11:30 AM - 11:45 AM
[ACG42-09] Estimating sediment discharge and its ratio to watershed volumes in the 109 first-class river watersheds in Japan
Keywords:Suspended solid, L-Q formula, Sediment management
1. Background and Objective
Sediment plays a key role in preserving natural resources, environment, and landscape, but it also has negative aspects that causes sediment disasters and environmental pollutions. Yamamoto (2014) had suggested comprehensive sediment management across the entire sediment system, rather than by individual areas in the system. Continuous observations of suspended solid (SS) in rivers are the first step for comprehensive sediment management, yet such data are scarce and limited even today.
We used the relationship between suspended sediment discharge L (SS × Q) and flow rate Q to calculate the SS discharge for each basin using discharge data, with the aim of visualizing the spatial distribution of sediment discharge from the Japanese archipelago as a fundamental sediment data. In addition, we investigated the effect of sediment discharge on the volumetric change at watershed scale.
2. Methods
We firstly collected the literatures containing the L-Q equations and graphs showing the relationship between SS discharge L and discharge Q at the lower-end points of the first-class rivers throughout Japan to obtain the L-Q equations for all the 109 watersheds. For rivers for which we could not obtain equations, we used the L'-Q' equation proposed by Takekawa and Nihei (2013). We then estimated the annual SS discharge for all the watersheds using the equations and observed discharge data.
In addition, we calculated the current watershed volumes from the watershed-mean elevation and watershed area for all the 109 watersheds, and the annual watershed volume increases from the mean uplift rates and watershed area for the watersheds by using the elevations of the electronic reference stations. Then we subtracted the annual SS discharges from the annual volume increases to obtain the net volumetric increases of the watershed and divided it by the watershed volume to obtain the ratio of the annual net volume increases to the watershed volumes.
3. Results
From our estimate of the spatial distribution of annual SS discharge based on the L-Q equation and river discharge data, we found that there are regional differences in sediment discharge. We found that there is a positive correlation between our results and those of previous studies. We also found that the SS discharge calculated by the L'-Q' equation of Takekawa and Nihei (2013) may underestimate SS discharge. From the calculation of the mean uplift of the watershed, we found that the average uplift rate is higher in the area around 40°N and in the central part of the Japanese archipelago. Our estimates of volumetric increases of the watersheds revealed that the volumetric increases are higher in the Hokkaido region, in areas around 40°N, and in the central part of Japan. Our estimates of the net volume increase of the basin per unit area are higher in the western Hokkaido, in the areas around 40°N, and in the Kinki region. Figure 1 shows the ratio of the annual net volume increase to the basin volume, which indicates that the area with the highest ratio is in the areas around 40°N.
4. Summary
We estimated the amount of SS discharge for all the 109 first-class river watersheds and analyzed the impact of sediment discharge on the increase or decrease in volume of the watersheds. As the result, we could visualize the status of SS discharges from the Japanese Archipelago and its ratio to the watershed volumes which would be useful as basic data for comprehensive sediment management in Japan. We are planning to discuss the accuracy of our estimates by comparing with available existing studies.
Sediment plays a key role in preserving natural resources, environment, and landscape, but it also has negative aspects that causes sediment disasters and environmental pollutions. Yamamoto (2014) had suggested comprehensive sediment management across the entire sediment system, rather than by individual areas in the system. Continuous observations of suspended solid (SS) in rivers are the first step for comprehensive sediment management, yet such data are scarce and limited even today.
We used the relationship between suspended sediment discharge L (SS × Q) and flow rate Q to calculate the SS discharge for each basin using discharge data, with the aim of visualizing the spatial distribution of sediment discharge from the Japanese archipelago as a fundamental sediment data. In addition, we investigated the effect of sediment discharge on the volumetric change at watershed scale.
2. Methods
We firstly collected the literatures containing the L-Q equations and graphs showing the relationship between SS discharge L and discharge Q at the lower-end points of the first-class rivers throughout Japan to obtain the L-Q equations for all the 109 watersheds. For rivers for which we could not obtain equations, we used the L'-Q' equation proposed by Takekawa and Nihei (2013). We then estimated the annual SS discharge for all the watersheds using the equations and observed discharge data.
In addition, we calculated the current watershed volumes from the watershed-mean elevation and watershed area for all the 109 watersheds, and the annual watershed volume increases from the mean uplift rates and watershed area for the watersheds by using the elevations of the electronic reference stations. Then we subtracted the annual SS discharges from the annual volume increases to obtain the net volumetric increases of the watershed and divided it by the watershed volume to obtain the ratio of the annual net volume increases to the watershed volumes.
3. Results
From our estimate of the spatial distribution of annual SS discharge based on the L-Q equation and river discharge data, we found that there are regional differences in sediment discharge. We found that there is a positive correlation between our results and those of previous studies. We also found that the SS discharge calculated by the L'-Q' equation of Takekawa and Nihei (2013) may underestimate SS discharge. From the calculation of the mean uplift of the watershed, we found that the average uplift rate is higher in the area around 40°N and in the central part of the Japanese archipelago. Our estimates of volumetric increases of the watersheds revealed that the volumetric increases are higher in the Hokkaido region, in areas around 40°N, and in the central part of Japan. Our estimates of the net volume increase of the basin per unit area are higher in the western Hokkaido, in the areas around 40°N, and in the Kinki region. Figure 1 shows the ratio of the annual net volume increase to the basin volume, which indicates that the area with the highest ratio is in the areas around 40°N.
4. Summary
We estimated the amount of SS discharge for all the 109 first-class river watersheds and analyzed the impact of sediment discharge on the increase or decrease in volume of the watersheds. As the result, we could visualize the status of SS discharges from the Japanese Archipelago and its ratio to the watershed volumes which would be useful as basic data for comprehensive sediment management in Japan. We are planning to discuss the accuracy of our estimates by comparing with available existing studies.