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Coping with water scarcity3In this chapter, we present technology options tohelp farmers to cope with water scarcity at the fieldlevel. The way to deal with reduced (irrigation orrain) water inflows to rice fields is to reduce thenonproductive outflows by seepage, percolation, orevaporation, while maintaining transpiration flows(as these contribute to crop growth). This can bedone at land preparation, at crop establishment, andduring the actual crop growth period.3.1 Land preparationLand preparation lays the foundation for the wholecropping season and it is important in any situationto “get the basics right.” Especially important forgood water management are field channels, landleveling, and tillage operations (puddling, and bundpreparation and maintenance).3.1.1 Field channelsMany irrigation systems in Asia have no fieldchannels (or “tertiary” irrigation or drainage channels)and water flows from one field into the otherthrough breaches in the bunds. This is called “plotto-plot”irrigation. The amount of water flowingin and out of a rice field cannot be controlled andfield-specific water management is not possible.This means that farmers may not be able to draintheir fields before harvest because water keepsflowing in from other fields. Also, they may not beable to have water flowing in if upstream farmersretain water in their fields or let their fields dryout to prepare for harvest. Moreover, a number oftechnologies to cope with water scarcity requiregood water control for individual fields (Chapter3.5). Finally, the water that continuously flowsthrough rice fields may remove valuable (fertilizer)nutrients. Constructing separate channels to conveywater to and from each field (or to a small groupof fields) greatly improves the individual control ofwater and is the recommended practice in any typeof irrigation system.3.1.2 Land levelingA well-leveled field is a prerequisite for good crophusbandry. When fields are not level, water maystagnate in depressions, whereas higher parts maybecome dry. This results in uneven crop emergenceand uneven early growth, uneven fertilizer distribution,and maybe extra weed problems. Informationon technologies for land leveling can be found atwww.knowledgebank.irri.org.3.1.3 Tillage: reducing soil permeabilitySeepage and percolation flows from rice fields aregoverned by the permeability (hydraulic conductivity)of their soils: their capacity to conduct waterdownward and sideward (Chapter 1.3). A rice fieldcan be compared to a bathtub: the material of a bathtubis impregnable and it holds water well—however,if you have only one hole (by removing theplug), the water runs out immediately. Rice fieldsjust need a few rat holes or leaky spots and they willrapidly lose water by seepage and percolation.Large amounts of water can be lost duringsoaking prior to puddling when large and deepcracks are present that favor rapid “by-pass flow”to below the root zone. Cabangon and Tuong (2000)showed the beneficial effects of additional shallowsoil tillage before land soaking to close the cracks:the amount of water used in wet land preparationwas reduced from about 350 mm to about 250 mm(Fig. 3.1).17
Water input (mm)400350300250200150100500ControlCracks plowedFig. 3.1. Effect of shallow tillage to fill cracks before soakingon water input during land preparation, Bulacan, Philippines.Data from Cabangon and Tuong (2000).Water input (mm)2,0001,8001,6001,4001,2001,0008006004002000No liningRainfallIrrigationLiningFig. 3.2. Effect on total water input of lining bunds withplastic in a field experiment at IRRI, Los Baños, Philippines.Data from Bouman et al (2005).Thorough puddling results in a good compactedplow sole that reduces permeability and percolationrates throughout the crop growing period(Chapter 1.3; De Datta 1981, Tuong et al 1994).The efficacy of puddling in reducing percolationdepends greatly on soil properties. Puddling maynot be effective in coarse soils, which do not haveenough fine clay particles to migrate downwardand fill up the cracks and pores in the plow sole.On the other hand, puddling is very efficient in claysoils that form cracks during the fallow period thatpenetrate the plow pan (Tuong et al 1994). Althoughpuddling reduces percolation rates of the soil, theaction of puddling itself consumes water, and thereis a trade-off between the amount of water used forpuddling and the amount of water “saved” duringthe crop growth period by reduced percolation rates.Puddling may not be necessary in heavy clay soilswith low vertical permeability or limited internaldrainage. In such soils, direct dry seeding on landthat is not puddled but tilled in a dry state is verywell possible with minimal percolation losses (Tabbalet al 2002; Chapter 3.2).Soil compaction using heavy machineryhas been shown to decrease soil permeability insandy soil with loamy subsoils with at least 5%clay (Harnpichitvitaya et al 2000). Although mostfarmers cannot afford to compact their soils, thistechnology may be feasible on a large scale withgovernment support.3.1.4 Bund preparation and maintenanceGood bunds are a prerequisite to limit seepageand underbund flows (Tuong et al 1994). To limitseepage losses, bunds should be well compactedand any cracks or rat holes should be plasteredwith mud at the beginning of the crop season.Make bunds high enough (at least 20 cm) to avoidoverbund flow during heavy rainfall. Small leveesof 5–10-cm height in the bunds can be used to keepponded water depth at that height. If more waterneeds to be stored, it is relatively simple to closethese levees. Researchers have used plastic sheets inbunds in field experiments to reduce seepage losses.For example, Bouman et al (2005) demonstrateda reduction of 450 mm in total water use in a ricefield by lining the bunds with plastic (Fig. 3.2).Although such measures are probably financiallynot attractive to farmers, the author came upon afarmer in the Mekong Delta in Vietnam who usedold plastic sheets to block seepage through veryleaky parts of his bunds.3.2 Crop establishmentMinimizing the turnaround time between landsoaking for wet land preparation and transplantingreduces the period when no crop is presentand when outflows of water from the field do notcontribute to production. Especially in large-scaleirrigation systems with plot-to-plot irrigation, waterlosses during the turnaround time can be very high.For instance, in the largest surface irrigation schemein Central Luzon, called UPRIIS (Upper PampangaRiver Integrated Irrigation System), it took up to63 days in a contiguous 145-ha block from the firstday of water delivery for land preparation until thewhole area was completely transplanted (Tabbalet al 2002). The total amount of water input dur-18
Page 2: Water Management in Irrigated Rice:
Page 5 and 6: PrefaceWorldwide, about 79 million
Page 8 and 9: Fig. 1.2. Water balance of a lowlan
Page 10 and 11: given in Table 1.1. Water losses by
Page 14 and 15: Distribution (%)1009080706050403020
Page 16 and 17: The plant-soil-water system22.1 Wat
Page 18 and 19: Ψ Air-100 MPaDemandLeafXylemStemRo
Page 20: tive to water deficit than cell enl
Page 25 and 26: Table 3.1b. Yield, water input, and
Page 27: Grain yield (t ha -1 )98A2002 20037
Page 30 and 31: Table 3.5. Water input (I = irrigat
Page 32 and 33: Practical implementationTemperate e
Page 34 and 35: Practical implementationSpecific in
Page 36 and 37: Table 3.8. Comparison of water use
Page 38: Flooded yield (t ha -1 )87A65432102
Page 41 and 42: consumption (Hamilton 2003). Many t
Page 44 and 45: Irrigation systems55.1 Water flows
Page 46 and 47: the main system is supply-driven, f
Page 48 and 49: Table 5.1. Area, water use, and tot
Page 50 and 51: Appendix: InstrumentationDetailed d
Page 52: 0.5 cm in diameter and spaced 2 cm
Page 55 and 56: Bronson KF, Singh U, Neue HU, Abao
Page 57 and 58: Lampayan RM, Bouman BAM, De Dios JL
Page 59: Uphoff N. 2007. Agroecological alte

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