Abstract
Background Infrequent rainfall and rare flash floods are the primary sources of water in semi-arid and arid regions and are the key aspects in the management of water supply. In these regions, rainfall varies temporally and spatially with infrequent high intensities and sporadic occurrences, making it impossible to systematically record measurements as a proper time series. Purpose The main purpose of this paper is to present temporal (risk assessment) and spatial (isohyet maps) methods for the evaluation of monthly rainfall water volumes and infiltration rates. Additionally, an aridity index based on rainfall and temperature measurements is proposed for use in semi-arid and arid regions. Method The limited data available for semi-arid and arid regions are treated using effective probabilistic and statistical methods. Calculation of the volume of rainfall is achieved through a simple hydrological procedure which involves examining monthly isohyet maps based on a set of rainfall measurements from weather stations in scattered locations. Calculation of rainfall and infiltration risks are performed according to the logarithmic normal probability distribution function. Furthermore, the potential monthly rainfall and infiltration rates are calculated using a set of pre-selected risk percentages. Results Aridity index, rainfall volume, and risk values are presented for Wadi Fatimah, which is located in the western Saudi Arabia. Potential rainfall and infiltration rates are calculated on a monthly scale using a set of pre-selected risk percentages. These simple calculations provide a sound basis for future studies on groundwater resource storage strategy, operation, and management. Conclusions Water management studies can be best advanced through simple but effective assessment methods, as presented in this paper with specific applications to the Wadi Fatimah drainage basin that joins the Red Sea.