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COMBATING DESERTIFICATION: WATER CONSERVATION AND WATER SAVING ISSUES IN AGRICULTURE LUIS SANTOS PEREIRA *
Jel classification : Q 250, Q 150 1. Desertification Abstract supporting the living of This paper refers to desertification in relation to other xeric regimes: aridity, an enormous part of the and Water Scar drought and man-made water shortage. Their definitions are introduced and world rural population city the respective water management issues discussed. The water conservation and provides for a large and water saving concepts are defined and the respective measures and prac fraction of the world's At the edge of the XXI tices are introduced focusing on man-made desertification and water shortage. century, the sustainable Issues for water conservation and saving in agriculture are then analysed fo food. Recognisable ef use of water is not only a cusing: crop management practices, soil management for water conservation, forts from innovation and priority question for water demand management in irrigated agriculture, and supply management for irri practice, or from funding scarce regions and for a gation. agencies and managers, are continuously provid griculture in particular, but Résumé for aB users' sectors and ing issues to improve irri Cet article aborde le thème de la désertification par rapport aux autres gation water manage regions. Imbalances be régimes xériques : l'aridité, la sécheresse et la pénurie d'eau produite par tween availability and de l 'homme. La définition de ces divers régimes est passée en revue afin d' ment, contro l negative mand, degradation of sur analyser les aspects relatifS à la gestion de l 'eau dans de telles conditions. Les impacts of irrigation, con face and groundwater concepts de conservation et d'économie d'eau sont discutés, en illustrant des serve and save water, use quality, inter-sectorial mesures et des pratiques pour la gestion des cultures, la gestion du sol orien low quality saline and tée à la conservation de l'eau, la gestion de la demande en agriculture ir wastewater, increase competition, inter-regional riguée et la gestion des approvisionnements en eau pour l'irrigation. and intemational conflicts, yields and farmers in- aB bring the water issues to the foreground. comes. Similarly, great Decades ago, water was viewed as a non-limited natural progress in engineering and management are producing resource because it was renewed every year in the course of new issues for water use and water quality contro l for non the seasons. Man progressively appropriated this resource agricultural uses, including domestic and industriaI uses. and used it with too few restrictions. Developments in con The sustainable use of water implies resource conservation, troBing and diverting surface waters, exploring groundwa environmental friendliness, technological appropriateness, ter, and in using water for a variety of purposes have been economie viability, and social acceptability of development made without enough care about conserving the natural re issues. The adoption of these sustainability facets is a pri source, avoiding wastes and misuses, and preserving its ority for using water in every human, economie and social quality. Thus, nowadays, water is becoming scarce not on activity particularly in water scarce regions. ly in arid and drought prone areas, but also in regions where Water scarcity may result from a range of phenomena. rainfaB is relatively abundant. Water scarcity is now viewed These may be produced by natural causes, may be induced under the perspective of the quantities available for eco by human activities, or may result from interaction ofboth, nomie, soci al and nature uses, and al so embraces the quali as indicated in Table 1 (Pere ira et al., 2002a). ty of water because degraded water resources become un available or at least lesser available for human and other Table 1. Xeric regimes more stringent uses. Xeric Nature- Man- Worldwide, agri culture is the sector with the highest de regimes produced induced mand for water, largely overtaking the demand from other Permanent Aridity Desertifica tion human and economie activities in water scarce regions. Temporary Drought Water shortage Thus, irrigated agri culture is often considered the main cause for water scarcity, and irrigation is accused of misuse Aridity is a nature-produced permanent imbalance in the of water, producing excessive water wastes, or degrading water availability consisting in lowaverage annual precipi the water and land quality. However, irrigated agriculture is tation, with high spatial and temporal variability, resulting in overaBlow moisture and low carrying capacity of the e * A,gricultural Engineering Research Center, Technical University of LlSbon cosystems.
