Sustainable intensification requires smarter, precision technologies for irrigation, and farming practices that use ecosystem approaches to conserve water
Crops are grown under a range of water management regimes, from simple soil tillage aimed at increasing the infiltration of rainfall, to sophisticated irrigation technologies and management. Of the estimated 1.4 billion ha of crop land worldwide, around 80 percent is rainfed and accounts for about 60 percent of global agricultural output1. Under rainfed conditions, water management attempts to control the amount of water available to a crop through the opportunistic deviation of the rainwater pathway towards enhanced moisture storage in the root zone. However, the timing of the water application is still dictated by rainfall patterns, not by the farmer.
Some 20 percent of the world’s cropped area is irrigated, and produces around 40 percent of total agricultural output1. Higher cropping intensities and higher average yields account for this level of productivity. By controlling both the amount and timing of water applied to crops, irrigation facilitates the concentration of inputs to boost land productivity. Farmers apply water to crops to stabilize and raise yields and to increase the number of crops grown per year. Globally, irrigated yields are two to three times greater than rainfed yields. Thus, a reliable and flexible supply of water is vital for high value, high-input cropping systems. However, the economic risk is also much greater than under lower input rainfed cropping. Irrigation can also produce negative consequences for the environment, including soil salinization and nitrate contamination of aquifers.
Growing pressure from competing demands for water, along with environmental imperatives, mean that agriculture must obtain “more crops from fewer drops” and with less environmental impact. That is a significant challenge, and implies that water management for sustainable crop production intensification will need to anticipate smarter, precision agriculture. It will also require water management in agriculture to become much more adept at accounting for its water use in economic, social and environmental terms.
Prospects for sustainable intensification vary considerably across different production systems, with different external drivers of demand. In general, however, the sustainability of intensified crop production, whether rainfed or irrigated, will depend on the adoption of ecosystem approaches such as conservation agriculture, along with other key practices, including use of high-yielding varieties and good quality seeds, and integrated pest management.
Rainfed cropping systems
Many crop varieties grown in rainfed systems are adapted to exploit moisture stored in the root zone. Rainfed systems can be further improved by, for example, using deep-rooting crops in rotation, adapting crops to develop a deeper rooting habit, increasing soil water storage capacity, improving water infiltration and minimizing evaporation through organic mulching. Capture of runoff from adjacent lands can also lengthen the duration of soil moisture availability. Improving the productivity of rainfed agriculture depends largely on improving husbandry across all aspects of crop management. Factors such as pests and limited availability of soil nutrients can limit yield more than water availability per se2, 3. The principles of reduced tillage, organic mulching and use of natural and managed biodiversity (described in Chapter 2, Farming systems) are fundamental to improved husbandry.
The scope for implementing SCPI under rainfed conditions will depend, therefore, on the use of ecosystem-based approaches that maximize moisture storage in the root zone. While these approaches can facilitate intensification, the system is still subject to the vagaries of rainfall. Climate change will increase the risks to crop production. Nowhere is the challenge of developing effective strategies for climate change adaptation more pressing than in rainfed agriculture4.
Other measures are needed, therefore, to allay farmers’ risk aversion. They include better seasonal and annual forecasting of rainfall and water availability and flood management, both to mitigate climate change and to improve the resilience of production systems. More elaborate water management interventions are possible to reduce the production risk, but not necessarily to further intensify rainfed production. For instance, there is scope to transition some rainfed cropping systems to low-input supplementary irrigation systems, in order to bridge short dry spells during critical growth stages5, but these are still reliant upon the timing and intensity of rainfall.
On-farm runoff management, including the use of water retaining bunds in cultivated areas, has been applied successfully in transitional climates, including the Mediterranean and parts of the Sahel, to extend soil moisture availability after each rain event. Off-farm runoff management, including the concentration of overland flow into shallow groundwater or farmer-managed storage, can allow for limited supplementary irrigation. However, when expanded over large areas, these interventions impact downstream users and overall river basin water budgets.
Extending the positive environmental and soil moisture conservation benefits of ecosystem approaches will often depend upon the level of farm mechanization, which is needed to take advantage of rainfall events. Simpler technologies, including opportunistic runoff farming, will remain inherently risky, particularly under more erratic rainfall regimes. They will also remain labour intensive.
Policymakers will need to assess accurately the relative contributions of rainfed and irrigated production at national level. If rainfed production can be stabilized by enhanced soil moisture storage, the physical and socio-economic circumstances under which this can occur need to be well identified and defined. The respective merits of low-intensity investments in SCPI across extensive rainfed systems and high intensity localized investments in full irrigation need careful socio-economic appraisal against development objectives.
