The hydrological function of organo-mineral soils in downstream flood risk

This project will seek to understand hydrological processes operating in upland organo-mineral soils and how their management and vegetation cover influences river flow peaks. This novel field, lab and modelling project will expand our knowledge on the function and hydrology of upland soils which are of high conservation value. The project will directly provide Natural England with urgently needed management decision-making evidence on upland soil management for flood peak reduction. Organo-mineral soils cover around 20 % of the UK, and are particularly common in upland areas with the main types being stagnohumic gleys and acid brown earths. Unlike peatlands, the function and hydrology of organo-mineral soils is globally very poorly understood with major gaps in the literature. These soils typically underlie upland heathland and grasslands in areas with high conservation value. It is unclear whether these soils are dominated by throughflow (and what their typical permeability range is), infiltration-excess overland flow or saturation-excess overland flow in different topographic contexts and rainfall events. It is also unclear how management of organo-mineral soils impacts their role in runoff generation.

There is an urgent need for evidence on 'nature-based' flood management solutions, particularly in UK uplands - source areas for the UK's major rivers. Recent modelling work on peatlands by U. Leeds has shown that controlling overland flow velocities by changing the surface cover conditions in key spatially identifiable parts of the catchment can play an important role in reducing flood peaks (by up to 20 % for some rainfall events). However, we do not have data from organo-mineral soils, which are likely to function quite differently to peat, to inform such modelling and so practitioners have limited basis for upland management decisions which may benefit those downstream at risk of flooding.

Study design: The above issues will be tackled through a combined monitoring, experimental and modelling approach providing PhD skills benefits involving: 1) Measure and determine the dominant hydrological pathways (e.g. overland flow, subsurface flow) through monitoring of flow volumes / rates though and over stagnohumic gleys and acid brown earths (controlling for slope position using the topographic index), moisture content & water-table depths; 2) permeability tests to measure the hydraulic conductivity (U. Leeds have a novel permeameter system allowing 25 soil samples to be tested simultaneously, allowing high throughput); 3) experimental overland flow velocity measurements by supplying water to plots and measuring the velocities for different slope angles, flow depths and vegetation conditions. The above three approaches will be used on areas of hillslope in Cumbria to test for differences in hydrological function related to soil condition and surface cover. The design will incorporate soils that have been assessed by Natural England to be in different states of degradation including high quality Atlantic Heath (dense dwarf shrubs with an understory of mosses), poor Atlantic heath, good acid grassland and acid grassland that has heavy grazing.

The three field and laboratory steps above will then enable the student to: 4) modify a new spatially-distributed version of TOPMODEL recently developed by U. Leeds; 5) use the model to test different spatial organo-mineral land-cover scenarios to test optimal solutions to reduce downstream flow peaks.