|
|
| (42 intermediate revisions by 5 users not shown) |
| Line 1: |
Line 1: |
| | | | |
| − | '''Wa''''''ter use efficiency in irrigated agriculture'''
| + | Consumed water is defined as water that has been evaporated, transpired, incorporated into products or crops, significantly contaminated or otherwise made unavailable to other water users. Water that has been withdrawn but not consumed and is returned to the system is called return flow ([http://www.fao.org/nr/water/aquastat/ http://www.fao.org/nr/water/aquastat/]). As shown in Figure 1, agriculture is the major user of consumptive water. Globally, around 50 percent of the water withdrawn for agriculture is consumed through evapotranspiration. Thus, the concept of water consumption mostly refers to evaporative losses. Non-consumptive water is almost entirely returned to the system. |
| | | | |
| − | {| cellspacing="0" cellpadding="0"
| + | = Facts and Figures = |
| − | |-
| + | |
| − | | height="159" |
| + | |
| − | {| width="100%" cellspacing="0" cellpadding="0"
| + | |
| − | |-
| + | |
| − | | <div>
| + | |
| | | | |
| − | </div> | + | While the world’s population tripled in the 20th century, the use of renewable water resources has grown six-fold. Within the next fifty years, the world population will increase by another 40 to 50 %. This population growth – coupled with industrialization and urbanization – will result in an increasing demand for water and will have serious consequences on the environment ([http://blogs.triplealearning.com/20117 http://blogs.triplealearning.com/20117]). Worldwide, agriculture accounts for 70% of all water consumption, compared to 20% for industry and 10% for domestic use. In industrialized nations, however, industries consume more than half of the water available for human use, while more than 90% is used for irrigation in developing countries, in particular [[Dryland_farming|in semi-arid or arid countries]] ([http://www.worldometers.info/water/ http://www.worldometers.info/water/]).<br/> |
| − | |}
| + | |
| | | | |
| − | |}
| + | [[File:Water uses and consumption by sector.jpg|thumb|left|188px|Figure 1: Water uses and consumption by sector, Shiklomanov, SHI and UNESCO 1999]] |
| − | The proportion of water absorbed by a crop and transpired by its leaves is called, in irrigated agriculture, the productive water use. This is the water that serves for crop growth resulting in yields. Therefore, the higher the productively used portion of water, the higher is its water use efficiency.
| + | |
| − | '''Content'''
| + | |
| | | | |
| − | Background
| + | Source: [http://www.unep.org/dewa/vitalwater/article43.html http://www.unep.org/dewa/vitalwater/article43.html] |
| | + | <p style="text-align: center"></p><p style="text-align: center"></p> |
| | + | [[File:Water uses by sector and country.jpg|thumb|left|193px|Figure 2: Water uses by sector and country, UNEP]] |
| | + | <p style="text-align: center"></p> |
| | + | <br/> |
| | | | |
| − | Water Use Efficiency – The Debate
| + | <br/> |
| | | | |
| − | Project example: Bolivia
| + | <br/> |
| | | | |
| − | Project example: Jordan
| + | <br/> |
| | | | |
| − | References
| + | <br/> |
| | | | |
| | + | <br/> |
| | | | |
| | + | <br/> |
| | | | |
| − | '''Background'''
| + | <br/> |
| | | | |
| − | Generally, in simple irrigation systems the efficiently used proportion of water adds up to less than 50%; frequently it ranges only between 30-40%. This low water use efficiency of ordinary irrigation systems has long been criticized by experts, with demands for improvements.
| + | <br/> |
| | | | |
| − | An increase of efficiency can be achieved by the following measures:
| + | <br/>Source: [http://www.unep.org/dewa/vitalwater/article49.html http://www.unep.org/dewa/vitalwater/article49.html] |
| − | *Applying water saving technologies like drop irrigation
| + | |
| − | *Optimizing management of the irrigation intervals
| + | |
| − | *Balancing of seasonal fluctuations of water availability by construction of water storage facilities on the farm or regional level
| + | |
| − | *Reducing the amount of evaporation by irrigation at night, sub-surface irrigation or mulching
| + | |
| − | *Cultivation of less water demanding crops, or cultivation of crops adapted to marginal quality
| + | |
| | | | |
| − | These measures may sound simple, yet they are not easy to implement. In many cases, the above measures do not even make sense, as they may imply other unforeseen disadvantages or require untenable preconditions. Particularly in the least developed countries, the following obstacles for implementation exist:
| + | <br/> |
| − | *Efficient technical systems are associated with high investment costs. Hence, they might be too expensive for individual farmers and small scale operating companies.