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Drought is a nature-produced but temporary imbalance of ter scarcity is common in semi-arid and sub-humid regions water availability, consisting of a persistent lower-than-av where population and economi c forces may make large de erage precipitation, of uncertain frequency, duration and mands on the local water resource, and where insufficient severity, of unpredictable or less-predictable occurrence, care is taken to protect the quality of the precious resource. resulting in diminished water resources availability, and re Therefore, water shortage may be considered as a first or duced carrying capacity ofthe ecosystems. Many other def inductive stage of desertification. initions of drought exist; e.g. the U. S. Weather Bureau de Water quality is an expanding problem. Surface waters fined drought (Dracup et al., 1980) as a lack of rainfall so become more polluted - low dissolved oxygen and in great as so long continued to affect injuriously the plant and creased fecal coliforms - in developing countries, this trend animallife of a pIace and to deplete water supplies both for being reversed in high income countries (The World Bank, domestic purposes and the operation of power plants espe 1992). Health problems are there particularly acute with cially in these regions where rainfall is normally sufficient 900 millions of people affected by diarrhoea each year and for such purposes. many other water vectored diseases are spread in the devel These definitions c1early state that drought is mainly due oping countries. This brings water shortage re1ated to water to the break down of the rainfall regime, which causes a se quality to the foreground. ries of consequences inc1uding agricultural and hydrologi Desertification should be al so seen under a broader per cal hazards associated with the severity and duration of spective where the space scale for land is wide and em rainfalliack. braces both agricultural and non-agriculturalland as well as Desertification is a man-induced permanent imbalance in the respective water resources. Then impacts relate not on the availability of water, which is combined with damaged ly to the natural resources and the land productivity but al soil, inappropriate land use, mining of groundwater, in so with the social and economic behaviour of populations creased flash flooding, loss of riparian ecosystems and a de living in the affected areas (Pereira et al., 2004). Assuming terioration of the carrying capacity of the ecosystems. Soil such a wide scale, it may be understood that when aridity erosion and salinity are commonly associated with deserti conditions prevail in a given area, desertification is fication. Desertification occurs in arid, semi-arid and sub favoured by droughts and by man-made water shortages s humid climates. Drought strongly aggravates the process of ince these xeric regimes increase the human pressure on the desertification when increasing the pressure on the dimin natural resources and, therefore, create conditions for the ished surface and groundwater resources. degradation ofland and the living conditions ofrelated pop Different definitions are used for desertification, many of ulations. them focusing on land degradation, so giving to water a rel ative1y less important role. This is the case for the definition 2. Wate~ Management Issues used by the UNCCD, where desertification refers to the Policies and practices of water management under water degradation of the land in arid, semi-arid and sub-humid scarcity must focus on specific objectives according to the zones resulting from various factors inc1uding c1imatic causes of water scarcity. An integrated technical and scien variation and human activities. However, these definitions tific approach is essential to develop and implement the need to be broadened in scope to focus attention on the wa management practices appropriate to deal with water s ter scarcity issues. When dealing with water scarcity situa carcity. Policies and practices must base on the assumption tions, it seems more appropriate to define desertification in that water will not become abundant, so that water man relation to the water imbalance produced by the misuse of agement policies and practices have to be appropriate to water and soil resources, so calling attention to the fact that cope with water scarcity. the misuse of water definitely is a cause for desertification. To cope with water scarcity means to live in agreement Water shortage is also a man-induced but temporary wa with the environmental conditions specific and dictated by ter imbalance which may result from generaI excessive ex limited available water resources. For millennia, civilisa ploitation of available resources. For example withdrawals tions developed in water scarce environments and the cul may ex ce ed groundwater recharge, surface reservoirs may turaI skills that made it sustainable to live under such con be of inadequate capacity and land use may have changed, ditions are an essential heritage of those nations and peo revising the local ecosystem and altering the infiltration and ples. Progress in the XX century questioned the traditional runoff characteristics. Degraded water quality is often asso indigenous know-how, which has often been replaced by ciated with water shortages and exacerbates the effects of modem technologies and management rules directly im water scarcity. There is no widely accepted definition for ported from other areas having different physical and social this water scarce regime and the term "water shortage" is environments. Water consumption and demand increased often used synonymously with water scarcity. However it is everywhere for domestic and urban uses, for agri culture important to recognise that water scarcity can result from and irrigation, for the industry and energy production, for human activity, either by over-use of the natural supply or recreation and leisure. However, such increase became par by degradation of the water quality. This man-induced wa- ticularly evident in the regions where water is scarce or, at
5 NEW MEDIT N. 1/2005
