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

More documents

Recommendations

Info

Irrigation systems55.1 Water flows in irrigation systemsIrrigated rice fields are characterized by large volumesof outflows by surface drainage, seepage, andpercolation (Chapter 1.3). Although these outflowsare losses from an individual field, there is greatscope for reuse of these flows within a landscapethat consists of many interconnected fields (Fig.5.1). Surface drainage and seepage water usuallyflow into downstream fields and the loss of onefield is the gain of another. At the bottom of atoposequence, these flows enter drains or ditches.However, farmers can use small pumps to lift waterfrom drains to irrigate fields that are inadequately,or not, serviced by irrigation canals. In many irrigationsystems in low-lying deltas or flood plainswith impeded drainage, the continuous percolationof water (from fields, but also from canals) has createdshallow groundwater tables close to the surfacethat may directly provide the rice crop with water(Chapter 1.4). Again, farmers can either directlypump water up from the shallow groundwater orpump groundwater when it becomes surface wateras it flows into creeks or drains.Irrigation channelSDDDDrainage channelPPPSPSPSGroundwaterinflowPumpPumpGroundwateroutflowFig. 5.1. Surface and subsurface water flows across a toposequence of rice fields. D = drainage (overbund flow), I = irrigation,P = percolation, S = seepage.39
Water reuse (10 6 m 3 )100908070605040302010005,000 10,000 15,000 20,000Area (ha)y = 0.0046x + 0.8885R 2 = 0.9049y = 0.0013x + 1.1412R 2 = 0.9174Fig. 5.2. Volume of reuse of surface water by check dams( ) and by groundwater pumping ( ) versus spatial scale inDistrict I of UPRIIS. The lines are linear regressions. Datafrom Hafeez (2003).Water productivity (g grain kg –1 water)0.200.180.160.140.120.100.080.060.040.020.000y = (7 × 10 –6 )x + 0.0561R 2 = 0.8465,000 10,000 15,000 20,000Area (ha)Fig. 5.3. Water productivity (WP IR; g rice grains kg –1 watersupplied (irrigation plus rainfall)) versus spatial scale inDistrict I of UPRIIS. The line is a linear regression. Theabsolute values of WP IRare low (compare with values inChapter 1.5) because a lot of water still flows out of the area(drainage water) but is reused by downstream irrigators. Datafrom Hafeez (2003).Recent studies of rice-based irrigation systemsin China and the Philippines indicate that manywater performance indicators (such as water productivity,fraction of applied water used by the crop)improve with increasing spatial scale because of thereuse of water (Hafeez 2003, Loeve et al 2004a,b).Much of this reuse is done informally by farmerswho take their own initiative to pump water, blockdrainage waterways, or construct small on-farm reservoirsfor secondary storage. Most of these farmersare found in tail-end portions of irrigation systemswhere water does not reach because too much wateris lost upstream (e.g., by upstream farmers takingtoo much water, by canal seepage losses, and byoperational losses). Hafeez et al (2007) reportedquantitative data on water reuse on 18,000 ha ofDistrict I of the rice-based Upper Pampanga RiverIntegrated Irrigation System (UPRIIS) in CentralLuzon, Philippines. A total of 16 check dams werefound for reuse of surface drainage water, and12% of all farmers owned a pump for groundwaterextraction. In the whole study area, 57% of all availablesurface water was reused by the check damsand 17% through pumping. The amount of waterpumped from the groundwater was about 30% ofthe groundwater recharge by percolation from ricefields. Figure 5.2 shows that the amount of waterreused by the check dams and by pumping increasedwith spatial scale (because, with increasing scale,the options for reuse increase). Because of thisincrease in water reuse with increasing scale, thewater productivity increased with spatial scale aswell (Fig. 5.3).Although water can be efficiently reused thisway, it does, however, come at a cost, especiallyto downstream farmers. The current debate on theimprovement of irrigation systems focuses on therelative benefits and costs of system modernizationvis-à-vis those of internal and (mostly informal)reuse of water. System modernization aims to improvethe irrigation system delivery infrastructureand operation scheme to supply each farmer withthe right amount of water at the right time. Gainsin water productivity are possible by providingmore reliable irrigation supplies, for example,through precision technology and the introductionof on-demand delivery of irrigation supplies (e.g.,Gleick 2000, Rosegrant 1997). The argument isthat when farmers have control over timing andamount of water supplies to their farm, they neednot take their turn in a fixed rotational scheduleof deliveries if the soil is still wet from rainfall.Matching system delivery and field-level demandneeds further research, as optimal scheduling ofirrigations is difficult when a large part of the cropwater requirement is met from rainfall. This isespecially true in large irrigation systems with aconsiderable time lag between diversion of waterat the source (river or reservoir) and its arrival atthe farmer’s gate. In some parts of China, although40
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 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 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

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?