Browsing by Author "Ichikawa, Yutaka"

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    Comparison of Forest Canopy Interception Models Combined with Penman-Monteith Equation.
    (2002) Mulungu, Deogratias M. M.; Shiiba, Michiharu; Ichikawa, Yutaka
    Three models of interception process: Rutter model with Deardorff's power function, Deardorff model and Modified Kondo model, each in combination with Penman-Monteith equation applied to the same climatic forcing over the year of simulation were compared. The modification of the Kondo's model done in our study includes the water balance component, between storms evaporation and transpiration including the power function, storage changes and the canopy drainage. The comparison was meant for assessing the simple Modified Kondo model developed in Japan and demonstration of the importance of the power function. The Penman-Monteith equation was the kernel for determination of evaporation and transpiration rates. Its use in this study was proposed because it has been widely used in Japan and in experimental sites and therefore can be used as a basis for comparison. Since much interest was on net rainfall, the control volume for the water accounting was between top of the canopy and above the ground surface and hence does not include soil moisture and transpiration. Results showed that forest canopy evaporation ranged from 22 to 29% of gross rainfall. Much model prediction differences were observed in winter, with lower rainfall intensity where wet canopy storages or rainfall did not meet the potential (atmospheric) evaporation demand. The annual net rainfall and transpiration ranged from 71 to 78% of gross rainfall and from 727 to 733 mm respectively. The adopted power function had significant impact on transpiration rate and small impact on evaporation rate for the Modified Kondo model. The Modified Kondo model predicted fairly close to the two models and therefore can be used for providing hourly input into hydrological models. The differences in the predicted hydrological fluxes resulted from the different model for mulations especially throughfall coefficients and drainage functions.
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    A Physically Based Distributed Subsurface-Surface Flow Dynamics Model for Forested Mountainous Catchments
    (Wiley, 2005) Mulungu, Deogratias M. M.; Ichikawa, Yutaka; Shiiba, Michiharu
    This study was designed to develop a physically based hydrological model to describe the hydrological processes within forested mountainous river basins. The model describes the relationships between hydrological fluxes and catchment characteristics that are influenced by topography and land cover. Hydrological processes representative of temperate basins in steep terrain that are incorporated in the model include intercepted rainfall, evaporation, transpiration, infiltration into macropores, partitioning between preferential flow and soil matrix flow, percolation, capillary rise, surface flow (saturation-excess and return flow), subsurface flow (preferential subsurface flow and baseflow) and spatial water-table dynamics. The soil-vegetation-atmosphere transfer scheme used was the single-layer Penman-Monteith model, although a two-layer model was also provided. The catchment characteristics include topography (elevation, topographic indices), slope and contributing area, where a digital elevation model provided flow direction on the steepest gradient flow path. The hydrological fluxes and catchment characteristics are modelled based on the variable source-area concept, which defines the dynamics of the watershed response. Flow generated on land for each sub-basin is routed to the river channel by a kinematic wave model. In the river channel, the combined flows from sub-basins are routed by the Muskingum-Cunge model to the river outlet; these comprise inputs to the river downstream. The model was applied to the Hikimi river basin in Japan. Spatial decadal values of the normalized difference vegetation index and leaf area index were used for the yearly simulations. Results were satisfactory, as indicated by model efficiency criteria, and analysis showed that the rainfall input is not representative of the orographic lifting induced rainfall in the mountainous Hikimi river basin. Also, a simple representation of the effects of preferential flow within the soil matrix flow has a slight significance for soil moisture status, but is insignificant for river flow estimations