Climate Smart Agriculture Sourcebook

Water management for Climate-Smart Agriculture

Production and Resources

Water management and climate change

Observed data and climate projections show that changes in water quantity and quality due to climate change are expected to negatively affect food security and increase the vulnerability of poor rural farmers, especially in arid and semi-arid areas. In many regions, agricultural production is already being adversely affected by climate change (FAO, 2016a). Higher temperatures, less reliable supplies of water and more frequent droughts and floods are likely to reduce yields in many areas.

Agriculture is clearly highly dependent on climate. However, there are many non-climatic factors that strongly influence agricultural production systems, such as expanding populations and urbanization, global economic growth, increasing competition for natural resources, agronomic management practices, technological innovations, and trade and food prices. These factors have more immediate impacts on water resources than those induced by climate change (Bates et al., 2008). For this reason, it is important to understand the current status of water management in agriculture before assessing the potential impacts of climate change.

B6 - 3.1. Water management in agriculture: status and trends

Between 1961 and 2011, global agricultural output more than tripled. The higher demand for food, fibre and other agricultural products has been met mostly by an increase in agricultural productivity. The expansion of agricultural land has remained relatively limited. Total cultivated land increased by only 12 percent between 1961 and 2009, but productivity more than doubled. The amount of land needed to produce food for one person has decreased from 0.45 hectares in 1961 to 0.22 hectares in 2009. During the same period, the extent of irrigated land more than doubled, increasing from 139 to 301 million hectares (FAO, 2011a). By providing farmers with access to water, irrigation has been a key factor in the intensification of agricultural production.

With the doubling of irrigated area, water withdrawals for agriculture have been rising sharply. Globally, agricultural water withdrawals represent 70 percent of all withdrawals. However, as water resources are very unevenly distributed, the impact of these withdrawals varies substantially between countries and regions. An increasing number of the world’s river basins have reached conditions of water scarcity through the combined demands of agriculture and other sectors. FAO (2011a) estimates that more than 40 percent of the world’s rural population lives in river basins that are classified as water scarce.

The high level of pressure on water resources has had serious repercussions for water users and the environment. Competition over water use is growing in river basins where there are no measures in place for arbitrating conflicts. Biodiversity is declining more rapidly for species that depend on freshwater ecosystems than for species from other types of ecosystems (Comprehensive Assessment, 2007). The large-scale public surface irrigation systems, which were built during the green revolution and dominated the landscape until the early 1980s, have had a profound impact on the flow of many rivers. Private investments, stimulated by the availability of cheap pumps and well-drilling capacity, have been used to tap groundwater. Consequently, aquifers are being depleted in countries with key agricultural production systems, including China, India, and the United States of America.

Water demand from cities and industries has been booming as a result of rapid economic growth in emerging economies. This growth has put pressure on irrigation schemes to release water for urban and industrial users. Pollution from agriculture, cities and industries has affected rivers and aquifers, and further reduced the amount of water available for use. The trends towards an increasing demand for water from all sectors is expected to continue in the coming decades as expanding populations and economic growth increase the consumption of food and manufactured goods.

Given the limits of water resources, the rate of expansion of land under irrigation is slowing substantially. FAO has projected that the global area equipped for irrigation may increase at a relatively low annual rate of 0.1 percent (Alesandratos and Bruinsma, 2012). At that rate, it would reach 337 million hectares in 2050, compared to around 325 million hectares in 2013. This represents a significant slowdown from the period between 1961 and 2009.

The role climate change will play with regards to water in agriculture must be considered in this context of rapid increases in water withdrawals, the degradation of water quality and the competition for water at all levels. Chapter B6-3 looks at the current state of knowledge about the impacts of climate change on water resources and the demand for these resources. These impacts are framed within the overall perspective of the current status, trends and challenges of water management in agriculture. Of particular interest are aspects of change that are associated with changing climatic conditions and require specific responses.

Climate action and the 2030 Agenda for Sustainable Development are closely connected. It will be difficult, if not impossible to eradicate poverty and end hunger without building resilience to climate change in smallholder agricultural production systems (FAO, 2016a). Sustainable Development Goal (SDG) 6 (Ensure availability and sustainable management of water and sanitation for all) has six targets that focus on improving access to drinking water and sanitation services; preserving water quality and reducing sources of pollution; strengthening integrated water resources management with the participation of the local population; and increasing efficiency in the use of water resources. By calling for substantial increases in water-use efficiency across all sectors, and the sustainable use of freshwater resources to reduce the number of people suffering from water scarcity, SDG 6 has a close relationship with SDG 2 (End hunger, achieve food security and improved nutrition and promote sustainable agriculture) and SDG 1 (End poverty in all its forms everywhere).

B6 – 3.2 Climate change impact on water in agriculture

Water is the prime channel through which the impacts of climate change on the world’s ecosystems and livelihoods will be felt. Climate change has the potential to affect every element in the water cycle (UN-Water, 2010). Agriculture will be affected by increased evaporative demand, changes in the amount of rainfall and rainfall patterns, and variations in river runoff and groundwater recharge, which are the two sources of water for irrigation (Figure 3.1).

