Katumi Musiake

A mathematical model for flood loss estimation

Abstract This paper introduces an integrated model for flood loss estimation in a river basin. The model is the combination of a physically based distributed hydrologic model and a distributed flood loss estimation model. The hydrologic model considers major processes of the water cycle through physically based governing equations, which are solved to simulate the propagation of water in each of these processes. It is designed to consider the man-made flood control structures, such as river embankments, retarding basins, etc. which affect flooding characteristics. The loss estimation model is formulated based on stage-damage relationships between different flood inundation parameters and landuse features. It calculates the economic loss to different landuse features based on the simulated flood parameters obtained from the hydrologic model for any flood event. A case study illustrates the real world application of the integrated model to a medium size river basin in Japan, which is frequently affected by floods. The simulated river discharge and surface inundation by the flood model show good agreement with the observations. Urban flood loss simulated by the loss estimation model with the simulated flood parameters agrees with the estimated damage using post flood surveyed parameters.

Global assessment of current water resources using total runoff integrating pathways

Abstract The anticipated water scarcity in the first half of this century is one of the international issues of most concern, which needs to be adequately addressed. However, even though the issue has an international impact and worldwide monitoring is critical, there are limited global estimates at present. In this study, annual water availability has been derived from annual runoff estimated by land surface models using total runoff integrating pathways (TRIP) with 0.5° by 0.5° longitude/latitude resolution globally. The global distribution of abstraction was estimated for each sector at the same spatial resolution based on country-based statistics of municipal water use, industrial water use, and agricultural intakes, using a global geographical information system with global distribution of population and irrigated crop land area. The total population under water stress estimated for 1995 corresponded very well with earlier estimates. However, the number is highly dependent on how one assumes the volume of water from upstream of a region, which can be considered as “available” water resources within the region. Therefore it is important, even for global scale analysis, to evaluate the regional water quality deterioration and the real consumption of water resources in the upper part of the stream, as well as the accessibility of water. Further studies should be promoted by an integrated approach to improve the accuracy of future projections on both the natural and social aspects of water resources.

Seasonal Change of the Diurnal Cycle of Precipitation over Japan and Malaysia

Abstract The diurnal cycle of precipitation is investigated using ground-based hourly observations for more than 10 years both in Japan and Malaysia. The diurnal cycle of precipitation in Japan is classified into three clusters. The first one has a peak in the morning, and the stations categorized into this cluster are located in coastal regions. The second cluster has two peaks in the morning and in the evening. These stations are located in an inland region. The morning peak in the above two clusters is dominant in June, when it is “baiu” in Japan. Baiu is the rainy season related to the southwest Asian monsoon. The third cluster is an exceptional case. No morning peak is observed in the stations of the third cluster and they have a comparatively strong evening peak. In the case of the Malay Peninsula, the inland region has a pronounced peak of rainfall at 1600 LST; the magnitude exceeds the mean of each month by 200%. This evening peak is too sharp to be represented by a 24-h-cycle sine wave decomposed...

Impact of Deforestation on Regional Precipitation over the Indochina Peninsula

It is now widely recognized that tropical deforestation can change the regional climate significantly. The increasing population and the spreading deforestation in the Indochina Peninsula, especially in Thailand, make it urgent to assess the effects of deforestation on the regional climate. Most of the previous numerical experiments generally have shown that decreases in precipitation occur as a result of deforestation. However, in most cases, these hydrometeorological changes have not been detected in observations. In this study, the nonparametric Mann‐Kendall rank test and linear regression analysis were applied to analyze precipitation data obtained over a period of more than 40 yr, for each month, at each meteorological station in Thailand. Significant decreases in precipitation over Thailand were detected only in the time series of monthly precipitation in September. Amounts of precipitation recorded at many meteorological stations in September have decreased by approximately 100 mm month21 (approximately 30% relative change) over the past three or four decades. Numerical experiments with a regional climate model based on the Regional Atmospheric Modeling System with a simple land surface scheme were carried out for the Indochina Peninsula. In these experiments, the type of vegetation in the northeastern part of Thailand was specified as either short vegetation (the current vegetation type) or forest (the former vegetation type). The experiments were carried out using the initial and boundary meteorological conditions of August and September in 1992‐94. The initial and boundary conditions were interpolated from the data of the National Centers for Environmental Prediction‐National Center for Atmospheric Research reanalysis. In these numerical experiments, a decrease in precipitation over the deforested area was obtained for September, but not for August. The magnitude of the mean decrease in precipitation over the whole deforested area in these experiments was 26 mm month21 (7% relative change), and the local maximum decrease was 88 mm month21 (29% relative change). Precipitation in the wet season over the Indochina Peninsula basically occurs under the influence of the Southeast Asian summer monsoon system. The strong summer monsoon westerlies bring abundant moisture to the Indochina Peninsula as a source of precipitation. The monsoon westerlies are the predominant external force influencing the regional climate. However, the strong westerlies over the Indochina Peninsula disappear in September, although that is typically the month of maximum precipitation. Accordingly, it is inferred that the effect of local deforestation appears significantly only in September because of the absence of this strong external force.