With regard to institutions, there is a need for re-organization and reinforcement of advisory services to farmers dependent on rainfed agriculture, and renewed efforts to promote crop insurance for small-scale producers. A sharper analysis of rainfall patterns and soil moisture deficits will be needed to stabilize production from existing rainfed systems under climate change impacts.
Irrigated cropping systems
The total area equipped for irrigation worldwide is now in excess of 300 million ha6, and the actual area harvested is estimated to be larger due to double and triple cropping. Most irrigation development has taken place in Asia, where rice production is practised on about 80 million ha, with yields averaging 5 tonnes per ha (compared to 2.3 tonnes per ha from the 54 million ha of rainfed lowland rice). In contrast, irrigated agriculture in Africa is practised on just 4 percent of cropped land, owing mainly to the lack of financial investment.
Irrigation is a commonly used platform for intensification because it offers a point at which to concentrate inputs. Making this sustainable intensification, however, depends on the location of water withdrawal and the adoption of ecosystem based approaches – such as soil conservation, use of improved varieties and integrated pest management – that are the basis of SCPI. The uniformity of distribution and the application efficiency of irrigation vary with the technology used to deliver water, the soil type and slope (most importantly its infiltration characteristic), and the quality of management.
Surface irrigation by border strip, basin or furrow is often less efficient and less uniform than overhead irrigation (e.g. sprinkler, drip, drip tape). Micro irrigation has been seen as a technological fix for the poor performance of field irrigation, and as a means of saving water. It is being adopted increasingly by commercial horticulturalists in both developed and developing countries, despite high capital costs.
Deficit irrigation and variants such as regulated deficit irrigation (RDI) are gaining hold in the commercial production of fruit trees and some field crops that respond positively to controlled water stress at critical growth stages. RDI is often practised in conjunction with micro-irrigation and “fertigation”, in which fertilizers are applied directly to the region where most of the plant’s roots develop. The practice has been adapted to simpler furrow irrigation in China. The benefits, in terms of reduced water inputs, are apparent but they will only be realized if the supply of water is highly reliable.
Knowledge-based precision irrigation that offers farmers reliable and flexible water application will be a major platform for SCPI. Automated systems have been tested using both solid set sprinklers and micro-irrigation, which involve using soil moisture sensing and crop canopy temperature to define the irrigation depths to be applied in different parts of the field. Precision irrigation and precision fertilizer application through irrigation water are both future possibilities for field crops and horticulture, but there are potential pitfalls. Recent computer simulations indicate that, in horticulture, salt management is a critical factor in sustainability.
The economics of irrigated agriculture are significant. The use of sprinkler and micro-irrigation technologies, as well as the automation of surface irrigation layouts, involve long term capital expenditure and operational budgets. Rain guns provide one of the cheapest capital options for large area overhead irrigation coverage, but tend to incur high operating costs. Other overhead irrigation systems have high capital costs and, without the support of production subsidies, are unsuited to smallholder cropping systems.
The service delivery of many public irrigation systems is less than optimal, owing to deficiencies in design, maintenance and management. There is considerable scope for modernizing systems and their management, through both institutional reform and the separation of irrigation service provision from broader oversight and the regulation of water resources.
Drainage is an essential, but often overlooked, complement to irrigation, especially where water tables are high and soil salinity is a constraint. Investment will be required in drainage to enhance the productivity and sustainability of irrigation systems and to ensure good management of farm inputs. However, enhanced drainage increases the risks of pollutants being exported, causing degradation in waterways and connected aquatic ecosystems.
Protected cropping, mostly in shade houses, is enjoying increasing popularity in many countries, including China and India, mainly for fruit, vegetable and flower production. In the long term, highly intensive closed cycle production systems, using conventional irrigation or hydroponic and aeroponic cultures, will become progressively more common, especially in peri-urban areas with strong markets and increasing water scarcity.
Using water for irrigation reduces instream flows, alters their timing, and creates conditions for shocks, such as toxic algal blooms. Secondary impacts include salinization and nutrient and pesticide pollution of water courses and water bodies. There are other environmental trade-offs from irrigated systems; rice paddies sequester higher levels of organic matter than dry land soils, and contribute less nitrate runoff and generate lower emissions of nitrous oxide (N2O). Offset against this are relatively large emissions of methane (from 3 to 10 percent of global emissions) and ammonia.
Crops normally use less than 50 percent of the irrigation water they receive, and irrigation systems that lie within a fully or over-allocated river basin have low efficiency. In accounting terms, it is necessary to distinguish how much water is depleted, both beneficially and unproductively. Beneficial depletion by crops – evapotranspiration – is the intent of irrigation: ideally, transpiration would account for all depletion, with zero evaporation from soil and water surfaces. There is some potential to improve water productivity by reducing non-productive evaporative losses.
