The difficulties of regional groundwater resources assessments in Arid areas

The paper discusses the hydrological characteristics of regional groundwater systems in arid areas. The difficulties of defining geological detail at large scales are raised and the importance of understanding lithological multi-layering and local structures aberrations. Emphasis is placed upon the variable character of hydraulic diffusivity and the importance of vertical hydraulic conductivity in intervening non-aquifer units. Recharge and discharge from the system are discussed together with the problem of groundwater flows in unstressed systems. The importance of using abstraction data for model calibration and the degree calibration that can be expected are noted. It is concluded that that models produced allow bulk flow interrogation of the regional systems and afford general principles of groundwater management and supply protection to be pursued, but not local impact details. It is stressed that the groundwater resources represented should not be interpreted beyond the depths of abstraction accessibility. (author)