| + | |
| − | *Needs-based irrigation management requires know-how regarding the water demand of specific crops. Conditions of the respective varieties and of crop locations must be identified.
| + | |
| − | *In surface irrigation, a certain amount of over-irrigation makes sense in order to avoid salinization (leaching requirement).
| + | |
| − | *The diminution of evaporation losses, as via optimal timing or irrigation at night, may collide with other exigencies. Especially in African countries, these measures can be dangerous due to roaming wild animals like elephants, hippos and snakes.
| + | |
| − | *Crops with a low water need usually have a low market price. Crop rotation not only depends on water consumption, but on various conditions of location and marketing.
| + | |
| | | | |
| − | As long as water is free of charge - as in most developing countries - the economic incentives to save water are too few to justify investment into new technologies or in advanced training.
| + | <br/> |
| | | | |
| − | In addition, there are some general aspects to be considered when the conservation of the water resources, and not only the profitability of a single farm or scheme, is in the focus. The conventional perception of water use efficiency focuses on the individual farm where, quite often, savings of irrigation water lead to an extension of the land irrigated on the farm. In the final analysis, more water may be used despite, or even due to, water saving technologies. Therefore, in developing countries with weak institutional structures, it is only in exceptional cases that the use of these technologies can contribute to a general reduction in the use of water resources across farms.
| + | <br/> |
| | | | |
| − | '''Water Use Efficiency - The Debate'''
| + | = Water consumption in agriculture = |
| | | | |
| − | For some years, Keller/Keller/Seckler (1996) and then later Giordano/Rijsberman/Saleth (eds.) (2006) from IWMI (International Water Management Institute) have called attention to the fact that improved water use efficiency on the farm level is not necessarily leading towards the conservation of water resources in general. Rather, improved efficiency of individual farms may even pander to a disparate distribution of water on the upper and lower course of a river if it is not combined with a reduction of water withdrawal by the same user.
| + | Annual global agricultural water consumption includes crop water, consumption for food, fiber and feed production (transpiration), plus evaporation losses from the soil and from open water associated with agriculture, such as rice fields, irrigation canals and reservoirs. |
| | | | |
| − | Irrigation plants are open systems, from where water returns into the catchment area and can be used by the downstream resident. Therefore, improved water use efficiency upstream can abate the availability of water downstream if the upstream resident extracts as much water as previously. This is a frequent scenario, as it makes economic sense to the upstream user to take advantage of the already existing concessions or pumping capacities. When the operator is able to extend his irrigated area, the extension of irrigation is economically reasonable.
| + | About 20% of the total 7,130 km3 of agriculture’s annual water consumption is ‘blue water’ – that is, water from [[Riverbed farming|rivers]], streams, lakes and groundwater for irrigation purposes which accounts for around 40% of the world’s production on around 20% of the cultivated land. Globally, irrigated crop yields are about 2.7 times those of rain-fed farming. Hence, due to increasing food demand, the importance of irrigation will most probably also increase in future. There is still potential for expansion of irrigation in places where sufficient water is available, particularly in sub-Saharan Africa and South America. |
| | | | |
| − | If the conservation of the water resources and the equitable distribution of water upstream and downstream are all in the focus of interest, the level of water abstraction of each farm must be considered and included into the strategies to improve the water use efficiency of the entire system through coordination of the users.
| + | Pathways to improve productivity and bridge the yield gap in irrigation include increasing the quantity, reliability and timing of water services; increasing the beneficial use of water withdrawn for irrigation; and increasing agronomic or economic productivity so that more output is obtained per unit of water consumed. Although there is still potential to increase the cropped area, some 5–7 million ha (0.6%) of farmland are lost annually because of accelerating land degradation and urbanization, which takes agricultural land out of production and reduces the number of farms as more people move to the cities. Increasing population means that the amount of cultivated land per person is also declining sharply: from 0.4 ha in 1961 to 0.2 ha in 2005 (WWAP, 2012).<ref name="WWAP">WWAP (UN-World Water Assessment Programme) (2012): The United Nations Water Development Report 4: Managing Water under Uncertainty and Risk. Paris. UNESCO</ref> |
| | | | |
| − | Single inefficient units at a water course may have a highly efficient use in total, if the return flow water is reused to a large extent. Such highly efficient systems do exist. For example, in Egypt many small scale farmers make use of the waters of the Nile in a very inefficient way; yet via the complete recirculation of the water, the water use efficiency of the whole user community is very high. Hence, the water productivity of the entire irrigation system, or what Keller calls the total effective efficiency, is excellent (see Keller et al., 1996: 5).
