given <strong>in</strong> Table 1.1. <strong>Water</strong> losses by seepage andpercolation account for about 25–50% of all water<strong>in</strong>puts <strong>in</strong> heavy soils with shallow groundwater tablesof 20–50-cm depth (Cabangon et al 2004, Donget al 2004), and 50–85% <strong>in</strong> coarse-textured soilswith deep groundwater tables of 1.5-m depth ormore (Sharma et al 2002, S<strong>in</strong>gh et al 2002). Thoughseepage and percolation are losses at the field level,they are often captured and reused downstream (justlike overbund flow; Chapter 5.1). The actual amountof water reuse <strong>in</strong> <strong>rice</strong>-based irrigation systems isnot known and is expected to vary widely amongirrigation systems.Daily seepage and percolation losses from theponded water do not occur <strong>in</strong> dryland crops suchas wheat and maize. Percolation of water below theroot zone can also occur <strong>in</strong> dryland crops when theamount of water <strong>in</strong>filtrat<strong>in</strong>g <strong>in</strong>to the soil (either afterheavy ra<strong>in</strong>fall or after irrigation) is larger than thestorage capacity of the root zone, but this is not adaily water flow as <strong>in</strong> lowland <strong>rice</strong>. Also, evaporationfrom ponded water surfaces is higher than fromsoil surfaces (as <strong>in</strong> dryland crops). Therefore, it isthe relatively large water flows by seepage, percolation,and evaporation that make lowland <strong>rice</strong>fields heavy “water users.” Total seasonal water<strong>in</strong>put to <strong>rice</strong> fields (ra<strong>in</strong>fall plus irrigation) can beup to 2–3 times more than for other cereals such aswheat or maize (Tuong et al 2005). It varies fromas little as 400 mm <strong>in</strong> heavy clay soils with shallowgroundwater tables (that directly supply waterfor crop transpiration) to more than 2,000 mm <strong>in</strong>coarse-textured (sandy or loamy) soils with deepgroundwater tables (Bouman and Tuong 2001,Cabangon et al 2004). Around 1,300–1,500 mm isa typical value for <strong>irrigated</strong> <strong>rice</strong> <strong>in</strong> Asia. Table 1.1lists some values for water <strong>in</strong>puts and daily seepageand percolation rates for lowland <strong>rice</strong> fields <strong>in</strong>Ch<strong>in</strong>a and the Philipp<strong>in</strong>es.It is useful to dist<strong>in</strong>guish between the wateroutflows from <strong>rice</strong> fields that can, <strong>in</strong> pr<strong>in</strong>ciple, bereused and those that cannot be reused. Nonreusableoutflows are called “depleted water” (Molden1997) and are evaporation and transpiration. Overbundflow, seepage, and percolation are generallyreusable flows. Only when these flows enter verydeep or sal<strong>in</strong>e groundwater (or sal<strong>in</strong>e surface water)from where they cannot be recovered are theynot reusable any more and they become depletionflows as well.1.4 Groundwater under <strong>rice</strong> fieldsThe role of groundwater <strong>in</strong> provid<strong>in</strong>g water to <strong>rice</strong>plants may be large, but it has been neglected <strong>in</strong>most studies of the <strong>rice</strong> water balance. Recent datacollection suggests that through the (decade- toage-old) practice of cont<strong>in</strong>uous flood<strong>in</strong>g, the largeamounts of percolat<strong>in</strong>g water have raised groundwatertables to very close to the surface. This isespecially true <strong>in</strong> soils with a heavy texture that arepoorly dra<strong>in</strong>ed <strong>in</strong> the subsoil, as is the case <strong>in</strong> manytraditional <strong>irrigated</strong> <strong>rice</strong> environments.Figure 1.3 gives the groundwater table measuredunder flooded <strong>rice</strong> fields at some sites <strong>in</strong> Ch<strong>in</strong>aand the Philipp<strong>in</strong>es. When the groundwater is lessthan 20 cm deep, it provides a “hidden” source ofwater to the <strong>rice</strong> crop as the roots of the plants candirectly take up water from the groundwater. Whenfor some reason fields are not flooded (Chapter3), capillary rise may reach <strong>in</strong>to the root zone andaga<strong>in</strong> provide extra water to the crop. In mostwater balance studies, the effect of groundwateron water supply is not taken <strong>in</strong>to account, and thebeneficial effect of water-sav<strong>in</strong>g technologies canbe overestimated. With shallow groundwater, cropgrowth with a small irrigation water supply can stillbe good because of the “hidden” water supply ofgroundwater.1.5 <strong>Rice</strong> water productivity<strong>Water</strong> productivity (WP) is a concept of partialproductivity and denotes the amount or value ofproduct (<strong>in</strong> our case, <strong>rice</strong> gra<strong>in</strong>s) over volume orvalue of water used. Discrepancies are large <strong>in</strong> reportedvalues of WP of <strong>rice</strong> (Tuong 1999). These arepartially caused by large variations <strong>in</strong> <strong>rice</strong> yields,with commonly reported values rang<strong>in</strong>g from 3 to8 tons per hectare. But the discrepancies are alsocaused by different understand<strong>in</strong>gs of the denom<strong>in</strong>ator(water used) <strong>in</strong> the computation of WP. Toavoid confusion created by different <strong>in</strong>terpretationsand computations of WP, it is important to clearlyspecify what k<strong>in</strong>d of WP we are referr<strong>in</strong>g to andhow it is derived. Common def<strong>in</strong>itions of WP areWP T: weight of gra<strong>in</strong>s over cumulative weightof water transpired.WP ET: weight of gra<strong>in</strong>s over cumulative weightof water evapotranspired.WP I: weight of gra<strong>in</strong>s over cumulative weightof water <strong>in</strong>puts by irrigation.