Figure 3.1. How climate change affects all the elements of the water cycle and its impact on agriculture

B6 - 3.2.1 Impact on water supply and demand

The Intergovernmental Panel on Climate Change’s (IPCC) Fifth Assessment Report, published in 2014, describes the expected impacts of climate change on water resources. Its findings are summarized in this section.

Renewable surface water and groundwater resources

Precipitation and evaporation are the main climatic phenomena affecting freshwater resources. Climate models predict decreases of renewable water resources in some regions and increases in others. In many areas, there is considerable uncertainty in this regard. However, there is high agreement and robust evidence that water resources will likely decrease in many mid-latitude and dry subtropical regions, and increase at high latitudes and in many humid mid-latitude regions. Even where increases are projected, there may be short-term shortages due to more variable streamflow caused by greater variability in precipitation. There may also be seasonal reductions in water supply due to reduced snow and ice storage. The availability of clean water may also be reduced by the negative impacts of climate change on water quality. 

The decrease of renewable surface water and groundwater resources in dry subtropical regions will intensify competition for water by different users (e.g. agriculture, ecosystems, settlements, industry, and energy production). This will have an impact on regional water, energy and food security. How changes in vegetation brought about by increasing concentrations of greenhouse gas in the atmosphere will affect water resources and irrigation requirements remains uncertain.

Floods and droughts

Climate change is also projected to alter the frequency and magnitude of floods and droughts. The impact is expected to vary from region to region. Despite limited evidence, there is high agreement that floods will increase over more than half of the globe, particularly in central and eastern Siberia, parts of Southeast Asia, including India, tropical Africa, and northern South America. Decreases in floods are projected in parts of northern and Eastern Europe, Anatolia, Central and East Asia, central North America, and southern South America. The Fifth Assessment Report stated with high confidence that since the mid-20th century, socio-economic losses from flooding have increased mainly due to greater exposure and vulnerability. There is limited evidence and medium agreement that global flood risk will increase partly due to climate change. 

In some regions, including southern Europe and West Africa, metereological droughts, which are characterized by significantly reduced rainfall, and agricultural droughts, which are characterized by significant declines in soil moisture that affect crop production, have become more frequent since 1950 (Seneviratne et al., 2012). Climate change is likely to increase the frequency and length of both kinds of droughts by the end of the 21st century. However, it is still uncertain what these rainfall and soil moisture deficits may mean for prolonged reductions of streamflow and lake and groundwater levels. Droughts are projected to intensify in southern Europe and the Mediterranean region, central Europe, central and southern North America, Central America, northeast Brazil, and southern Africa. 

Water quality

There is medium evidence and high agreement that climate change negatively impacts freshwater ecosystems by changing streamflow and water quality. Climate change affects the quality of water through a complex set of natural processes and anthropogenic factors.

There is medium evidence and high agreement that climate change will likely reduce raw water quality, which will pose risks to drinking water quality even with conventional treatment. The sources of these risks are increased temperatures, higher levels of sediment, nutrient and pollutant loading due to heavy rainfall, the reduced dilution of pollutants during droughts, and the disruption of treatment facilities during floods.

Streamflow seasonality

In regions with snowfall, climate change has already altered the seasonality of streamflow. There is robust evidence and high agreement that this variability will increase. Except in very cold regions, warming in the last decades has reduced the spring maximum snow depth and brought forward the spring maximum of snowmelt discharge. Smaller snowmelt floods, increased winter flows, and reduced summer low flows have all been noted. River ice in Arctic rivers has been observed to break up earlier.

Groundwater recharge

The impact of climate change on groundwater recharge is difficult to predict. Changes in precipitation intensity will affect the amount of total runoff that recharges groundwater. In humid areas, increased precipitation intensity may decrease groundwater recharge because the infiltration capacity of the soil will be insufficient.  In semi-arid areas, increased precipitation intensity may increase groundwater recharge due to the faster rate of percolation through the root zone, which will reduce evapotranspiration.

Crop water demand

Changes in climate will affect the water demand of crops grown in both irrigated and rainfed systems. An increase in temperatures will trigger a higher demand for water for evapotranspiration by crops and natural vegetation, which will lead to more rapid depletion of soil moisture. This scenario, combined with changes in rainfall patterns may lead to more frequent crop failures. The Fifth Assessment Report provides different projections on the effect of climate change in crop water demands. In general, projections show that the water demand to produce a given amount of food on either irrigated or rainfed systems will increase in many regions due to climate change. By the 2080s, there is high confidence that irrigation demand will increase significantly in many areas – perhaps by more than 40 percent across Europe, the United States of America, and parts of Asia. Other regions, including major irrigated areas in India, Pakistan, and southeastern China, may experience a slight decrease in irrigation demand, as a result of higher precipitation. However, this is only projected in some climate change scenarios. Wada et al. (2013) suggest an increase in irrigation demand by the 2080s, with a global average increase of 7 to 21 percent, depending on the emissions scenario, and with pronounced regional variations. By contrast, Zhang and Cai (2013), predict a slight global decrease in crop water deficits in both irrigated and rainfed areas by the 2080s. The decrease can be explained partly by a smaller difference between daily maximum and minimum temperatures. Where poor soil is not a limiting factor, increases in crop water productivity due to carbon dioxide fertilization might partly moderate the adverse effects of climate change, and potentially reduce global irrigation water demand (Konzmann et al., 2013).