least, not abundant. Therefore, the man- made xeric support measures are also required complementing reactive regimes are now adding to the natural water scarce condi measures. Among preparedness and reactive measures, wa tions, generally aggravating the existing situation. ter pricing and penalty policies may contribute to the effec Solving water management problems faced in water s tiveness of drought management measures. Last but not carce regions call for innovative issues to cope with water least, the awareness of populations about drought water s scarcity. Innovation includes the adaptation ofthe tradition carcity must be developed to create a worldwide behaviour al know-how to the nowadays challenges, the adaptation of oriented to reduce water wastes, induce water savings and the extemal available technologies to the prevailing physi favour water conservation practises. cal and soci al conditions, and the creation of new and well Because desertification and water shortage are man-in adapted technologies and management issues. Innovation duced and are associated to problems such as soil erosion, must assume that man has to cope with the environrnental land degradation, mainly through salinisation, over-ex constraints and that engineering and managerial solutions ploitation of soil and water resources, and water quality must be specific to the existing causes for water scarcity, in degradation, require that policies and measures be oriented cluding the man-induced desertification and water shortage. to solve the existing problems. Therefore, combating deser Drought is often considered together with desertification tification and water shortage includes: since droughts favour mining of groundwater to face the • re-establishing the environrnental balance in the use of lack ofrainfall and surface waters, and the use oflow qual natural resources, ity and non-conventional water resources is then maximal, • enforcement of integrated land and water planning and which may contribute to land degradation. management policies, Water management under drought requires measures and • restoring the soil quality, including the adoption of so il policies that are common to other water scarcity regimes and water conservation measures, such as those oriented to avoid water wastes, reduce de • defining new policies for water allocations favouring re mand and make water use more efficient. But the peculiar duced demand and water conservation practises, characteristics of droughts imply specific measures. Inap • controlling ground-water abstractions and developing propriate water management to cope with droughts is a way measures for recharging the aquifers, to induce desertification. A better understanding of • minimising water wastes, droughts is essential to develop tools for predictionlfore • combating soil and water salinisation, casting of drought initiation and ending, at least to clearly • controlling water withdrawals, recognise when a drought has begun or has come to an end. • adopting policies and practices for water quality manage This is essential to timely and appropriately enforcing ment, and drought mitigation measures. An adequate lead-time - the • installing policies of economic incentives to users, includ period between the release of the prediction and the onset ing relative to ' water prices, and penalties for misuse and of the predicted hazard - is more important than the accura abuse of natural resources. cy of the prediction because it makes it possible that deci Coping with water scarcity requires that measures and sion and policy makers implement in time the policies and policies of water management be in line with the challeng measures required to mitigate the effects of drought. In case ing and widely accepted concept of sustainable develop of agriculture the lead-time is essential to make it possible ment. It calls for new approaches on development and, that farmers take the decisions required, including relative therefore, on water and so il resources management. This is to cropping pattems and cultivation systems. Early waming not only a question of new technologies and management does not provi de a lead-time as long as prediction but it def issues for allocating and controlling the water and land us initely helps the timeliness of decisions. es. There is the need for considering the driving forces com Because droughts are nearly unpredictable, preparedness manding the pressures on the resource itself, the behaviour measures are paramount to cope with droughts. As they of the users, and the human and social objectives. Despite have pervasive long-term effects and their severity may be the enormous progress of technological and managerial very high, appropriate reactive measures are required. The tools that are becoming available to improve management, break in the natural water supply implies changes in water we still have many gaps in knowledge and, mainly, in trans allocation and delivery policies, as well as in the manage ferring scientific and technological knowledge into prac ment ofwater supply and irrigation systems. Water supplies tice. Thus, adopting the concept of sustainability, water may be reinforced using non-conventional waters, includ management to cope with water scarcity has to: ing wastewater and low quality waters, thus requiring the • be based on the knowledge of processes which can lead to enforcement of appropriate policies to control the respec resource degradation and to the conservation of natural tive impacts. Diminished water supplies during droughts re resources; quire that farmers and other users be able to adopt reduced • include resource allocation to non productive uses such as demand and efficient water use practices. Incomes of farm natural ecosystems; ers and other users may be drastically reduced, so financial • consider technological development not only for respond-
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ing to production objectives but to control resource degra mental conditions that prevail in a given area or region. dation and environmental impacts; However they have many facets that are common to alI x • value the non-productive uses of the land and water; eric regimes while the corresponding measures and prac • prioritise the processes which help degradation processes tices should receive different importance and priority ac to be reversed; cording to the water scarcity regime being considered . • include institutional solutions which support the enforce Those measures and practices are analysed by Pereira et al. ment of policies and rules and the social acceptability of (2002a) with reference to their relative importance for each decisions and measures for land and water management; xenc regIme. and • rely on clear objectives focusing not only on the natural 3.2. Water conservation and water saving to resources by themselves and the economics of the returns cope with desertification and water from their use, but embracing also the human needs and shortage aspirations. Desertification and water shortage are man induced and Sustainable development facets base the issues referred refer to problems such as land degradation by soil erosion above. Resource conservation and environmental friendli and salinisation, over