3.3.3 Runoff and Surface Water Flow
There are a few isolated perennial surface water flows originating from the upper reaches of wadies in the northern Oman Mountains. These flows basically occur only during storms and only for short duration of hours to a few days after storm events. Flows usually are violent, with sharp flood peaks and heavy loads of sediments. These flows, which result from heavy rain, occur only a few times a year, and there can be periods of two or more dry years with no runoff (Ali Al-abri, 1997).
There are two Wadi gauges installed within the study area. Table (3.2) summarizes the details of these gauges and their locations are shown in Figure (3.3). All the details of these gauges are given in Appendix (B).
The Wadi gauging sites in the Wadi Tanuf catchment consist of automatic gauges with datalogger recorder, pressure transducer and graphical chart recorder and manometer system near the upstream end of the full reservoir area an automatic recording gauge at the Dam to measure reservoir water levels. The Wadi Tanuf gauge located at the upper end of the reservoir area was installed, including auxiliary crst-stage gauges, prior to the construction of the Tanuf Dam. The Wadi Tanuf Dam gauge is a float gauge located upstream of the right abutment of the dam and is accessible from the right bank. A staff gauge is also mounted at the right abutment of the dam wingwall. The recorder in the float gauge shelter is an Otto graphical recorder with continuous chart, cable- activated by a float and weight.
Records of wadi flow have been collected at the upper Wadi Tanuf monitoring site since 1986 and from the dam site gauge since 1989.The Wadi flow records since 1986 have been generally good and complete, with exception of some short periods of lost record. Good records of reservoir water levels have been obtained during flood events at the Wadi Tanuf dam site, except for periods when the intake of the first gauge system was blocked by silt. Records of daily discharges, some instantaneous discharges, gauge heights of flood hydrographs and peak discharges determined by indirect methods have been obtained in digital and hardcopy forms from the Ministry Surface Water Department for the upper Wadi Tanuf gauge site.
Initial reviews of the available data indicate that additional Wadi gauges may be recommended between the Tanuf Dam and the Nizwa area to more accurately measure Wadi flows from the reservoir area and recharge from surface water flows in the downstream recharge area.
Most Wadi flow occurs in few events during the year. The mean annual volume for the Wadi Tanuf Wadi gauge is about 4.9 x 10 m³ from 1986 to 1995 (MWR SWD). The highest instantaneous discharge that has been recorded at the upper Tanuf Wadi gauge was 2290 m³/s in 1992(Cansult Limited, 1996).
The average annual ratio of runoff to rainfall for the period of 1986 to 1995 has been computed by (Cansult Limited, 1996) as 9.2%. The range varied from 0.1% (1991) to 27.7% (1992). Records of antecedent moisture conditions are not well defined in the upper catchment areas as well as the recharge area downstream of the Tanuf Dam.
Figure (3.6) shows total and average annual inflows measured at the Wadi Tanuf gauging station upstream of the dam for the period of record.