10:45 AM - 12:15 PM
[ACG42-P03] 100 years of water and sediment observational research at the Ecohydrology Research Institute, The University of Tokyo
Keywords:experimental catchment, gauging weir, water and sediment transport, 100's anniversary, denuded mountain
Ecohydrology Research Institute, Graduate School of Agricultural and Life Sciences, The University of Tokyo celebrated 100 years since its establishment in 2022. About 100 years ago, Professor Kitaro Moroto of Tokyo Imperial University established experimental watersheds to measure water discharge in Seto City, Aichi Prefecture, where mountains had been devastated by excessive use of wood resources. In this presentation, we will review the results of research on water and sediment that has been conducted since its establishment and introduce what has been learned through nearly 100 years of continuous observation.
Major Research Outcome
The major achievement is the establishment of a highly accurate discharge observation method and accumulation of data in small mountain watersheds with steep topography and humid climatic conditions. Because of the large changes in flow rate, a gauging weir with a characteristic structure of multiple rectangular notches was devised and installed, and data has been obtained with almost the same method for nearly 100 years. The results of the continuous and persistent work to remove the sediment that accumulated during heavy rainfall in the pools and to remove fallen leaves resulted in the observation data with few missing data points. Accumulated observation data revealed an increase in evapotranspiration and a decrease in direct runoff with forest recovery. Detailed observations have yielded knowledge about the mechanism of water movement in hillslopes, such as the dominance of preferential subsurface flow during rainfall, forming flood flows, which is now common knowledge in the field of hillslope hydrology but was not known worldwide at the time.
What we have learned from nearly 100 years of continuous observation
It was found that continuous observation in one watershed for nearly 50 years over 20 to 30 years is necessary to reveal changes in watershed flow (e.g., water balance and flood runoff) associated with forest restoration. This was probably due to the fact that the effect of changes in precipitation and other conditions were large, while the effect of changes in vegetation were not so large. In other words, after more than 50 years of observation, the various effects of forest vegetation on runoff from the watershed could be extracted. On the other hand, records of sediment dredging at the weir pond had been conducted to maintain the function of the weir since the establishment of the experimental watershed showed that the amount of soil erosion decreased significantly as soon as the forest recovered on the devastated hill, followed by a significant decrease in sediment discharge from the watershed. The sediment transport pattern on the devastated hillslopes had been “surface flow and surface erosion type”. However, after 100 years and the forest recovered and the soil developed, it was found that the sediment transport pattern on the devastated hillslopes changed to "subsurface flow and collapse type”.
Forest vegetation recover in about 20 years after hillside works and forest succession takes decades to hundreds of years. While the development of the topsoil layer takes tens to hundreds of years. In addition, local communities have changed dramatically over the past 100 years. The dynamics of water and sediment movement in the watershed change over time due to the combination of such events of different time scales and it should be reflected to long term data. In the future, the data will be more important for assessing and predicting the effects of climate change on water resources, occurring on time scales of several years to several decades.
Major Research Outcome
The major achievement is the establishment of a highly accurate discharge observation method and accumulation of data in small mountain watersheds with steep topography and humid climatic conditions. Because of the large changes in flow rate, a gauging weir with a characteristic structure of multiple rectangular notches was devised and installed, and data has been obtained with almost the same method for nearly 100 years. The results of the continuous and persistent work to remove the sediment that accumulated during heavy rainfall in the pools and to remove fallen leaves resulted in the observation data with few missing data points. Accumulated observation data revealed an increase in evapotranspiration and a decrease in direct runoff with forest recovery. Detailed observations have yielded knowledge about the mechanism of water movement in hillslopes, such as the dominance of preferential subsurface flow during rainfall, forming flood flows, which is now common knowledge in the field of hillslope hydrology but was not known worldwide at the time.
What we have learned from nearly 100 years of continuous observation
It was found that continuous observation in one watershed for nearly 50 years over 20 to 30 years is necessary to reveal changes in watershed flow (e.g., water balance and flood runoff) associated with forest restoration. This was probably due to the fact that the effect of changes in precipitation and other conditions were large, while the effect of changes in vegetation were not so large. In other words, after more than 50 years of observation, the various effects of forest vegetation on runoff from the watershed could be extracted. On the other hand, records of sediment dredging at the weir pond had been conducted to maintain the function of the weir since the establishment of the experimental watershed showed that the amount of soil erosion decreased significantly as soon as the forest recovered on the devastated hill, followed by a significant decrease in sediment discharge from the watershed. The sediment transport pattern on the devastated hillslopes had been “surface flow and surface erosion type”. However, after 100 years and the forest recovered and the soil developed, it was found that the sediment transport pattern on the devastated hillslopes changed to "subsurface flow and collapse type”.
Forest vegetation recover in about 20 years after hillside works and forest succession takes decades to hundreds of years. While the development of the topsoil layer takes tens to hundreds of years. In addition, local communities have changed dramatically over the past 100 years. The dynamics of water and sediment movement in the watershed change over time due to the combination of such events of different time scales and it should be reflected to long term data. In the future, the data will be more important for assessing and predicting the effects of climate change on water resources, occurring on time scales of several years to several decades.