exploitation of soil and water re ness are main issues in relation to water and soil. Appropri sources, and water quality degradation. Therefore, they re ate technologies constitute an essential challenge to achieve quire that water conservation and water saving policies and policies and practices welI adapted for problem solving. measures combine with others oriented to solve the existing The economic viability of measures to be implemented is a problems (Pereira et al. , 2002a). Water conservation to condition for their adoption and a reason for financial in combat desertification and water shortage should include: centives. The need for appropriate institutional develop (a) land and water uses planning and management for re-es- ments is considered as a pre-condition for implementation tablishing the environmental balance of the natural and ofinnovative issues, oftechnological, managerial, econom man-made ecosystems, as welI as in the use of natural ic or social nature. FinalIy, the social acceptability of meas resources, ures to cope with water scarcity is particularly related with (b) appropriate reservoir and groundwater management the development of the social awareness on water scarcity rules, including artificial recharge and water quality p problems and the inherent issues. reservation measures, One promising issue is to consider water not only as a (c) water allocation policies that favour social and efficient natural renewable resource but also as a social, environ uses ofwater, including criteria relative to water quality, mental and economi c good. Valuing water as an economic, (d) measures and practices for water quality management marketable good may be insufficient since water acts not and focusing on the control of related impacts on the en only for producing but is also supporting other natural re vironment, -including monitoring, sources and plays a major role in the human and sociallife, (e) soil and water conservation practices in rainfed and irri particularly when scarcity gives it a peculiar value. A cou gated agriculture, namely for erosion control, pled environmental, economic, and social approach is (f) soil and crop management practices for restoring the soil therefore required in valuing water in water scarce environ quality, including so il moisture conservation, ments. (g) combating so il and water salinisation, (h) strengthening the institutional framework for the imple 3. Water Conservation and Water Saving mentation and enforcement of required policies and measures, and 3. 1. Concepts (i) developing the public awareness for combating deserti The terrns water conservation and water savings are gen fication and water shortage. eralIy associated with the management of water resources Water saving policies and practices play al so a funda under scarcity. Water conservation is used to every policy, menta l role in combating water scarcity due to deserti fica managerial measure or user practice that aims at conserving tion and water shortage. As for droughts, several measures or preserving the water resources, as welI as combating the and practices are generalIy common with those for coping degradation ofthe water resource, including its quality. Wa with aridity (Pereira et al. , 2002a). Others should be pur ter saving is adopted to every policy, managerial measure or posefulIy focusing the combat against desertification and user practice that aims at limiting or controlling the water water shortage: demand and use for any specific finality, including the (a) changes in crops and crop patterns for reducing the irri avoidance of water wastes and the misuse of water. In the gation demand and taking advantage of soil moisture practice both perspectives are complementary and inter-re conservation, lated. Although it is not easy to distinguish among them, (b) cropping and irrigation practices oriented to control these terrns are not synonymous. non-point source pollution by agro-chemicals, fertilisers Water conservation and water saving measures and prac and erosion sediments, tices should be specific of the xeric regime and environ- (c) adoption of irrigation and drainage practices favouring
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salinity management and reduced water demand, soil management. Yield responses to supplemental irriga (d) water recycling in industry and energy generation, and tion are also considered when water is available during the control of effiuents quality to avoid pollution of surface water stress criticaI periods. and ground-water, Water conservation in rainfed agri culture mainly refers to (e) enforcing specific water price policies in re1ation to wa crop and soil management practices. Water conservation ter use performances and the quality of effiuents to practices and techniques in crop management generalIy re favour adoption of practices that contribute to restoring late to three main approaches: the environrnental balances, • Crop management to minimi se the risks of crop failure (f) enforcing a policy of incentives and penalties in relation and to increase the chances for successful crop yielding to users' adoption of tools and practices that contribute using the available rainfall. They refer to the selection of to restoring the water and soil quality and resource con crops, crop varieties, planting dates, harvesting, and the servation, use of supplemental irrigation for yield saving. (g) developing campaigns for end-users to adopt water sav • Techniques and practices for controlling the water stress ing and environrnentalIy friendly tools and practices. effects. They include soil management practices, the se lection and management of the crop rotations and inter 4. Water Conservation and Saving in cropping, the adoption of small planting densities, and Agriculture appropriate fertilising strategies . • Practices referring to increase soil water availability and 4. 