Groundwater depth (cm)20AGroundwater depth (cm)20B00–20–20–40–60–80–40–60–80Panicle<strong>in</strong>itiationFlower<strong>in</strong>gHarvest–10018 Jan 7 Feb 27 Feb 18 Mar 7 Apr 27 AprDayGroundwater depth (cm)C–10016 Jun 11 Jul 5 Aug 30 Aug 7 Sep 19 OctDayGroundwater depth (cm)2020–20100–60–100–140–180–220–260–300–10–20–30–40–50–60–70–80–90–10014 Jan 13 Feb 15 Mar 14 Apr 14 Jan 13 Feb 15 Mar 14 AprDayDayFig. 1.3. Groundwater depth under flooded <strong>rice</strong> at Tuanl<strong>in</strong>, Hubei Prov<strong>in</strong>ce (A), and Changle (B), Beij<strong>in</strong>g, Ch<strong>in</strong>a, <strong>in</strong> 2002.Adapted from Belder et al (2004) and unpublished data from Ch<strong>in</strong>a Agricultural University/IRRI. Groundwater depth underflooded <strong>rice</strong> <strong>in</strong> Dolores (C) and Gabaldon (D) <strong>in</strong> 2002, Central Luzon, Philipp<strong>in</strong>es. Adapted from Lampayan et al (2005).DWP IR: weight of gra<strong>in</strong>s over cumulative weightof water <strong>in</strong>puts by irrigation and ra<strong>in</strong>.WP TOT: weight of gra<strong>in</strong>s over cumulative weightof all water <strong>in</strong>puts by irrigation, ra<strong>in</strong>,and capillary rise.Breeders are <strong>in</strong>terested <strong>in</strong> the productivity ofthe amount of transpired water (WP T), whereasfarmers and irrigation eng<strong>in</strong>eers/managers are <strong>in</strong>terested<strong>in</strong> optimiz<strong>in</strong>g the productivity of irrigationwater (WP I). To regional water resource planners,who are <strong>in</strong>terested <strong>in</strong> the amount of food that canbe produced by total water resources (ra<strong>in</strong>fall andirrigation water) <strong>in</strong> the region, water productivitywith respect to the total water <strong>in</strong>put by irrigationand ra<strong>in</strong>fall (WP IR) or to the total amount of waterthat can no longer be reused (WP ET) may be morerelevant.Modern <strong>rice</strong> varieties, when grown underflooded conditions, have similar water productivitywith respect to transpiration (WP T) as other C 3cereals such as wheat, at about 2 g gra<strong>in</strong> kg –1 watertranspired (Bouman and Tuong 2001, Tuong et al2005). The few available data <strong>in</strong>dicate that waterproductivity with respect to evapotranspiration isalso similar to that of wheat, rang<strong>in</strong>g from 0.6 to1.6 g gra<strong>in</strong> kg –1 of evapotranspired water, with amean of 1.1 g gra<strong>in</strong> kg –1 (Tuong et al 2005, Zwartand Bastiaanssen 2004, Fig. 1.4A). Comparedwith wheat, the higher evaporation rates from thewater layer <strong>in</strong> <strong>rice</strong> than from the underly<strong>in</strong>g soil <strong>in</strong>wheat are apparently compensated for by the higheryields of <strong>rice</strong>. For maize, be<strong>in</strong>g a C 4crop, the waterproductivity with respect to evapotranspiration ishigher, rang<strong>in</strong>g from 1.1 to 2.7 g gra<strong>in</strong> kg –1 water,with a mean of 1.8 g gra<strong>in</strong> kg –1 . <strong>Water</strong> productivityof <strong>rice</strong> with respect to total water <strong>in</strong>put (irrigationplus ra<strong>in</strong>fall) ranges from 0.2 to 1.2 g gra<strong>in</strong> kg –1water, with 0.4 as the average value, which is abouthalf that of wheat (Tuong et al 2005, Fig. 1.4B).Compar<strong>in</strong>g WP among seasons and locationscan be mislead<strong>in</strong>g because of differences <strong>in</strong> climaticyield potential, evaporative demands from
- 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 14 and 15: Distribution (%)1009080706050403020
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- Page 25 and 26: Table 3.1b. Yield, water input, and
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- Page 36 and 37: Table 3.8. Comparison of water use
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- Page 41 and 42: consumption (Hamilton 2003). Many t
- Page 44 and 45: Irrigation systems55.1 Water flows
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- Page 48 and 49: Table 5.1. Area, water use, and tot
- Page 50 and 51: Appendix: InstrumentationDetailed d
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- Page 57 and 58: Lampayan RM, Bouman BAM, De Dios JL
- Page 59: Uphoff N. 2007. Agroecological alte