Catchment subsurface water storage, mixing and flowpaths: implications for land cover change as a natural flood management strategy

Efforts are increasing globally to harness the potential of forests to alter catchment water runoff and storage dynamics as a ‘natural flood management’ (NFM) strategy, particularly given a projected rise in the frequency and severity of floods with climate change. Despite decades of research on forest hydrology, knowledge of how forests and land use control catchment runoff is still limited, especially in relation to important, though less investigated, subsurface runoff processes. This PhD research aimed to examine how forest cover interacts with soils and geology to influence runoff pathways at different spatial and temporal scales, focusing on the 67 km2 Eddleston Water NFM pilot site in the Scottish Borders. At the catchment scale, isotopic (2H and 18O)and geochemical tracers (Acid Neutralising Capacity (ANC)), conductivity and pH) were used to investigate whether forest cover is a significant control on water storage and mixing over seasonal and storm event timescales. At the hillslope scale, dense subsurface monitoring (soil moisture, groundwater and time-lapse electrical resistivity tomography (ERT)) compared improved grassland to an across-slope forest strip, similar to those promoted in NFM schemes to control runoff, to reveal water storage potential in soil underneath the forest and the downslope extent of any impacts on subsurface hydrological dynamics. The results revealed complex interactions between land cover and runoff processes at different scales. At the catchment scale, soil type and superficial geology were found to be more dominant controls on catchment storage over seasonal timescales, with land cover playing a secondary role. Dynamic storage estimates for headwater catchments underlain predominantly by glacial till were low, ranging from ~16 mm to 46 mm, and were correlated with low mean transit times, ranging from ~130 to ~210 days. There were no differences in these estimates, within the bounds of error, between catchments with up to 90% forest cover and those with much lower cover (<50%). However, there were significant differences compared to steeper catchments with low glacial till cover. In these catchments dynamic storage estimates ranged from ~160 mm to~200 mm, and were correlated with high mean transit times, ranging from ~320 to~370 days. At the storm event timescale, and comparing two adjacent catchments with similar superficial geology and soils but differences in land cover, forest cover reduced the event water runoff fraction for four high flow events. The fraction of event water runoff at peak discharge during the largest event monitored was 0.37 for the forested catchment but 0.54 for the adjacent partially forested catchment. A third catchment, with minimal glacial till and low forest cover, demonstrated very different dynamics, with much lower runoff ratios for all events, higher groundwater fractions (0.21-0.55 at peak), and ‘double-peak’ hydrographs, illustrating the impacts of geology on runoff processes. Similar relative differences in runoff fractions were found between catchments across the three winter events, with differences between storms greater than differences between catchments. These findings suggest that while catchment characteristics mediate event responses, the characteristics of the event(rainfall depth, intensity and antecedent conditions) may dominate responses, though it was not possible to disaggregate the different event characteristics with this dataset. The hillslope scale work identified significant differences in subsurface moisture dynamics underneath the forest strip over seasonal timescales: drying of the forest soils was greater, and extended deeper and for longer into the autumn compared to the adjacent grassland soils. Water table levels were also persistently lower in the forest and the forest soils responded less frequently to storm events. Downslope of the forest, soil moisture dynamics were similar to those in other grassland areas and no significant differences were observed beyond 15 m downslope, suggesting minimal impact of the forest at shallow depths downslope. The depth to the water table was greater downslope of the forest compared to other grassland areas, but during the wettest conditions there was evidence of upslope-downslope water table connectivity beneath the forest. The results indicate that forest strips provide only limited additional subsurface storage of rainfall inputs in flood events after dry conditions in this temperate catchment setting. In summary, the research results show that while forests have some seasonal impacts on subsurface moisture dynamics, soil type and underlying superficial geology are primary controls on catchment storage and mixing in temperate upland environments, suggesting limited impacts of changing land use. At storm event timescales increased forest cover has some impact on reducing the amount of event water runoff, but event characteristics are a more dominant control, so forest cover alone is unlikely to lead to significant reductions in peak flows during large flood events. Strategically placed forest cover, such as field boundary planting on hillslopes has some impacts on subsurface moisture dynamics but the effects are spatially limited and not present in winter periods. The processes leading to these findings appear to be similar at the catchment and hillslope scales. From an NFM policy perspective the findings suggest that while tree planting is not a flood management panacea, it may have benefits in certain situations, as well as significant co-benefits. This implies a need for a change in emphasis within flood risk management policy, which ‘mainstreams’ tree planting as a flood risk strategy into wider policy processes to create multifunctional landscapes. There are still many unknowns about the impacts of land cover on hydrological processes, particularly in the subsurface, and there is a need for enhanced research on these processes. This will also help to reduce some of the large uncertainties surrounding the impacts of NFM, which remain one of the key barriers to its wider implementation

Land use partnerships for addressing climate change: What are they, why use them, and how do they work?