1. Crop patterns and cropping pradices control of soil water losses by evaporation and transpira To cope with water scarcity, crops and crop varieties may tion by weeds, as welI as cultivation techniques to reduce be se1ected taking into consideration their tolerance to the transpiration and evaporation losses water stress conditions characterising the environments 4.2. Soil management for water conservation where they are cultivated. In generaI, these crops corre spond to centuries of domestication of plants native to these Soil management practices are since long known to have environments. However, new crops and new varieties have positive impacts in water conservation. However, results been introduced for the market not considering the prevail depend upon the soil physical and chemical characteristics, ing environmental conditions and contributed to degrade the land forms and geomorphology, the climate and the im the so il. But fortunately also other crop varieties are being plements used. AlI factors interact, thus creating variable introduced in water scarcity areas during the last decades responses. folIowing scientific plant breeding and improvement pro Soil management practices for water conservation are grammes. summarised in Tables 2 and 3. They are often common with Mechanisms of crop resistance to water stress include those for soil con~ervation, i.e. not only provi de for aug drought escape, avoidance and tolerance according to the menting the soil moisture availability for plant growth but nature of physiological and morphological processes and also contribute to control erosion and soil chemical degra characteristics involved. Despite progress in plant breeding dation. In many cases they contribute to improve the soil research, including genetic manipulation, results are stilI quality. Because these practices produce changes in soil in somewhat beyond expectations: filtration and soil water storage, they may produce re1ative • it is too large the number of genes and characters with po ly important changes in the hydrologic balance at the local, tential for altering the plant responses to water stress, thus field scale. making genetic manipulation difficult and slowly respon The soil management practices for water conservation Sive, can be grouped as folIows: • benefits of plant responses to water stress are commonly • Improving water retention at the soil surface to control contradictory to yield responses, which create additional surface runoff and increase the time opportunity for infil difficulties for plant breeding and improvement pro tration. Since the time opportunity for infiltration is larg grams, moreover for the adoption by farmers of water er, the volume ofwater retained in the soil is increased; so stress resistance but low yield varieties, more water becomes available for crop roots extraction. • crop responses to water stress are influenced by other en Because runoff is decreased the erosion by the surface vironrnental and cultivation conditions, so making adap water is al so controlled. Practices included in this group tive field tests more demanding and difficult. are tilIage methods for increasing the so il surface rough Due to limitations of plant breeding programmes, re ness and those relative to the creation of contour furrows, search is also oriented to evaluate the photosynthetic, bio as welI as surface mulching for roughness augmentation. mass and yield water use efficiency of crops (Steduto, • Increasing the soil water storage capacity by modifying 1996) commonly used in arid zones, mainly in connection the soil physical properties that determine the soil water with improved crop management techniques and practices. reservoir and soil water redistribution throughout the soil Among the most important practices are those relative to profile. Techniques include the control of soil com the se1ection of planting dates, responses to fertilisers, and paction, the destruction of soil impermeable layers, the
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Table 2. Soi! management techniques for augmenting sail infiltration and storage aggregates and aggregates stability. These techniques are common with Soil management techniques Benefits Effective ness soil conservation since they fight a Water retention on the soil gainst the destruction of soil aggre surface and runoff control gates, which plays a main role in soil • Soil surfaee ti Il age eultivation Storage of rainfall excess in mi ero High (for low slopes) erOSlOn. far inereased surfaee · depress ions, larger time • Controlling evaporation from the roughness opportu nity for i nfiltration soil by using mulches to limit the High Contour and graded furrows Runoff and erosion control, and amount of solar radiation available · · i ncreased ti me fo r i nf i Itratio n Residues and e rop mu leh ing at the soil surface, and using tillage • Increased surface roughness and High • · infiltration that limit the water fluxes to the e Furrow dikes • Rain water storage in furrow High vaporating surface. · basins and i ncreased infiltration, • Runoff contro l through water har Bed surfaee profile • Variable • Runoff contro I and i ncreased vesting. Differently from the above, infiltration runoff is not decreased but can be Increasing soil infiltration rates augmented in the area where it is Organic ma tter for improving • Improved soi I aggregates and High produced, and is controlled to be in · aggregation increased infiltration filtrated downstream where the crop Conservati o n tiIIage · • Preserve soi I aggregates and High is established. As described in Sec infiltration rates Mulches, crop residues tion of Chapter 6, water harvesting · • Soil surface protection, higher High embraces a variety of techniques infi Itration reduced erosion · Traffic control well proved in arid regions, includ • Decreased soil compaction and Variable ing spate irrigation, water spreading · Chemical s fo r aggregates improved water penetration and runoff farroing. • Preserve soi I aggregates and High • Runoff contro l in sloping areas by infiltration rates conditions Increasing the soil water storage modifying the Iand forro, thus de capacity creasing the velocity of the surface Loosening tillage runoff and increasing the time op · Increased soi I porosity, and soil Variable Subsoil ing for destroying natural · water transmission and retention portunity for water infiltration, thus · or plough made hardpans Improved water transmission and High to become availabie for crops use. · storage, and deepening the soil These techniques are mostly known depth explo itable by crop roots Deep tillage/profile modificat ion as soil conservation practices and in · in presence of cl ay hori zons Increased water penetration and High · soil depth exploitable by roots clude terraces, contour ridges, and Chemical and physical treatments Increased infiltration and High · of sa lt-affected soi I s strip cropping, which are in the ori · avai lable soi I water Deep soil treatm ents Variable gin of very beautiful landscapes in · Deeper roots and water storage · volume many regions of the world · Adding fine materials and Increase water retention in the soil Economie li mits hydrophilic chemicals to · profile 5. Water Savings and sandy/coarse soils Economie li mits Increase wa ter tra nsm ission and Mixing fine and coarse horizons · retention Conservation in Irrigated · Limited · Asp halt ba rriers in sand y soi Is · Decrease deep perco lation Agriculture: Demand Ma Compacting