Hydrophysical processes governing brownification: A case study of Lake Bolmen, Sweden

Access to water of good quality and quantity has become more challenging because of a changing climate, as well as an increase in the use of natural resources, which has lead to altered water chemistry. One of these changes is known as brownification, resulting from a higher organic matter content causing a yellow-brown color of surface waters. Brownification has a manifold of side effects for the ecosystem, but also requires more efforts at drinking water treatment plants to remediate the discoloring of the water. Substantial research has been carried out to increase the understanding and knowledge of the complex process of brownification, in many cases focusing on the biological aspects. The significance of water movement as described in hydrology and hydrodynamics for the process of brownification, including the generation and transport of organic matter content, has been less investigated. With rising energy prices, higher demands on limiting the CO2 emissions, and requirements for increased resilience of vulnerable infrastructure, such as drinking water treatment plants, the demand to better understand the physical processes governing brownification has grown.
This thesis investigates links between increased levels of organic matter and catchment hydrology using Lake Bolmen in south Sweden as a case study. Moreover, internal hydrodynamic processes of the lake were analyzed and related to the process of brownification. This was achieved by compiling existing data on catchment and lake properties, but also through additional field measurements. Hydrological balances and hydrodynamic modeling were employed to understand the system and to simulate the effects of climate change on brownification and to analyze possible management strategies and measures. Altered precipitation patterns, and related increases in surface runoff, have proven to be a main driver for seasonal and long-term change in brownification. Also, significant modifications in land use during the last centuries are an important driver. The application of a surface wave model indicated that resuspension influences the water color in a lake, possibly becoming an increasing problem in the future with less ice cover due to climate change. A validated box model describing the transport patterns in the lake, was used to simulate different scenarios and their impact on the brownification. These scenarios included changes in the climate forcing and different mitigation measures applied in the catchment or in the lake. Currently existing knowledge regarding measures to prevent further increase of organic matter, and resulting yellow-brown water, were critically reviewed and analyzed in the context of field studies of wetlands.