In this paper, the spurious oscillation phenomena were mathematically analyzed that occur in the numerical simulation process of quasi-three dimensional groundwater flow model by the finite element method (FEM). It was shown that FEM causes spurious oscillation solution under the condition of small time step Δt or large element size L due to non-diagonal storage matrix. Then the criterion on the selection of Δt and L was identified to avoid the spurious oscillation. Two methods with diagonal element storage matrix—the storage-lumped finite element method (SFEM) and the triangular mesh finite difference method (TFDM) that have been used as the methods free from spurious oscillation were compared with the Theis analytical solution. The results indicate that TFDM is the best with respect to stability and convergence.

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Flood runoff analysis by the storage routing models normally requires estimation of the effective rainfall as an input, which is computed by use of runoff coefficient or filtering separations of runoff-component. The previous study proposed a storage routing model coupled with the loss mechanism, which can calculate the total hydrograph from the observed rainfall instead of using the effective rainfall. The model can accommodate a nonlinear relationship between the storage and discharge. The present study proposes a two-step storage routing model in which the first storage function regulates the surface/subsurface runoff while the second the returning flow from the infiltration through the ground. The model also regulates the evapotranspiration as a proportion derived from meteorological method. In this paper, infiltration and evapotranspiration of the watershed are separately taken into account for loss mechanisms in the rainfall-runoff process. The model developed in this study was applied to 1998's flood records and long-term runoff records for eleven years from rivers in Hokkaido, Japan. The results clearly show that the proposed model appears to provide better reproduction of the hydrograph than the single storage routing model, which was previously developed. The proposed model has the advantage of the real-time flood forecasting, because the hydrologic data are directly processed. The model is also helpful for the runoff separation estimation. Five model parameters are easily calculated by the Newton-Raphson method. The long-term runoff analysis in the study watershed showed that the ground water level is good accordance with secondary storage height calculated by the model.

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The La Plata River Basin is one of the largest international river basin in the world, with an area of about 3.1 million km2. It includes parts of five countries (Argentina, Bolivia, Brazil, Paraguay and Uruguay) and its water resources are essential for their economic development. Together with reservoir development for hydropower, extensive deforestation, intensive agriculture practices and large urban development took place in the Parana, Paraguay and Uruguay basins affecting environmental conditions and raising important issues concerning water resources use and conservation which can be illustrated by the occurrence of blue green algae bloom, excessive sedimentation, and related water quality degradations. Therefore the need to promote participatory and cooperative efforts among water resources stakeholders as well as systematic interchange of information and experiences on common regional problems among organizations and experts throughout the basin devoted to water resources use and management, was claimed by researchers and managers gathered in the I and II International Workshops on Regional Approaches for Reservoir Development and Management in the La Plata River Basin held in 1991 and 1994, respectively. As a concrete response to said need, efforts carried out by a number of organizations from various countries within the Basin, with the support of international and national governmental organizations including Japanese Governmental and Non-Governmental Organization, succeeded in the foundation of La Plata River Basin Environmental Research and Management Network (RIGA), carried out in March 2001, within the framework of the III International Workshop, which was precisely one of the short term activities included in RIGA Action Plan.

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