Dieter Prinz and Dr.Amir H.Malik
Institute of Water Resources Management,Hydraulic and Rural Engineering,Section of Rural Engineering,University of Karlsruhe,D-76128 Karlsruhe,Germany
1 Introduction
Water is already a scarce commodity in many parts of the world-and will be of even shorter supply in future:Due to an ever increasing world population,improving standard of living,irregularities caused by global climate change and growing water pollution,the world water problems aggravating day by day.Especially the drier parts of the tropics and subtropics,but also European countries,experience severe water supply problems-and agriculture will be hit hard-est.Agriculture utilizes globally about 70%of all the water managed by man,and about 80% of the water used in the developing world（Prinz,2000）.At the same time,the competition between the various sectors-agriculture,communities,industry,nature,becomes stiffer and agriculture will be the loser in the run for scarce water resources,as the output per unit water is of significantly lower value than in the other economic sectors.On the other hand,the need for more food asks also for more irrigation water,therefore we have to find ways of growing more food with less water（Agarwal,2001）.But it is not only a problem of water quantity,but of quality due to increasing pollution,too.
2 General overview
Water conservation in agriculture may be defined as the application of measures designed
（1）To improve the availability of water for agricultural purposes（“Supply Management”）,
（2）To reduce the present size of water demand（“Demand Management”）,and
（3）To keep water resources from being polluted or wasted.
The solutions found must be sustainable and possible negative effects e.g.on nature have to be avoided.
（1）Rainwater management:Rainwater management can be either“in-situ moisture conservation”or“Water Harvesting”.
（2）In-situ moisture conservation:Its main aim is to minimise the runoff losses and to in-crease the available soil moisture for crop growth.
（3）The water retention within the field allows higher infiltration rates and thus results（in dry areas）in better crop yields.
（4）Water Harvesting is defined as“the collection and concentration of rainfall（and overland flow）and its use for the irrigation of crops,pastures and trees for domestic and livestock consumption”.The water storage can be done in the soil matrix or in a reservoir（Prinz and Wolfer,1998）.
Fig.1 The atmosphere-soil-plant-water-system（by Dupriez ＆ De Leener,1992,altered）
（5） Rain and surface runoff management serves also the purposes of soil conservation,a prerequisite for water conservation,and flood control.
The term“efficient use of water”is a very critical one:The efficiency might be defined as“unit of water used by crops to produce one unit of dry matter”or“…to produce one unit of harvested produce”.
When water is in short supply,farmers are very much interested in increasing the efficiency by limiting unproductive water losses（evaporation from soil,surface runoff,seepage）.Even if the farmers can increase the efficiency per unit water available in their fields,this does not necessarily improve general,regional water efficiency.
Fig.2 Water losses under semiarid conditions（Average values）（Source:Rockstroem ＆ Falkenmark,altered）
Reduced seepage e.g.means a lower recharge of groundwater,less surface runoffmeans reduced surface water flows and fewer opportunities for using water further downstream,including less water supply for river valleys and wetlands.Therefore，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（Fig.2）.
3 Measures in rain-fed agriculture
Globally,rain-fed agriculture is practised on 83% of cultivated land and supplies more than 60% of the world's food（FAO,2001）.In-situ water conservation,often combined with water harvesting measures,can contribute significantly making better use of the rain in dry areas to supply the crops during the rainy season with sufficient water.One element of in-situ moisture conservation is the curbing of runoff losses,which can amount to 30%～50%of rainfall on sloping grounds.
3.1 Reduction of runoff losses
Runoff losses can be minimized e.g.by
（1）Tying ridges:A tied ridge system can double crop,yields in semi-arid areas,while simultaneously preventing soil erosion.The water storing capacity in a tied ridge system amounts to 40～70 mm（Fig.3）.
Fig.3 Tied ridges（Source:Prinz and Malik,2001）
Fig.4 Stone lines in North Africa（Source:FAO）
（2）Cultivating crops in furrows:The runoff can be minimised by cultivation in furrows①on hilly land as well as in lowlands，②for rainfed or for irrigated cropping，③with or with-out water conservation.
Advantages are①As ridges dry out easily,moisture retention below furrow remains optimal，②No runoff and no soil erosion occur.
Disadvantages are①Danger of water logging and anaerobic conditions，②Compaction of furrow bottoms by tractor traffic may worsen water logging and③Transmission of diseases
Construction of earth basins and sunken beds to cultivate crops in them.A very remark-able example is the Zay technique（Chritchley et al.,1992）.Pits of 30 cm diameter are dug,90cm apart and 15～20cm deep.An experiment in Niger showed a doubling of yield from 600kg/ha on a（manured）field without Zay to 1200 kg/ha on fields with Zay pits（Fatondji et al.,2001）.
