What is Optimisation /Efficiency of Water Use in Agriculture?
Improving the water use efficiency in agriculture means to effectively increase the crop yield whilst minimising water use. Water saving agriculture implies the combination of agronomic, physiological, biotechnological/genetic and engineering approaches.
Agricultural water consumption can be optimised by improving existing irrigation systems, enhancing the water use efficiency of crops, and by properly maintaining the existing systems to avoid malfunctioning. Regarding crop irrigation, optimal water efficiency means to minimise water losses due to evaporation, evapotranspiration, runoff or subsurface drainage.Improving the water use efficiency of crops means selecting crops that are adapted to the respective climate, e.g. crops that are drought tolerant or adapted to dry climates.
The optimisation of water use in agriculture can sometimes be achieved by simple means and is also of economic importance (water savings). However, farmers need to be motivated by the right incentives and policies and may require technical assistance.
This factsheet will provide an overview of the technical options in order to use water more efficiently in agriculture. In the factsheets policies and legal framework requirements and building an institutional framework you will find related information to capacity building tools, incentives and legislative frameworks.
Surface irrigation stands for a large group of irrigation methods in which water is distributed by gravity over the surface of the field. Water is typically introduced at the highest point or along the edge of a field, which allows covering the field by overland flow. Historically, surface irrigation has been the most common method of irrigating agricultural land. The defining feature of surface irrigation methods is that the soil is used as the transport medium (as opposed to pipelines or through the air, as with sprinklers). The soil is also controlling the depth infiltrated over time.Surface irrigation methods contain two basic categories: ponding (surface water pooled in a puddle) and moving water. The moving water methods require some runoff or ponding to guarantee adequate infiltration at the lower end of the field. In order to avoid water loss due to evaporation, it is important not to irrigate the crops during the day but in the early morning or at night. The better the quality of the soil is, the less is the unnecessary runoff and the better the infiltration into the soil and therefore the use for the crops.
Smallscale bucket drip irrigation system.
These irrigation technique systems have low requirements for infrastructure and technical equipment but need high labour inputs. Irrigation by using water cans is to be found for example in periurban agriculture around large cities in some African countries. Smallscale drip irrigation system with buckets is one of the very waterefficient manual irrigation methods. It consists of two drip lines, each 15 to 30 m long, and a 20litre bucket for the water. Each of the drip lines is connected to a filter to remove any particles that may clog the drip nozzles. The bucket is supported on a bucket stand, with the bottom of the bucket at least 1 m above the planting surface. For example, a bucket system requiring 2 to 4 buckets of water per day can irrigate 100 to 200 plants with a spacing of 30 cm between the rows. For crops such as onions or carrots, the number of plants can be as many as the bed can accommodate.
Automatic, Nonelectric Irrigation (Ropes, Buckets)
In addition to the common manual watering by bucket, an automated, natural version of this technique also exists. Using plain polyester ropes combined with a prepared ground mixture can be used to water plants from a vessel filled with water. The ground mixture would need to be made depending on the plant itself, yet would mostly consist of black potting soil, vermiculite and perlite. This irrigation system is often used for tree establishment in dry climates.
Sprinkler Irrigation is a method of using irrigation water is similar to rainfall. Water is distributed through a system of pipes usually by pumping. The water is sprayed into the air and irrigates the entire soil surface through spray heads. Sprinklers provide efficient coverage for small to large areas and are suitable for all types of properties. Furthermore, it is adaptable to nearly all irrigable soils since sprinklers are available in a wide range of discharge capacity. Sprinkler irrigation is appropriate to any farmable slope, whether uniform or undulating. The lateral pipes supplying water to the sprinklers should always be laid out along the land contour whenever possible. This will minimise the pressure changes at the sprinklers and provide a uniform irrigation. Sprinklers are best suited to sandy soils with high infiltration rates although they are adaptable to most soils. The average application rate from the sprinklers (in mm/hour) is always chosen to be less than the basic infiltration rate of the soil so that surface ponding and runoff can be avoided. Evaporation is very high with this kind of irrigation technique but can be minimised when practiced during the night or early morning like in surface irrigation.
