Field measurements in the Sa'dah area showed significant differences in transmissivity (Table 1), which reflect the variations in the hydraulic permeability and thickness.2S Also the wide range of values for storativity reflects the variability of the confining pressure due to differences in lithology and thickness of both the aquifer and the overlying formations, particularly toward the south-eastern edge of the system. However, direct and indirect recharge has been reported in the southern Sa'dah-Najran area where average rainfall is 250-300 mm/yr. In Yemen, it is estimated that a total recharge (natural sources plus irrigation return) of 17.7 MCM/yr30 occurs in this escarpment zone, which is equivalent to 7.9 mm/yr.3' although lower (4.4 mm/yr,32 3.2 mm/yr33) values have been suggested.