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

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