sandy soils • Slowing water penetration and Variable nagement · controlli ng deep percolation Control of acidity by liming, and More intensive and deep rooting, and High 5.1. Generai aspects salin ity by gypsum application improvement of aggregation Demand management for lIT1ga- use of soil additives that improve water retention, the cor tion includes practices and manage rection of soil salinity to improve soil water availability ment decisions of multiple nature: agronomie, economie, for the plants as well as the soil physical conditions, and and technical. Objectives concern a reduction of irrigation the use of organic additives to favour soil aggregates. By requirements, the adoption of practices leading to water favouring the quantity of water that can be redistributed savings in irrigation, both reducing the demand for water at through the soil profile, and the amount of water that in the farro, and an increase in yields and income per unit of filtrates and is subtracted to runoff, these practices al so water used. contribute to controi soil erosion. On farro management ofwater under scarcity can be sum • Increasing so il infiltration by combating the processes of marised as follows: soil crusting and soil sealing, including surface tillage • Reduced water demand methods, soil correctives and mulching for improving soil - Select low demand crop varieties / crop patterns
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Table 3. Soil management techniques forcontrolling evaporation and capturing runoff sideration of the factors influencing the hydraulic processes, the water infiltration Soil management techn iques Benefits Effective n ess and the uniformity of water application Control of soil evaporation to the entire field. • Crop residues and stones mulching • Decrease energy avai lable on soil Very high surface for evaporation 5.2. Water conservation and • Su rface tillage • Control of upward water fluxes to High saving in surface irrigation the s oi! surface Chemical surfactants • Decrease capi lIary rise Economie costs Several irrigation methods are used in practice. The mai n ones are basin irriga Water harvesting (a rid lands) tion, furrow irrigation and border irriga • M icro-watersheds • Runoff collectio n to be infiltrated High in the cropped area tion. In traditional systems, the water • Ru noff farmi ng • Maximise ru noff in collection High control is carried out manually according areas to be infiltrated i n the to the ability of the irrigator, so it is diffi cropped area cult to control "how much" water is ap • Water spreading • Spate irrigat ion by diverti ng flood High runoff plied and over-irrigation often occurs. In • M icro water-harvesti ng • Planting in the fu rrow bottom to High modemised systems, fields are often pre maximise rainfall infiltration cision levelled, and some form of control Runoff control in sloping areas of discharge such as siphons, gated pipes, • Terracing • Reduce r unoff a nd i ncrease Very High lay-flat tubes or gates, and some form of infi Itration automation including surge flow valves, • Contour ridges • Runoff sto rage and increase High cablegation or automated canal gates are infiltration used. Thus, in these systems, it is possi • Stri p cropping • Runoff retardation for in filtrati on High ble to control "how much" water should be applied at a given irrigation. - Adopt deficit irrigation In surface irrigation, the uniformity mainly depends upon • Water saving / conservation the system variables such as unit inflow discharges, field - Improve irrigation systems uniformity and management length and slope, which can not be easily modified by the - Reuse water spills and runoff irrigator. Re is in control of the management variable time - Surface mulch for controlling evaporation from soil of cut-off, but the latter depends also on other system vari - Soil tillage for augmenting the soil water reserve ables, namely those determining the advance time. The ap • Higher yields per unit of water plication efficiency depends upon the same variables as u - Adopt best farming practices niformity, and upon the farm management variables time of - Avoid crop stress at criticaI periods cut-off and soil water deficit at time of irrigation. The • Higher farmer incomes farmer ability plays a major role in controlling the manage - Farm for high quality products ment variables but his capability to achieve higher per - Select cash crops formances is definitely limited by the system characteristics Often, issues for irrigation demand management mainly and, often, by off-farm delivery decisions. This means that refer to irrigation scheduling giving a minor role to the irri it is not enough to tell the farmers to adopt target manage gation methods. Rowever, a conjunctive approach is re ment rules when the off- and on-farm system constraints quired. Irrigation scheduling is the farmers decision process are not identified and measures are not taken to improve the relative to "when" to irrigate and "how much" water to ap irrigation system. ply to a crop. The irrigation method concems "how" that Improvements in surface irrigation systems that help to desired water depth is applied to the field. The crop growth cope with water scarcity are in larger number and depend phase, its sensitivity to water stress, the climatic demand by upon field conditions (Pereira et al., 2002a and b). They re the atmosphere, and the water availability in the soil deter late to changes in system variables to improve water ad mine when to apply an irrigation. The frequency of irriga vance conditions, increase uniformity of water distribution tion depends upon the irrigation method, i.e. on the depths in the entire field, and control of deep percolation and ofwater which are typically associated with the on-farm ir runoff. They include land levelling, tools to control inflow rigation system. Both the irrigation method and the irriga discharges and volumes, farm hydraulic equipment for wa tion scheduling are inter-related. ter distribution and automation, surge-flow, cablegation and Irrigation scheduling requires knowledge about crop wa cut-off systems, as well as automation. When water savings ter requirements and yield responses to water, the con are required, improvements consist of irrigation with alter straints specific to each irrigation method and irrigation e nate furrows, adopting anticipated cut-off, and reuse oftail quipment, limitations relative to the water supply system, water runoff. and financial and economic implications of the irrigation Field evaluations play a fundamental role in improving practice. To improve the irrigation method requires the con- surface irrigation systems, as they provide information for
