Wealthier farmers have enjoyed more benefits than poorer farmers, and new (in some areas, severe) environmental problems have emerged, because of badly managed irrigation programs: water-logging triggered by over irrigation, the drying up of downstream areas – including productive wetlands and grazing areas, depletion of and falling groundwater tables in some areas, as well as salinisation of the upper soil layers and the spread of certain diseases (e.g. malaria, bilharzia).
In this sub-section, we will look more in-depth at irrigation programs, and examples of how small-scale farmers can benefit from them. Irrigation systems come in many different forms, using different sources of water, channeling water differently, and they range from small- to large-scale. There are three main categories of irrigation, all of which are relevant to small-scale farmers: surface irrigation, groundwater irrigation and flood-based farming systems.
Surface irrigation systems
Surface irrigation diverts water from rivers, lakes and springs by gravity, to the irrigated area. The area served by the irrigation canal system is called the “command area” – but with water shortage not all of it may actually be irrigated. The size of a single command area may vary from a few hectares to over one million hectares. The smaller systems are often farmer-built and farmer-managed, whereas the larger systems usually involve a separate, usually public, management agency – with farmers still responsible for the water distribution at the field level.
There are several ways to distribute water within an irrigation command area. In some areas there is a clear “on demand” element, which makes it possible for farmers to influence the time and volume of water they receive at their farm. Other systems, such flexibility is missing. In these surface irrigation systems an important issue is the duration of the irrigation cycle – the time that it takes between two irrigation turns. If this is very long it becomes difficult to grow crops that require regular watering, such as vegetables.
Another important factor in understanding irrigation systems is the irrigation duty: the amount of irrigation water available per hectare. Obviously this needs to be seen in relation to other sources of water that are available at different times of the year: rainfall, run-off water, groundwater and even dew, but particularly in arid areas irrigation water may be the main source of water in the important crop seasons. There are several main approaches to the distribution of water in an irrigation command.
One is so-called ‘protective irrigation’. Here surface irrigation is spread thinly – the objective being to provide protective water supply and prevent crop failure over a large area. In such systems, the irrigation duty is low and there tends to be much competition for water between upstream and downstream farmers.
In other areas, irrigation duties are higher and shortage is not a constant factor. High irrigation duties also increase the risk of water-logging, however. In water-logging, water levels in irrigation systems have risen so much that they saturate the root zones of the plants.
Many plants cannot grow under such circumstances. Rice is a major exception, although the success of the so called SRI (System of Rice Intensification) programs suggests that rice yields will be higher, if the paddy fields are not water-logged (see Box 4).
While water is delivered through gravity canals in surface irrigation, water in groundwater irrigation water is pumped. This can be done with manual devices (treadle pumps, rope pumps and traditional lifting devices) or mechanical pumps. At shallow depths – up to 7 metres, centrifugal pumps are common. Beyond this depth, other types of pumps, in particular submersible pumps, are commonly used. Using groundwater for irrigation makes it possible to carry out “precision water” supply. Using water exactly when it is required by crops.
In surface irrigation, this is not always possible as an individual farmer often. Cannot change the timing of water delivery to his or her farm. In fact, groundwater irrigation has increased tremendously and has very much been promoted by the Green Revolution. Which some argue even made it possible. These days, most water in many large-scale surface irrigation systems comes from (shallow) groundwater. This is called “conjunctive” use in which excess surface water ends up as shallow groundwater and is as such reused.
In principle, this combined use of surface and shallow groundwater can bring the best of both worlds:. Surface irrigation water that is not used ends up as shallow groundwater and is reused; the groundwater component makes it possible for water to be applied to the crops. At the time when it is most required and the regular surface supplies make sure. The shallow groundwater levels stay intact. In Box 5, water-use efficiency in small irrigation systems is discussed further.
What kinds of irrigation systems do you know about that work well for small farmers in your region?
The third type of irrigation systems concerns flood-based farming. Here, flood water is used to moisten the land and grow crops. There are several forms of flood-based farming systems.
One is recession agriculture, in which flood plains or lake banks are inundated during the rainy season and. Crops are planted as the water recedes. A variation on this concerns inundation canals that branch out of a river and. Start to flow only if water levels in the river have reached a certain level.
Another form of flood-based farming is spate irrigation, which is common in semi-arid areas. In spate irrigation short-term floods are diverted and spread over a land area. For irrigation, range land regeneration, filling drinking water ponds or recharge.