Water management in irrigated rice - Rice Knowledge Bank ...

or increasing the internal drainage rate may improvecrop growth and yield (Ramasamy et al 1997). Theunderlying reason may be improved soil aerationor the removal of toxic substances.The first response option to decreasing wateravailability would be to check “the basics.” Theamount of water loss that can be reduced dependson the initial condition of the rice field; if the basicsare right from the start, there is not much that canbe done any more. With progressing water scarcity,establishment options alternative to transplantingcan be considered if the turnaround time betweensoaking and transplanting is relatively large, suchas in some large-scale irrigation systems. If communityseedbeds or a commercial provider ofseedlings could be organized, this would be theleast water-consuming method of getting a cropestablished. Seedlings could get transplanted a fewdays after land soaking and puddling only, whilethe large-scale raising of seedlings would ensurean efficient use of water during that period (mainfields do not yet have to be soaked). Both direct wetand direct dry seeding are alternative options. Dryseeding will be effective only in relatively clayeyand impermeable soils that don’t need puddling toreduce the permeability any more.With further increasing water scarcity, watermanagement practices during the whole growingseason need to be considered. Instead of keeping a5−10-cm depth of ponded water during the growingseason, the depth can be reduced to around 3 cm.This will reduce the hydrostatic pressure and minimizeseepage and percolation losses. In saturatedsoil culture (SSC), the depth of ponded water isreduced to 0−1 cm. Around flowering, from 1 weekbefore to 1 week after the peak of flowering, pondedwater should best be kept at 5-cm depth to avoid anypossible water stress that could result in severe yieldloss. The practice of SSC would require frequent(once in 2 days) applications of small amounts ofirrigation water, and hence require a high level ofcontrol over irrigation water. The practice of safeAWD can reduce water losses by a small to considerableamount without a yield penalty. To whatextent water losses can be reduced under SSC orAWD depends mainly on soil type and depth ofthe groundwater table: with a heavy clay soil andshallow groundwater (10−40 cm deep), water lossesare small to start with and reductions in water lossesare equally small. With more loamy or sandy soilsand/or deeper groundwater tables, reductions inwater losses can be higher, but the risk of a reductionin yield also becomes higher. If water is gettingso scarce that “safe AWD” is no longer possible,the periods between irrigation will have to becomelonger (letting the water in the field water tubes godeeper than 15 cm) and yield loss becomes inevitable.All forms of AWD require water control by thefarmer. With own water sources, such as tubewells,this is not a problem. In community-based or largescaleirrigation systems, a communal approach toAWD is required in which delivery of water togroups of farmers is scheduled to realize a certainpattern of AWD. Irrigation system upgrading ormodernization may be required to do this, or smallstorage facilities (such as on-farm reservoirs) mayprovide the required water control (Chapter 5).With still further increasing water scarcity,yield of lowland rice under AWD will continue togo down. At a certain point, aerobic rice systemsbecome a viable alternative. How much less wateris used under aerobic conditions than under floodedconditions depends mostly on the seepage and percolation(SP) losses under flooded conditions and onthe deep percolation losses of irrigation water underaerobic conditions. Typical SP rates of flooded ricefields are given in Table 1.1 in Chapter 1.3. Underaerobic conditions, the amount of deep percolationdepends on the combination of soil water-holdingcapacity and method of irrigation, and is reflectedin the irrigation application efficiency (EA). With aprecise dosage and timing of irrigation in relation tocrop transpiration and soil water-holding capacity,the EA in flash-flood irrigation can be up to 60%(Doorenbos and Pruit 1984). If furrow irrigation (orraised beds) is used, the EA can go up to 70%, andwith sprinkler irrigation up to 80% or more. Assumingan average growth duration of 100 days, andmean ET values for rice, we can roughly calculatethe “break-even” point for SP rates in flooded fieldsthat would result in similar water requirements inaerobic fields with different irrigation methods(Table 3.8). When the SP rate in flooded rice is 3.5mm d −1 or higher, aerobic systems with flash-floodirrigation will require less water, and, if the SPrate is 0.5 mm d −1 or lower, only aerobic systemswith sprinkler irrigation require less water. Whenaerobic rice systems are direct (dry) seeded, as is thetypical target technology, an additional amount ofwater input can be saved by forgoing the wet landpreparation. An example of the cross-over point interms of water availability where aerobic rice gives30