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Arid-site recharge, while generally low, can be highly variable. Recharge under similar climate and soil conditions but with different plant cover and topography can vary from zero to more than the annual precipitation. Simple estimates of recharge based on fixed fractions of annual precipitation are misleading because they do not reflect the plant and soil factors controlling recharge. Detailed water balance models, successful for irrigated agriculture, fail to predict evapotranspiration accurately under conditions where plants suffer seasonal water stress and cover is sparse. Recharge, when estimated as a residual in water balance models, may be in error by as much as an order of magnitude. Similar errors can occur when soil water flow models are used with measured o r estimated soil hydraulic conductivities and tension gradients. Lysimetry and tracer tests offer the best hope for evaluating recharge at arid sites, particularly in siting waste disposal facilities, where reliable recharge estimates are needed. Quantification of drainage using lysimetry over several years under a given set of soil, plant, and climate conditions for a specific site can provide a basis for calibrating models for recharge prediction. Tracer tests using such long-lived tracers as '"CI or perhaps stable isotopes ("0, deuterium) can provide qualitative estimates of recent recharge at a given site. KEY WOKIX Rechargc Watcr halancc Lysimetry Traccr tcsts

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Consideration of the partial pressure of H2O in the Martian atmosphere and the range of mean annual temperatures at the Martian surface suggests that the occurrence of ground ice in equilibrium with the atmosphere is restricted to latitudes poleward of ±40°. However, there is a growing body of morphologic evidence that indicates that substantial quantities of ground ice may have been present in the equatorial regolith throughout Martian geologic time. The accepted explanation for this apparent contradiction has been that the H2O found near the equator is a relic, emplaced very early in Martian geologic history (>3.5 b.y. ago) and under substantially different climatic conditions. It is generally believed that this fossil ground ice layer has been preserved to the present day by the diffusion‐limiting properties of a relatively shallow layer ( ≲10 m) of fine‐grained regolith. To evaluate this hypothesis, the lifetime of a 200‐m layer of ground ice, buried below 100 m of ice‐free regolith, has been examined for the latitudes between ±30° on Mars. Twelve model pore size distributions, representative of silt‐ and clay‐type soils found on Earth, were selected to simulate the pore structure of the Martian regolith. The parallel pore model of gaseous diffusion was then used to calculate the flux of escaping H2O molecules from the buried ground ice layer. The potential effects of such factors as depth of burial, the Martian geothermal gradient, regolith porosity, adsorption, surface diffusion, and climatic change were also considered. Based on our analysis, the most important factors affecting the stability of equatorial ground ice appear to be (1) soil structure, (2) the magnitude of the geothermal gradient, and (3) the climatic desorption of CO2 from the regolith. We conclude that while the conditions necessary for the long‐term survival of ground ice may exist in isolated regions within the equatorial regolith, on the global scale it appears unlikely that a fossil ground ice layer has been preserved throughout Martian geologic history. If massive ground ice does presently exist in the equatorial regolith, then its presence may imply the existence of some continuous or periodic process of replenishment.

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