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10: Irrigation

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10

 Irrigation

Irrigation Decisions

There are many ways to apply irrigation water. Several factors must be considered when choosing if, how, and how much to irrigate. The choice of irrigation system will depend on:

• The type of blue water source(s) accessible – How close is the blue water source to the field, and how will it be moved? Is it groundwater, surface water, captured rainwater, or another source? What quantity is available for sustainable use? Make no assumptions about the abundance of a water source: it is important to thoroughly assess the supply and consult with other users in the watershed before installing an irrigation scheme.

• Quality of the water source(s) – What water quality is available? Low-quality or sediment-rich water will clog pipes and pumps, so it is not suitable for certain irrigation systems. Saline water needs to be managed using specific techniques. Water quality is discussed further in Chapter 12.

• The energy source that will be used to move water – If there is a sufficient difference in elevation between the water source and the field, irrigation can be powered by gravity. Otherwise, some form of energy input will be required to move water from the source to the field, and to distribute it over the cultivated area. This could be human or animal power, or energy from fuel combustion in the case of mechanical pumps.

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6: Soil-focused Strategies: Reducing Water Loss

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6

Soil-focused Strategies:

Reducing Water Loss

Chapter 2 introduced the concepts of productive and unproductive water uses within the overall farm water budget. Recall that the only fully productive use of water is crop transpiration (T), which is supplied by readily available soil water stored within the root zone. Typically, the percentage of rainfall that ultimately translates into transpiration is very low, in most cases between 15% and 30%.1 Unproductive water uses, including e­ vaporation, runoff, weed growth and deep percolation result in the loss of the remaining portion of the water budget. Loss percentages vary widely by context – in extreme cases, the combined forces of evaporation, runoff and deep percolation can consume more than 90% of the rainwater falling on the field.2

In order to improve rainwater productivity, farm management practices must seek to shift the way that water inputs from rain are partitioned among these competing uses. The goal is to promote infiltration and reduce water losses as much as possible, leaving more water available for use in crop transpiration.

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7: Rainwater Harvesting

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7

Rainwater Harvesting

Unfortunately, it is not feasible for a farmer to decide to ‘turn on the rain’ to water his fields at the time of his choosing and in the amount needed. To maximize his water supply, he will need to effectively capture all of the rain that falls on the field, and if possible intercept and collect excess rainwater falling on the surrounding area. In purely rainfed farming where no blue water sources are exploited, these are the only available water inputs. Even when surface or groundwater is accessible for use as supplemental irrigation, it is most efficient to first optimize the effectiveness of rainwater before moving water from other sources.

Rain that drains from the field instead of infiltrating becomes runoff. Left unchecked, runoff leads to significant water loss, wasting up to 40% of rainwater inputs.1 Rain runoff is also the principal cause of soil erosion. Rather than allowing runoff to leave the field, landforms such as basins, bunds, and gullies can be used to intercept and direct it toward the base of crop plants where it is needed. The use of land-­shaping to capture, direct, and concentrate rainwater is commonly known as rainwater harvesting (RWH). Because they discourage the rapid outflow of runoff, rainwater harvesting techniques are also erosion prevention measures.

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1: Key Concepts

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1

Key Concepts

The Water Cycle

Water exists in a variety of forms and on a number of levels in the ecosystem.

Surface water flows in rivers, lakes and swamps, while groundwater flows underground through aquifers found at various depths within the soil and rock layers of the subsurface. Water stored in the top layers of surface water bodies, soils, and the ocean evaporates when heated by the sun. Evaporated water

­becomes water vapor, which makes the air humid, and vapor trapped in clouds will condense to become rain under the right conditions. When rain falls, a portion is absorbed into the soil, where it will either infiltrate toward groundwater aquifers or remain in reserve as soil moisture. The remaining rainfall will run off the surface of the land, flowing downhill into lakes and rivers and eventually the ocean. Water that moves through plants from the soil will ultimately be transpired into the air, becoming vapor once again. Figure 1.1 outlines the water cycle.

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5: Climate Outlook

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5

Climate Outlook

The earth’s climate is changing. Farmers, especially those working on rainfed projects in the drier parts of the world, have likely already felt the effects of global climate change, in the form of shifted rainy seasons, heavier storms, or extended dry periods during the growing season. In fact, smallholder farmers and laborers working on rainfed farms in the developing world are among those most directly affected by global climate change. At the same time, the ingenuity and traditional environmental knowledge that small farmers possess will be a key tool for adaptation to new climate realities.

Changes in water availability – in the form of rainfall patterns, green water storage, surface water flows, and water quality – are some of the most visible effects of climate change. Because the unpredictability of these resources is only expected to increase in as time goes by, it is important to learn to anticipate and adapt to new conditions before their effects become too pronounced.

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