Summary of "Chloride mass-balance method for estimating ground water recharge in arid areas: examples from western Saudi Arabia"

Jennifer Richardson


Report for Water Resources or Hydrologic Cycle April 28, 1997

In deciding how to use our ground water resources, it is important to have knowledge of how long it will take before the water we use today will be replenished. Hence, studying models for that are used for estimating ground water recharge is important, especially in dry areas, since it would seem logical that this recharge will take much longer in places that lack rainfall. In this article, a method for estimating ground water recharge in arid areas is discussed and critiqued in terms of study done in western Saudi Arabia.

The method of estimating ground water recharge discussed in this article is based on the chloride concentrations in the ground water. For this method, it is assumed that chloride only enters the ground water through precipitation and that chloride is conserved in the system, i.e. it doesn’t react and disappear when mixed with other components of ground water. These assumptions make the amount of rainfall that recharges the ground water proportional to the chloride concentrations in the ground water and the rainfall. It is further assumed that steady-state conditions are maintained in the system, so that average concentrations and rainfall amounts may be used in the calculations. Finally, it is also assumed that no surface runoff leaves the aquifer area, that no recycling of the chloride occurs in the ground water, and that ground water does not evaporate upgradient from the sampling points used to determine chlorine concentrations. These assumptions are also necessary for the concentrations of chlorine to be proportional to the ground water recharge rate. In this article, these assumptions were shown to be basically valid for test area in western Saudi Arabia.

These assumptions make this method of estimating ground water recharge easy to use. By using averages, modeling rainfall with time and spatial differences is avoided. Since precipitation in these arid areas is extremely variable, this greatly simplifies the estimation procedure. In addition, this method requires no special instruments or data collection procedures, making it inexpensive. In this paper, the authors wished to test whether this method could yield valuable information, since it would be an easy and inexpensive method of estimation if it has any accuracy.

The test area for this paper was 13 alluvial-filled valleys of the Asir and Hijaz mountains of western Saudi Arabia, an area which is arid to semi-arid. There is a great deal of variation of landscape and precipitation patterns in this area, which make it exactly the kind of area for which this method would be valuable.

Using annual average rainfall data and average chlorine concentrations in the rainfall and in the groundwater, the recharge rate was calculated for each of the 13 systems. Recharge as a percent of precipitation ranged from 1 to 20%, but most of the values were around 3 to 4%. Though the authors would have liked to have compared these numbers with results from some sort of physically-based set of measurements for the ground water recharge in this geographical area, no such results were available. Instead, they used data from other semi-arid to arid areas. Most other semi-arid to arid regions for which there are physically-based measurements estimate that ground water recharge is on the order of 1 to 7% of the total precipitation. These measurements seem to support the results that the authors of this paper obtained for their study. Hence, the authors concluded that the chlorine mass-balance method is useful for quick approximations of recharge rates in semi-arid to arid areas, including western Saudi Arabia. They did caution, however, that there might be significant errors in the estimations due to the lack of data they have about this regions actual ground water recharge rate.

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