Drip irrigation is a technique in which water flows through a filter into special drip pipes, with emitters located at different spacing. Water is distributed through the emitters directly into the soil near the plants through a special slowrelease device. If the drip irrigation system is properly designed, installed, and managed, drip irrigation may help achieve water conservation by reducing evaporation and deep drainage. Compared to other types of irrigation systems such as flood or overhead sprinklers, water can be more precisely applied to the plant roots. In addition, drip can eliminate many diseases that are spread through water contact with the foliage. Finally, in areas where water supplies are severely restricted, there may be no actual water savings, but rather simply an increase in production while using the same amount of water as before. In very arid regions or on sandy soils, the trick is to apply the irrigation water as slowly as possible. Irrigation scheduling can be managed precisely to meet crop demands, holding the promise of increased yield and quality.
Drip irrigation is adaptable to any farmable slope and is suitable for most soils. On clay soils water must be applied slowly to avoid surface water ponding and runoff. On sandy soils higher emitter discharge rates will be needed to ensure adequate lateral wetting of the soil.
Subsurface drip irrigation.
Subsurface drip irrigation is a variation of the conventional surface drip irrigation technique. It is using water more efficiently than traditional irrigation techniques like surface irrigation by minimising evaporation. The laterals (also used in conventional drip irrigation) are buried in a depth below the soil surface depending mostly on the tillage practices and the crop to be irrigated. Subsurface drip irrigation can be understood as the oldest modern irrigation method. Subsurface irrigation applies water directly to the
plant‘s root zone at a rate closelyth.The soilmatchingtypeandcrops th planted determine the instalment depth of subsurface drip irrigation systems. Subsurface drip irrigation has
shown great potential for increasing crop yield and uniformity, while decreasing the use of water and the environmental impact.
Spate irrigation is a water management system that is unique to semiarid regions. It is found in the Middle East, North Africa, West Asia, East Africa and parts of Latin America. Floodwater from mountain catchments is distributed to riverbeds (socalled wadis, the Arabic term for valley, referring to a dry riverbed that contains water only during times of heavy rain) and spread over large areas. Spate irrigation systems contain a big risk and uncertainty. The uncertainty comes both from the unpredictable nature of the floods and the frequent changes of the riverbeds from which the water is diverted. It is often the poorest segments of the rural population whose livelihood and food security depends on spate irrigation systems. However, over time, considerable local wisdom has developed in organising spate systems and managing both the flood water and the heavy sediment loads that go along with it by constructing spurs and bunds. These spurs and bunds are generally made in such a way that the main diversion structures in the river break when floods are too big. Breaking of diversion structures also serves to maintain the floodwater entitlements of downstream landowners and therefore helps to reduce the upstreamdownstream water conflicts. Some of the larger spate irrigation rank among the largest farmermanaged irrigation systems in the world. The structures are sometimes spectacular: earthen bunds, spanning the width of a river, or extensive spurs made of brushwood and stones. Spate systems are made in such a way that ideally the largest floods are kept away from the command area. Very large floods would create considerable damage as they would destroy flood diversion channels and cause rivers to shift.
Agricultural Reuse of Rainwater, Storm water and Reclaimed Water
Rainwater and storm water harvesting can be defined as an irrigation method for inducing, collecting, storing and conserving local surface runoff for agriculture in arid and semiarid regions. Rainfall has four dimensions regarding irrigation. Rainfall induces surface flow on the runoff area. At the lower end of the slope, runoff is collected in the basin area, where a major portion infiltrates and is stored in the root zone. When infiltration has ceased, the stored soil water is conserved In rain water harvesting for agriculture, three different groups of techniques can be distinguished:
Flood water harvesting from far away, large catchments (e.g. spate irrigation);
Rain water harvesting from macrocatchment systems utilising the runoff from a nearby slope for agricultural purposes
Rainwater harvesting from microcatchment where the water from an adjacent, small catchment is used for cropping (e.g. roof rainwater harvesting which can also be used for drinking water. For further information, see also roof top rainwater harvesting in urban or rural areas.