A hillslope-based hydrological model using catchment area and width functions

Abstract A flow-interval hillslope discretization scheme is proposed for catchment hydrological modelling. By this scheme, a two-dimensional catchment is simplified into a one-dimensional cascade of flow intervals linked by the main stream. Each flow interval comprises a set of parallel hillslopes. The hillslope is the fundamental computational unit in the hydrological model providing lateral inflow to the main stream. The size of hillslope is determined by the catchment area and width functions. Catchment runoff is the total of hillslope responses through the river routing. Tests in four Japanese catchments showed that the model performed well on simulating the overall water balance, general flow pattern, and daily and hourly hydrographs of a whole catchment, as well as simultaneous simulation in different subcatchments. Characteristics of catchment hydrological responses and model applicability are discussed.

Comparison of different distributed hydrological models for characterization of catchment spatial variability.

Most physically-based hydrological models rely on either spatially distributed or lumped characterizations of topography and other spatial variations. Three disturbed hydrological models with different treatments of topography, namely the MIKE SHE, TOPMODEL and GB model (geomorphology-based hydrological model) are discussed in this paper. A regular square grid system is used by MIKE SHE for representation of catchment spatial variability and as the fundamental computation units. TOPMODEL characterizes the topography using a distributed topographic index, ln(a/tan β). The areas with the same ln(a/tan β) are assumed to be hydrologically similar. Then, the catchment is divided into a number of segments corresponding to the topographic index intervals. The GB model employs the catchment area function and width function to lump the topography and divide the catchment into a series of flow interval-hillslopes. The catchment spatial variations are averaged over each flow interval and represented by one-dimensional distribution functions with respect to the flow distance from the catchment outlet. The hillslope elements are the fundamental computational units in the GB model. The above three models are applied to the Seki River in Japan. The model structure and performance are compared and discussed in the paper. Copyright

Flood inundation simulation in a river basin using a physically based distributed hydrologic model.

A physically based distributed hydrologic model is developed in this study for flood inundation simulation combining newly developed overland and channel network flow simulation models with evapotranspiration, unsaturated zone and saturated zone models. The overland flow and river flow models are validated individually with test data, and then coupled with other models. The model can take fine resolution spatial data as input preserving spatial heterogeneity of physical characteristics of a river basin. River embankments play an important role in flood prevention. The model can incorporate river embankment data in flood inundation simulation. The model is applied in a river catchment in Japan to simulate a flood event in 1996. Outputs from the model show good agreements with observed flood hydrographs and surveyed flood inundation. Copyright

Spatial rainfall distribution at a storm event in mountainous regions, estimated by orography and wind direction

In this paper the orographical effect for rainfall distribution at a storm event is discussed. The main effect is represented by the ground-forced ascending current of air. A three-dimensional numerical model of the hydrostatic atmosphere, including basin orography, is used to estimate the ascending current area for any wind direction. The authors also propose a new concept named directional probability of storm event (DiPoSE). A severe storm is more likely to occur when the wind blows from a high DiPoSE direction rather than from a low DiPoSE direction. The wind direction at a storm event is determined from a DiPoSE analysis and is used in the numerical model. The calculated ascending air region is regarded as a severe rainfall area in mountainous river basins. The results are examined and compared with observations.

River network solution for a distributed hydrological model and applications.

A simultaneous solution for one-dimensional unsteady flow routing for a network of rivers has been developed, which can be used either with a complete distributed hydrological model, a simple rainfall-runoff model or as a stand alone river routing model. Either dynamic or kinematic solution schemes can be selected to simulate the river flows. The river network is either generated from the Digital Elevation Model (DEM) or directly input to the model. The model can handle any number of upstream channels and computational points. A sparse matrix solution algorithm is used to solve the 2N×2N matrix resulting from N nodes in the network. A submodule generates the initial water depth and discharge at each computational point from equilibrium discharge in the absence of observed initial conditions. The model is applied in three sub-catchments of the Chao Phraya river basin, Thailand, considering three different conditions. The simulated results show good agreement with observed discharges and provide insight to water level fluctuations, especially where tributaries join the main channel. Copyright

The effects of heterogeneity within an area on areally averaged evaporation

Unresolved heterogeneity in a numerical simulation has a significant impact on the hydrological land surface processes, and the areally averaged quantities do not give the actual values of different phenomena. In the present study, the equation for computing the areally averaged evaporation taking account of the two hierarchical distributions of the physical quantities, namely subarea and intrasubarea scale distributions, and its components are explained based on the equation. Using the derived equation, numerical experiments to identify the effect of the intrasubarea scale distribution of the temperature and soil moisture content at soil surface are conducted. The estimated evaporation by the mean parameter method can be in the opposite direction because of a slight underestimation of the area representative saturated specific humidity. The effect of the soil moisture distribution is significantly larger than that of the temperature distribution. Normal and uniform distributions with the same mean value and variance of the soil moisture have different effects on the area representative evaporation and the difference between the two reaches some 10%.