Policy brief produced to inform the Scottish Government's Regional Land Use Partnership programme.Reducing greenhouse gas emissions and achieving climate change adaptation objectives in the land sector will rely on effective collaboration bridging scales and sectors. Many approaches to ‘partnership’ working have been developed in the sector, working at different scales and focussed on a range of issues. Scotland has committed to the development of Regional Land Use Partnerships (RLUPs) to help deliver a more integrated approach to land use change and management, and meet its target of net zero by 2045. Stakeholders have different expectations about what RLUPs can deliver and how they might function. Success will rely in part on there being a clear vision for how they work. This brief explores how existing land use partnerships work and the learning they provide for how RLUPs might be designed to meet their multiple objectives

Natural capital assessment in landscape-scale land use planning: how it works and key challenges

Policy makers, land managers and investors are increasingly interested in considering nature – and the services it provides – in decision making. It is expected that quantifying and valuing natural assets will help drive decisions that are more environmentally, economically, and socially sustainable, and help address the climate and biodiversity emergencies. Government and private sector actors are increasingly looking to better recognise this ‘natural capital’ in public and corporate policies. This is driving a thriving industry of assessment frameworks, methodologies, and tools to quantify natural capital ‘assets’ ranging from stored carbon to biodiversity. But the sheer number of approaches and the language of natural capital can be confusing. This makes it hard to know which tools may for improve decision-making. This brief introduces natural capital terminology, provides a summary of the ‘natural capital approach’, outlines its relevance to strategic, landscape scale land use planning, and introduces some of the tools being developed to support the approach

Natural flood management, lag time and catchment scale:Results from an empirical nested catchment study

Natural flood management (NFM) techniques attract much interest in flood risk management science, not least because their effectiveness remains subject to considerable uncertainty, particularly at larger catchment and event scales. This derives from a paucity of empirical studies which can offer either longitudinal or comparison data sets in which changes can be observed. The Eddleston catchment study, with 13 stream gauges operated continuously over 9 years, is based on both longitudinal and comparison data sets. Two years of baseline monitoring have been followed by 7 years of further monitoring after a range of NFM interventions across the 69 km2 catchment. This study has examined changes in lag as an index of hydrological response which avoids dependence on potentially significant uncertainties in flow data. Headwater catchments up to 26 km2 showed significant delays in lag of 2.6–7.3 hr in catchments provided with leaky wood structures, on‐line ponds and riparian planting, while larger catchments downstream and those treated with riparian planting alone did not. Two control catchments failed to show any such changes. The findings provide important evidence of the catchment scale at which NFM can be effective and suggest that effects may increase with event magnitude

The impact of across-slope forest strips on hillslope subsurface hydrological dynamics

Forest cover has a significant effect on hillslope hydrological processes through its influence on the water balance and flow paths. However, knowledge of how spatial patterns of forest plots control hillslope hydrological dynamics is still poor. The aim of this study was to examine the impact of an across-slope forest strip on sub-surface soil moisture and groundwater dynamics, to give insights into how the structure and orientation of forest cover influences hillslope hydrology. Soil moisture and groundwater dynamics were compared on two transects spanning the same elevation on a 9° hillslope in a temperate UK upland catchment. One transect was located on improved grassland; the other was also on improved grassland but included a 14 m wide strip of 27-year-old mixed forest. Sub-surface moisture dynamics were investigated upslope, underneath and downslope of the forest over 2 years at seasonal and rainfall event timescales. Continuous data from point-based soil moisture sensors and piezometers installed at 0.15, 0.6 and 2.5 m depth were combined with seasonal (~bi-monthly) time-lapse electrical resistivity tomography (ERT) surveys. Significant differences were identified in sub-surface moisture dynamics underneath the forest strip over seasonal timescales: drying of the forest soils was greater, and extended deeper and for longer into the autumn compared to the adjacent grassland soils. Water table levels were also persistently lower in the forest and the forest soils responded less frequently to rainfall events. Downslope of the forest, soil moisture dynamics were similar to those in other grassland areas and no significant differences were observed beyond 15 m downslope, suggesting minimal impact of the forest at shallow depths downslope. Groundwater levels were lower downslope of the forest compared to other grassland areas, but during the wettest conditions there was evidence of upslope-downslope water table connectivity beneath the forest. The results indicate that forest strips in this environment provide only limited additional sub-surface storage of rainfall inputs in flood events after dry conditions in this temperate catchment setting

Investigating the role of groundwater in catchment functioning in the Eddleston Catchment, Scotland

BGS have been investigating the role of groundwater in catchment functioning for the past 10 years in the Eddleston Research Catchment – a tributary of the River Tweed, located in the Scottish Borders. The research is part of a wider initiative funded by the Scottish Government examining the evidence for the efficacy of natural flood management and river restoration measures. Here we give a brief summary of several of the experiments undertaken: (1) exploring the coupling of an upland floodplain aquifer with the river and hillslope; (2) examining soil permeability and infiltration in different land uses and superficial geology; (3) monitoring groundwater flow and soil moisture changes underneath a forest strip; and (4) using tracers to measure the partitioning between groundwater flow, soil water and event runoff during storm events. The research experiments reinforce the importance of subsurface conditions, and in particular geology in shaping the response of catchments to rainfall. Groundwater plays an important, but often unrecognised role in mediating catchment flows, and variability in superficial geology often exerts a larger control on flooding than land use