Dieter Prinz and A.H.MalikInstitute of Water Resources Management,Hydraulic and Rural Engineering,University of Karlsruhe,D-76128 Karlsruhe,Germany1 IntroductionIn the Developing World,agriculture counts for about 80% of total water withdrawal（Prinz,2000）.Agriculture is under enormous pressure by the other sectors of the economy,as the economic value created per unit water is lowest in agriculture.At the same time,growing populations make it necessary to produce more food and fibre and to ensure that crop yields per unit land continue to rise.Therefore,developing countries will have to find ways of growing more food with less water.But there is not only the problem of water quantity but additionally the one of deteriorating quality of water.There are many modern and traditional or combined technologies available to improve water conservation development（Agarwal,2001,Rijsberman,2001）.There is the need①to define the aim（under given frame work conditions）；②to analyze the losses（conveyance,distribution,application a.o.losses）；③to identify the most promising water conservation methods and techniques,and,eventually；④to integrate the various elements within a（soil and）water conservation strategy（Emerson,1998,UNEP,1998）.2 Measures applied in irrigated agricultureIrrigation water,if in ample supply,reduces considerable the risk of agricultural production and allows yields double as high as the yields which can be obtained from rain-fed agriculture（FAO,2001）.Roughly 40% of the food is produced on irrigated land,on 17% of the total cultivated land.The water needed for crops amounts to 1000～3000 m3per ton of cereal harvested.With other words,it takes one to three tones of water to grow 1kg of rice.As mentioned before,the losses of water have to be covered as well as the measures to increase the efficiency of water use in irrigation.We have to distinguish between recoverable water losses and unrecoverable water losses;the latter ones are those quantities of water lost to the atmosphere,to saline aquifers or to the sea.Recoverable losses include:seepage,surface runoff,operational losses and losses due to deep percolation.Fig.1 Water losses within a surface irrigation system in a semi-arid regionThe water losses could be（Fig.1）conveyance losses.distribution losses（e.g.canal evaporation,percolation and operation losses）orfield-ditch losses,field-application losses which depend on soil,irrigation method and management,Reduction of conveyance and distribution losses33%～50% of water diverted for irrigation is lost‘en route"：The conveyance and distribution losses are enormous.By lining the canal system or by conveying the water in pipes,these losses can be significantly reduced.It should be kept in mind,that at least the seepage losses are in most cases recoverable losses;the water might be lifted up from the groundwater layer downstream.Seepage losses occur in unlined main canals,in the canal distribution system and in field ditches.Most of the water is lost in unlined main canals.The questions which arise here are the following:What quantities of water are lost?Are these losses easy to recover?What feasible lining methods could be used?How high are the costs of lining and of maintenance?What are the benefits of the saved water?Could the unlined canal be used in the rainy season to recharge an aquifer?Reduction of application lossesApplication losses are either surface runoff losses or percolation losses,often summarised as“operational losses”.The water applied,should be sufficient to wet the volume of root penetration,but should not go beyond.Numerous technical means are available to apply exactly the amount of water needed,but financial and labour problems,in large irrigation schemes also management problems,hinder this.Operational losses depend on：①the chosen distribution system；②the available regulating and mechanical facilities；③the skill and discipline of the operator.3 Crop root zone depthThe active root zone of the crop（if water is not a constraint）depends on：①crop type；②its stage of maturity；③soil conditions（Fig.2）.Fig.2 The water applied should be sufficient to wet the volume of root penetration（but not more）Use of efficient irrigation methodsThe large differences in water efficiency between the various irrigation methods are quite well known:Traditional surface irrigation generally achieves only around 40% efficiency,sprinkler irrigation can be 70%～80% efficient and drip irrigation might reach over 90% efficiency（Wolf and Stein,1998,Fig.3）.Modern irrigation technology could in theory save about half of the water presently consumed in irrigation,but technical,economic and socio-cultural factors hinder the transformation of theory into practice.These methods are arranged according to their average efficiency.The surface irrigation techniques are：①basin irrigation，②furrow irrigation and③border irrigation.Fig.3 The five basic methods of applying water to the soilThese techniques have one drawback in common:the uneven water application over the irrigated area.Water infiltration is much greater at the top end of the field than the bottom because of the longer opportunity time at the top end and this results in high deep percolation.Surge irrigation（Fig.4）has been shown to markedly improve the efficiency of water application.It is the practice of intermittently stopping and starting water flows across a field.Fig.4 Water percolation in surge and continuous flowAnother method is the sprinkler irrigation.Low Energy Precision Application（LEPA）center pivots are one of the most efficient irrigation methods available today because they offer both high water application efficiency and low operating pressure.The water is applied near the ground surface below canopy.Drip/trickle irrigation is characterised by the following:low flow rate,long duration irrigation,frequent irrigation,water applied near or into the plant"s root zone,and low-pressure delivery system shigh investment costs and maintenance demand.In drip irrigation some techniques were developed which are not as efficient as the more costly ones,but which allow even small farmers a very high efficiency e.g.in vegetable cropping:the drip bucket irrigation is such a low cost and relatively efficient technique,applied already on thousands of farms in East Africa（Prinz and Malik,2001）.3.1 Case study:Growing More Rice with Less Water（China）Water Efficient Irrigation TechniquesOne method to save water in irrigated rice cultivation is the intermittent（submerged）irrigation.This example shows real water saving and increase of production.Production levels remained stable over the time