Land cover influence on catchment scale subsurface water storage investigated by multiple methods: Implications for UK Natural Flood Management

Study region: United Kingdom (UK). Study Focus: 'Natural flood management' (NFM) schemes manipulating land use and other catchment features to control runoff are increasingly promoted across the UK. Catchment water storage and mixing processes influence runoff, but our understanding of the effects of land cover change on these processes is still limited. This study combined hydrometric, isotopic and geochemical measurements to investigate land cover versus potential topographic, soil and geological controls. It compared storage-discharge dynamics in nine nested catchments within a 67 km2 managed upland catchment in southern Scotland. Storage and mixing dynamics were characterised from hydrometric data using recession analysis and from isotopic data using mean transit time and young water fraction estimates. To give information on water sources, groundwater fraction was estimated from end member mixing analysis based on acid neutralising capacity. New hydrological insights: The analysis showed low but variable sub-catchment scale dynamic storage (16-200 mm), mean transit times (134-370 days) and groundwater fractions (0.20-0.52 of annual stream runoff). Soil hydraulic conductivity was most significantly positively correlated with storage and mixing measures, whilst percentage forest cover was inversely correlated. Any effects of forest cover on increasing catchment infiltration and storage are masked by soil hydraulic properties even in the most responsive catchments. This highlights the importance of understanding dominant controls on catchment storage when using tree planting as a flood management strategy.