Hydrology and Water Resources Engineering

Tomohito Yamada

Journey of Water around the Earth

Tomohito Yamada , Associate Professor

Faculty of Engineering, Graduate School of Engineering

High school : Chiba Prefecture Sakura High School

Academic background : Doctorate from the University of Tokyo

Research areas
Hydrometeorology, hydroclimatology, water resources engineering
Research keywords
global water circulation, land-atmosphere interaction, rainfall, evapotranspiration, soil water

What is your goal?

I carry out research to clarify everything regarding the role of water in natural phenomena. Of the approximately 1.4 billion km3 of water said to exist on Earth, freshwater that the human race can actually use accounts for as little as approximately 0.8%. All mankind, approximately 6 billion people, share this limited amount of water resources for their livelihood. Improving the accuracy of forecasting the amount of water resources is not only indispensable for mankind to live an enriched life but also important in mitigating water-related disasters, such as floods and drought. But how can we forecast the amount of water resources with improved accuracy? Each day, water travels to and from the atmosphere, ocean, and land through the processes of rainfall and evapotranspiration.

Fig. 1 Global Water Circulation System

Fig. 2 Conceptual Diagram of Soil Water Content and Rainfall/Evapotranspiration Processes (Land-Atmosphere Interaction)


Fig. 3 Atmospheric Temperature Forecast Accuracy From Initial Information on Soil Water. (a) and (d): Forecast Results with Initial Information on Soil Water; (b) and (e): Observation Values; (c) and (f): Forecast Results without Initial Information on Soil Water.

Fig. 4 Conceptual Diagram of the Global Climate Model (From NOAA


Fig. 5 A Group of Earth Observation Satellites (A-Train, Fig. 5(a)) and the Global Rainfall Distribution Obtained through Observations by Artificial Satellites (Fig. 5(b))

Fig. 6 Observation of Soil Water, Rainfall, and Evapotranspiration (Thailand)


This water movement on a global scale is known as global water circulation (Fig. 1). In previous studies, it was regarded as important to improve the observation accuracy of atmospheric and sea water temperatures. However, we cannot overlook the land on which we humans live either. For example, soil water content, which indicates ground humidity, increases as a result of rainfall but decreases by evapotranspiration (Fig. 2). While the amount of soil water is slight compared to that of sea water (Fig. 1), our research is making it increasingly clear that its impact on weather cannot be overlooked.

Let us look into how the initial information on soil water affects forecast accuracy regarding atmospheric temperature in reference to the summer of 1988, when severe drought hit North America. It is generally said that weather forecast accuracy decreases significantly if the forecast is made approximately 10 or more days in advance of the target day. Here, we are going to see the results for the 16-30 days following the start of forecast. Figures 3(b) and 3(e) illustrate the observation values, wherein the more warm-colored an area on the map is, the warmer the area was that year. In the forecasts made using only the initial information on atmospheric and sea water temperatures (Figs. 3(c) and 3(f)), the number of warm-colored areas are fewer than on maps based on observation values, meaning that the drought was not predicted. On the other hand, one can see that the soil water information on the forecast start day can be used to improve the drought forecast accuracy (Figs. 3(a) and 3(d)). I believe that in the future, the expansion and sophistication of observation networks around the world for data such as soil water content, amount of rainfall, and evapotranspiration will enable us clarify the global water circulation system and secure a stable amount of water resources for mankind.


How do you carry out research?

A numerical model known as the global climate model is needed to grasp weather at all locations around the globe. In carrying out research, we use this as a numerical model on a large-scale computer known as a supercomputer (Fig. 4). However, the global water circulation system cannot be clarified without observation data. Thus, as illustrated in Figure 5, using data from multiple earth observation satellites in combination with the global climate model is indispensable. On the other hand, we carry out hydrometeorological observations in the field to help clarify the water circulation system (Fig. 6).


What is your next goal?

I chase after water as it travels around the globe, and its various physical mechanisms always surprise me. Going forward, I hope to carry out comprehensive research on the entire array of topics from the microphysical process of rain formation to the water circulation system on a global scale, thereby helping solve the water resource issues facing the human race