VAPOR FLOW CONTRIBUTIONS TO ECO-HYDROLOGY IN DRY LAND CRITICAL ZONES

When the temperature of land surface and plants is lower than that of air and deeper soils and rocks, water vapor moves toward the ground and plant surfaces where dew maybe formed depending on prevailing dew point and wind speed. Some plants are able to directly absorb the dew and vapor flow while the soil can readily absorb both. Certain animals such as desert beetles and ants harvest water from the morning fog using their hydrophobic hairs on the back. Recently, it is also realized that the dew and vapor flow can be a life-saving amount of water for plant survival in at critical times in the critical zones, such as at the end of the dry season in southern Sierra areas of California, a Mediterranean climate zone where there is almost no precipitation from May to September.

Researches are being conducted to quantify the amount of dew and plant-absorbed water from near-surface vapor flow in arid and semi-arid regions. Quantitative leaf water absorption and desorption functions were derived based on laboratory experiments on turf grasses. Results show that plant leaves absorb and release water at different speeds depending on species and varieties. The “ideal” native plants in the dry climate zones can quickly absorb water and slowly release it. This water-holding capacity of plant is characterized by the absorption and desorption coefficients in the functions for plant physiology studies and water balance simulations.

Field studies are conducted to measure the dynamic vapor flow processes from atmosphere to soil surface and from groundwater table to soil surface in the vadose zone. Results show that dew is usually formed on soil and plant surfaces during the daily hours when the temperature gradients are inverted toward the soil surface. The amount of dew harvested using gravels on the soil surface was enough to support water melon agriculture on deserts. The vapor flow can be effectively intercepted by artificially seeded plants in semi-arid regions of China forming new forests.