الملخص الإنجليزي
Sand dunes occupy large areas of Ash Sharqiah Sands and Empty Quarter in Oman. Accumulation of aeolian sand dunes also takes place in some cities (e.g. Buwshar dunes in Muscat). In Oman, like other arid-zone countries, sand dunes receive sparse rainfall events. The infiltration and percolation of rainwater might be trapped within sand dunes and create zones of high water saturation inside. Heterogeneity of soil and layered (stratified) texture of the dune bodies reduces infiltration, evaporation and redistribution of soil moisture. In order to have a good understanding of sand dunes hydrology, it is important to investigate moisture and temperature fluxes in the desert sand dunes. In this thesis, the main objective was to study a combined effect of a nonflat topography (dune slope) and capillary barrier (created by topsoil textural structuring) on evaporation from a saturated zone beneath (e.g. a perched water table or retained wet spot in the dune interior). Buwshar's sand dunes (Oman) were selected to estimate evaporation from desert sand dunes. During field visits, microstratification (lamination) on outcropping cliffs, with layers' thickness of 1-2 mm and tilted at a slope of about 35 ° was described. Loose sand and consolidated rock samples were collected for laboratory experiments. Microscopic analysis was conducted to determine porosity and mineral composition. The evaporation rate and cumulative evaporation from vertical sand columns, 20-40 cm tall, capped by porous stratified "plugs", about 3 cm thick, were measured. Numerically, HYDRUS-2D software was used to simulate the column experiments by considering trapezoidal flow tubes with and without a tilted stratified "plug". Transient distributions of the pressure head, moisture content, and Darcian flux were obtained. Laboratory and microscopic analysis evidenced that the micro-stratified structures, common in sand dunes, have particle sizes about two times larger than loose sands from the same area. This validates the potential of a coarser cover of stratification to act as a capillary barrier against up-ward fluxes of evaporating water. The experiments and simulations showed that in the columns, which were plugged by a thin natural capillary barrier "plug", evaporation was impeded and reduced as compared with un-plugged columns of equivalent size (0.9 cm/day for un-plugged and 0.1 cm/day for plugged column, in one experiment). Hence, this "plugging" can preserve moisture in the dunes during drought periods. Further understanding of the role of micro-stratified structures of the dunes in motion of moisture is needed for more complex stratification patterns, boundary conditions in modeled flow domains, and transpiration by desert plant roots, and other factors linking models and real dunes.
Keywords: Capillary barrier; Dune hydrology; Evaporation; HYDRUS-2D modeling; Soil column experiment; Micro-stratified structure.
