Hiroshi Ishidaira

Monotonic trend and step changes in Japanese precipitation

For the purpose of detecting the possible long-term trends of Japanese precipitation, both parametric t-test and nonparametric Mann–Kendall and Mann–Whitney techniques are applied to the spatially averaged precipitation time series over Japan. The results indicate that although several step changes occurred in Japanese precipitation, the time series did not exhibit significant evidence of monotonic trend during the past century. When a step change is present, the number of observations required for detecting the trend of a given magnitude at a specified significance and power level is investigated with the power function of the t-test. Results indicate that if the magnitude of the step change reaches one or two times of its standard deviation, the previous 50-year of record together with 5 years or more of new data will be available for detecting the possible trend. This conclusion may be helpful for the detection of step changes in the regions where the precipitation has near-normal distributions.

Introduction of block-wise use of TOPMODEL and Muskingum-Cunge method for the hydro-environmental simulation of a large ungauged basin

Abstract For the sustainable management of water quantity and quality, a hydrological model that can simulate the hydro-environmental dynamics of river basins at arbitrary locations is valuable. There are several streamflow simulation models suitable for such a purpose. Yet the lack of data commonly poses a serious problem for their application. This is a serious problem encountered for the sustainable management of water resources at a basin scale. This paper describes a combination of TOPMODEL with the Muskingum-Cunge flow routing method to overcome the problem at least partially. The resultant block-wise version of TOPMODEL is used to simulate daily flows in the data-rich 3570 km2 Fuji-kawa basin, central Honshu, Japan and in the data-poor 20 750 km2 northern part of Minjiang basin above Dujiangyan, Sichuan Province, China. The results demonstrate the potential use of the model for hydro-environmental simulation in a large data-poor basin.

Development and application of a new algorithm for automated pit removal for grid DEMs

Abstract Pits and flat surfaces in raster digital elevation models (DEMs) have been unavoidable obstacles in the extraction of drainage networks. A new automatic method of “pits filling” is presented that eliminates all pits and flat areas simultaneously. Spatially distributed elevation increments of pits are computed. Based on this new approach, drainage networks of more than 10 medium- to large-sized basins have been extracted, including the Mekong River and the Yangtze River. Overlaying the computed drainage networks onto the digitized actual stream networks, good visual consistency was obtained. Besides, the calculated flow pattern over a flat area has been shown to be convergent. The results indicate that the proposed algorithm is easy to use and applicable to DEMs with a wide range of terrain relief and resolution.