It is evident that all three groups of rainwater harvesting for agriculture techniques need different geographic settings for an appropriate implementation. In addition to topography, the runoff properties of the surface and the infiltration rates are important natural parameters for the implementation of any water harvesting system. Furthermore, the soil types of the runon areas and the depth of the soil layer in the cropping areas are important factors that influence the outcome. Additionally, socioeconomic factors have to be taken into due consideration. With a growing scarcity of freshwater resources in arid and semiarid regions and the everincreasing demand for more efficient food production for larger populations, the importance of wastewater for irrigation increases and is more widely acknowledged. Wastewater has long been used as a resource in agriculture. The use of contaminated water in agriculture, which may be intentional or accidental, can be managed through the implementation of various barriers, which reduce the risk to both crop viability and human health. Today, an estimated 20 million hectares (7%) of land is irrigated using wastewater worldwide, particularly in arid and semiarid regions and urban areas where unpolluted water is a scarce resource and the water and nutrient values of wastewater represent important, droughtresistant resources for farmers Wastewater is often the only source of water for irrigation in these areas. Even in regions where other water sources exist, small farmers often prefer wastewater due to its high nutrient content, which reduces or even eliminates the need for expensive chemical fertilisers. Wastewater reuse is likely to become more widely practised, and it is already becoming incorporated into some national water resources management plans. Reuse can take place at a local level (e.g. fertigation, greywater towers or vertical gardens) or at a centralised level (e.g. aquaculture). The wastewater used in irrigation can be taken from different sources. It can be completely untreated municipal, pretreated municipal or industrial wastewater, or particularly or fully purified wastewater treated biologically). In 2006, the World Health Organisation has edited a large curriculum of guidelines for the save use of excreta and wastewater (WHO 2006) in agriculture (Vol. II), in aquaculture (Vol. III) as well as the save use of excreta and greywater (Vol. IV). Volume I of these guidelines gives an overview on policy and regulatory aspects (see Further Readings). In any case, the reuse of wastewater is not only beneficial for crop production but generally also implies an improvement of the water quality (e.g. nutrients are transferred to the plants, bacteria killed by the sun or predators, etc.). However, the institutionalisation of reuse of wastewaters is important in order to avoid health risk and negative environmental impacts.
Aquaculture is another alternative to improve the water use efficiency in agriculture. Aquaculture is the farming of freshwater and saltwater organisms such as fish, crustaceans and aquatic plants. Aquaculture can be combined with the reuse of wastewater (municipal, industrial or agricultural wastewater from feedstock). Nutrients contained in the wastewater are removed by feeding animals or plants, which can be harvested. Pathogens can also be removed by natural dieoff, solar disinfection (in shallow ponds) or predation (even though the effluent is not pathogenically safe). Interactions between crops and livestock are considered crucial to the sustainable development of agriculture. The combination of aquaculture and wastewater reuse allows optimising the water use for farming of aquatic animals and plants for food production all by increasing the quality of the wastewater effluent. Typical Aquaculture systems can be used for plants (aquaculture plants) or animals such as fish or crustaceans (aquaculture animals).
By choosing the appropriate crop for production can reduce the water used for irrigation to a great extent. The better the crop is adapted to the existing climate, topography and soil condition, the less water is used for the irrigation. Growing a different crop each year (crop rotation) prevents organic matter loss, improves soil structure and reduces the incidence of weeds and pests. Generally, the longer the rotation, the better. Crop rotations can also lead to greater efficiency in soil water utilisation. For example, deeprooted crops following shallow crops can take advantage of the extra reserve of deep moisture, which was unavailable to the shallow rooted crop. Crop rotation also improves the soil structure and thus its water retention capacity. Cover crop is important to protect the surface of the soil from evaporation, erosion and drying out. A cover crop should be established as soon as possible after harvesting short season vegetables. Annual or cereal rye is good cover crops for longer season vegetables because they grow well in cooler weather (such as in autumn and early spring), and are also good at taking up excess fertiliser.
The two major causes for loosing water from cropping systems include evaporation and transpiration. Evaporation losses occur directly from the soil, while transpiration losses are through plants. A plant can be pictured as a pump, drawing water from the soil and moving it to the leaves where it is lost to the atmosphere through tiny openings. The water losses of soils to the atmosphere by either evaporation or plant transpiration are usually described as evapotranspiration. Evapotranspiration values are highest when the soil is near field capacity and the air is warm, dry and moving. The potential evapotranspiration (PET) is the maximum amount of water that could evaporate and/or transpire when moisture is not limiting. When the PET is high, plants must draw heavily on soil water and transpiration can occur faster than the plants can draw water from the soil, which may eventually cause wilting.
Some typical insitu moisture conservation techniques are micro catchments, broad beds and furrows or contour bunds. You can find more information in TNAU (2008).
The organic matter content of the soil has a considerable influence on many of the physical, biological and chemical properties of soil and thus also its structure and water retention and holding capacity, nutrient content, biological activity and aeration. Intensive crop production often returns little organic matter to the soil. However, there are several approaches however, to maintaining or improving organic matter content. These include spreading compost (e.g. garden compost, humanure, terra preta etc.) or animal manure, reducing tillage, green manuring and practicing good crop rotations. See also the factsheets use of urine agriculture at large or small scale and fertigation.