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Mahad Baawain and Jamal Abu Ashour Department of Civil Engineering, Sultan Qaboos University, Muscat, Sultanate of Oman Abstract This study is an introductory stage for a current research project on the sustainability of groundwater in Salalah Area, Sultanate of Oman. The Salalah plain extends over a 253 km2 area to the north of the Omani coastline of the Arabian Sea to the Mountains of Dhofar. The area is recharged by rainfall on the mountains and on the plain. The annual recharge is estimated at 40 Mm3yr-1. About 94% of this amount comes from the mountains. Water is being abstracted from the Salalah aquifers to meet the increasing domestic and agricultural demands of the city. Average annual discharge is estimated at 44 Mm3yr-1. Agriculture consumes about 87% of this amount. From these estimates, it is obvious that the area will face a major crisis if these rates are maintained or increased and no management plans are implemented. Over abstraction from the aquifers caused saline water intrusion in addition to the decline of groundwater storage. Present studies showed that the salinity of water is increasing but not to the extent to prevent its use for agriculture. However, this rate of increase is alarming of a serious quality problem the area might face in few years from now. Policies have to be implemented to protect the Salalah area groundwater and the supplying aquifers, i.e. the Jebel. Other solutions to enhance the infiltration rates and resist the saline water intrusion, such as constructing dams and the use of treated wastewater, are required. Introduction Salalah, the southern capital of Oman, is on the southern coast of Sultanate of Oman. It lies on a flat coastal plain to the north of which rise the Jebel or mountains. Water is supplied to this town through two well fields situated to the north of the coastal plain. The main Salalah well field was drilled in 1970's. Due to the increase in water demand, the Sa'ada well field was drilled in 1986 with 3 wells to the north of the Salalah well field. The region can be divided into three main areas: the Salalah coastal plain, the Jebel and the Nejd. The plain extends to about 10 km from the coast with an elevation rising to 100 m. The Jebel, a mountain rising to 1000 m above sea level, borders the plain and extends along the southern coast of Oman. The range is about 20 km wide north of Salalah. The Nejd, a flat arid area, lies inland of the Jebel at an elevation of about 650 m above sea level and slopes gently to the north. There is extensive agriculture on the coastal plain and there are a large number of private groundwater abstractors in the area to provide water for irrigation. Other water supply for this area comes from perennial sprigs that issue from the edge of the Jebel. These are tapped off near the source and channelled to several large farms. Salalah area has a very different climate from the rest of the country. It has a hot and arid climate during the winter and a cooler summer monsoon period (Kharif) between June and September. The monsoon winds come from the south and result in persistent fog and light rain, much of the rain falling on the southern slopes of the Jebel. These winds are largely the result of the extreme summer heating that occurs in the vast inland desert of the Rub Al Khali or Empty Quarter (Schemenauer, 1989). The monsoon clouds are prevented from moving inland by mountains and are pushed against the Jebel forming extensive areas of high elevation fog. The cloud bank associated with the monsoon is about 140 km long and less than 80 km wide (Price et al, 1988). However, heavy rains may occur at any season during rare cyclonic storms. The regional geology of Salalah area is characterized by two distinct regions separated by a major fault, which divides the Salalah Plain from the Jebel. The Jebel consists of a thick sequence of limestone with some shales and rises to 1000 m above sea level. There are four main limestones units: the Tertiary Dammam, Ru and Umm Er Rahhuma, which form Hadramaut Group, and the Cretaceous limestones. Salalah plain has a uniform slope of approximately 0.01 and is crossed by a number of shallow wadi systems (Flint and Rippon, 1986). Methodology The available data from the Jebel and the plain were collected from different locations as shown in Table 1. Several studies have been made to estimate the recharge and discharge rates on this area along different periods since 1976. The recharge of the fresh groundwater of the Salalah plain can be derived from four alternative mechanisms. Subsurface flow from the Jebel, the infiltration of springs at the foot of the Jebel and the infiltration of rain falling on the plain directly are the main recharge mechanisms. Infiltration due to wadi flood flow is the fourth mechanisms, though it is of much less importance due to its rarity. This study compares some of these previous results of estimating the recharge and the abstraction rates in the Salalah area. These studies cover the Salalah plain and the Jebel. Salalah plain area is 253 km2 and the Jebel area is 536 km2. The average results will be determined for final comparison in order to determine an average rate for the difference between the inflows and outflows of the groundwater. Groundwater quality will be checked to identify its possible uses. The main issue of discussion will be the salinity of the groundwater since the area is near the southern coast of Oman. The amount of inflow and outflow to the sea will be determined in order to estimate the saline water intrusion. In addition, electrical conductivity for several samples taken from Dames and Moore study collected in 1991 will be compared with samples taken from similar locations in 1983. Then, the quality of groundwater can be determined and thus the use of this water. Results Studies about the groundwater in Salalah were done by several consultant companies along the past 30 years for the Ministry of Water Resources. Some of these companies are: Sir William Halcrow & Partners (1977), in Tetratech (1978), John Taylor & Sons (1976), in Tetratech (1978), Hydrotechnica (1986), Dames and Moore (1992) and Entec Europe Limited (1998). Table 2 shows estimates of groundwater inflow of the Jebel by the mentioned companies and an average value for all these studies.  