Why irrigation is needed, and the methods used

Overview

Arid and a semi-arid environment, fossil water and basin or flood irrigation.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is irrigation?

   Model answer

   The artificial application of water to land in order to grow crops, supplementing or replacing rainfall. In arid and semi-arid tropical environments it means capturing water from a river, a reservoir, a well or an aquifer and delivering it to fields that would otherwise be too dry to cultivate, or too dry to cultivate reliably.

2. 2

   Distinguish between an arid and a semi-arid environment.

   Model answer

   An arid environment receives less than about 250 mm of rainfall a year and potential evapotranspiration exceeds precipitation in every month, so without irrigation cultivation is impossible except at oases. A semi-arid environment receives roughly 250-500 mm, enough for grazing and for drought-resistant crops in a good year, but the rainfall is seasonal and highly variable, so harvests fail regularly. The distinction matters for management because semi-arid land can be farmed without irrigation but only at high risk, whereas arid land cannot be farmed at all without it.

3. 3

   Why is the water balance, and not just the rainfall total, the reason irrigation is needed?

   Model answer

   Because what limits plant growth is the difference between water coming in and water going out. In these environments high temperatures, intense insolation, low humidity and often strong winds give a very high potential evapotranspiration, frequently over 2000 mm a year against rainfall of a few hundred millimetres. There is therefore a large permanent soil moisture deficit: even the rain that does fall is quickly evaporated or transpired, and little is stored in the soil for later use. Irrigation exists to close that deficit.

4. 4

   Why is the variability of rainfall as much of a problem for farmers as its low total?

   Model answer

   A farmer can plan around a low but reliable rainfall by choosing suitable crops and a suitable planting date. What cannot be planned around is a wet season that arrives weeks late, ends early, or fails altogether, which is characteristic of the desert margin because it lies at the limit of the ITCZ's seasonal reach. Variability means that seed, labour and any purchased inputs may be lost entirely in a failed year, which discourages investment and traps farmers in subsistence. Irrigation converts an unreliable supply into a controlled one.

5. 5

   What sources of water are available for irrigation in arid and semi-arid regions?

   Model answer

   Exotic rivers, meaning rivers that rise in a wetter region and flow across the drylands, which are usually the largest and most reliable source and are often regulated by a dam and reservoir. Groundwater, pumped from shallow wells or from deep aquifers by boreholes and tube wells. Seasonal surface flow, captured behind small earth dams or in tanks and bunds during the wet season and released later. Recycled or treated waste water near urban areas. And, where funds allow, desalinated sea water, though its energy cost restricts it mainly to high-income arid states.

6. 6

   What is fossil water, and why does relying on it raise a question of sustainability?

   Model answer

   Water held in a deep aquifer that was recharged under a wetter climate thousands of years ago and receives negligible recharge under present conditions. Extracting it is effectively mining a finite resource rather than using a renewable flow: the water table falls year on year, pumping costs rise as the lift increases, shallower wells run dry, and in coastal areas saline water can be drawn into the aquifer. A scheme based on fossil water may raise output impressively for a few decades and then leave the region with neither the water nor the traditional livelihoods it replaced.

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