A Case Study from System H, Sri Lanka

42 Results &DiscussionTable 4. Total rainfall received, water duty, and field water use efficiency dIhg maha 1984/85.1985/86 and 1 9W7inwareaSeasonRainfaWater dutyField water(Lglham)use. effiiency@u/ac.fi)1984/85'(feet) 1.9(cm) 59.04.0121.03890 231985/86'19WT(feet) 2.3(cm) 69.0(feet) 1.1(cm) 33.04.2128.03890 23I 1116.0 3.8 456621'Initialappmachbcoordinatcd approachSourcw. Flow Monitoning Unit - System H: Statistical Unit - Mahaweli Economic Agency.The equation used to calculate Actual Equivalent Water Duty was:Actual Equivalent Water Duty =Total water suppliedTotal area cultivated X [(%RBE x 1.75) + RDLHG]Table5 showsthecomparisonbetweentheActualEquivalentWaterDutyandrainfallforthethreeblocksduringthrecmha seasons. Within a season, the. variation among blocks was presented as the range of given values. The interblwkvariation was minimal for maha 1986/87, when the coordinated approach was used, showing equimbfity in water supply(rainfall plus water duty).WATER DUTY AND RICE YIELD DURING YALA (DRY SEASON)Rice yield, water duty and field water use efficiency during the past few years are presented in Table 6. Water useefficiency during yula was about half of that in maha. This low water use efficiency in yulu was due to the. low rice yieldassociated with that season.To achieve a yula rice yield comparable with that in mha, the water duty supplied during yda should be higher thanin maha. This higher water demand may be associated with a low groundwater table and high evapotransphtion. It hasbeen recommended that there be a diversification during yalu towards crops that have low percolation requiremenubecause lowland rice dtivatioa reqh flood inigation and ifs wdcr quirement Wore includes born ev- ' 'nand percolation. It is not possible to analyze this Situation further using the available data bcuruse the water dutypresented includes rice and other field crops together.