| + | Predicting future water demand for agriculture is fraught with uncertainty; alongside population growth, water demand depends on what and how much people eat, on uncertainties in seasonal climatic variations, efficiency of agriculture production processes, grown crop types and yields, among other factors. Although projections vary considerably, future global agricultural water consumption (including both rain-fed and irrigated agriculture) is estimated to increase by about 19% to 8,515 km3 per year in 2050 (Comprehensive Assessment of Water Management in Agriculture, 2007). The Food and Agriculture Organization of the United Nations (FAO) estimates an 11% increase in irrigation water consumption from 2008 to 2050. This is expected to increase by about 5% the present water withdrawal for irrigation of 2,740 km3. |
| | | | |
| − | In order to elaborate effective strategies encompassing the effective efficiency of the entire water course, the following questions have to be answered:
| + | Although this seems a modest increase, much of it will occur in regions already suffering from water scarcity (FAO, 2011a) as Figure 3 illustrates. |
| − | #Is the backflow into the water course complete or are there losses in sinks, where the water cannot be regained?
| + | |
| − | #How is the use of water upstream and downstream adjusted? Can coordination be improved?
| + | |
| − | #How often can the return flow water be reused without too much contamination by residues of pesticides and fertilizer? Can the application of those inputs be reduced?
| + | |
| | | | |
| − | Irrigation policies and a good watershed management must have an eye on both the increase of water use efficiency on farm-level and on the whole watershed. Licensed amounts of water abstraction for each user, taking into consideration the backflow of water, can be a very efficient instrument for regulation.
| + | [[File:UN WWAP.jpg|thumb|left|276px|Figure 3: Surface water and groundwater withdrawal, UN WWAP 2012]] |
| | | | |
| − | The stronger a water course is in use, the more important the coordination among users and the conservation of the water quality becomes. Furthermore, a prior condition for the sustainable use of water resources is regulation by local or regional authorities and institutions, who can act effectively for the whole water catchment area.
| + | <br/> |
| | | | |
| − | Alongside the management and technical issues, the scheme or resource-oriented political and institutional aspects have to be included to enable higher water use efficiency to happen. But these political and institutional facets will only improve the situation if they are in the interests of the main stakeholders. Unfortunately, this is frequently not the case. Inadequate transparency with regard to water allocation and use, and the associated inefficiencies, often pave the way for officials to make money illegally through preferential allocation of water (petty corruption). Hence, increasing transparency and accountability in the management of irrigation can yield significant efficiency gains and water savings.
| + | <br/> |
| | | | |
| − | '''Project example: Sahel'''
| + | Source: UN WWAP - World Water Assessment Programme (in Cosgrove et al., 2012:8)<ref name="Cosgrove">Cosgrove, Catherine E. and William J. (2012): The Dynamics of Global Water Futures 2011-2050. Report on the findings of Phase One of the UNESCO-WWAP Water Scenarios Project to 2050. UN-World Water Assessment Programme.</ref> |
| | | | |
| − | Water-spreading weirs for the development of degraded dry river valleys
| + | <br/> |
| | | | |
| − | A very successful example for an activity in a whole region was the construction of water-spreading weirs in degraded dry river valleys in the Sahel. It took place for a period of twelve years in Niger, Burkina Faso and Chad. The Cooperating Partners of German Development Cooperation were GIZ and KfW.
| + | <br/> |
| | | | |
| − | The water-spreading weirs are constructed in a way that they span the entire valley. They consist of a spillway in the riverbed and lateral abutments and wings. Floodwaters are spread above the weir and will at a certain state overflow the wings and slowly flow to the riverbed behind the weir. Thus the basic runoff and sedimentation process in the site is changed. Erosion will be reduced; sedimentation and infiltration of the water into the ground will be increased. In most cases the groundwater table rises within a few years.
