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given in Table 1.1. Water losses by seepage andpercolation account for about 25–50% of all waterinputs in heavy soils with shallow groundwater tablesof 20–50-cm depth (Cabangon et al 2004, Donget al 2004), and 50–85% in coarse-textured soilswith deep groundwater tables of 1.5-m depth ormore (Sharma et al 2002, Singh 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 in rice-based irrigation systems isnot known and is expected to vary widely amongirrigation systems.Daily seepage and percolation losses from theponded water do not occur in dryland crops suchas wheat and maize. Percolation of water below theroot zone can also occur in dryland crops when theamount of water infiltrating into the soil (either afterheavy rainfall or after irrigation) is larger than thestorage capacity of the root zone, but this is not adaily water flow as in lowland rice. Also, evaporationfrom ponded water surfaces is higher than fromsoil surfaces (as in dryland crops). Therefore, it isthe relatively large water flows by seepage, percolation,and evaporation that make lowland ricefields heavy “water users.” Total seasonal waterinput to rice fields (rainfall 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 in heavy clay soils with shallowgroundwater tables (that directly supply waterfor crop transpiration) to more than 2,000 mm incoarse-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 irrigated rice in Asia. Table 1.1lists some values for water inputs and daily seepageand percolation rates for lowland rice fields inChina and the Philippines.It is useful to distinguish between the wateroutflows from rice fields that can, in principle, 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 saline groundwater (or saline surface water)from where they cannot be recovered are theynot reusable any more and they become depletionflows as well.1.4 Groundwater under rice fieldsThe role of groundwater in providing water to riceplants may be large, but it has been neglected inmost studies of the rice water balance. Recent datacollection suggests that through the (decade- toage-old) practice of continuous flooding, the largeamounts of percolating water have raised groundwatertables to very close to the surface. This isespecially true in soils with a heavy texture that arepoorly drained in the subsoil, as is the case in manytraditional irrigated rice environments.Figure 1.3 gives the groundwater table measuredunder flooded rice fields at some sites in Chinaand the Philippines. When the groundwater is lessthan 20 cm deep, it provides a “hidden” source ofwater to the rice 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 into the root zone andagain provide extra water to the crop. In mostwater balance studies, the effect of groundwateron water supply is not taken into account, and thebeneficial effect of water-saving 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 Rice water productivityWater productivity (WP) is a concept of partialproductivity and denotes the amount or value ofproduct (in our case, rice grains) over volume orvalue of water used. Discrepancies are large in reportedvalues of WP of rice (Tuong 1999). These arepartially caused by large variations in rice yields,with commonly reported values ranging from 3 to8 tons per hectare. But the discrepancies are alsocaused by different understandings of the denominator(water used) in the computation of WP. Toavoid confusion created by different interpretationsand computations of WP, it is important to clearlyspecify what kind of WP we are referring to andhow it is derived. Common definitions of WP areWP T: weight of grains over cumulative weightof water transpired.WP ET: weight of grains over cumulative weightof water evapotranspired.WP I: weight of grains over cumulative weightof water inputs by irrigation.
Groundwater depth (cm)20AGroundwater depth (cm)20B00–20–20–40–60–80–40–60–80PanicleinitiationFloweringHarvest–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 rice at Tuanlin, Hubei Province (A), and Changle (B), Beijing, China, in 2002.Adapted from Belder et al (2004) and unpublished data from China Agricultural University/IRRI. Groundwater depth underflooded rice in Dolores (C) and Gabaldon (D) in 2002, Central Luzon, Philippines. Adapted from Lampayan et al (2005).DWP IR: weight of grains over cumulative weightof water inputs by irrigation and rain.WP TOT: weight of grains over cumulative weightof all water inputs by irrigation, rain,and capillary rise.Breeders are interested in the productivity ofthe amount of transpired water (WP T), whereasfarmers and irrigation engineers/managers are interestedin optimizing the productivity of irrigationwater (WP I). To regional water resource planners,who are interested in the amount of food that canbe produced by total water resources (rainfall andirrigation water) in the region, water productivitywith respect to the total water input by irrigationand rainfall (WP IR) or to the total amount of waterthat can no longer be reused (WP ET) may be morerelevant.Modern rice 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 grain kg –1 watertranspired (Bouman and Tuong 2001, Tuong et al2005). The few available data indicate that waterproductivity with respect to evapotranspiration isalso similar to that of wheat, ranging from 0.6 to1.6 g grain kg –1 of evapotranspired water, with amean of 1.1 g grain kg –1 (Tuong et al 2005, Zwartand Bastiaanssen 2004, Fig. 1.4A). Comparedwith wheat, the higher evaporation rates from thewater layer in rice than from the underlying soil inwheat are apparently compensated for by the higheryields of rice. For maize, being a C 4crop, the waterproductivity with respect to evapotranspiration ishigher, ranging from 1.1 to 2.7 g grain kg –1 water,with a mean of 1.8 g grain kg –1 . Water productivityof rice with respect to total water input (irrigationplus rainfall) ranges from 0.2 to 1.2 g grain kg –1water, with 0.4 as the average value, which is abouthalf that of wheat (Tuong et al 2005, Fig. 1.4B).Comparing WP among seasons and locationscan be misleading because of differences in 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
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 23 and 24: Water input (mm)4003503002502001501
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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