256
E.-S. E. Omran
following five main factors responsible for the waterlogging and salinity problem
in Dakhla Basin, the observation and physical identification of the waterlogged and
salt soil.
The primary causes responsible for the problem of waterlogging and salinity
are hardpans, which are a dense soil layer located usually below the highest layer
of topsoil. There are different kinds of hardpans in Dakhla Basin as claypans and
clacipans, all of which share the common characteristic of being a distinct soil layer
that is largely water-impermeable. Some hardpans are produced by soil deposits that
fuse and bind the soil particles. These deposits can range from silica dissolved to
iron oxide and matrices that form calcium carbonate. In the study area, hardpan can
be a problem by preventing water drainage and restricting the growth of plant roots.
Caliche is a sedimentary rock, a hardened natural carbonate cement that binds other
materials like gravel, sand, clay, and silt. A claypan is a thick, compact, gradually
permeable layer in the subsoil with a much higher content of clay than the surrounding
material from which a sharply defined boundary separates it. Usually, when dry,
claypans are hard, and when wet, plastic and sticky. They limit or slow down the
movement of water through the soil.
Inadequate drainage is a second cause. Waterlogging, of course, depends mainly
on soil physical properties, depth and slope, and subsoil bedrock properties, but
the position of natural drainage patterns in closed and playa drainage basins is an
important factor. Where there is essentially irrigation-excess water and sub-surface
flow, it will accumulate under cultivated playa surfaces, especially if an impermeable
rock underlines it. Some of the main causes of irrigation system misrepresentation
are lack of surface and sub-surface drainage, poor drainage system maintenance,
over-irrigation and increasing water-intensive plants. While both the formulation of
irrigation projects and drainage should go hand in hand, drainage as an essential
component was not given the attention it deserved. Given the low topography of
the Oasis and the lack of any natural streams in some places and rainwater falling
over the land, excess irrigation water in the absence of proper drainage percolates
continuously down raising the water table. The sand ridges covering the whole district
were flattened for agriculture. Because of its natural slope, these ridges enabled rapid
drainage of excess rainwater.
The third cause is faulty irrigation systems. The depletion of groundwater quality
also results from over-irrigation. The soil profile percolating water contains most of
the salts left behind by the phenomenon of evaporation and transpiration. As water
passes through the soil’s profile, extra salts can be dissolved. In addition, some salts
may be precipitated in the soil, while some salt ions are transferred in the water added
to the soil. Proper irrigation involves applying only enough water to re-wet the soil
where the crops have removed it, but not so much that the soil just above the hardpan
begins to waterlog. When the soil in which they emerge is waterlogged, the roots
soon die.
Lastly, groundwater overpumping is a major contributor to waterlogging growth
in Dakhla Basin. Like all oases, water is Dakhla’s key component. Prehistoric lakes
once occupied the majority of this oasis’ agricultural area. In the hope of establishing
E.-S. E. Omran
following five main factors responsible for the waterlogging and salinity problem
in Dakhla Basin, the observation and physical identification of the waterlogged and
salt soil.
The primary causes responsible for the problem of waterlogging and salinity
are hardpans, which are a dense soil layer located usually below the highest layer
of topsoil. There are different kinds of hardpans in Dakhla Basin as claypans and
clacipans, all of which share the common characteristic of being a distinct soil layer
that is largely water-impermeable. Some hardpans are produced by soil deposits that
fuse and bind the soil particles. These deposits can range from silica dissolved to
iron oxide and matrices that form calcium carbonate. In the study area, hardpan can
be a problem by preventing water drainage and restricting the growth of plant roots.
Caliche is a sedimentary rock, a hardened natural carbonate cement that binds other
materials like gravel, sand, clay, and silt. A claypan is a thick, compact, gradually
permeable layer in the subsoil with a much higher content of clay than the surrounding
material from which a sharply defined boundary separates it. Usually, when dry,
claypans are hard, and when wet, plastic and sticky. They limit or slow down the
movement of water through the soil.
Inadequate drainage is a second cause. Waterlogging, of course, depends mainly
on soil physical properties, depth and slope, and subsoil bedrock properties, but
the position of natural drainage patterns in closed and playa drainage basins is an
important factor. Where there is essentially irrigation-excess water and sub-surface
flow, it will accumulate under cultivated playa surfaces, especially if an impermeable
rock underlines it. Some of the main causes of irrigation system misrepresentation
are lack of surface and sub-surface drainage, poor drainage system maintenance,
over-irrigation and increasing water-intensive plants. While both the formulation of
irrigation projects and drainage should go hand in hand, drainage as an essential
component was not given the attention it deserved. Given the low topography of
the Oasis and the lack of any natural streams in some places and rainwater falling
over the land, excess irrigation water in the absence of proper drainage percolates
continuously down raising the water table. The sand ridges covering the whole district
were flattened for agriculture. Because of its natural slope, these ridges enabled rapid
drainage of excess rainwater.
The third cause is faulty irrigation systems. The depletion of groundwater quality
also results from over-irrigation. The soil profile percolating water contains most of
the salts left behind by the phenomenon of evaporation and transpiration. As water
passes through the soil’s profile, extra salts can be dissolved. In addition, some salts
may be precipitated in the soil, while some salt ions are transferred in the water added
to the soil. Proper irrigation involves applying only enough water to re-wet the soil
where the crops have removed it, but not so much that the soil just above the hardpan
begins to waterlog. When the soil in which they emerge is waterlogged, the roots
soon die.
Lastly, groundwater overpumping is a major contributor to waterlogging growth
in Dakhla Basin. Like all oases, water is Dakhla’s key component. Prehistoric lakes
once occupied the majority of this oasis’ agricultural area. In the hope of establishing