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.