Impacts on Flood Risk from Land Use Strategies for Coping with Climate Change – An Assessment using the Time-Area Method

Abstract: Climate change places pressure on rural land in terms of carbon sequestration, biomass for substitution and adaptation of crops. Land use can both positively and negatively influence the runoff regime, therefor a catchment perspective is important when developing climate strategies. This is also important with respect to flood-risk that will increase with climate change. Here I investigate if there is the potential to improve the representation of impact from rural land use during conventional hydrological modeling using the simple Time-Area Method, an advantage over other expensive and time consuming models. Using this method I have further assessed the impact on flood risk due to external effects from climate strategies in the rural sector. I have further developed the rural component of an existing MIKE URBAN model over a small village, Eneryda, Sweden. The focus has been on constructing more detailed runoff coefficients (φ) for rural areas. Through a literature search and results from an integrated land use model, Dyna-CLUE, I constructed four rural climate strategy scenarios for year 2050: A2, B2, Substitution and Carbon storage. The impact from the different climate strategies on the flood risk was then analyzed in MIKE URBAN and MIKE FLOOD. My results show that there is a potential to present the general impact from rural land use through the Time-Area Method. The influence from rural land is of importance for flood risk in Eneryda, prolonging the duration of over pressure in the sewer system. The total sum from the applied φ is valid for both the west and east catchments in Eneryda during the validation of a ̴10 year rain event (r2 0.9633 and 0.8691). The individual φ are higher then what is conventionally applied in Sweden, but the values used are supported for conditions with high soil moisture in other countries. Results from a MIKE SHE model also supports the proportional difference in my φ values for mixed forest, agriculture and clear-cut. The substantial disadvantage with my model is the inability to distinguish the effect of different runoff processes on how they contribute to the resulting hydrograph. Rural climate strategies can have external effects on flood risk in Eneryda. A substitution strategy for example, including intensive forestry, is projected to increase the flood risk compared to a climate strategy favoring carbon storage. When both are exposed to a 100-year (24 hour) rain event, the substitution strategy increases the water volume by 22%, giving a 1 hour longer and 2.5% larger flooded area, and increasing the severity of flood depth by a few centimeters. This shows that local adaptation of strategies and the use of best forestry and agricultural practices are needed to not increase the risk of flooding. The impact from rural land use also highlights the importance of working with flood measures using a wider perspective than considering only grey infrastructure as land use change over time. Land use and the runoff regime can also change quickly due to direct effects from weather events. Recently, substantial storm damage of the forest in the area has made the current land use the worst case scenario for floods in this thesis. My results therefore stress the importance of adaptation strategies in the forest sector. Keywords: environmental science, physical geography, climate strategies, land use, hydrological modeling, flooding