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The aim of the present study was to evaluate fog (cloud water) deposition on vegetation and its contribution to the balance of water and energy in semi-arid and arid regions. Fog deposition onto the woodlands in semi-arid environments in Saudi Arabia was estimated using a detailed land surface model (SOLVEG) that included fog deposition on vegetation in meteorological conditions simulated using a three-dimensional mesoscale meteorological model (MM5). MM5 proved capable of reproducing meteorological variables and successfully detected fog events, although it did overestimate the liquid water content of the fog. Despite the margin of an error due to that overestimation. SOLVEG calculations indicated that fog deposition provides a more effective source of water for plant growth than the heavy rainfall typically observed in semi-arid and arid areas. Water evaporation from the surfaces of the leaves (leaf surface water) eases the plant’s water stress by reducing the need for transpiration, even if the leaf surface water due to fog deposition does not drip onto the soil and is thus not a source of water for the plants. Latent heat being released through evaporation from the leaf surface water and soil reduces the temperature of the soil and leaves during the daytime, which in turn then eases heat stress in woodlands. Moreover, the decreased transpiration rates that result from the increase in evaporation from the leaf surface water due to fog deposition is an effective source of water for use in photosynthesis for the plants suffering from severe water stress in semi-arid and arid environments. Our results suggest that fog deposition can affect the accuracy of the water and heat exchange that take place in the atmosphere-land interface in semi-arid and arid regions. (C) 2009 Elsevier B.V. All rights reserved.