Simulated changes in water levels in the Minjur Aquifer, Riyadh area, Saudi Arabia

A two-dimensional finite-difference computer model of the Minjur aquifer in the Riyadh area of Saudi Arabia was developed to simulate and predict drawdown in an aquifer which, in 1979, supplied more than 80 percent of all water used in this capital area. The Minjur was modeled as a confined aquifer recharged by leakage through underlying confining beds. Pumpage was simulated for a 23-year period during which it increased from 2,160 to 220 ,000 cubic meters per day. At Riyadh, the top of the Minjur aquifer is about 1200 meters below land surface. Aquifer thickness within the 40,000-square-kilometer study area ranges from 74 to 138 meters inclusively. Values for aquifer characteristics used in the model are: transmissivity, from 0.001 to 0.0062 square meters per second; storage coefficients, 0.07 for the unconfined, and 0.0004 for the confined sectors of the model area. A value of 0.02 meters per second was used to represent the vertical conductivity of the confining bed. Predictive simulations indicate that if the Minjur aquifer in the Riyadh area is stressed by estimated future pumpages, static water-level declines ranging from 10 up to and including 90 meters, depending on location, will occur from 1980 through 1999. Simulations also demonstrated that pumping levels in the new Buwayb well field will decline substantially from 1980 through 1981. (USGS)