If the most recent studies were considered, Dames and Moore (1992) and Entec Europe Limited (1998), a better comparison can be established since they took more considerable factors in their studies. Figure 1 shows this comparison and average value for their results. The annual average rainfall on the plain is around 110 mm whereas in the Jebel it is about 260 mm (Dames and Moore, 1992). The annual average rainfall is 84.8 mm on the plain and 275 mm on the Jebel (Entec, 1998). The fractions of recharge due to rainfall are 2.5% from the plain and 25% from the Jebel according to Dames and Moore study, compared to 20% and 35% in the Entec study as shown in Table 3. The saline water intrusion leads to a concern about the water quality knowing the high demand needed for irrigation.  Groundwater Quality The results from Dames and Moore show decline in the storage water of 7.9 Mm3yr-1. This is considered high compared to Entec estimate of 1.7 Mm3yr-1. However, both studies prove that saline water intrusion exists in the area. Figure 2 shows some electrical conductivity of pumped groundwater in 1983 and 1991 (Dames and Moore, 1992). The increase in saline water intrusion does not appear to have significant affect on the EC profile in the main freshwater zone which has an average EC of about 1000 mScm-1. Sea water intrusion is expected to continue its increasing which will cause serious problems if no solutions are implemented. Conclusions The study covers a total area of 789 km2, 253 km2 of Salalah Plain and 536 km2 of the Jebel. Studies have evaluated the situation in the Salalah area since 1970's. Some records are since 1940's such as the one in Salalah airport area. The locations of the records gauges and monitoring wells are mainly along the well fileds with others from different locations on Salalah plain and the Jebel. Some records of very high value are likely to be biased. However, recent studies are of more reliability since they consider several important parameters such as infiltration from different sources, impact of fog in enhancing precipitation, evaporation and evapotranspiration, leakage from water supply network, geology of the area, etc. The average value for the recharge to the groundwater is 37.5 Mm3yr-1 from Jebel inflow and 2.45 Mm3yr-1 from rainfall. The total discharge is about 44 Mm3yr-1. Thus, the groundwater in Salalah catchment is extensively declining and a policy will be required to protect against the high pumping which makes 87% of the discharge. Another policy is required to protect the groundwater of the Jebel since it makes 93.9% of the recharge. This recharge may reach over along term an average of 45 Mm3yr-1. This groundwater must support both well fields and the extensive abstraction of irrigation water on the plain.   There is some saline water intrusion in addition to the decline of groundwater storage. Sea water intrusion is a problem of concern since lots of farms are situated along the coastal line of Salalah area. So, additional solutions should be provided to resist this problem. The use of treated wastewater and construction of several dams to enhance the filtration rates from the rainfall may appear to be valid solutions.  References . Dames and Moore, 1992. Water and Wastewater Masterplan for Salalah: Dhofar Municipality, Final Report. . Dames and Moore, 1992. Investigation of The Quality of Groundwater Abstracted from the Salalah Plain: Dhofar Municipality, Final Report. . Entec Europe Limited, 1998. Consultancy Services for The Study of Development Activities on Groundwater Quality of Wadi Adai, Al Khawd and Salalah Well field Protection Zones, Contract No 96-2133, Final Report. Volume 4, Hydrogeology and Modelling, Salalah. Ministry of Water Resources. . Flint, R. B. and Roppin, P. W., 1986. Geological Reassessment of the Salalah Plain Aquifer. Sultanate of Oman, Council for Conservation of Environment and Water Resources. Public Authority for Water Resources Report. . Halcrow, Sir William Halcrow and Partners, 1977. Surveys and Investigations for Land and Water Resources Development in Dhofar, Annex 2- Water Resources and Irrigation. Ministry of Agriculture and Fisheries. . Hydrotechnica, 1986. Robat-Awqad Supply Scheme, Report of pre-feasibility Study. . Price, M. R. S., Ahmed Bin Hamoud Al Harthy, Whitcombe, R. P., 1988. Fog Moisture and its Ecological Effects in Oman. Proc. Arid Lands Today and Tomorrow Conf., E. E. Whitehead et all (eds.), Westview Press, Boulder, Colo., 69-88. (881). . Schemenauer, R. S., 1989. Report to the World Meteorological Organization Technical Coordination Division on the Potential for Fog Water Collection in the Dhofar Region of Southern of Oman. Cloud Physics Research Division, Atmospheric Environment Service, Environment Canada. . Taylor, J. and Sons, 1978. Well field Completion Report Presenting Information Collected on the Water Resources of the Salalah Plain. |