Medium 9781780646862

Sustainable Water Management in Smallholder Farming: Theory and Practice

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Water is critical to all human activities, but access to this crucial resource is increasingly limited by competition and the effects of climate change. In agriculture, water management is key to ensuring good and sustained crop yields, maintaining soil health, and safeguarding the long-term viability of the land.ÊWater management is especially challenging on smallholder farms in resource-poor areas, which tend to be primarily rainfed and thus highly dependent on unreliable rainfall patterns. Sustainable practices can help farmers promote the development of soils, plants and field surfaces to allow maximum retention of water between rains, and encourage the efficient use of each drop of water applied as irrigation. Especially useful for farmers' groups, agricultural extension workers, NGOs, students and researchers working with farmers in dryland areas, this comprehensive yet concise book is a practical and accessible resource for anyone interested in sustainable water management.

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1: Key Concepts

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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.

 

2: Goals of Agricultural Water Management

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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.

 

3: Soil and Water

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Soil and Water

More than anything else, the key to enhancing resilience and promoting water availability for crop growth lies in the proper care of farm soils. In fertile regions, the native soil underlying forests, brush or grasslands tends to be naturally ‘healthy’, that is, rich in nutrients with good structure and organic matter content. When land is cleared for farming, soil can quickly lose its ‘healthy’ qualities, especially if farming practices employed do not encourage its regeneration. Without proper management, agricultural soils can become completely depleted in as little as a few decades or even a few years after clearing, depending on the nature of the land and its use.1

Some negative effects that agriculture can have on the soil include:

nutrient mining (continual removal of nutrients without renewal); breakdown of organic matter; loss of water holding capacity; compaction; erosion; surface sealing (crusting); and deterioration of natural habitat for soil organisms (microorganisms, insects, and worms).

 

4: Plants and Water

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Plants and Water

Water availability is a principal limiting factor to plant development and crop yield. In laboratory studies, the total dry matter production of crop plants has been shown to have a linear relationship to water uptake: the more water used, the more yield produced, up to the point where the full plant water requirement is met.1

Water plays several roles in plant development and crop production:

1. Water is the principal transport mechanism for moving essential nutrients, minerals and dissolved carbohydrates through plant tissues. Water moves from regions of low to high potential, pulling it from the soil into roots, upward through plant tissues, and out through the leaf surface into the atmosphere in a continuous sequence driven by transpiration. As it moves through the plant, water delivers essential elements from roots to shoots and leaves where they are used in plant metabolic processes.

2. Water is a critical reactant in chemical reactions occurring in plant cells.

 

5: Climate Outlook

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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.

 

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.

 

6: Soil-focused Strategies: Reducing Water Loss

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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.

 

7: Rainwater Harvesting

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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.

 

8: Crop-focused Strategies: Using Available Water Wisely

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Crop-focused Strategies: Using

Available Water Wisely

The plant also has a role to play in water productivity. Crop management techniques can enhance the plant’s ability to use soil water efficiently, further reinforcing the benefit of the soil and water management practices described in

Chapters 6 and 7. Cropping systems can also be designed to encourage drought resistance and mitigate the harmful effects of dry periods. Drought resistance ultimately improves water productivity because it increases the potential for producing crop yields in seasons disrupted by dry spells.

Cropping patterns have a considerable impact on water productivity. Crop layouts and plant combinations can be calibrated to produce a maximum amount of product (or income) from within available water supplies. Often, this involves rotating between different plant varieties with complementary water and nutrient requirements.

Drought resistance can also be enhanced by selecting drought-resistant cultivars, and by training the physiological adaptation mechanisms of existing crops to access scarce soil water or improve the efficiency of the conversion process that transforms it into product.

 

9: Conservation Agriculture

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Conservation Agriculture

‘Conservation agriculture’, also known as ‘conservation farming’ or ‘no-till farming’, is a farming style that bucks against long-held understandings of the importance of tillage for land preparation. Conservation farming is an integrated set of practices that seeks instead to minimize soil disturbance, while promoting the continuous recycling of organic matter and nutrients through the plant–soil system. When effectively adopted, these practices have the potential to increase crop yields with respect to conventional farming, all while requiring less work.1 By supporting the natural cycles of soil regeneration in parallel with crop production, conservation agriculture also promotes the long-term health of the farm ecosystem. It is promoted as a form of sustainable land management.2

Conservation agriculture (commonly abbreviated as CA) has expanded rapidly over the past 20–30 years, with the strongest growth in Brazil and throughout South America.3 It is practiced across humid, sub-humid, and semiarid climate zones. Though still relatively uncommon in sub-Saharan Africa, some countries in the dry tropics of East and Southern Africa have seen an expansion in the number of farms converting to conservation farming over the past two decades.4 As the benefits of CA become visible, governments, international organizations and farmers’ groups are now moving to promote conservation agriculture among poor farmers struggling with poor soil health and excessive erosion.

 

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

 

10: Irrigation

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 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.

 

11: Irrigation Scheduling

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Irrigation Scheduling

Goals of Irrigation Scheduling

As introduced in Chapter 3, only a portion of the water held in the soil is available to plant roots. The soil itself will have a certain water holding capacity (field capacity), above which point no new water can be stored. Within that capacity, part of the water (below wilting point) is never accessible to plants. Stored soil water between field capacity and the wilting point is called available water. Of this quantity, only part (typically 20–80%) will be readily available to the crop plant, and this proportion will vary by species.1 The goal of irrigation scheduling is to ensure that the soil always maintains some degree of readily available water within the crop’s root zone. Below the critical point of minimum readily available water, the plant suffers irreversible water stress and its yield is reduced.2

Soil water content conditions that lie below field capacity but above this critical point are considered to fall within the optimal range of soil moisture for plant development (Fig. 11.1).

 

12: Water Sources for Agriculture

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Water Sources for Agriculture

Especially in dryland environments, the fundamental prerequisite for irrigation

­development is a good and reliable source of water. When assessing a water source for use in agriculture, it is relevant to consider both the quantity and quality that will be accessible over time. If one of these factors proves to be insufficient, the harm caused by an irrigation project could outweigh its benefit in the long term.

This chapter addresses the character and quality of water sources commonly used to supply irrigation projects. It also introduces some water contaminants of special concern, and provides a brief overview of water-lifting devices traditionally employed on small farms.

Water Quantity

The quantity of water readily available from a blue water source is difficult to assess with accuracy, since water levels, stream size and groundwater flows fluctuate significantly over time. Climate, rainfall, and the activities of upstream users all have a strong influence on the quantity of water that will be accessible from a blue water source at a given point in time.

 

Summary of Key Points

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Summary of Key Points

 Dry spells during the growing season, and not total rainfall deficits or droughts, are the principal cause of water deficit on most rainfed farms.

 The impact of dry spells on crop yields can be mitigated by adopting soil and water conservation practices, harvesting rainfall, applying supplemental irrigation, and/or practicing conservation agriculture.

 In many dryland areas, over half of the rain that falls is not captured by the soil but is lost as runoff, evaporation, deep percolation, and evaporation.

 The capacity of field soils to hold water is closely related to organic matter content and soil type.

 Soil organic matter content can be enhanced by providing soil cover, recycling plant residues into the soil, and planting several varieties of crop.

 Cover crops and green manures cover the soil while acting as natural fertilizer.

 Rainwater runoff can be beneficially harvested to provide additional water inputs.

 

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