| + | = Impact of climate change on water availability and use = |
| | | | |
| − | Furthermore agricultural production may be expanded and diversified. In many cases a second or even third crop cycle in the year becomes possible.
| + | Climate change leads to new uncertainties concerning future water demand through different water-using sectors. For example, global warming suggests increased energy demands for air conditioning, while higher evapotranspiration rates could increase future demands for agriculture. The precise impacts of climate change on water in specific locations remain uncertain, especially at the local or river basin level. |
| | | | |
| − | The implementation is undertaken in synchronized steps:
| + | Under different scenarios of the Intergovernmental Panel on Climate Change (IPCC), regions may become ‘drier’ or ‘wetter’, as there are a variety of possible ways in which climate change may impact the hydrological cycle in different areas and at different times. The uncertainties generated by climate change add a global dimension to the challenges of water resources management, as efforts to effectively manage water locally may be impeded by climate-induced hydrological impacts or increasing demands (WWAP, 2012, p.24).<ref name="WWAP">WWAP (UN-World Water Assessment Programme) (2012): The United Nations Water Development Report 4: Managing Water under Uncertainty and Risk. Paris. UNESCO</ref> |
| − | #Identification of geographically suitable sites
| + | |
| − | #Information of the respective villages, technical services and authorities
| + | |
| − | #Submission of a written request by interested communities
| + | |
| − | #Intermediate examination
| + | |
| − | #Feasibility study
| + | |
| − | #Final approval of the construction
| + | |
| − | #Technical study
| + | |
| − | #Construction performed with intensive manual labor
| + | |
| − | #Training of local craftsmen for maintenance
| + | |
| − | #Handing over to a local committee or administrative structure
| + | |
| | | | |
| − | An intensive participation by the communities is the principle of the project in order to transfer the responsibility as soon as possible. To ensure that the management committee or a local structure is able to function after the end of the project is crucial for the success and the sustainability.
| + | = Influence of agriculture on the hydrological cycle = |
| | | | |
| | + | As water is a renewable resource, agricultural consumptive water uses are not entirely lost; they return within the larger and smaller water cycle. Water as a resource is primarily provided by precipitation from the large weather systems. As Millán (2012) points out, the amount of available water depends on the location of the watershed, and can result from different types of precipitation. For instance, isotopic studies of rain in tropical rainforests (Brazil) show that approximately 65% of the water precipitated in any one day during the rainy season comes from water precipitated on the three previous days. This suggests that at the beginning of the wet season there is a massive inflow of water vapor evaporated from the oceans, and afterwards the water is basically recycled (re-circulated) between the soil-forest and the atmosphere, producing a daily afternoon-evening shower.<ref name="Millan">Millán M. Millán: Water begets water, and vegetation is the midwife. CEAM, Valencia, Spain: Green Week Brussels, 22-25 May, 2012. http://ec.europa.eu/environment/greenweek2012/sites/default/files/2-3_millan.pdf [2013-02-19].</ref><br/>However, water as “renewable” must be understood in a spatiotemporal way. As water resources can be drawn upon beyond their ‘renewable’ capacity per unit of time, their management can become unsustainable. This is already the case for West Asia and North Africa, where withdrawals as a percentage of internal renewable water resources have exceeded 75%; and southern Asia and the Caucasus and Central Asia which have nearly reached 60%, the threshold signaling water scarcity. An addition significant problem is the change of water quality, which is affected by most types of water uses through chemical, microbiological and thermal pollution (WWAP, 2012, p.8).<ref name="WWAP">WWAP (UN-World Water Assessment Programme) (2012): The United Nations Water Development Report 4: Managing Water under Uncertainty and Risk. Paris. UNESCO.</ref> Hence, not only climate change has an impact on the hydrological cycle and our water resources, but also the way we manage water has significant backlash. In concrete terms, about 40% of the total global runoff to the oceans has been captured for human use, with groundwater being used faster than it is being replenished in most dry areas of the world. Humans have extensively altered river systems through impoundments and diversions to meet their water, energy, and transportation needs: dams are holding back about 15% of the total annual river runoff globally (Steffan et al., 2004, p.113, Nillson et al., 2005).<ref name="Nilsson">Nilsson, C., C. A. Reidy, et al. (2005). "Fragmentation and Flow Regulation of the World's Large River Systems." Science 308 (5720): 405-408. </ref> |