（3）Lines of stones and trash,bunds and ridges（Fig.4）.
（4）The application of Fanya Juu techniques,catching the rain in uphill or downhill trenches and inducing the establishment of terraces over a longer period（Fig.5）.
Fig.5 There are two different Fanya Juu techniques to arrange ridge ＆ basin:Either the ridge is located up or downhill of the basin.（Source:Prinzand Malik,2001）.
（5）Construction of ditches and basin to retain water in the field（Fig.6）
（6）Contour farming is also well established measures in soil and water conservation.
Contour farming means that tillage,planting and other farm operations are done along the contour（Fig.7）,by impeding the down-slope flow water,allowing the water to infiltrate and avoiding erosion.
Contour farming on slopes of 4%～6% can reduce water loss（runoff）by 50% and soil loss by about 50%,compared to up and down hill cultivation.
Fig.6 Retention ditches
Fig.7 Contour farming.（S:urce:WOCAT.2000）
（7）Strip Cropping:Strip cropping means dividing land into alternative strips of soil and water conserving crops and crops with a high soil erosion/runoff risk（Fig.8）.The strips should be of equal widths and not too wide to allow uncontrollable quantities of runoff to accumulate.
Vegetation strips either of grasses or hedges slow down runoff water and force to deposit its load.The vegetation acts as filter.Over the years,terraces might develop.
Aside of reducing runoff losses the fol-lowing measures can be applied：
（8）Conservation tiliage:Conventional tillage has a number of advantages（it opens and turns the soil,it is rather effective in sup-pressing weeds,etc.）,but in dry areas it has also many disadvantages：①it leads to higher evaporation；②it brings oxygen into the soil,thus accelerating humus degradation；③may lead to lower infiltration rates（after a few rainfall events）.
Fig.8 Planting vegetation strips（Source:WOCAT,2000）
Conservation tillage aims at soil amelioration and rooting depth improvement to overcome water supply problems in dry areas.Soil management in water conservation means：①improving soil moisture intake；②reducing evaporation losses from the soil；③improving water holding capacity of soils；④avoiding compaction；⑤reducing deep percolation losses；⑥creating an optimal root environment.
（9）Construction of the various kinds of terraces:Terraces cut slopes into segments,reducing slope angle and length of slope（Fig.9）.Bench terraces are the most ancient form of terrace.Beach terraces as well as channel terraces impound rainfall and runoff and encourage water to infiltrate into the soil.
Fig.9 Different types of terraces（Source:Prinz and Malik,2001）
3.2 Improving crop selection
It is well established,that different crops need rather different quantities of water to pro-duce a yield.Rice,e.g.is a very water intensive crop,using twice as much water per hectare as wheat（FAO 200 1）.When farmers decide to switch from rice cultivation to any so called upland crop,this will save substantial amounts of water.But the cultivation of less water demanding crops than rice is not the only measure suggested in this respect:
using crops of high water use efficiency
using well adapted,high yielding varieties;short-strawed wheat gives double or triple yield per unit water in comparison to the traditional varieties.
3.3 Improving crop husbandry
In areas with a short rainy season,the right timing of the crop is a decisive feature,as well as the type of cropping,e.g.relay or sequential cropping.
A high water use efficiency can only be achieved if all the other growth factors are kept near the optimum.Important elements in this respect are：①nursery techniques with optimal water supply,but little percolation；②optimal seeding density and seeding methods,e.g.“dry seeding”；③the optimal association of crops；④crop protection,to avoid any suffering of the crops from pests and diseases；⑤supply of nutrients,i.e.the manuring and applying mineral fertiliser；⑥timely weeding of the crop to avoid water losses by unwanted plants.
Fig.10 shows an example of water production function for maize under Kenyan conditions,illustrating the dependency of the yield level under given rainfall conditions from the level of management.
Fig.10 Water-production function for maize in Machakos District,Kenya.（Source:Beets,1990）
Combining cropping with animal husbandry
According to Cape（1995）it needs 700 l of water to produce one litre of milk.This water,of course,is needed to produce fodder plants and one kg of alfalfa hay needs roughly 600 1 of water.The combination of cropping and raising animals,e.g.cows,can be very water efficient as can be the cultivation of fishes in rice fields（aquaculture）.
