A statistical comparison of spatio‐temporal surface moisture patterns beneath a semi‐natural grassland and permanent pasture: From drought to saturation

Abstract

Some 60% of the agricultural land in the UK is grassland. This is mostly located in the wetter uplands of the west and north, with the majority intensively managed as permanent pasture. Despite its extent, there is a lack of knowledge regarding how agricultural practices have altered the hydrological behaviour of the underlying soils relative to the adjacent moorland covered by semi‐natural grassland. Near‐surface soil moisture content is an expression of the changes that have taken place and is critical in the generation of flood‐producing overland flows. This study aims to develop a pioneering paired‐plot approach, producing 1,536 moisture measurements at each of the monitoring dates throughout the studied year, that were subsequently analysed by a comparison of frequency distributions, visual cum geostatistical investigation of spatial patterns and mixed‐effects regression modelling. The analysis demonstrated that the practices taking place in the pasture (ploughing, re‐seeding and drainage) reduced the natural diversity in moisture patterns. Compared to adjacent moorland, the topsoil dried much faster in spring with the effects requiring offset with moisture from slurry applications in summer. With the onset of autumn rains, these applications then made the topsoil wetter than the moorland, heightening the likelihood of flood‐producing overland flow. During the sampling within one such storm event, the adjacent moorland was almost as wet as the pasture with both visibly generating overland flow. These contrasts in soil moisture were statistically significant throughout. Further, they highlight the need to scale‐up the monitoring with numerous plot pairs to see if the observed highly dynamic, contrasting behaviour is present at the landscape scale. Such research is fundamental to designing appropriate agricultural interventions to deliver sustainable sward production for livestock or methods of mitigating overland‐flow incidence that would otherwise heighten flood risk or threaten water quality in rivers.

The effect of hedgerow wild‐margins on topsoil hydraulic properties, and overland‐flow incidence, magnitude and water‐quality

“…This is combined with streamflow from the nearby Back Greenriggs flume, and precipitation from the Back Greenriggs rain‐gauge (see Figure 1). An Antecedent Precipitation Index (API) which started in January 2019 with a 0.99 decay factor (see Wallace & Chappell, 2020) is also given, with initial conditions having no effect from the beginning of the monitoring period…”

“…Highly similar soil pH is likely because of similar soils, as well as plot‐proximity and identical historical management. The fairly neutral conditions (Table 1) are likely because of historic liming (Holland et al, 2018; Wallace & Chappell, 2020). Acidic soils have increased sensitivity to disturbance, and therefore a greater likelihood of reduced topsoil permeability and increased overland‐flow likelihood (Chappell, Stobbs, Ternan, & Williams, 1996).…”

“…This is combined with streamflow from the nearby Back Greenriggs flume, and precipitation from the Back Greenriggs rain-gauge (see Figure 1). An Antecedent Precipitation Index (API) which started in January 2019 with a 0.99 decay factor (see Wallace & Chappell, 2020) is also given, with initial conditions having no effect from the beginning of the monitoring period rainfall), 11.21 mm (16.6% of rainfall) and 27.51 mm (40.7% of rainfall) of overland-flow from plots P1, M1 and M2, respectively. Over the whole 11-month record, overland-flow in P1 amounted to 5.2% of total rainfall, while the adjacent hedge-margin plot (M1) produced a much smaller 1% of total rainfall.…”

“…Organic matter likely increases aggregate and structural stability, thereby increasing permeability and reducing overland-flow likelihood, as well as increasing water-holding capacity (Chandler & Chappell, 2008;Chaney & Swift, 1984 Highly similar soil pH is likely because of similar soils, as well as plot-proximity and identical historical management. The fairly neutral conditions (Table 1) are likely because of historic liming (Holland et al, 2018;Wallace & Chappell, 2020). Acidic soils have increased sensitivity to disturbance, and therefore a greater likelihood of reduced topsoil permeability and increased overland-flow likelihood (Chappell, Stobbs, Ternan, & Williams, 1996).…”

Comparing fine‐scale structural and hydrologic connectivity within unimproved and improved grassland

“…Although hydrographs were not measured here, other work from woodland ecosystems shows that where flow pathways through landscapes become more disconnected and friction due to surface roughness increases, the peak of the hydrograph is delayed (Papanicolaou et al, 2018;Thomas & Nisbet, 2007). The lengthier flow paths within the M. caerulea tussocks may thus result in greater residence time of water within fields enhancing processes of soil infiltration, root uptake and evapotranspiration (Wallace & Chappell, 2020). This residence time and uptake of surface water will likely be dependent on the time of year which would need further exploration.…”