| | | | |
| | + | = References and further reading = |
| | | | |
| − | '''Project example: Bolivia'''
| + | <references /> |
| | | | |
| − | Title of the Project: SIRIC (Subprograma de Inversiónes en Riego Intercomunal)
| + | BMZ, GIZ, KfW (2012): Water-spreading weirs for the development of degraded dry river valleys. Experience from the Sahel. Published by GIZ and KfW. [http://www.giz.de/Themen/de/dokumente/E-Water-spreading-weirs.pdf http://www.giz.de/Themen/de/dokumente/E-Water-spreading-weirs.pdf] [2013-02-19]. |
| | | | |
| − | Overall objectives:
| + | Food and Agriculture Organization of the United Nations (FAO): AQUASTAT. [http://www.fao.org/nr/aquastat http://www.fao.org/nr/aquastat]. |
| − | *Plan and implement medium-sized irrigation projects in the regions of Chuquisaca, Cochabamba Santa Cruz and Tarija
| + | |
| − | *Raise income of small-scale farmers in the region
| + | |
| | | | |
| − | Project time: 2005-2015
| + | Nilsson, C., C. A. Reidy, et al. (2005): Fragmentation and Flow Regulation of the World's Large River Systems. Science 308 (5720): 405-408. |
| | | | |
| − | Cooperating Partners: KfW/ GIZ
| + | Rost, S., D. Gerten, A. Bondeau, W. Luncht, J. Rohwer, and S. Schaphoff (2008): Agricultural green and blue water consumption and its influence on the global water system Water Resour. Res., 44, W09405, doi: 10.1029/2007WR006331. |
| | | | |
| − | Approach
| + | Steffen, W., A. Sanderson, et al. (2003): Global Change and the Earth System. Springer. |
| | | | |
| − | The participating farmers are closely involved in the planning and construction of the irrigation systems, and then trained in how to use these systems. This takes the form of cooperation in some case, in others of a financial contribution. Altogether, five to six individual projects can support nearly 2,000 families (corresponding to some 8,500 people) on an area of about 3,000 hectares. The approach consists of:
| + | Shiklomanov, I. A., State Hydrological Institute (SHI St. Petersburg) and United Nations Educational, Scientific and Cultural Organisation (UNESCO, Paris) 1999. |
| − | *Assuring the financing, planning and implementation of the individual projects,
| + | |
| − | *Supporting training for participating farmers, e.g. trainings in technical detail planning and quality control for irrigation projects. In this way, the available local know-how gained from practical work is sustainably enhanced,
| + | |
| − | *Advising the Bolivia's Ministry of the Environment and Water on elaborating general guidelines for the irrigation sector, and on planning and implementing water catchment protection measures.
| + | |
| | | | |
| | + | WWAP (UN-World Water Assessment Programme) (2012): The United Nations Water Development Report 4: Managing Water under Uncertainty and Risk. Paris. UNESCO. [http://www.unesco.org/new/fileadmin/MULTIMEDIA/HQ/SC/images/00_WWAP_A4_FLYER_PICA_WEB_290212_01.pdf http://www.unesco.org/new/fileadmin/MULTIMEDIA/HQ/SC/images/00_WWAP_A4_FLYER_PICA_WEB_290212_01.pdf] [2013-02-19]. |
| | | | |
| | + | The World's Water 2008-2009: Global Water Outlook to 2025. Published by the International Food Policy Research Institute (IFPRI) and the International Water Management Institute (IWMI). [http://www.ifpri.org/sites/default/files/pubs/pubs/fpr/fprwater2025.pdf http://www.ifpri.org/sites/default/files/pubs/pubs/fpr/fprwater2025.pdf] [2013-02-19]. |
| | | | |
| − | '''Project example: Jordan'''
| + | [[Category:Excellent]] |
| − | | + | [[Category:Economics]] |
| − | Brackish Water Project (BWP), Jordan Valley
| + | |
| − | | + | |
| − | The objective of the agricultural component of the project was the improvement of management and practices when brackish water is used for irrigation. Guidelines have been compiled to serve farmers and agricultural extension agents as a source of appropriate know-how that can be applied in the field.
