Predicting Climate Change Impacts On Reservoirs And Water Supply

Climate change is a growing concern, especially the uncertainty of how it will impact the environment. Climate conditions and temperatures vary by region and the effects of climate change will not be uniform. The current understanding of climate change impacts on water quality and surface water resources implies there will be an impact on reservoir water supplies, and consequently, on consumers. Future water treatment process design should consider climate change projections in system design and operations. Assessment of the Impacts of Climate Change on Reservoir Water Quality, a research report by The Water Research Foundation, investigated how risk propagates and changes with a changing climate. The objective of the project was to develop and test an approach that would provide an improved, quantitative understanding of the potential impacts of future climate change on reservoirs.

The research team believes it is imperative to understand the impacts of climate change to reservoirs as part of larger watershed systems. Operational changes of reservoir management may need to be considered with a changing climate. The Intergovernmental Panel on Climate Change (IPCC) states that the main catalyst for climate change is human activity. In addition, water resource availability is expected to decrease across the board, but especially in semi-arid regions. Another water quality risk for potable water production from climate change is a potential increase in the growth of algae and the frequency of cyanobacterial blooms in reservoirs that will have downstream impacts. The research’s aim is to help find ways to proactively mitigate adverse effects to water and provide utilities with tools that may fit their capabilities.

Freshwater resources are especially vulnerable to the consequences of a changing climate. Reservoirs represent an element in the supply chain from catchment to tap where impacts on water quality may affect security and a safe water supply. The research team assesses the most likely climate-related risks to water quality are algal growth, increased turbidity, and increased dissolved organic carbon (DOC) loads. To understand the potential impacts of climate change, the research team used an integrated modeling scheme to examine three reservoirs used for potable water supply. Each location represents a very different type of climate and were selected based on their role in the potable water supply and the availability of past data. The chosen locations are the Occoquan Reservoir (Virginia, USA), Hsin-Shan Reservoir (Taiwan), and Myponga Reservoir (Australia). Researchers determined applying this approach to very different climate zones was determined to be the best option to quantify potential impacts and draw comparisons into the characteristics and resilience of individual water supply systems.

The Occoquan Watershed is located in Northern Virginia. The Upper Broad Run and Middle Broad Run watersheds are in the northwestern part of the Watershed and drain into Lake Manassas, a man-made reservoir that is a key drinking water source for the surrounding county. The climate in this area is characterized as temperate, experiencing four distinct seasons. Researchers chose two projection models based on the median of mean annual precipitations and mean annual surface air temperature to represent the upper and lower limits of other models, and also represent a “hot and wet” and “cool and dry” weather condition. Based on the projection models, the research team deduced future thermal stratification in Lake Manassas will expand and will be intensified due to climate change. In addition, higher amounts of streamflow are expected in the future, resulting in an increased amount of nutrient runoff. However, Lake Manassas could dampen the negative effects of climate change within the watershed and increase the resiliency of the region of the whole Occoquan Watershed. 

The Hsin-Shan Reservoir is in Northern Taiwan and is the largest drinking water source for Northern Taiwan. The reservoir is a humid, sub-tropical climate and experiences more distinct seasons compared to other reservoirs in Taiwan. In a sub-tropical climate, Taiwan is subject to tropical cyclone activity. This case study assessed the impacts of climate change on water quality in the near term (2020–2039) and long term (2080–2099). The water quality in the reservoir is very vulnerable to climate change due to the small, deep size of the reservoir — the adverse impacts to water quality will come from thermal stratification. The researchers believe the observed increase in atmospheric water vapor and increased surface water temperature from a warming climate indicates increased intense tropical cyclone activity. A rise in atmospheric temperature was the major factor that will lower the quality of water in Hsin-Shan Reservoir, as it will reduce dissolved oxygen concentrations and release more phosphorus from sediment.

The Myponga Reservoir, in Southern Adelaide, Australia, receives water from a natural catchment. The region has a Mediterranean climate with hot, dry summers and mild winters. The water is treated by a nearby water treatment plant using a conventional treatment process that includes flocculation and chlorination. The results indicated a higher consumptive demand is likely to have a large impact on water quality. Based on the modeling simulations, the researchers believe the Myponga River in the future is less likely to supply water to the Myponga Reservoir than in the past, as increasing temperatures may lead to decreased rainfall. Reduced inflow from the catchment and increased evaporation will place future pressure on water systems. Similarly, nutrient loading is expected to decrease as a result of reductions in concentration and volume. However, a decrease in nutrient loading may not mean less productivity in the reservoir since it is expected it will be maintained by internal nutrient cycling.

Taking the overall impacts from each site, the research team notes it will be equally important to actively manage watersheds to prevent and manage contaminant runoff. An integrated-modeling approach can help inform business-related future risks associated with catchment-derived nutrients, DOC, and microbial contamination. Based on the research findings, there are several general conclusions from each location. First, surface water temperatures will increase where air temperatures increase. This temperature increase can also affect nutrient dynamics based on stratification behavior, and increase phytoplankton productivity. Moving forward with projected impacts, the research team recommends destratification approaches will be important for future design and system operations, as well as ways to intercept and manage contaminant runoff. Ultimately, utilities will need to develop more aggressive strategies for engagement to reduce the amount of in-stream and nonpoint source nutrient loads.