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Preamble to Part 2

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Preamble to Part 2

Although agricultural literature devotes much of its attention to irrigation methods and equipment, the vast majority of farms across the world, and especially across the tropics, are exclusively rainfed. Globally, rainfed agriculture represents 80% of all cultivated farmland, on which 60% of the world’s food crops are grown.1 In sub-Saharan Africa over 95% of all farmland is purely rainfed, and a full 90% of all crops are produced in this way. Rainfed farming also dominates in Latin America (90% of farmed land), South Asia (60%), East Asia

(65%) and the Middle East and North Africa (MENA) countries (75%).2 Most of the world’s grain crops are purely rainfed.3

These statistics, however, reflect a misconception that there is a clear dividing line between ‘rainfed’ and ‘irrigated’ agriculture projects. In fact, the distinction is rarely so cut and dry. As introduced in Chapter 1, farming includes a continuum of water management practices spanning from purely rainfed to purely irrigated agriculture, and most projects lie somewhere between these two extremes. Just as many rainfed projects incorporate some degree of irrigation in order to mitigate dry spells, irrigated agriculture projects should also strive to make the best use of available rainfall in order to minimize blue water withdrawals.

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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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Preamble to Part 3

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Preamble to Part 3

What is Irrigation?

Irrigation is the term used to describe any type of water application to agricultural fields that is administered by artificial means (as opposed to natural means such as rain, floods, and runoff). Irrigation exists in many forms, and involves widely varying levels of technology. On small farms in developing countries, irrigation is primarily human-, or animal-powered, and systems are designed around locally available resources. On larger commercial farms, mechanical irrigation is used to reduce the labor required to apply water to large plots. These installations carry a high capital cost but are often more efficient and easier to calibrate than simpler methods.

Full irrigation is the practice of applying water to the field at regular intervals throughout the growing season in order to maintain a desired level of available soil water. Supplemental irrigation, as introduced in Chapter 7, is the selective application of water to primarily rainfed fields when rainfall is insufficient to protect the plants against water stress. Application methods for supplemental irrigation are much the same as full irrigation methods, though they generally lie toward the low end of the technology spectrum. On smallholder farms in the semi-arid tropics, supplemental irrigation is usually applied using low-cost surface irrigation.1 Supplemental irrigation is a useful complement to the soil and water management practices outlined in Part 2, and significant productivity improvements have been observed in cases where these strategies are implemented together.2 Because water is only applied when needed, the efficiency of crop water use under supplemental irrigation is much higher than it is for fully irrigated crops.3

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2: Goals of Agricultural Water Management

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2

Goals of Agricultural Water

Management

Water is a vital component of every agricultural project. In addition to supporting plant growth, water is critical to maintaining soil health and promoting the overall ecological well-being of the land, which are essential in ensuring the long-term viability of the farm. In this book, the term soil and water management practices is used to designate the range of farming practices that influence the way in which water flows through the farm environment and is transformed into crop yields. This category includes methods of water application, but also cropping systems, soil management practices, and land use patterns. The purpose of this publication is to define and explain sound practices for managing water in the cultivation of field crops. While the management of soil and water resources is equally important in other agricultural categories such as livestock rearing, aquaculture and forestry, these remain outside of the scope of this book.

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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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