3.4 Reduction of transpiration losses
Transpiration of plants counts for the largest share of the water used by a crop stand.The high evaporative demand of the atmosphere determines largely the amount of water lost by transpiration.If the air suction is higher than the water supply of the plant,the closing of the leave pores（stomata）is a counter measure,but a measure which lowers productivity.The grower can interfere by two means：①by reducing wind speed,e.g.by planting shelter belts and②by planting crops in greenhouses or in plastic tunnels.Measure 2 is the cultivation of crops under shade trees or in shade houses.The breeders have already succeeded in breeding a number of new grain varieties with lower transpiration rates per kg of dry matter produced.
3.5 Reduction of evaporation losses
According to Hudson（1987）,shading the surface can substantially reduce the evaporation losses.The more of less unproductive evaporation from soil and water surfaces should be reduced wherever possible.Proven measures are e.g.:
（1）The application of mulch layers of organic origin or as plastic mulch；
（2）The use of certain cover crops,which need less water for transpiration than they save from evaporation;
（3）The use of conservation tillage which disturbs to a lesser degree the ground,but disturbs the capillary rise of the water to the surface;
（4）Various systems of agro-forestry produce shade and reduce temperatures for the annual crops grown below.Shade trees often belong to the Leguminosae family；
（5）Contour hedges and shelter belts reduce wind speed;
（6）Greenhouses and tunnels reduce not only transpiration but also significantly the evaporation from the ground.
3.6 Reduction of percolation losses
Percolation losses occur as well under rain-fed agriculture as under irrigation,from soils to groundwater layers as well as from ponds and reservoirs to deeper layers.
To improve soil management in regard to water efficiency,the following measures are recommended：
（1）Improving rain water intake by keeping an open soil surface,by mulching with organic material and by keeping a high organic content in the soil and a good soil structure.
（2）Improving soil water holding capacity by keeping a high level of soil fertility（high percentage of organic material,good structure etc.）.
（3）Avoiding compacted layers,e.g.a plough pan,allowing water to reach deeper soil layers and to increase the wetted soil volume.
（4）Optimising the root environment:A soil environment is optimal for plant growth,contributing to a high water application efficiency,if it is deep enough,if the soil is well structured and well supplied with nutrients,if it contains a high water storage capacity,has no hard pans or stone layers,has no salt and toxic element accumulation and has no drainage problems.
4 Future Outlook
Future research fields will include biotechnology,bioengineering and plant breeding which should be employed to arrive at species and varieties with a significant lower water demand.Future research should be multidisciplinary.
References
[1]Agarwal A.Increasing water harvesting and water conservation is the only way to ensure food security.Down to Earth,Vol 10,No.3,June 30,2001.
[2]Beets W.C.Raising and sustaining productivity of smallholders farming systems in the tropics.AgBe Publishing,Holland.1990.
[3]Cape J.Irrigation Research:Past,present and future.Irrigation Australia.1995,Vol.10,30～32.
[4]Chritchley W.,Reij C.and Turner S.D.:Soil and water conservation in Sub-Saharan Africa:to-wards sustainable production by the rural poor.IFAD,Rome and CDCS,Amsterdam.1992.
[5]FAO.Crops and drops:Making the best use of land and water.Food and Agriculture Organisation（FAO）,Rome.2001.
[6]Fatondji D.Martius C.and Vlek P.A traditional technique for land rehabilitation in Niger.Newsletter No.8,Zentrum fÜr Entwicklungsforschung/Center for Development Research,Universität Bonn,September,2001.
[7]Hudson N.W.Soil and water conservation in semi-arid areas.Food and Agriculture Organisation（FAO）Soil Bulletin,1987,57 pp.109～122.
[8]Prinz D.Global and European water challenges in the 21stcentury.Keynote Speech,3rd Inter-Regional Conference on Environment-Water，“Water Resources Management in the 21st Century”，1-3 June 2000,Budapest/Hungary.Proceedings,2000,p.247～254.
[9]Prinz,D.and Malik,A.H.Water Conservation in Agriculture,FAO Training Course,Draft Version on CD ROM,FAO,Rom.2001.
[10]Prinz D.and Wolfer S.Opportunities to Ease Water Scarcity（Water Conservation Techniques and Approaches）.Proceedings,International Conference on“World Water Resources at the Beginning of the 21stCentury”03-06 June 1998.Paris.UNESCO-IHP,Paris.1998.
[11]UNEP.Sourcebook of Alternative Technologies for Freshwater Augmentation Some Asian Countries.UNEP,Paperback,1998,p.223.
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