The Fifth Assessment Report states with medium confidence that climate change will increase the interannual variability of crop yields in many regions. The differences in yield and yield variability between rainfed and irrigated land may also increase with changes in climate and greater climate variability.

Sea level

There has been significant progress in the understanding of sea level rise since the release of the IPPC's Fourth Assessment Report in 2007. It is virtually certain that sea level will continue to rise during the 21st century and beyond. For all scenarios, the rate of 21st century of global mean sea level rise is very likely to exceed the average rate during the 20th century. The expected rise in sea levels will affect agriculture in coastal areas, particularly river deltas. Higher sea levels combined with variations in the distribution of runoff and more frequent floods in upstream areas, will result in an increased incidence of flooding and saltwater intrusion in estuaries and aquifers. This will affect some of the world's most productive agricultural areas.

B6 - 3.3 Combined impacts climate change and non-climatic drivers of change in water for agriculture

Rapid increases in water withdrawals driven by intensified agricultural production, overall economic development, population growth and urbanization have modified the water balance in many watersheds and aquifers. Unsustainable water withdrawals and pollution are threatening ecosystems and the livelihoods of rural communities in an increasing number of river basins and aquifers. 

The extent to which changing climatic conditions will affect the water cycle and agriculture will be determined by non-climatic drivers of change. In arid and semi-arid areas, climate change will place additional burdens on already stretched water resources. However, in these environments, agriculture will first need to respond to the challenges posed by increasing human pressures on water resources. In other places, climate change will be the main factor driving changes in water resources and will necessitate specific climate-smart responses. Table 3.1 shows the relative importance of climatic and non-climatic factors in determining changes in water resources for agriculture. The impacts of climate change will vary from one agricultural system to another. It is important that climate-smart strategies take into account the overall socio-economic and environmental setting in which they are to be implemented.

Of particular relevance is the time frame for the projected impacts of climate change and its relation to the speed of change driven by development. Annual changes in runoff and recharge due to climate change are expected to occur at a slower pace than changes caused by anthropogenic demands for water. However, changes in climate variability and extreme events associated with climate change may already be having impacts on water resources for agriculture, and deserve particular attention when preparing short- and medium-term responses.

Table 3.1. Impacts of climate change and non-climatic drivers of change on water resources for agriculture

Type of hydrological change

Impact from

Non-climatic drivers

Climate change

Change in annual precipitation 

No or minor impact

Expected to increase globally during the 21st century, with potentially significant spatial variations

Interannual precipitation variability

No impact

Expected to increase everywhere

Seasonal precipitation 

No impact

Expected to increase everywhere

Agricultural droughts 

Limited impact: some agricultural practices can deplete soil moisture faster than natural vegetation

Moisture stress to generally increase as a result of increasing variability of rainfall distribution (i.e. longer periods without rain) and increasing temperatures.

Exposure to floods

Moderate impact: flood intensity and impact can be exacerbated by changes in land use and unplanned development in alluvial plains

Percentage of global population annually exposed expected to increase.

Snow and glacier melt

Limited impact through deposit of pollutants and change in the reflecting power of the surface (albedo)

Rising temperatures lead to accelerated snow and glacier melt with initial increases in river flow followed by decreases.

Change in river discharge

High impact in water scarce areas, where reservoir construction and water diversion for agriculture and other uses are modifying runoff regimes and reducing annual flow. Large-scale water conservation measures also have an impact on river discharge

Increased variability as a result of changes in rainfall patterns. Changes in snow and glacier melt induce changes in seasonal patterns of runoff. Changes in annual runoff expected to vary from region to region.

Change in groundwater resources

High impact: large-scale development of infrastructure to withdraw groundwater resources in many regions are already threatening the sustainability of aquifers in many dry areas. 

Varies as a function of changes in rainfall volumes and distribution. 

Increase evapotranspiration

Limited impact in agriculture: some crops have higher evapotranspiration rates than natural systems, other less

Increases as temperatures rise

Water quality (in rivers, lakes and aquifers)

High impact from pollution in highly developed areas

Moderate impact due to increased  temperatures

Salinity in rivers and aquifers

High impact from water withdrawal in highly developed areas, mostly in arid regions.

Potentially high impact where sea water level rise combines with reduced runoff and increased withdrawal