Basin level improvements in water productivity focus on minimizing non-beneficial depletion7. However, the downstream impacts of increased water depletion for agriculture are not neutral: there is evidence of big reductions in annual runoff from “improved” upper catchments that have adopted extensive water harvesting in parts of peninsular India8.
Water management is a key factor in minimizing nitrogen losses and export from farms. In freely drained soils, nitrification is partially interrupted, resulting in the emission of N2O, whereas in saturated (anoxic) conditions, ammonium compounds and urea are partially converted to ammonia, typically in rice cultivation. Atmospheric losses from urea can occur, therefore, as both ammonia and N2O are released during wetting and drying cycles in irrigation. N is required in nitrate form for uptake at the root, but can easily move elsewhere in solution. A number of protected and slow release fertilizer compounds are under development for different situations (see Chapter 3, Soil health).
The dynamics of phosphate mobilization and movement in drains and waterways are complex. Phosphate export from agriculture can occur in irrigated systems if erosive flow rates are used in furrow irrigation, or if sodic soils disperse. Phosphate, and to a lesser extent nitrate, can be trapped by buffer strips located at the ends of fields and along rivers, which prevents them from reaching waterways. Hence, a combination of good irrigation management, recycling of tailwater and the incorporation of phosphate in the soil can reduce phosphate export from irrigated lands to close to zero.
The sustainability of intensified irrigated agriculture depends on minimizing off-farm externalities, such as salinization and export of pollutants, and the maintenance of soil health and growing conditions. That should be the primary focus of farm level practice, technology and decision-making, and reinforces the need for depletion water accounting and wiser water allocation at basin and catchment scales, and a better understanding of the hydrological interactions between different production systems.
The way forward
Sustainable agriculture on irrigated land – and also across the range of rainfed and improved rainfed production systems – involves trade-offs in land use, water sharing in the broadest sense, and the maintenance of supporting ecosystem services. These trade-offs are becoming more complex and have significant social, economic and political importance.
The overall governance of land and water allocations will strongly influence the scale of longer term investment in irrigated SCPI, particularly given the higher capital and input costs associated with irrigated production. Competing demands for water from other economic sectors and from environmental services and amenities will continue to grow. Water management in agriculture will need to cope with less water per hectare of land and will also have to internalize the cost of pollution from agricultural land.
With regard to policy, the nature of agriculture is changing in many countries, as the pace of rural outmigration and urbanization accelerates. Policy incentives that focus on the most pressing environmental externalities, while leveraging individual farmer’s profit motives, have a greater chance of success.
For example, where agrochemical pollution of rivers and aquatic ecosystems has reached crisis point, a ban on dangerous chemicals could be accompanied by measures to raise fertilizer prices, provide farmers with objective advice on dosage rates, and remove perverse incentives to apply fertilizer excessively. Follow-up measures might promote management at “required or recommended” levels, and seek alternative approaches to higher productivity with more modest use of external inputs. In that case, more public investment would be needed to improve the monitoring of ecosystem conditions.
In the future, fertigation technology (including use of liquid fertilizers), deficit irrigation and wastewater-reuse will be better integrated within irrigation systems. While the introduction of a new technology into irrigated cropping systems has high entry costs and requires institutional arrangements for operation and maintenance, the use of precision irrigation is now global. Farmers in developing countries are already adopting low-head drip kits for niche markets, such as horticulture. In addition, the availability of cheap, plastic moulded products and plastic sheeting for plasticulture is likely to expand. However, the broad-scale adoption of alternatives, such as solar technologies, or the avoidance of polluting technologies, will need the support of regulatory measures and effective policing of compliance.
Shortcomings in governance of some irrigation investments have led to financial irregularities in capital funding, rent-seeking in management and operation, and poor co-ordination among agencies responsible for providing irrigation services to the farmer. Innovative approaches are required to create institutional frameworks that promote agricultural and water development, and at the same time safeguard the environment. There remains considerable potential to harness and learn from local initiatives in institutional development, to manage the externalities of intensification, and to reduce or avoid transaction costs. Solutions are more likely to be knowledge-rich than technology-intensive.
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3. Sadras, V.O. & Angus, J.F. 2006. Benchmarking water use efficiency of rainfed wheat in dry environments. Australian Journal of Agricultural Research, 57: 847–856.
4. UNDP. 2006. Human Development Report 2006. New York, USA.
5. Wani, S.P., Rockstrom, J. & Oweis, T., eds. 2009. Rainfed agriculture: Unlocking the potential. Comprehensive Assessment of Water Management in Agriculture 7. Wallingford, UK, CABI Publishing.
6. FAO. 2011. AQUASTAT statistical database (www.fao. org/nr/water/aquastat/main/ index.stm).
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8. Batchelor, C., Singh, A., Rama Rao, M.S. & Butterworth, J. 2005. Mitigating the potential unintended impacts of water harvesting. UK, Department for International Development.
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