Effectiveness of Adaptation Options for Multi-Purpose Reservoir Operation to Climate Change: A Case Study in Japanese River Basins
Daisuke Nohara
Kyoto University
Yoshinobu Sato
Ehime University
Tetsuya Sumi
Kyoto University
Introduction
There is concern that hydrological systems would be changed due to climate change associated with global warming. This change in hydrological systems may have significant impacts on hydrological cycles in river basins. Extreme drought or flood events that are beyond the design level of infrastructures for water resources management may occur more frequently in the future. Seasonal fluctuation in river streamflow may be changed due to changes in the seasonal precipitation pattern or snowfall and snowmelt processes. These changes can degrade the effectiveness of infrastructures for water resources management, which are usually designed based on the current hydrological conditions, given the historical observational data of precipitation or river streamflow. Impacts of climate change on water resources management are therefore considered significant.
On the other hand, sedimentation has been observed in many reservoirs all over the world. Sedimentation reduces the active storage capacity of a reservoir, which results in degradation of its capability for flood control, water supply, or power generation. Reservoir sedimentation inevitably progresses with the operation of the reservoir unless countermeasures for sedimentation are properly taken. Adequate reservoir sediment management is important to improve the sustainability of reservoirs, which is also key to securing the function of reservoirs for water resources management in the future under the changing climate. Thus, the impacts of sedimentation need to be carefully considered when climate change impacts on reservoir systems are investigated.
From the viewpoint described above, various studies have been conducted to investigate the impacts of climate change on water resources. Sato et al. (2009) assessed the impacts of climate change on water-related natural hazards in the Yoshino River basin in Japan using the output of a super-high-resolution atmospheric general circulation model (AGCM). Hunukumbura and Tachikawa (2012) estimated the potential impacts of climate change on the discharge of the Chao Phraya River basin in Thailand by using a distributed flow routing model. Ranzi et al. (2017) analyzed the impacts of changes in both climate and land use on river flow in the Southern Alps region. These studies have pointed out that river discharges can change significantly due to the effect of climate change, which makes water resources management more challenging in the future.
Impacts of climate change on water resources systems such as reservoirs have also been investigated in recent years. Kim et al. (2009) analyzed the adaptability of the Yagisawa Reservoir in the Upper Tone River in Japan. Soito and Freitas (2011) investigated the impacts of climate change on reservoir management for hydropower to clarify a potential method for adaptation. Terada et al. (2015) evaluated the impacts of climate change and reservoir sedimentation on the operation of the Makio Reservoir in the Kiso River basin in Japan for water supply and hydropower generation. They found that the annual total hydropower generation can be maintained regardless of the sedimentation scenarios and flow-regime changes due to climate change while water supply operation of the reservoir would be seriously affected due to a loss of active storage volume by reservoir sedimentation and changes in the seasonal availability of river discharge. Although these studies have highlighted the possible impacts of climate change on reservoir management, assessments of climate change impacts on reservoir operation need to be conducted for each river basin because impacts can be varied according to regional characteristics such as seasonality in precipitation and river flow, or in water use. Potential adaptation measures against climate change impacts also need to be evaluated with a scientific approach to clarify effective adaptation options to climate change in water resources management through reservoirs.
Considering the circumstances described above, impacts of climate change on the operation of reservoir systems for water use were investigated in this study to provide information on possible changes in drought risks from the present to the future to identify potential adaptation measures. First, practices and current issues in reservoir management in Japan are discussed to provide a better understanding of the potential impacts of climate change on Japanese reservoir systems. Assessments are then carried out for two Japanese river basins, the Yoshino River basin and the Mogami River basin, which have different hydrological characteristics from each other, using the output of a high-resolution AGCM. Impacts of climate change on river flow and reservoir operation are investigated through hydrological simulation coupled with a model of reservoir system’s operation. The effectiveness of possible adaptation options for drought management is also analyzed for the Yoshino River basin.
Background
Reservoir Management Practices in Japan
The Japanese archipelago is located off the eastern coast of the Eurasian continent and surrounded by the Pacific and the Japan Sea. Due to the hilly topographical condition with high mountains surrounded by sea, the islands receive an abundance of precipitation in most regions (average annual precipitation: 1700 mm). The archipelago is affected by the East Asia Monsoon. This brings a warm rainy summer and cold dry winter to the Pacific side of the islands. However, due to the effect of the northwest wind and warm ocean current in the Japan Sea, the islands on the Japan Sea side receive a lot of snowfall in winter as well.
Although the amount of surface water supply such as rainfall or snowfall is not small in most regions in Japan, the available water resource is much less than that received in river basins. River waters flow out to the sea very quickly because rivers are steep due to the hilly geographical condition. Temporal variation in rainfall is large, which makes it difficult to utilize river water as water resources in a stable manner. Therefore, many reservoirs have been constructed to stabilize surface water use by regulating river flow for water use as well as flood control. Multi-purpose reservoirs, which are constructed and operated for multiple purposes such as flood control, water supply, power generation, or environmental flow, play an important role in Japan where both floods and droughts occur frequently due to a great variation in precipitation.
A typical example of the allocation of the storage capacity of a multi-purpose reservoir is shown in Figure 6.1. The storage capacity of a reservoir is largely divided into three capacities, namely, flood control capacity for flood management, water use capacity for water supply or power generation, and sediment storage capacity for the countermeasure against sedimentation. Storage capacities for water supply and power generation are also divided in some multi-purpose reservoirs. Such an allocation of storage capacity is determined during the planning process before dam construction considering the needs for those functions in that river basin and cost allocation.
Figure 6.1 Example of allocation of the storage capacity of multi-purpose reservoirs.
