FOSSIL WATER OR RENEWABLE RESOURCE - THE CASE FOR ONE ARABIAN AQUIFER

The eastern side of Saudi-Arabia is underlain by an extensive limeston e aquifer known as the Umm Er Radhuma (UER), The UER outcrops over a d istance of some 1200 km from north to south in the central area and di ps to the east where it is overlain by confining deposits, The rainfal l is low with a large temporal and spatial variability, and a mean ann ual value of less than 200 mm, Before the present work, it was suspect ed that all water in the UER was fossil water and that abstraction con stituted mining the resource, A three-year field study of the water an d soil resources of the region was undertaken, the results of which st rongly contest this assumption, Much of the outcrop zone is covered by the coarse sands of the Ad Dahna dunes, where, periodically, large te mporary lakes form after rainfall, Isotope studies indicate that some recharge is taking place in this region. A hydrological model was set up, based on extensive field measurements of infiltration, runoff, and evaporation (using energy balance instrumentation). The model allowed for four cases of recharge in three broad climatological zones, Thirt y years of historical data were fed into the model to obtain estimates of recharge, Along the coastal region in the east the aquifer is conf ined and the piezometric level is above the ground level, Large salt p ans exist where fissures in the confining layers permit the aquifer wa ter to rise to ground level, These pans, known as sabkhas, act as disc harge areas by evaporation, The rate of evaporation from the saline su rface was measured using the energy balance method, and an estimate wa s made of the annual water loss, Despite wide annual variations in the recharge, it was found that over the thirty-year period the recharge was of a very similar magnitude to the sabkha discharge, This indicate s that the aquifer is more or less in balance and that abstractions fo r irrigation are not necessarily detrimental to the UER.