In a typical RAA, little precipitation and large potential evaporation coincide causing little runoff. It is widely known that single events are very important in runoff generating processes of arid zones and the efficient runoff is affected by transmission losses (El-Hames and Richards, 1998; Lange et al., 2000). But in some hyper-arid area, single events which can induce efficient runoff seldom occur. There is some research in central Asia which report that the recurrence interval of the rainstorm ranges from 15 years to 30 years in many endoreic basins such as the Hami, Turpan and Tarim basin and the largest single precipitation intensity usually ranges from 10-30 millimeters per day in arid area and below 10 millimeters in hyper-arid area (Wang, 1982). Light rain usually can not induce efficient runoff in dry condition because of dry soil storage. Although there are insufficient experiments about the groundwater recharge from infrequent rainstorms, it is commonly accepted that only the area where water table depth is less than five meters can be efficiently recharged from rainfall when the single precipitation is greater than ten millimeters (Chen and Qu, 1992). According to the groundwater budget study in the Gansu Corridor, less than two percent of the groundwater recharge comes from precipitation (Chen and Qu, 1992).
Since rainstorm is not infrequent, we might deduce arbitrarily that runoff generating processes are not significant in a typical RAA, hydrological modeling would therefore focus on runoff-evaporation processes.