The storage capacity for each objective is managed independently in principle, which means that water cannot be stored in the flood control capacity except when the reservoir carries out flood control operation. There is more chance of floods in river basins in Japan from early summer through the fall, which is typically from June to October, due to the effect of typhoons from the Pacific and frontal systems developed by the East Asia Monsoon. The flood control capacity of many multi-purpose reservoirs in Japan is therefore enlarged in that period by reducing the capacity for water use to have more capability for flood control. Although this is very effective for reducing flood risks in the river basins, this may also make drought management in summer more challenging because water must be supplied to the downstream from the reduced water use capacity in summer where water demand for irrigation is high.
Current issues: floods, droughts, and sedimentation
Although multi-purpose reservoirs enhance the level of water resources management in the river basins, various water-related disasters have still occurred in recent years in Japan. Especially, extreme flood events have occurred in the past several years. Prolonged heavy rainfall hit broad regions across western Japan in July in 2018 because of an active seasonal frontal system (called “Baiu Front”) hovering over western Japan, causing severe floods. The total rainfall over five days ranged from 400 to 1850 mm in most parts of western Japan, and record rainfall was observed in many places. The maximal amounts of 48- and 72-hour rainfall topped historical records in 120 stations of the Automated Meteorological Data Acquisition System (AMeDAS) of Japan Meteorological Agency (JMA, 2018) across Japan. The prolonged heavy rainfall caused water-related disasters such as flood inundation, landslides, and debris flows in many places. The number of fatalities rose to 237 (The Cabinet Office of Japan, 2019), which is the worst record of the number of victims killed by a single event of heavy rain disaster in Japan in the last three decades. Floods occurred in 315 rivers because of inundation, levee breach, or failure of sewage systems, leaving approximately 28,500 houses flooded and 18,500 damaged (Cabinet Office of Japan, 2019).
Wide-ranging deep inundation occurred because of levee breaches especially in Mabi Town in the Oda River basin, a tributary of the Takahashi River in Okayama Prefecture, leaving 51 victims. Deep inundation also occurred in Seiyo and Ozu cities, both of which are located in the Hiji River basin in Ehime Prefecture, killing nine people who lived along the river. Those flood damages were caused even though there are reservoirs for flood control in the upstream of the basins because the flood control capacity of those reservoirs was not sufficient to keep the water level in the downstream rivers at safe levels due to prolonged rainfall beyond their design levels (Nohara et al., 2020). These flood events highlighted the need for enlargement of the flood control capacity in reservoirs, while it is difficult to simply increase the flood control capacity of a reservoir by reducing its water use capacity as those river basins are located in relatively dry areas surrounded by two major mountain ranges and therefore prone to drought.
Although the frequency of severe droughts has become less than that of extreme flood events recently, serious droughts have occurred in some river basins in Japan. For example, a record shortage in precipitation caused severe droughts widely across Japan in 1994. Droughts often occurred in river basins in Shikoku Island also in these
15 years, including severe droughts in the Yoshino River basin in 2005, 2007-2009, and 2013. There is also concern that extreme droughts could occur more frequently in Japanese river basins in the future due to climate change, so drought management through reservoirs is expected to play a more significant role in the future. Reservoir operation for power generation is also considered more and more important because hydropower is regarded as a very important renewable energy resource especially after the Fukushima Daiichi Nuclear Disaster caused by the great earthquake and tsunami in Tohoku, Japan, in 2011.
Since many reservoirs have already been constructed in Japan, there are not many sites suitable for constructing a new reservoir in Japanese river basins. It is therefore important to maintain or enlarge the capability of existing reservoirs for water resources management. Sediment management of reservoirs is one of the key issues for the sustainable management of existing reservoirs. The sediment yield is, however, high in some river basins in Japan due to their topographical, geomorphological, and hydrological conditions, especially in the mountainous basins where upper reaches lie on tectonic lines (Sumi, 2006). Sedimentation issues are therefore serious in those river basins. The cumulative storage loss due to sedimentation reaches 60%-80% of the total storage capacity in some hydroelectric reservoirs that have been operational for more than 50 years (Sumi & Kantoush, 2018). Various sediment management techniques have been implemented in those reservoirs to avoid degradation of their effective storage capacity by sedimentation. They include sediment removal, sediment sluicing or flushing, and sediment bypassing by the use of sediment bypass tunnels (Figure 6.2).
Typical expected impacts of climate change on water resources management in Japan
Although the effects of climate change on water resources can vary from region to region in Japan, general characteristics of expected impacts of climate change can be discussed as follows. The frequency of heavy rainfall is expected to increase as global warming progresses. It is reported that even the strong hourly
Figure 6.2 Koshibu Dam and the outlet of its sediment bypass tunnel in the Tenryu River basin, Japan.
precipitation will increase in the near future almost linearly with temperature increase (Kitoh et al., 2009). It is also reported that typhoon-related rainfall could increase in the areas where a typhoon hit is not common under the current climate (Kanada et al., 2017).
On the other hand, the temperature rise will strongly affect snowpack and snowmelt especially in the high mountains or the north part of Japan (Sato et al., 2013; Katsuyama et al., 2017). The river flow regime in those areas can largely be affected by this change in the processes of snowpack and snowmelt, especially in spring and summer. This could have an adverse impact on water resources management because much water is taken from the rivers or reservoirs for irrigation in those seasons under the current climate conditions. Coupled with sedimentation progress in reservoirs, impacts of climate change on water use operation of reservoirs could become even greater with the degradation of available storage capacity.
Although the general aspects of climate change impacts can be described as above, particular impacts can vary from one river basin to another. Detailed investigations on impacts of climate change on river basins and reservoirs in concern are therefore needed to identify measures for mitigation or adaptation to climate change. Examples of case studies on impact assessment of climate change on operation of reservoir systems in particular river basins are introduced in Section 6.4.
