Analytical estimation show low depth-independent water loss due to vapor flux from deep aquifers

Recent articles have provided estimates of evaporative flux from water tables in deserts that span 5 orders of magnitude. In this paper, we present an analytical calculation that indicates aquifer vapor flux to be limited to 0.01 mm/yr for sites where there is negligible recharge and the water table is well over 20 m below the surface. This value arises from the geothermal gradient, and therefore, is nearly independent of the actual depth of the aquifer. The value is in agreement with several numerical studies, but is 500 times lower than recently reported experimental values, and 100 times larger than an earlier analytical estimate. Plain Language Summary The amount of water that is lost from aquifers under desert conditions is important to managing water resources in these areas of scarcity. The current scientific literature presents estimates which differ by a factor of 50,000, leaving a great deal of uncertainty. By looking at the key physical driver we obtained a simple estimate of this water loss, 10 grams of water per square meter per year, which agreed with estimates based computer models, but was 500 times lower than a recently reported experimental measurement, and 100 times higher than an earlier physically-based analysis that contained an important error in assumptions. Because the change in earth's temperature is consistent with depth, and this is the driver of the water loss, this estimate is almost "universal," holding true for any desert water table that is more than 20 m below the surface of the earth.