period in spite of this massive shift of water（see Fig.5,Table 1）out of agriculture.Growing more rice with less water improves also the productivity of water.This was made possible through pol-icy,management,and technological changes（Rijsberman,2001）.Fig.5 Water quantities used for domestic,industrial and hydropower purposes increased 10 times during the period 1976 and 1996,with subsequent reduction of water quantities for irrigationTable 1 Changes in land and land productivity in Zhanghe Irrigation District,China（1966～1998）Source:Rijsberman 2001.3.2 Subsurface Irrigation Techniques3.2.1 Pitcher irrigationPitcher irrigation or“Pot Irrigation”is a traditional,extreme efficient form of irrigation.The technique is particularly suitable to the irrigation of vegetables（like legumes,beans,water-melons,etc.）in home ardens（Fig.6）.The clay pots can either be installed independently,i.e.the water supply is done by the farmers manually,one by one,or interconnected:the water supply is done by using a water reservoir,which can be a tank,a well,etc.3.2.2 Surface trickle irrigationFig.6 Clay pots used forpitcher irrigationTo reduce deep percolation losses in subsurface irrigation,a trench is dug and a layer of plastic sheets is positioned below the trickler pipe（Fig.7）.3.2.3 Vertical Pipe MethodA very simple but effective method for ree cultivation is the vertical pipe method:Provided sufficient water storage capacity in the root one is given,a larger quantity of water is applied through the vertical pipe to supply the tree with water for 2～4 weeks（Fig.8）.Fig.7 Subsurface trickle irrigation with reduced percolation lossesFig.8 Vertical pipe methodThe water efficiency depends not only on the methodWolf and Stein（1998）cite a study made in Israel by Hagan（1994）,who found surface irrigation to be 70% water efficient but drip irrigation only 42%～56%.This deviation from generally believed figures is due to differences in the available underlying conditions.For the farms using surface irrigation,water was in very short supply and therefore it had to be used as efficiently as possible.Drip irrigation on the other hand has been used under conditions of sufficient water to grow crops of high market value.Low cost of water and high market prices did not give any incentive to the farmers to use water efficiently.3.3 More rational use of irrigation water3.3.1 Supplemental irrigation（SI）It is the application of small quantities of irrigation water to essentially rain-fed crops in times when the demand can not be covered by rainfall（Oweis,1997）.SI is usually practiced usually in the wetter part of the dry areas with 300 to 600 mm annual rainfall in order to improve and stabilise yields（Oweis et al.,2001）.Supplemental irrigation might be taken from groundwater or from excess water stored during the rainy season.3.3.2 Deficit irrigationAnother technique which allows a very high water use efficiency under fully irrigated conditions is deficit irrigation.The deficit irrigation is the distribution of limited amounts of irrigation water to satisfy essential water needs of plants.The water supply is reduced in less critical periods of water demand by the crop and supply of full amount of water during stress-sensitive periods.A similar technique is the“intermittent submerged irrigation technique”for rice（ISI）.This technique has been promoted in China but it is now applied in many rice growing areas world wide.Up to 20% of the irrigation water can be saved,if the paddy crop is not grown under submerged conditions through out the main growing season,but only intermittently.The phases where submerged conditions are recommended are those“sensitive”stages mentioned earlier.Precondition for deficit irrigation management is the knowledge of the sensitive periods of the crop/the variety in question.3.3.3 AquaculturePaddy croppingcan be combined with fish raising if certain preconditions are given（Fig.9）.This allows a multiple use of water and hence water saving per unit produce.Fig.9 Combining paddy cropping with fish raisingCase study:Drip Irrigation Systems（DIS）in IndiaDrip Irrigation Systems in India are being praticised since 1970,being used on a limited scale in Tamil Nadu,Karnataka,Kerala and Maharashtra States,mainly for high value,horticultural crops like coconut,coffee,grape and vegetable production without the benefit of any subsidies from the governments.At Rahuri,in Maharashtra State,the use of drip irrigation of pomegranates,grown in gravely soils,resulted in a savings of about 44%（as compared to conventional check basin irrigation systems）with a further water savings of about 14% when compared to un-mulched plots.The capital costs involved are high compared to conventional irrigation systems,but the labor and operational costs are low in India.The net result is that the benefit-cost ratio for DIS is very favourable compared to conventional systems since the payback period for investment very short.The cost of using drip irrigation system is summarized in the Table 1.With DIS in India,there was an improvement in crop yields and savings in water use of between 18% and 40%.Consequently,there was a substantial improvement in the water use efficiency that ranged up to three times that of conventional surface irrigation methods,even with the use of poor quality irrigation water.See Table 2 for water savings and increased yields achieved using drip irrigation in Indian case.Table 2 Drip irrigation cost;water savings v.increased yield achieved in IndiaSource:Saksena 2000.4 Improving water availabilityAs mentioned earlier,aquifer depletion is a common problem in many dry areas of the world.Many techniques have been developed to artificially recharge aquifers to sustain the water table and to allow further control of pumping water to cover the water needs of humans and crops.One interesting example of combined basin irrigation with groundwater recharge is reported from Uttar Pradesh Province in India（IWMI,2002）.In the monsoon season,surface water is diverted through an unlined canal system to provide farmers with irrigation water for rice crops.Around 60% of the irrigation water applied is used by the plants,most of the remaining 40%filters through the soil to recharge the groundwater.Combined with seepage from unlined canals those“losses”provide farmers with groundwater to irrigate dry season crops.The research showed,that the water table in the study area,which had been progressively declining,has been raised from an average of 12m below ground level to an average of6.5m.5 SummaryWe need information on crop water requirements.Losses have to be a
