Quantifying and modelling spatio-temporal flood-mitigation, drought-resilience, and water-quality benefits provided by grassland interventions in the Eden Catchment (North-West England, UK)

Wallace, Ethan and Chappell, Nick and Tych, Wlodek and Quinton, John (2022) Quantifying and modelling spatio-temporal flood-mitigation, drought-resilience, and water-quality benefits provided by grassland interventions in the Eden Catchment (North-West England, UK). PhD thesis, Lancaster University.

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Abstract

Overland flow and rapid shallow-subsurface flow are hydrological pathways capable of quickly transporting precipitation to the stream network. These rapid pathways can greatly increase the flood-risk within a catchment, as well as cause water-quality degradation through the transport of harmful pollutants and pathogens, alongside encouraging other serious issues such as soil erosion, damage to infrastructure, mass siltation, and reduced agronomic efficiency. Given the economic and humanitarian costs associated with flooding and water-quality deterioration throughout the globe, efforts are needed to effectively control and limit such hydrological pathways. Grasslands, specifically those used for intensive agriculture such as improved-pastures and meadows, encompass a large percentage of the land-use in many regions of the world. Many improved-pasture- and meadow-dominated catchments are prone to both flooding and water-quality issues, with concerns also growing regarding their resilience to drought. With such widespread international prevalence, incorporating grasslands within a land-use management framework could well be an effective method of mitigating against floods, droughts and water-quality deterioration in such areas. Despite such an extensive presence globally, the hydrological understanding of grasslands is extremely underdeveloped, with the understanding of surface hydrodynamics and their controls on overland flow and rapid shallow-subsurface flow extremely limited. The aim of this thesis was to quantify the observable hydrological change for several widespread grassland farming features and practices (interventions) that likely alter the spatio-temporal dynamics of overland flow and rapid shallow-subsurface flow and their hydrological controls. This was conducted by paired-plot experimentation, controlled experiments, and subsequent statistical and physics-based analysis and modelling of three improved-pasture- and meadow-dominated sub-catchments of the Eden catchment, North-West England, United Kingdom. The four investigated interventions were semi-natural grasslands (Chapter 4), blade-aeration (Chapter 5), hedgerow wild-margins (Chapter 6), and dry-stone walls (Chapter 7). In Chapter 4, surface moisture patterns were compared between a semi-natural grassland and a bordering improved-pasture/silage field. Converting semi-natural grasslands into improved-pasture was shown to significantly reduce the natural diversity in surface soil moisture patterns, causing substantially more uniform responses to hydrological stresses. Improved-pastures were shown to naturally dry faster than neighbouring semi-natural grasslands in spring, although slurry applications were shown to offset this drying in summer and improve drought resilience. Slurry wetted improved-pasture became wetter than the semi-natural grassland at the beginning of the autumnal rains however, heightening the likelihood of overland flow and rapid shallow-subsurface flows. During sampling within a storm, both semi-natural grassland and improved-pasture were shown to visibly produce overland flow. In Chapter 5, blade aeration was conducted on subsections of two improved-pasture and silage fields. Blade aeration was shown to significantly improve the topsoil permeability of an improved-pasture/silage field and significantly improve the penetration resistance between 5 cm – 15 cm, as well as substantially reduce the likelihood of infiltration-excess overland flow. Improvements were only seen in one of the two investigated sites however, with the alternate location showing no notable changes to permeability or infiltration-excess overland flow likelihood, and a significant increase in penetration resistance at 10 cm. In Chapter 6, hedge-margin overland flow plots were compared against overland flow plots within an immediately adjacent improved-pasture. Hedge-margins were shown to significantly improve topsoil permeability and soil physico-chemical properties compared to the adjoining improved-pasture. Hedge-margins were slower to produce overland flow, requiring an equal or increased amount of saturation than the improved-pasture, and resultantly produced a lower total overland flow volume. Hedge-margins were also found to release more nitrate, nitrate-nitrite and loose sediment in comparison to the improved-pastures in a ‘wash-off’ experiment, and therefore may store more potential contaminants on the surface and possibly offer water-quality benefits. In Chapter 7, the effect that dry-stone walls have on topsoil wetness was assessed by measuring soil volumetric wetness during saturated and near-saturated conditions above and below several dry-stone walls throughout the landscape within a large number of sloped improved-pastures. Dry-stone walls were shown to predominantly have an inconsistent and insignificant effect on soil volumetric wetness, although a possible rain shadow effect on a very localised scale (up to 3 m from the dry-stone wall) was observed. The thesis has successfully quantified the role of four widespread grassland interventions present in improved-pasture and meadow dominated catchments throughout the globe in relation to how they alter surface hydrodynamics, specifically overland flow and rapid shallow-subsurface flow. This research has resultantly improved the understanding of the hydrological functioning of agricultural grasslands specifically in relation to flood-risk and water-quality, with some advances in drought-resilience also made. The thesis finally highlights several key areas for future hydrological research in relation to furthering the understanding of grassland hydrology, as well as offers improvements and advice for both hydrometric observations, experimental designs, as well as broader hydrological modelling, and hydrological and environmental science.