Yasuto Tachikawa

Real-time stage and discharge estimation by a stochastic-dynamic flood routing model.

A dynamic-stochastic model for flood forecasting based on the Kalman filtering theory is described. To reduce the computational time that the Kalman filter involves, this model is performed using the reduced rank square root algorithm, which approximates the covariance matrix by a matrix of lower rank. By using the special banded structure of the Jacobian matrix of the discretization of the Saint Venant equations by the four points Preissman scheme, this algorithm is made computationally efficient for a flood routing model. The noise terms are treated as coloured noise processes, with one term for every grid element. An application to estimate the state of a one-channel reach is described. Copyright

Ensemble Kalman Filtering and Particle Filtering in a Lag-Time Window for Short-Term Streamflow Forecasting with a Distributed Hydrologic Model

AbstractThe performance of the ensemble Kalman filter (EnKF) and the particle filter (PF) is assessed for short-term streamflow forecasting with a distributed hydrologic model, namely, the water and energy transfer processes (WEP) model. To mitigate the drawbacks of conventional filters, the ensemble square root filter (EnSRF) and the regularized particle filter (RPF) are implemented. For both the EnSRF and the RPF, sequential data assimilation is performed within a lag-time window to consider the response times of internal hydrologic processes. The proposed methods are applied to two catchments in Japan and Korea to assess their performance. The model ensembles are perturbed by the noise of the soil moisture content and are assimilated with streamflow observations. The forecasting accuracy of both the EnSRF and the RPF is improved when sufficient lag-time windows are provided. The EnSRF is sensitive to the length of the lag-time window and has a limited ability to forecast within short lead times, wherea...

Over 5,000 Years of Ensemble Future Climate Simulations by 60-km Global and 20-km Regional Atmospheric Models

AbstractAn unprecedentedly large ensemble of climate simulations with a 60-km atmospheric general circulation model and dynamical downscaling with a 20-km regional climate model has been performed to obtain probabilistic future projections of low-frequency local-scale events. The climate of the latter half of the twentieth century, the climate 4 K warmer than the preindustrial climate, and the climate of the latter half of the twentieth century without historical trends associated with the anthropogenic effect are each simulated for more than 5,000 years. From large ensemble simulations, probabilistic future changes in extreme events are available directly without using any statistical models. The atmospheric models are highly skillful in representing localized extreme events, such as heavy precipitation and tropical cyclones. Moreover, mean climate changes in the models are consistent with those in phase 5 of the Coupled Model Intercomparison Project (CMIP5) ensembles. Therefore, the results enable the a...

Interaction between Topographic and Process Parameters due to the Spatial Resolution of DEMs in Distributed Rainfall-Runoff Modeling

Selecting an appropriate digital elevation model (DEM) resolution is an essential part of distributed rainfall-runoff modeling since the resolution affects parameter values and, in turn, leads to predictive uncertainty. Moreover, the DEM resolution directly determines the computational workload required for model simulation. This study conducted several experiments to clarify the interaction between topographic and process parameters due to the spatial resolution of DEMs in distributed rainfall-runoff modeling. First, five different spatial resolutions (from 50 m to 1 km) were used to analyze the effects of DEM resolution on the topographic and process parameters of a distributed rainfall-runoff model [kinematic wave method for subsurface and surface runoff (KWMSS)]. Second, parameter compatibility was tested with regard to the sensitivity of model performance to optimal parameter values for each DEM, by applying the best-performing parameter combinations for each resolution to the models based on differi...

3D simulation of soft geo‐objects

In terms of geometry, existing approaches for 3D GIS modelling can be classified into three types: surface‐based, volume‐based, and hybrids. These approaches are suitable for representing rigid geo‐objects, but so far there are no ideal GIS modelling methods for simulation of soft geo‐objects such as flowing water and soil. In this study, a GIS flow element (FE) and GIS soft voxel (SV) were used for simulation of overland flow and soil erosion. The length of a GIS FE is directly proportional to the velocity of overland flow, while the inclination of a GIS FE represents the direction of overland flow. The GIS SV is generated by implicit functions, which can be utilized to create a smoothed surface. This study reveals that GIS FEs can realistically represent the dynamics and stochastics of soft geo‐objects, while GIS SVs can simulate deformation of soft geo‐objects. Moreover, both of these approaches can reflect geoscientific laws because the animation of 3D scenarios is controlled by geoscientific models.

