Oman is an arid country has an overall rainfall average below 100 mm/yr with high evaporation rate ranging between 1700 mm/yr in the southern part to 2100 mm/yr in the northern part and 3000 mm/yr in the interior. Groundwater is the main source of water use representing 92 % of the total renewable water resources. Rainfall and wadi flow are the main source for recharging the groundwater aquifers. Storm and flood events are rare, 90 % or more of the time is dry. Although, tropical storms occur from time to time in the Arabian sea but they rarely exceed the tropical cyclone. On 5-6 June 2007, Oman was subjected to the strongest tropical cyclone ever been recorded on it’s recent history since 1890. This exceptional prevailing climate produced a heavy rainfall, concentrated on the eastern coastal area of Northern Oman, particularly Muscat and As Sharqiyah Assessment regions. This leads to a precipitation of the highest recorded intensity for one day 714 mm at Jabal Al Asfar. The heavy rainfall accompanied with high flash floods caused massive devastation, including various infrastructures, roads, communication lines, fresh water supply lines in addition to destroying many farms, building and private properties with estimated damages cost of 1,500 million Rial Omani, 3,900 million US$ which equal to 13% of the GDP (MNE, 2006). The objective of this paper is to estimate the amount of groundwater recharge occurred as a result of Gonu cyclone. The study area covered (16,477 km2) representing both Muscat and As Sharqiyah assessment areas in Northern Oman. During Gonu event (24 hours), a rainfall depth of 240 mm fall over the study area which is more than two and half the annual average of the country. The rainfall volume measured was 3,672 Mm3 equal to 19 % of the long term annual average volume overall Oman (19,526 Mm3), the surface runoff measured (losses to the sea) was 1870 Mm3 which represent eight times the annual average (211 Mm3). The calculated recharge for the shallow alluvium and fracture limestone aquifers were 1,686 Mm3 which is five times the long term annual average recharge for the study area (325 Mm3) and approximately 1,5 times the annual average recharge for the Sultanate. These figures indicate that arid regions, for only one day, may subjected to exceptional climate condition that may cause flash floods with severe damages and on the other hand, play an important role on the national water resources budget. Also, the recorded hydrological data during the event spot the light on the necessity of taking into accounts the reconsideration of rainfall and flood frequency analysis methods applied to Oman, the redesign of recharge dams according to the flood peak discharge measured and the redesign of main infrastructures.

Oman is an arid country has an overall rainfall average below 100 mm/yr with high evaporation rate ranging between 1700 mm/yr in the southern part to 2100 mm/yr in the northern part and 3000 mm/yr in the interior. Groundwater is the main source of water use representing 92 % of the total renewabl...