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

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