الملخص الإنجليزي
Sand dunes in the deserts of arid regions, in particular, groundwater in aquifers
underlying the sand fields (e.g. Al-Sharqiyah Sands preciously known as
Wahiba Sands) has been studied since 1970th. However, several aspects of
dunes hydrology in Oman have not been investigated: the moisture storage and
fluxes in dunes' vadose zone, ecohydrology of vegetated dunes hillslopes, the
potential of using dune fields as sites of artificial recharge of underlying
aquifers, mimicking shapes and internal heterogeneity of natural dunes for
designing and engineering of dune-type porous composites as structures for
greening drylands.
The occurrence of scattered vegetation on the topsoil of desert sands in Oman,
despite the limited-sporadic rainfall pattern (<50 mm/year, 80% of it
evaporates), deserves special attention. Even in the harsh desert environment,
still, sparse vegetation survives on the dunes' slopes and interdunal areas.
Understanding the interplay between the vegetation spatial patterns and the soil
moisture in sand dune areas is important as it determines the ecohydrology of
the desert environment. In this thesis, the main objective was to study the
relationship between soil-moisture dynamics in the vadose zone of sand dunes,
giving special consideration to the capability of the sand dunes to act as
engineered self-irrigation units.
The diurnal pore water motion was studied in Ash Sharqiah Sands (linear dunes
separated by valleys, 1-2 km apart, the water table depth in wells located in the
interdental area is about 50 m under the valley, as of December 2018).
Vegetation pattern, density, and distribution were explored and described.
Measurements of volumetric moisture content ϴv were taken by 5TE sensors
(Decagon Devices) inserted at three different depths (5, 15, and 25 cm) across
four different locations (up-crest, crest, slope, and interdunal area), at an
interval of 125 m. Results have shown that denser and greener shrubs exist in
the upper part of the dune slope compared to the lower part.
In addition, a pilot investigation of the vadose zone and topsoil of small-size
coastal dunes (Nabkhas) of arid regions was done in Al-Hail North, Muscat.
The moisture content redistribution after a 13 mm rainfall event was monitored.
Three different Nabkhas, N1, N2, and N3, were investigated. The length (L)
and height (H) of each Nabkha were measured. Profile excavations were made
for three Nabkhas, and soil samples were collected. Decagon EC-05 sensors
were inserted at the depths (0, 20 cm) in the vertical excavations of N1, N2,
and N3 to monitor the diurnal variations of ϴv along with the Nabkha vertical
profile. The variation of ϴv shows a significant increase of ϴv in the top sensor
immediately after the rain event. While the bottom sensor has a trivial increase
with time. The value of ϴv in the top sensor kept increasing and reached the
maximum ϴv= 0.1 m3
/m3
on the last day of the rain event. This spatial and
temporal variation of ϴv in Nabkhas is also strongly affected by the
condensation of water from the humid ambient air and around the native
shrubs. Besides, it can catch and store the evaporated moisture from the
shallow water table beneath (i.e. in sabkha landforms).
Interdunal areas were explored and tested for managed aquifer recharge (MAR)
if natural wadis or small-size constructed channels are used as surface water
spreading systems. These channels would both transport and seep water into
the vadose zone and enhance the amount of moisture content there. A coupled
surface-subsurface flow was experimentally and numerically studied in
application to surface water harvesting through small-size channels. In the
field, experiments were done at the crest and slope of a selected dune,
triangular and rectangular channels. By applying a constant discharge at the
inlet of the channel, the length, L, of the water "jets" propagating until their
complete seepage extinction has been measured. Numerically, using
HYDRUS2D, saturated-unsaturated transient infiltration was simulated by
considering a cross-section of triangular channels of different bank slopes λ to
find the maximum total volume of infiltrated water from the channel.
Understanding the eco-hydrology of interior and coastal dunes, helped to
design mini-dunes (≤ 1 m in height) which were tested as sustainable irrigation
units. They can intercept moisture from humid air or even from the capillary
fringe and shallow water tables (e.g. in sabkha landforms). Thus, ornamental
plants or crops can be cultivated at the crest of the mini-dune with minimum
irrigation frequencies. Two SMDs of a conical shape (150 cm in diameter and
45 cm high) were constructed in the field under a controlled water table. SMDs
were similar to natural mini-dunes "Nabkhas". The moisture content variation
was monitored for five months. Then, the experimental results were compared
with HYDRUS2D models. The measured and simulated values of ϴv showed
a perfect agreement at the top layer of SMD. The SMDs can be used for
cultivating plants in arid regions, especially in areas affected by the water
tables rise.
The importance and uniqueness of this study is that it examines diurnal
moisture content oscillations in Oman's interior and coastal dunes. Based on
experimental and numerical simulations of coupled surface-subsurface flow in
dunal channels of varied shapes and topographic gradients, the research is a
novel investigation of how to utilize dunes for potential MAR projects. The
construction of the designed mini-dune (SMDs) that act as self-irrigation units
was aided by understanding the relationship between soil-moisture dynamics
and vegetation patterns.