Integrating the NRCS Runoff Curve Number in Delineation of Hydrologic Homogeneous Regions

Cluster analysis, the generally used technique in delineation of hydrologic homogeneous regions for regional flood frequency analysis, requires standardization of flood governing attributes, and hence makes use of large subjective consideration. A new technique is developed for the delineation with no need of standardizing the attributes. The technique uses Natural Resources Conservation Services runoff curve number as a starting point in proposing the hydrologic regions. Five numbers of hydrologic regions were proposed inside the Nepalese territory by superimposing monsoon rainfall map over the runoff curve number map. The L -moment based regional hydrologic homogeneity test led finalization of hydrologic regions with minor adjustments. Heterogeneity measure value, which is a regional homogeneity test parameter representing dispersion in sample moment coefficients among the basins, was found within or near acceptable limit in each of the region. The use of the regionalization for estimating extreme flood...

A FLOOD RISK CURVE DEVELOPMENT USING CONDITIONAL PROBABILITY DISTRIBUTION OF RAINFALL ON DURATION

To design integrated flood risk management, flood risk assessment based on the economic damage is essential. In previous studies, the authors have developed a method to estimate a flood risk curve considering spatio-temporal distribution of rainfall by applying the Synthesis Probability Method (SPM) that estimates probability function of annual maximum flood peak discharge (AMF). Recently, the SPM was improved by introducing the relation between total rainfall and the duration time, and the estimated probability distribution of AMF with the improved SPM showed a better agreement with the probability distribution of observed AMF. This study extended the improved SPM to flood risk curve development and estimated a flood risk curve of the Ayabe city in the Yura-gawa River basin.

COST-BENEFIT ANALYSIS OF LANDUSE REGULATION STRATEGY BASED ON FLOOD RISK ASSESSMENT USING A LOCATION EQUILIBRIUM MODEL INCORPORATING HOUSEHOLD INCOME DISTRIBUTION

流域管理的治水対策の費用便益評価を行う枠組みの中で，世帯の所得分布を考慮できるように立地均衡モデルを拡張し，これを用いて寝屋川流域における土地利用規制の費用便益評価を行うとともに，土地利用規制が世帯に与える影響を所得の違いに応じて分析した．その結果，土地利用規制の影響は各所得層で異なることが明らかになった．所得に対する費用の割合は所得が低いほど大きく，低所得層の負担が大きくなることがわかった．また，再現期間で5年（時間45mm程度）相当の弱いレベルの土地利用規制で総便益が最大となり，再現期間で25年以上の強い土地利用規制を実施した場合は，費用が便益を上回る結果となった．以上より，本流域では，比較的高い頻度で浸水する地区の住宅地としての利用は避けた方がよい可能性のあることが明らかとなった．

COST-BENEFIT ANALYSIS OF BUILDING REGULATION BASED ON FLOOD RISK ASSESSMENT AND ITS COMPARISON TO LAND USE REGULATION

本研究では，大阪地域と東京地域を対象として，水災害危険度に基づいて建築規制を実施した場合に生じる費用と便益を比較することで，建築規制の利害得失や適用性について検討した．その結果，いずれの地域においても，今回検討した範囲では建築規制の総便益は正となり，建築規制は水防災対策として一定程度の適用性を有していること，大阪地域のほうが東京地域より建築規制の総便益が大きくなり，比較的少ない負担で効果的に水災害被害額を減少させうることが明らかとなった．さらに，同じ地区を規制対象とする建築規制と土地利用規制を比較したところ，建築規制のほうが土地利用規制より総便益が若干大きいか同程度となることがわかった．このことから，建築規制は床下浸水を許容する土地利用規制とほぼ同程度の適用性を有していると考えられる．

INFLUENCE OF TRIBUTARY WATER QUALITY CHARACTERISTICS AND COLD WATER COUNTERMEASURE ON WATER QUALITY IN THE OKUTAMA RESERVOIR

Change in the plankton and the increased phytoplankton confirmed in the Okutama reservoir indicate the progress of eutrophication. This paper discusses the factors deteriorating the water quality of the reservoir and aggravating eutrophication. The principal component analysis is used for the factorial experiment of the eutrophication. The results suggest that four principal components can explain the factors and influenced water quality. In addition, This paper examined correlations between the principal component scores for 24 years and countermeasures for the water quality preservation and against the cold water. Finally, it can be concluded that the factors that influenced the water quality of the reservoir most were the river water temperature rise and the cold water countermeasure.