Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

M.Ing. / Please refer to full text to view abstract

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This thesis examines the hydrological processes of a restored coastal dune slack at St. Fergus, north-east Scotland. The fundamental hydrological processes of the saturated and unsaturated zones of these habitats is relatively poorly understood yet these ecosystems are under increasing pressure from industrial and recreational development and the possible impact of climate change. Within a 1.2km² catchment, a detailed field investigation was initiated at differing spatial scales to examine; (i) the spatial and temporal variability in soil moisture retention and movement through the unsaturated zone; (ii) the behaviour of the water table and; (iii) the main hydrogeochemical processes over the year October 1996 to September 1997. The field research was augmented with the development of a groundwater flow model of the site which was used in a predictive role to examine the possible implications of future climate change on the groundwater regime of the dune slack and the impact of this on the ecology and geomorphology of the dune system. The field and modelling results were integrated to provide a relatively holistic interpretation of the hydrological processes of the dune slack system. The soils of all study sites were relatively similar in their retention and temporal variability of soil moisture over the year. Soil moisture contents were generally very low, which was predominantly related to the low organic matter content and highly porous sediment. These soil properties resulted in great temporal variability in soil moisture content over the year with no pattern of higher retention in the winter and lower in the summer. Groundwater behaviour was also relatively homogeneous in the catchment, though local scale, transient processes and phenomena appeared to be important. The main influences on groundwater levels and direction of flow included; (i) the groundwater recharge processes (influenced by variable specific yield phenomena, preferential flow paths and the height of the capillary fringe); (ii) the hydrogeology of the site, particularly the complex plateau region; (iii) site topography; (iv) the shallow water table and; (v) terminal encroachment on to the dune slack. The modelling procedure highlighted the need for an integrated field and modelling approach as problems encountered were resolved through interpretation of the field data. Issues raised focused on scale and scaling in groundwater modelling, the validation of such models and the importance of good spatially distributed field data.

The effect of removing soil water stress on fine root growth and the pattern of water use was studied by irrigating a small plot (20 m x 20 m) of 40 year old Sitka spruce. A control plot was established close by. Soil water content and soil matric potential were measured independently on both plots at several depths. Root growth was measured using monthly coring for the surface horizons (c. 10 cm depth) and ingrowth cores to study rooting at depth. In the absence of irrigation the 2 plots had similar water regimes. The irrigation kept matric potential > 20 kPa and water content > 20%. Areas between trees were wetter than areas close to stems and wetted up more during periods of rainfall. This is in contrast to young plantations where stemflow accounts for a much larger proportion of precipitation reaching the forest floor. At field capacity most of the water uptake was from near the surface but as the profile dries the proportion from depth increases, at the end of a dry period 50% of uptake was from the bottom half of the rooted zone. Water uptake from below the rooted zone was small. Irrigation had a significant effect on root tip density and on small root mass but not on fine and dead root masses. Root tip density stayed high through the summer with irrigation but fell on the control plot, the number of tips began to increase earlier on the control plot. This is interpreted as irrigation increasing the longevity of tips from 2-3 months (control) to 5 months. Rewetting of the soil in autumn appears to act as a cue for an increase in root growth. Irrigation had no effect on root growth at depth and there was no difference in growth at different depths on either plot.

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