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TRANSBOUNDARY WATER MANAGEMENTAnnual change of groundwater level in the observation well at Kikuyo3050004000Groundwater level altitude (m)201030002000Annual rainfall (mm)100001982 1988 1994 2000 2006Annual rainfall Monthly average level Trend0Source: T. Tanakastores major groundwater resources in the Kumamoto region andis developed for water resources as a huge groundwater reservoirwhich has relatively high local precipitation of around 2,200 mmper year and highly permeable pyroclastic deposits.In a geologically unique area called the groundwater pool, a lacustrinedeposit layer separating the two aquifers allows rainwater andirrigation water to recharge directly into the No. 2 aquifer system. 1The groundwater recharged in this area flows toward to the southwest,flowing out into the lake of Ezu and many other locations inKumamoto City. The groundwater supplies 100 per cent of the waterfor the 670,000 residents of Kumamoto City, which is a prefecturalgovernment. In this regard, the No. 2 aquifer in the Kumamotoregion is a transboundary aquifer crossing regional administrativeboundaries. This is a typical feature in transboundary aquifer distributionobserved in many other Japanese provinces.Kumamoto City started to measure groundwater levels in the1980s through a network of observation wells. In the 25 yearsbetween 1982 and 2006, the groundwater level declined 4.4 m,an average decline of 0.18 m/year. This trend of groundwaterlevel decline is also observed in the other 12 observation wellslocated in the upland area of the region. 2 The discharge of springwater in the representative spring lake of Ezu has also diminishedby approximately 15 per cent during the last 15 years from450,000 m 3 /day to 380,000 m 3 /day. In the 1950s, it was approximately1 million m 3 /day. 3 On the other hand, total withdrawalsof groundwater in the region have been reduced, mainly due toa considerable decrease in groundwater extraction for industrialand agricultural uses. The amount of city water supplied is almostconstant, and now accounts for more than 60 per cent of totalgroundwater consumption. 4 These facts indicate that the region’sgroundwater resources have decreased due to a fallinggroundwater recharge rate in the Kumamoto region.The major reason for this decreasing groundwaterrecharge rate is considered to be land use change inthe past 30 years due to rapid urbanization.In the Kumamoto region, sources of groundwaterrecharge to the reservoir are mainly attributed to thesurrounding mountain regions, forests, grasslands andpaddy fields. Among them, the groundwater rechargerate from the paddy fields is estimated at about 46per cent annually. 5 Therefore, the most effectivemeasure to increase groundwater in the region is touse the paddy fields through collaboration with localfarmers. In this regard, the Kumamoto Prefecture andKumamoto City have created a unique funding systemto encourage artificial groundwater recharge projectsthrough abandoned paddy fields in neighbouringtowns outside Kumamoto City, for the sustainable useand management of regional groundwater resourcesand the preservation of this precious resource forfuture generations.A Conference on Utilizing Rice Paddies forGroundwater Recharge consisting of the KumamotoPrefecture, Kumamoto City, two relevant localgovernments, four land improving districts and JapanAgricultural Cooperatives (JA) was established in2004 to promote the funding system for implementingthe artificial groundwater recharge project throughthe abandoned rice paddy fields. This groundwatermanagement system is regulated by the Kumamoto[ 89 ]