On the spatial and temporal distribution of near surface soil moisture across a low-arctic tundra hillslope

Abstract

Recent changes in global climate are having an unprecedented effect on the Arctic environment. This thesis examines the surface hydrology of a heterogeneous low-Arctic Tundra heath hillslope site near Abisko, Northern Sweden (68 17'N, 18 51’E) from May 2003 to September 2005, The study centred on two grids; on a hillslope (10800 m(^2)) and a flatter site (20000 m(^2)) with average slopes of approximately 10 and 7 respectively. Spatial soil moisture measurements, at 5 cm depth and 10 m grid spacing, were carried out in May, June, July, August, September 2003 and May, July, August, September 2004. Time series of soil moisture and temperature (at 5 cm and 10 cm depths) were also recorded at half hourly intervals between June 2003 and September 2005. Spatial surveys of topography, snow depth, soil properties, soil depth and vegetation height were also undertaken producing a high resolution dataset. Variogram analysis of the spatial soil moisture patterns shows seasonal variation. Ranges of between 50 m and 55 m were obtained, with the variograms having distinct sills and nuggets. The nugget values fall within the range expected due to measurement error. Directional variograms demonstrate that, on the hillslope, topography is an important factor driving the soil moisture distribution during the spring snowmelt period. This is not the case during the drier summer months. Conversely, soil moisture patterns show continuity and connectivity when the soils are wet during the spring snowmelt period. The topography of the site influences spatial snow distribution, vegetation community structure and soil properties. Topographic index values, derived from digital elevation models, were not correlated well with observed soil moisture values due, in part, to heterogeneous soil properties. This study suggests that the processes driving soil moisture variability for this area switch between topographical controls (when soils are wet) and local controls such as soil properties and evapotranspiration (when soils are dry).A Monte-Carlo parameter sweep method was employed, using observed soil moisture values, to give effective soil parameters values for use in a land surface model (JULES). The ID model did not perform well when modelling soil moisture values during the freeze and thaw periods on the hillslope since the effects of lateral water movement could not be accounted for. Comparisons with the flatter site were much more favourable. It is clear that the hydrology of high latitudes are driven by the same underlying processes as in other parts of the world; the redistribution of water due to topography when soils are wet and the vertical water fluxes due to soil, vegetation and climate when the soils are dry.