The Republic of Yemen occupies the southwestern tip of the Arabian Peninsula
on the Red Sea, opposite Ethiopia, and extends along the southern part
of the Arabian Peninsula on the Golf of Aden and the Indian Ocean. Saudi
Arabia is to the north and Oman to the east. The country is about the size
of France. A narrow coastal plain in the south gives way to a mountainous
region and then a plateau area. Some of the interior highlands attain a
height of 3600 metres above sea level. Ta'iz is the one of the most important
cities in Yemen, but has a population of only 350 000. The national capital
is Aden, a port city on the Indian Ocean. The national currency is the
Rial. The practiced religion is Islam, both Sunni and Shi'i. The principal
language is Arabic. Ta'iz is two hours ahead of Greenwich Mean Time.
The climate varies according to altitude. The coastal plain is hot and
dusty throughout most of the year. The highlands are warm in summer and
during winter, from October to March, nights can be very cold in the mountains.
Annual rainfall is extremely low and temperatures, particularly in summer,
are very high. Rainfall depends on two mechanisms, the Red Sea Convergence
Zone (RSCZ) and the Intertropical Convergence Zone (ITCZ). The RSCZ, whose
influence is most noticeable in the west of the country, is most active
between March and May. The ITCZ reaches Yemen in July-September, moving
north and then south again, thus having a greater influence in the south.
Both the RSCZ and the ITCZ produce precipitation in convective storms of
high intensity and limited duration. The ITCZ storms have greater aerial
extent than do those of the RSCZ. The annual rainfall distribution is the
result of these rainfall mechanisms and the orographic influence of the
mountains.
Ta'iz is located within the catchment of the Wadi Rasyan (Rasyan river),
in an area forming a low plateau within the Southern Highlands of Yemen.
The plateau lies in a faulted graben 25 km wide, aligned east-west, and
descending from 1500 m above sea level in the east to 900 m in the west,
in a series of step faulted volcanics. A flat, loess-covered plateau, forming
the surface water divide between the Red Sea and the Indian Ocean occupies
the eastern edge of the area. To the north and south the graben is bound
by highlands rising to 3000m composed respectively of volcanic rocks and
granite. These highlands receive among the highest quantities of rainfall
in the Arabian Peninsula (Handley and Dottridge, 1997). The volcanic rocks
of the Ta'iz graben comprise a thick sequence of fractured basic and acidic
lavas, tuff and ash, which form an unconfined aquifer. Storage is low,
mostly limited to fractures. Hydraulic conductivity is also low, typically
0.04 m/d. This unit is not considered adequate as an aquifer to supply
the city of Ta'iz, due to its low storage, and the low probability of intersecting
significant fracture conduits. In recent years the water supply of Ta'iz
has been derived from nearby alluvial deposits. Pumping tests in the alluvium
indicated that the hydraulic conductivity for 70% of wells would range
between 4.5 and 61 m/d, averaging 16.5 m/d (Handley and Dottridge, 1997).
Some parts of the alluvial aquifer, which were originally in a confined
condition, are now unconfined due to excessive drawdowns. The exploration
of another alluvial aquifer at Al Hayma, 15 km north of Ta'iz, took place
in the 1970's, and subsequent development followed in the 1980's. Increasing
abstraction due to population growth led to water declines and an eventual
sharp drop in 1995. Cretaceous sandstone (Tawilah formation) outcrops to
the north of the graben and is thought to occur throughout the graben beneath
the volcanic sequence. The sandstone is approximately 200 m thick. Pumping
tests in six exploratory boreholes indicated hydraulic conductivity of
between 1 and 8 m/d and an unconfined storage coefficient of around 7%
(Handley and Dottridge, 1997). The sandstone is unexploited and represents
the most promising future resource, although its recharge pattern has not
been determined and groundwater mining may take place. The sandstone has
been metamorphosed to quartzite, with a resultant reduction in porosity,
in areas affected by volcanic intrusions and dykes.