| + | |
| − | | + | |
| − | The project was executed in a period of four years (2000 – 2003). The cooperating Partners were the Jordan Valley Authority (JVA), individual farmers and the GIZ.
| + | |
| − | | + | |
| − | The following activities have been carried out:
| + | |
| − | *Monitoring and recording of irrigation practices along the Jordan river,
| + | |
| − | *Interviews and discussions with selected farmers and extension agents,
| + | |
| − | *Measurements by project staff (water and soil quality, yields etc.),
| + | |
| − | *Creation of a data bank,
| + | |
| − | *Identification and evaluation of local experiences and successful practices,
| + | |
| − | *Continuous scientific update and reviewing by researchers,
| + | |
| − | *Elaboration of guidelines,
| + | |
| − | *Promotion and distribution of the guidelines.
| + | |
| − | | + | |
| − | '''References'''
| + | |
| − | | + | |
| − | BMZ, Federal Ministry for Economic Cooperation and Development<br/>Water-spreading weirs for the development of degraded dry river valleys<br/>Experience from the Sahel, published by GIZ and KfW
| + | |
| − | | + | |
| − | GIZ (2010): Water Saving Irrigation. Briefing Note. Division of Rural Development
| + | |
| − | | + | |
| − | GTZ (2003): Brackish Water Project – Guidelines for Brackish Water Irrigation in the Jordan Valley
| + | |
| − | | + | |
| − | GIZ/VAG Armaturen GmbH/Institute for Ecopreneurship/ Institute for water and river<br/>basin management: Guidelines for Water Loss Reduction: A Focus on Pressure Management
| + | |
| − | | + | |
| − | Keller, Andrew/ Jack Keller/ David Seckler (1996): Integrated Resource Systems: Theory and Policy Implications, Research Report 3. International Irrigation Management Institute (IIMI), Colombo
| + | |
| − | | + | |
| − | Giordano, Meredith A., Frank Rijsberman, R. Maria Saleth (2006) (eds.): “More Crop per Drop”: Revisting a Research Paradigm. Results and Synthesis of IWMI’s Research: 1996-2005, IWMI, Sri Lanka, Colombo, ISBN: 1843391120
| + | |
| − | | + | |
| − | [http://www.waterlossreduction.com www.waterlossreduction.com] | + | |
Consumed water is defined as water that has been evaporated, transpired, incorporated into products or crops, significantly contaminated or otherwise made unavailable to other water users. Water that has been withdrawn but not consumed and is returned to the system is called return flow (http://www.fao.org/nr/water/aquastat/). As shown in Figure 1, agriculture is the major user of consumptive water. Globally, around 50 percent of the water withdrawn for agriculture is consumed through evapotranspiration. Thus, the concept of water consumption mostly refers to evaporative losses. Non-consumptive water is almost entirely returned to the system.
While the world’s population tripled in the 20th century, the use of renewable water resources has grown six-fold. Within the next fifty years, the world population will increase by another 40 to 50 %. This population growth – coupled with industrialization and urbanization – will result in an increasing demand for water and will have serious consequences on the environment (http://blogs.triplealearning.com/20117). Worldwide, agriculture accounts for 70% of all water consumption, compared to 20% for industry and 10% for domestic use. In industrialized nations, however, industries consume more than half of the water available for human use, while more than 90% is used for irrigation in developing countries, in particular in semi-arid or arid countries (http://www.worldometers.info/water/).
Annual global agricultural water consumption includes crop water, consumption for food, fiber and feed production (transpiration), plus evaporation losses from the soil and from open water associated with agriculture, such as rice fields, irrigation canals and reservoirs.
About 20% of the total 7,130 km3 of agriculture’s annual water consumption is ‘blue water’ – that is, water from rivers, streams, lakes and groundwater for irrigation purposes which accounts for around 40% of the world’s production on around 20% of the cultivated land. Globally, irrigated crop yields are about 2.7 times those of rain-fed farming. Hence, due to increasing food demand, the importance of irrigation will most probably also increase in future. There is still potential for expansion of irrigation in places where sufficient water is available, particularly in sub-Saharan Africa and South America.
