where they can cause serious harm to the plants and even prevent the plants from growing.

Another danger of insufficient drainage is waterlogging. In waterlogging, the water table is raised by excess irrigation water faster than it can lower itself by natural processes. Eventually, the water table is so high that it is above the level of the roots of the crops, which seriously interferes with the growth of the plants. Additionally, when the water table is this high, more evaporation occurs, which leaves behind more salts, thus exacerbating the problem of salination.

To prevent salination and waterlogging, adequate drainage is essential. Excess surface water can be removed from the fields using deep ditches that run along the fields, and excess ground water can be removed using either underground drainage lines or pumped drainage wells.