Upland Flood Mitigation by Managing Infiltration

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Student thesis: Doctoral ThesisDoctor of Philosophy

Original languageEnglish
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Award date2020
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Abstract

Climate change models predict an increased risk of future extreme weather events and increased risk of fluvial flooding. This has led to increased interest in natural flood mitigation solutions. The UK uplands provide natural flood mitigation, absorbing and storing rainfall and slowing the flow of water to lowland catchments. Infiltration is key to this ability of uplands to absorb precipitation. While studies have shown a link between upland land use management and changes to soil hydrology, the nature of this link remains unclear. There is conflicting evidence about the effects of upland grassland management on infiltration. In lowland catchments there is also increased interest in grass species that can adapt to extreme climate conditions. This is a relatively new area of research and further insight is needed into the effects of flooding on novel grass varieties A glasshouse experiment established novel grass varieties within mesocosms and investigated the impacts of flooding on the growth of these plants, including changes in infiltration and CO2 flux. No
differences were found in responses of the different grass varieties to flooding, but the study highlighted the impacts of flooding on grass and soil, and the need for effective flood mitigation. Field studies on upland grassland sites in mid Wales explored the impact of management including grazing, hay cutting, fertiliser inputs and liming application, on water regulation, CO2 flux and
biodiversity. Data collection revealed different management treatments did impact on these ecosystem services. Hydraulic conductivity was higher in improved grassland, which was maintained by fertiliser and liming inputs, compared to grassland without inputs. There were seasonal changes in management impacts, which were affected by wet winters and a dry summer.
An investigation into the impacts of sheep grazing and hay cutting on soil compaction in upland grassland found that these management processes caused soil compaction and that infiltration rates decreased when they were applied. Management to improve infiltration was explored. Mechanical aeration of soil improved infiltration rates in the short term, while results suggested that grazing exclusion and management for earthworms provide a more sustainable long-term solution. The effects of liming on upland grassland were investigated and the results highlighted the importance of liming to maintain earthworm populations and infiltration rates. A comparison of grassland sites with variable dates of last lime application showed that grassland sites that had been
improved and limed in the past but were now declining had lower infiltration rates than those limed regularly. The results suggested fertiliser inputs and liming on improved grassland could improve infiltration. The results from these studies demonstrate that infiltration rates are higher on improved upland
grassland, where stocking rates are carefully managed, than on declining grassland. Compaction from sheep grazing can reduce infiltration rates, but grazing exclusion can allow rates to recover. The results highlight the importance of earthworms in maintaining infiltration and demonstrate that
liming is a key to maintaining soil pH on improved grassland and supporting earthworm populations. The findings have implications for future management of the uplands for flood mitigation.