Pathways to improve productivity and bridge the yield gap in irrigation include increasing the quantity, reliability and timing of water services; increasing the beneficial use of water withdrawn for irrigation; and increasing agronomic or economic productivity so that more output is obtained per unit of water consumed. Although there is still potential to increase the cropped area, some 5–7 million ha (0.6%) of farmland are lost annually because of accelerating land degradation and urbanization, which takes agricultural land out of production and reduces the number of farms as more people move to the cities. Increasing population means that the amount of cultivated land per person is also declining sharply: from 0.4 ha in 1961 to 0.2 ha in 2005 (WWAP, 2012).[1]
Predicting future water demand for agriculture is fraught with uncertainty; alongside population growth, water demand depends on what and how much people eat, on uncertainties in seasonal climatic variations, efficiency of agriculture production processes, grown crop types and yields, among other factors. Although projections vary considerably, future global agricultural water consumption (including both rain-fed and irrigated agriculture) is estimated to increase by about 19% to 8,515 km3 per year in 2050 (Comprehensive Assessment of Water Management in Agriculture, 2007). The Food and Agriculture Organization of the United Nations (FAO) estimates an 11% increase in irrigation water consumption from 2008 to 2050. This is expected to increase by about 5% the present water withdrawal for irrigation of 2,740 km3.
Although this seems a modest increase, much of it will occur in regions already suffering from water scarcity (FAO, 2011a) as Figure 3 illustrates.
Climate change leads to new uncertainties concerning future water demand through different water-using sectors. For example, global warming suggests increased energy demands for air conditioning, while higher evapotranspiration rates could increase future demands for agriculture. The precise impacts of climate change on water in specific locations remain uncertain, especially at the local or river basin level.
Under different scenarios of the Intergovernmental Panel on Climate Change (IPCC), regions may become ‘drier’ or ‘wetter’, as there are a variety of possible ways in which climate change may impact the hydrological cycle in different areas and at different times. The uncertainties generated by climate change add a global dimension to the challenges of water resources management, as efforts to effectively manage water locally may be impeded by climate-induced hydrological impacts or increasing demands (WWAP, 2012, p.24).[1]
As water is a renewable resource, agricultural consumptive water uses are not entirely lost; they return within the larger and smaller water cycle. Water as a resource is primarily provided by precipitation from the large weather systems. As Millán (2012) points out, the amount of available water depends on the location of the watershed, and can result from different types of precipitation. For instance, isotopic studies of rain in tropical rainforests (Brazil) show that approximately 65% of the water precipitated in any one day during the rainy season comes from water precipitated on the three previous days. This suggests that at the beginning of the wet season there is a massive inflow of water vapor evaporated from the oceans, and afterwards the water is basically recycled (re-circulated) between the soil-forest and the atmosphere, producing a daily afternoon-evening shower.[3]
However, water as “renewable” must be understood in a spatiotemporal way. As water resources can be drawn upon beyond their ‘renewable’ capacity per unit of time, their management can become unsustainable. This is already the case for West Asia and North Africa, where withdrawals as a percentage of internal renewable water resources have exceeded 75%; and southern Asia and the Caucasus and Central Asia which have nearly reached 60%, the threshold signaling water scarcity. An addition significant problem is the change of water quality, which is affected by most types of water uses through chemical, microbiological and thermal pollution (WWAP, 2012, p.8).[1] Hence, not only climate change has an impact on the hydrological cycle and our water resources, but also the way we manage water has significant backlash. In concrete terms, about 40% of the total global runoff to the oceans has been captured for human use, with groundwater being used faster than it is being replenished in most dry areas of the world. Humans have extensively altered river systems through impoundments and diversions to meet their water, energy, and transportation needs: dams are holding back about 15% of the total annual river runoff globally (Steffan et al., 2004, p.113, Nillson et al., 2005).[4]
Nilsson, C., C. A. Reidy, et al. (2005): Fragmentation and Flow Regulation of the World's Large River Systems. Science 308 (5720): 405-408.
Rost, S., D. Gerten, A. Bondeau, W. Luncht, J. Rohwer, and S. Schaphoff (2008): Agricultural green and blue water consumption and its influence on the global water system Water Resour. Res., 44, W09405, doi: 10.1029/2007WR006331.
Steffen, W., A. Sanderson, et al. (2003): Global Change and the Earth System. Springer.
Shiklomanov, I. A., State Hydrological Institute (SHI St. Petersburg) and United Nations Educational, Scientific and Cultural Organisation (UNESCO, Paris) 1999.
