Rainfed production dominates agriculture, covering about 80 per cent of total cropland. Farmers, particularly small-scale farmers, have limited influence over the amount and timing of water. The main challenge is to manage and adapt to weather variability, temperatures and rainfall patterns.
Global analyses estimate that extreme weather events affecting rainfall and temperature can explain 18–43 per cent of yield variation for key crops, including maize, rice, soybean and wheat. As water shortages increase, and population and economic growth accelerate, there will be pressure on all agricultural systems, especially rainfed ones, to use water more productively.
While the challenge of addressing water shortages remains the same in both low-input and high-input rainfed production systems, what differs is their capacity to address it. Farmers in high-input systems can more easily invest in improved water management and agronomic practices to ensure the most efficient use of scarce rainfall.
Yields in rainfed agriculture remain lower than those in irrigated areas, and substantial yield gaps persist globally and regionally. Such gaps are expected to largely mirror the classification of low-input and high-input systems. There is great opportunity to increase yields in Africa, Eastern and South-western Europe, and parts of Asia, where gaps are largely due to a combination of water and nutrient shortages.
In temperate regions, such as Western Europe and Northern America, where a substantial amount of cropland is rainfed and largely high-input, yields of cereals often exceed 6 tonnes per hectare, against a global average of 4 tonnes per hectare.
In Central and Western Europe, supplemental irrigation maintains yields during dry summers. Yields in Eastern Europe remain lower, suggesting that unlocking the vast potential of the region will depend on new agricultural water management and technological change.
While some countries in tropical areas often exceed 5 tonnes per hectare for cereals, others do not surpass 2 tonnes per hectare. This suggests that the biophysical constraints causing low yields in rainfed farming, particularly in tropical low-income countries, can be overcome, inter alia, by appropriate water management, combined with best agronomic practices.
Making best use of rainfall for improved rainfed crop productivity
There are two broad strategies for increasing yields in rainfed agriculture: (i) collecting or harvesting more water, infiltrating it into the root zone; and (ii) conserving water by increasing plant uptake capacity and/or reducing root-zone evaporation and drainage losses.
Where the issue is excess water, strategies focus instead on relocating practices to divert it. Figure 16 illustrates options, described along a continuum from production fully dependent on rainfall to situations in which farmers rely partly on supplemental irrigation.
Key to making the best use of rainwater are soil and water conservation technologies – first box on the left in Figure 16 – which control the water available to a crop by affecting the water content in the root zone.
Terracing, agroforestry, contour cultivation and conservation agriculture can modify and enhance soil-water content to retain moisture and prevent erosion. Organic mulching, a natural or artificially spread layer of plant residues or other organic materials on the surface of the soil, can also minimise evaporation.
As residues decay over time, they increase the water-holding capacity of soil, improving efficiency. Organic mulching also provides soils with nutrients and restricts weed growth by blocking light penetration of the soil surface, contributing to increased water efficiency.
Water harvesting involves collecting rainwater or runoff (see the Collection box in Figure 16), which can either be diverted directly, spread on fields, or collected and stored. Effective water harvesting – combined with best agronomic practices – can boost crop yields, especially during low rainfall.
Combined with small-scale on-farm ponds, water harvesting can also integrate fish production and livestock watering with crop production. These are more climate-resilient measures and offer greater income to small-scale farmers.
Water harvesting
A distinction is often made between in situ water harvesting, which refers to the capture of local rainfall on farmland, and ex situ water harvesting, which refers to rainfall capture outside the farm. Ex situ water harvesting uses water to mitigate dry spells, protect springs, recharge groundwater, enable off-season irrigation and permit multiple uses.
These practices include surface microdams, subsurface tanks, ponds, and diversion and recharging structures. Communities or individual farmers usually manage these systems, and they require information, training and awareness raising to properly implement and maintain these practices.
For example, in Tigray, a water-constrained region in northern Ethiopia, the government has prioritised different ex situ systems, the majority run by individual farmers. These have helped increase crop and livestock productivity, crop diversification and access to water points.
However, outcomes depend on farmer and stakeholder participation during planning, implementation and utilisation. In the Sahel, FAO is implementing the “One million cisterns” programme to promote rainwater harvesting and storage systems for vulnerable communities.
The objective is to allow millions of people in the Sahel, especially women, access to safe drinking water, enhance agricultural production, improve food and nutrition security, and strengthen their resilience.
Water collected through harvesting can be later applied as supplemental irrigation when rainfall is scarce (Box 9). In situ water harvesting covers different technologies – microcatchments, bunds, broad-beds and furrows – as well as management options such as tillage or adding organic matter.
Supplemental irrigation
Where rainfall is insufficient, supplemental irrigation provides essential soil moisture and, thus, increases water productivity. If supplemental irrigation were applied to all rainfed cropland, global cereal production could be increased by 35 per cent, the largest potential being in Africa and Asia.
Even relatively small supplemental irrigation can lead to substantial increases in crop yield. An example from the Syrian Arab Republic shows yield improvements of up to 400 per cent.
In the State of West Bengal, India, small rainwater storage ponds for supplemental irrigation have doubled mustard yields and increased paddy yields by 20 per cent. They have also increased farmers’ incomes by 34 per cent. More farmers are considering cultivating a range of highly profitable vegetables during the dry season. The approach has also released more water for gardening, livestock, raising fish and domestic uses.
In Zimbabwe, supplemental irrigation reduces the risk of complete crop failure from 20 per cent to 7 per cent, and increases water productivity by almost one-third, especially when combined with inorganic nitrogen. Therefore, supplemental irrigation is a key strategy, despite still being underused, for unlocking rainfed yield potential and water productivity.
Agronomic practices
Combining water conservation and harvesting can be highly effective. Rost et al estimate that a 25 per cent reduction in evaporation and a 25 per cent collection of runoff could increase crop production by 19 per cent.
Jägermeyr et al have shown that soil moisture conservation alone could boost global rainfed kilocalorie production by 3–14 per cent. The authors also found that a combination of in situ and ex situ water harvesting could further increase kilocalorie production by 7–24 per cent. Under the ambitious scenario (all measures combined, including irrigation expansion), this could increase global kilocalorie crop production by 41 per cent.
Access to cost-effective rainwater management and supplemental irrigation technologies can give farmers in rainfed holdings the security to invest in fertilisers and high-yielding varieties. Aside from water management, the performance of a crop is the result of inherent attributes (i.e. genetic gains, as with improved varieties) and agronomic practices, including various inputs. Without agronomic practices, in situ water harvesting and soil and water conservation may generate only marginal, if any, crop yield gains.
Water relocation is another important supplement to water harvesting and conservation (last box on the right in Figure 16). Farmers combine harvesting and conservation with drainage to avoid floods during heavy rainfall, while terracing systems can also work as drainage structures on sloping cropland.
Almost 20 per cent of global cropland is suitable for water harvesting, and for soil and water conservation, with hotspots in large parts of Eastern Africa and South-eastern Asia. Water harvesting in these cropland areas can increase production by 60–100 per cent.
These practices may reduce surface and groundwater flows; therefore, water accounting should precede any implementation. In many rainfed areas, efforts towards sustainable rainfed production have been in place for decades.
In Ethiopia, public investments, farmer in-kind contributions through labour and international development inputs have gone into soil and water conservation for more than 40 years. As a result, about 20 per cent of the country’s cropland employs terracing. The extent of cropland under improved management practices at the local and global levels remains unknown. Global data are also scarce for agricultural areas equipped for surface and subsurface drainage.
Source: http://www.fao.org
