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Variation in wilting point among functional soil classes for hydrological modelling of the Nant-y-Brwyn catchment

By Haydn Johnson and Ed C. Rowe


The WADES project is assessing the impact of climate change on ecological services of moorlands, such as plant biodiversity and flood and drought regulation. o achieve this by producing a hydrological model based on the functional soil classes of the Nant-y-Brwyn catchment, North Wales. Using hypotheses suggesting how climate change may alter these soil classes, it is then possible to model the hydrological implications.\ud This report describes the collating of data that the hydrological model will require to run. The structure of the PDM model chosen to model the catchment area was analysed, and soil moisture capacity and field capacity were identified as key parameters. Potentially useful field measurements were identified, including taking soil core samples to calculate soil moisture capacities and the polythene sheet method for field capacity measurement. Fieldwork to obtain soil cores from four functional soil classes (deep peat with true blanket bog vegetation; organomineral soils with 10-40 cm or < 10 cm organic layers; and flushed soils dominated by groundwater efflux) was undertaken in the Nant-y-Brwyn catchment. Soil cores were then analysed in the laboratory. Soils were dried to water potentials near (above and below) the Wilting Point (–1.5 MPa), and measurements of actual water potential in the sample were taken using a WP4 Dewpoint PotentiaMeter machine. The samples were then weighed, dried and reweighed to calculate soil moisture content. A linear regression was fitted to the water potential vs. water content plot for each soil core, and used to calculate the expected water content at wilting point.\ud Results showed an as-expected general trend of higher soil moisture contents for less negative water potentials. This was less conclusive amongst the Flush soils however, where significant scatter was observed, along with the largest range of average soil moisture contents of all soil classes. Measurement errors are possible, but the Flush soil type was relatively heterogeneous and so uncertainty in the measurement is unsurprising. Bog soils had the smallest range of average soil moisture contents (0.08g H2O g-1 dried soil), indicating more homogenous soil characteristics, despite having the greatest average soil moisture at wilting point, 0.98 g H2O g-1 dried soil. Wet and dry soil classes were found to have smaller but similar average soil moisture values at wilting point, at 0.91 and 0.94 g H2O g-1 dried soil, respectively.\ud Proposals have been made as to how the hydrological soil characteristics discovered would be represented in the PDM model, with focus on the proportional distribution of the soil classes in the catchment being incorporated into the PDM’s Pareto distribution parameter.\u

Topics: Hydrology
Publisher: NERC/Centre for Ecology & Hydrology
Year: 2011
OAI identifier:

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