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design and for advising irrigators on how to improve their efficiency mainly depends upon the same system variables systems and practices. Field evaluations may play a major and upon the management variables related to the duration role in the adoption of water saving irrigation by farmers and frequency of the irrigation. Therefore, the farmer may because the respective management rules are based on the improve the application efficiency when adopting appropri actual farm performances. ate irrigation schedules but performances are limited by the Improved design and modelling are al so of great impor system constraints. tance because these may lead to change the field and sys The quality of design and equipment are essential because tem characteristics to favour improved distribution unifor they dictate the characteristics of the systems that deter mity. In fact design provides for the selection of the best mine the system performances. Design should be made for combination of field sizes, slope, inflow discharges, and high performances, not for reduced water use. A well-de time of application to provide for controlled deep percola signed system can be managed for water saving. Field e tion and runoff, and for application of deficit irrigation. valuations al so play an important role in advising farmers, creating information for design of new systems, for quality 5.3. Water conservation and saving in sprin- control of design and services, and for establishing water kler irrigation systems conservation and saving practices. Main sprinkler systems are set systems, travelling guns, Improvements in micro-irrigation systems mainly con and continuous move latemls. The irrigation uniformity es cern: sentially depends on variables characterising the system, • the achievement of high uniformities of pressures and which are set at the design phase. Similarly, the efficiency discharges along the operating sets, depends upon the same system variables as uniformity, and • the selection of drip and microspray emitters according upon the management variables concerning the duration to soil conditions, field slopes and crops grown, and the frequency of the irrigation events. In generaI, the • the adoption of system characteristics including au performance indicator having greater relation to enhanced tomation and fertigation that make easier to adopt deficit ir management, to controlled water application and to higher rigation. productivity of the water is the distribution uniformity 5.5. Irrigation scheduling and deficit irrigation (DV), while efficiency often follows DV and largely de pends upon the irrigation scheduling applied. The irrigator Research has provided a large variety of tools to support can do little to improve the uniformity of irrigation and is improved irrigation scheduling, i.e. the timeliness of irriga constrained by the system characteristics to improve the ir tion and the adequateness of volumes applied. Irrigation rigation performances. scheduling tools include the observation of soil water con The improvement of sprinkler irrigation systems to cope tent and or potential, of plant water status or related indica with water scarcity includes a variety of measures (Pereira tors, of meteorological variables relative to evapotranspira et al., 2002a and b) aiming at: tion, and the use of models and remote sensing. Many of • providing for the conduits and laterals system to be able these tools are still applicable in research only. Most of to produce adequate uniformity in pressure and discharge them require technical support by extension officers or distribution within the operating set, technical advisers, and the expertise of the farmers. There • adapting the system variables relative to the sprinklers fore, large limitations occur for their use in the farmers' to the soil infiltration and slope characteristics and wind practice. conditions, When surface irrigated areas are supplied from collective • making the system management able to cope with lim irrigation canal systems, farm irrigation scheduling depends ited water availability. upon the delivery schedule, e.g. rate, duration and frequen The quality of design play a major role in making the sys cy, which are dictated by the system operational policies. tems able to be managed for water savings. Field evalua Discharge and duration impose constraints to the volume of tions provide go od advice to farmers to improve manage application, and frequency determines the timing of irriga ment and to introduce limited changes in the system, as tion. Surface irrigation delivery systems are often rigid and well as useful information to designers and to the quality the time interval between successive deliveries is too long. control of design and services. Then, farmers apply all water that is made available and over-irrigation is often practised. Improving the on-farm ir 5.4. Water saving and conservation with mi rigation systems should go together with the improvement croirrigation systems of the delivery system to allow more flexibility in selecting the appropriate inflow rates and supply time. Microirrigation mainly include drip, micro-sprinkling, Irrigation scheduling can only be applied when the farm and sub-irrigation systems. Microirrigation uniformity, as ers have control on the timing and duration of irrigation. for sprinkler systems, depends upon system variables, i.e., Then, irrigation can be managed in relation to farmers' tar with exception of maintenance, the farmer can do very lit gets. tle to achieve good distribution uniformity. The application Deficit irrigation is an optimising strategy under which
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Fi gure 1. Relative evapotranspiration (---) and the re~ative yield 1055 (- ) of the su irrigation volume applied also decreases. gar beet crop to different deficit irrigation strategles 'I} . Vigia for average ( .~ )é Fig. 2 illustrates that the gross margins high ( ___ ) and very high ( -O- ) demand condltlOns under center-plvot Imga- per unit surface steadily decrease with tion (Rodrigues et al., 2003) the irrigation volumes and that the gross
100 l ' ...... ~ ·;.' n 100 margins per unit water appliedare main f tained or slightly increase until a mini [ r mum threshold of water applied and then ..- '., i ~.~: -'~-- ' 'y ...... ) _ 75., ...... " i -::.ll'f'-- ...... ~. ",;..' ... ., " . - - - . . .. . 75~- sharply decrease. This result indicates ~ - , . ~tl!.;: :.Jlf- .. ~ ~. ----~ that after such threshold is attained it is ;il . ,.m- .,>.,;- , . .Q I-w l - ~ .: -: ~ ." ., ..... ~. 50'0_ no more feasible to reduce irrigation. . ,.l.~ . . .1 '" ......
