The followings introduce representative research themes of our group. In addition, studies on snow-melting processes and riverine ecosystems are launched recently and they are going to grow to be a major scientific themes in our group. We encourage future students to bring your new field for expanding our research capacity. Students from other disciplines are welcome for this reason.
We are experiencing population declines in the country side of Japan for the first time in the modern time under the declines of national budgets. As the countermeasure of this issue, we are exploring the possibility of "planned inundation" as the watershed flood control strategy for the next century. In terms of virtual experiments with an improved rainfall-runoff-inundation model, we are exploring the possibility of "planned inundation" in depopulation area in Japan. We are expecting the "planned inundation" will be an alternative and yet realistic approach of watershed planned control for our sustainable future.
For more details, please refer the following articles:
Ugajin, R., Yokoo, Y., Sayama, T.: A virtual experiment on the effectiveness of planned inundation as a watershed flood control strategy in a declining population area, Japanese Journal of Japan Society of Civil Engineers, Special Issue (Hydraulic Engineering), 81(16), 24-16010. (In Japanese with English abstract)
When and how sediments and nutrients are transported from land to sea? To answer for this question, we need direct and indirect observation and monitoring of river water, because data is the origin of hydrology as a discipline of field-based sciences. We therefore established our observation sites in and around Fukushima prefecture in Japan, so that we can regularly visit there to feel the breathing and pluses of the nature. We believe such data would be highly precious not only for the understandings of nature, but also for the reliable ground truth data for the validations of numerical models. In addition, we hope that such data can connect hydrology and ocean science as a united water science that helps to understand our mother earth.
For more details, please refer the following articles:
Data-based modeling approach of runoff processes have not been succeeded yet, although thousands of runoff models (regardless of spatially distributed or lumped model structure) have been suggested and tested by today. This scientific inability made it impossible to develop a runoff model based only on observed precipitation and discharge data. After Kirchner (2009, WRR), the situation have changed and many hydrologists started to develop and runoff model based only on observed data. With the successes, they have also succeeded to obtain the ability to explain fundamental hydrological processes such as precipitation as the input to the system, storage as the state variables of the system, and evapotranspiration and discharge as the output from the system. These advances seem insignificant for some people, we recognize that hydrologists have obtained ability to (1) simply explain hydrological process with minimum assumptions and (2) predict dominant hydrological processes of a watershed. Currently, we attempt to modify the methodology of Kirchner (2009, WRR) so that we can apply it in Asian watersheds over a mosaic of different geological setting under humid climate.
For more details, please refer the following articles:
Even in humid watersheds, mountainous recharge is crucial for fresh water resource planning and managements. From a viewpoint of national security for resources, it is necessary to grasp where the vital recharge forests are in the nation. Then how can we point such vital recharge forests? What is the characteristics of mountainous recharge areas in water-rich watersheds? This research attempts to specify such characteristics based on hydrological and physio-graphic data analysis. Our pilot study revealed typical physio-graphic watershed characteristics that enhance subsurface water recharge in mountainous areas.
For more details, please refer the following articles:
Kobayashi, S., Yokoo, Y., Kawasaki, M. (2012) Toward estimations of watershed-scale storage and storage potential in Japanese mountaious watersheds, Annual Journal of Hydraulic Engineering, JSCE, Vol.56, pp.I_475-I_480 (In Japanese with English abstract).
Yokoo, Y., Oki, T., Kawasaki, M., Sakata, K. (2011) Toward a mapping of groundwater recharge potential: Testing a doughty flow based approach, Annual Journal of Hydraulic Engineering, JSCE, Vol.55, pp.S385-S390 (In Japanese with English abstract).
A flow duration curve (FDC) is a cumulative distribution curve of time series data of daily stream flow. By sorting the daily flow in the order of it's magnitude, normally with an annual time series data, we can draw a FDC. As such, a FDC exhibits a characteristic curve for an individual watershed. Climate, soil, geology, topography, vegetation, human activity, and all the other factors would affect the shape of a FDC. Here we face a question such as what the relative roles of climate, soil, geology, topography, vegetation, and human activities for a single FDC of a watershed? Or how does they change with such watershed characteristics? For answering to these questions, we attempt data-based approach and model-based approach.
For more details, please refer the following articles:
Leong, C., Yokoo, Y. (2021) A step toward global scale applicability and transferability of flow duration curve studies: A flow duration curve review (2000-2020), Journal of Hydrology, 603, B, 126984. DOI: 10.1016/j.jhydrol.2021.126984.
Komatsu, K., Yokoo, Y. (2013) Exploring climatic characteristics and the shapes of flowduration curves in Japanese watersheds, Annual Journal of Hydraulic Engineering, JSCE, Vol.57, pp.I-481-I-486 (In Japanese with English abstract).
Yokoo, Y., Oki, T. (2009) How should we classify surface geology to best explain the shapes of flow duration curves in headwater basins?, Annual Journal of Hydraulic Engineering, JSCE, Vol.53, pp.463-468 (In Japanese with English abstract).
Yokoo, Y., Udo, K. (2007) Exploration of geographic watershed characteristics shaping flow duration curve, Annual Journal of Hydraulic Engineering, JSCE, Vol.51, pp.373-378 (In Japanese with English abstract).
We can learn watershed response to precipitation and roles of climatic and geographic characteristics of a watershed by means of analyses of observational data. This is a traditional approach for understanding the functions of watersheds with different climatic and geographic characteristics. This research theme took opposite approach to understand the functions of watersheds. Starting from assumptions of watershed responses and governing equations, we perform sensitivity analysis on the controls of climatic and geographic characteristics on watershed responses on a numerical watersheds. Taking this approach would enrich our understandings on hydrological responses of watersheds as an alternative of the traditional data based approach and would accelerate the advances of hydrological science.
For more details, please refer the following articles:
Yokoo, Y., Oki, T. (2010) Effects of climate, topography, soil, geology and land use on flow regimes in Japanese mountaious watersheds, Annual Journal of Hydraulic Engineering, JSCE, Vol.54, pp.469-474 (In Japanese with English abstract).
Yokoo, Y., Sivapalan, M. (2006) Effect of Capillary Rise on Subsurface Runoff in the REW Modeling Framework, Annual Journal of Hydraulic Engineering, JSCE, Vol.50, pp.55-59.