Ground water in the shallow aquifers around the city are the only renewable
water sources available to the people of Ta'iz. Alluvial aquifer thickness
generally ranges between 20-200 m. Groundwater from the shallow alluvials
is used mainly for domestic purposes and limited irrigation. Fossil groundwater
aquifers are the other (non-renewable) source of water available to the
people of Ta'iz. The development of well fields in the alluvial deposits
was planned to provide adequate household water supply for the town's population.
During the 1980's, extensive development, rapid population growth, and
substantial improvements in the standard of living have all led to an imbalance
between increasing water demand and existing limited water resources. It
is estimated that Yemen experienced a 20-30% annual increase in water demand
between 1980 and 1990 for domestic and industrial purposes. Substantial
increases in agricultural water use, ranging from 15 to 75% are also estimated
(Abdulrazzak, 1995). In the development of the alluvial aquifers to provide
household water supply, it was assumed that all agricultural abstraction
would cease. Instead farmers proceeded to deepen their wells, to keep pace
with declining water levels. Users have drilled to supply irrigation water
for a second, valuable cash crop, within the same growing season as their
rainfed subsistence crop.
Domestic demand doubled between 1980 and 1990, a trend which is expected
to repeat by 2000, and again by 2010. Agricultural water demand is largely
met through diversion of flood water and groundwater exploitation of shallow
alluvial aquifers. Rainfall produces a relatively dependable source of
surface water that is used for large-scale terrace farming and flood irrigation.
However, increased use of modern pumping technology has resulted in uncontrolled
drilling and extraction, causing water consumption for agricultural purposes
to increase. According to Handley and Dottridge (1997), there were 25 agricultural
wells in 1975, which were estimated to irrigate 522 ha. By 1985, the total
area under irrigation had increased by 25 ha. In 1996 however, the number
of dug wells was 142. Of these, 110 were dry, and 23 were operational.
The area under irrigation had dropped to less than 500 ha. In addition
to the dug wells, 118 wells had been drilled into the Al Hayma valley,
33 of which were dry and 40 were operational while 56 supplied water to
Ta'iz. It is estimated that seepage of more saline groundwater into the
Al Hayma valley from the east and human activity will increase the chloride
content of the groundwater (Handley and Dottridge, 1997).
Ta'iz is faced with water shortages due to limited natural water resources,
rising demand in all sectors, and the lack of comprehensive planning efforts.
Improvements in the standard of living coupled with the absence of conservation
programs and ambiguous agricultural policies have resulted in dramatic
increases in water requirements. The development of water supplies through
desalinization plants and the exploitation of groundwater are only band-aid
solutions. The absence of regulations, the provision of interest-free loans,
the desire to become overly productive in certain foodstuffs, as well as
other agricultural incentives have resulted in substantial increases in
irrigated area. If current trends in water utilization are allowed to continue
in the absence of regulations and long-term planning, the imbalance between
limited supplies and rapidly growing demand will become even greater, possibly
resulting in groundwater contamination and mining of groundwater reserves.
Efficient use of the limited resources available requires the development
of a comprehensive water plan with clear policies and objectives and good
alternatives to cope with water deficit. The implementation of these policies
must be treated with urgency. In addition to the establishment of sustainable
development policies, it is important to establish a monitoring network,
as an urgent priority, such that recommendations and development policies
be based on accurate and current information. Conservation practices must
be stressed in all sectors. The city of Ta'iz, through the exploitation
of the Taliwah sandstone, has begun a new phase of water supply. This formation
has very little area of outcrop, and although good yields are expected,
the extent of recharge remains to be proven. The importation of water for
recharge purposes could be explored as a means of increasing renewability
of this ground water resource.
Abdulrazzak, M.J. 1995. Water supplies Versus Demand in Countries of Arabian
Peninsula. Journal of Water Resources Management and Planning. 121 (3):227-234.
Farquharson, F.A.K., D.T. Plinston, and J.V. Sutcliff. 1996. Rainfall
and Runoff in Yemen. Hydrological Sciences Journal/Journal des Sciences
Hydrologique. 41 (5):797-810.
Handley, C. and J. Dottridge. 1997. Causes and consequences of extreme
water shortage in Yemen. Groundwater in the Urban Environment: Problems,
processes and Management. J. Chilton et. al. (eds).