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voir and conveyance system, which is a pre-condition for and to irrigate by night reliability of deliveries, and • maintenance and management • the development of new sources of water supplies to cope - effective systems maintenance to avoid spills and leaks with extreme conditions of water scarcity, as is the case - water metering (flow depth and discharge) for droughts. - monitoring system performances (physical, environ- Supply management is also considered under the per mental and servi ce) spective of systems operation for delivery scheduling. The - personnel training following aspects may be considered: - farmers' information • hydrometeorological networks, data bases and informa The effective combination of demand and supply man tion systems to produce appropriate information for ef agement requires however appropriate information, train fective implementation and exploration of real ti me oper ing and farmers' participation. On the one hand, there is the ation of irrigation systems, need for training and assistance to farmers to improve on • agrometeorological irrigation information systems includ farm irrigation systems and scheduling, and to implement ing tools for farmers to access to information, often com water conservation and demand reducing practices. On the prising GIS, to support local or regional irrigation man other hand, measures have to be adopted to implement ef agement programs, fective farmers' participation in decisions relative to supply • real time reservoir operation and management tools, management. This includes the information to farmers on • decision support systems serving the reservoir operation characteristics and limitations of the water supply systems. and the water system management, and to help users to s Finally, the training of irrigation operation agents and irri elect water use options, including crop patterns, irrigation gation extension officers is a must. scheduling and irrigation systems, • automation and remote control for regulation of irrigation References reservoirs and conveyance and distribution systems, and Dracup, J. A., K. S. Lie, E. D. Paulson, 1980, On the definition of • planning for droughts to establish allocation and delivery droughts, Water Resources Research, 16(2): 297-302. policies, and operation rules. English, Musick, J.T , Murty, V.V.N., 1990. Deficit irrigation. In: The management of irrigation and multi-purpose supply G.J. Hoffman, TA Howell. and K.H. Solomon (Eds) Manage systems for coping with water scarcity includes: ment of Farm Irrigation Systems. ASAE, St. Joseph, MI, pp. 631 - • use/reuse of low quality water, treated wastewater and 663 . saline water, Pereira, LS, Cordery, I, Iacovides, I, 2002a. Coping with Water S • conjunctive use of surface water, groundwater and non carcity. UNESCO IHP VI, Technical Documents in Hydrology conventional waters, No. 58, UNESCO, Paris, 267 p. (available on line: http://unes • improved reservoirs operation mainly adopting doc.unesco.org/images/OO 12/00 1278/127846e.pdf). - information systems, including remote sensing, GIS, Pereira LS, Oweis T, Zairi A, 2002b. Irrigation management un and operation models der water scarcity. Agric. Water Manag. 57: 175-206. - hydrological forecasting and drought watch systems, Pereira L.S., Louro v., Rosario L., Almeida A, 2004. Desertifi - optimisation, risk, and decision models cation, territory and people, a holistic approach in the Portuguese • improved management of conveyance and distribution context. In: J.L. Rubio et al. (eds.) Desertification in the Mediter ranean Region: a Security Issue. NATO Sc.Com., Kluwer, Dor systems through drecht (in press). - information systems as a base source for management, - canal lining, Rodrigues, P.N. , T Machado, L.S. Pereira, J.L. Teixeira, H. El A mami, A Zairi, 2003. Feasibility of deficit irrigation with center - improved regulation and control of canal systems for pivot to cope with limited water supp1ies in Alentejo, Portugal. higher flexibility in deliveries and high servlce per In: G. Rossi, A Cancelliere, L. S. Pereira, T Oweis, M. formance, Shatanawi, A. Zairi (Eds.) Tools for Drought Mitigation in - low pressure pipe distributors for avoiding spills and Mediterranean Regions. Kluwer, Dordrecht, pp. 203-222. leakage, and easy delivery scheduling, Steduto, P., 1996. Water use efficiency. In. L.S. Pereira, R. Fed - change from supply oriented to demand oriented deliv des, J.R. Gilley, and B. Lesaffre (Eds) Sustainability of Irrigated ery schedules, Agriculture. Kluwer, Dordrecht: 193-209. - intermediate storage (in canal, reservoirs, farm ponds) The World Bank, 1992. World Development Report 1992: Devel • involve farmers in delivery scheduling decisions opment and the Environment. Oxford University Press, New Y • use incentive water prices to induce farmers to save water ork.
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