Interception of Vapor Flow near Soil Surface for Water Conservation and Drought Alleviation

Abstract:

Liquid and vapor flow of water in soil and the eventual vaporization of all waters near the soil surface are mechanisms controlling the near-surface evaporation. Interception and prevention of the vapor form of flow is critical for soil water conservation and drought alleviation in the arid and semiarid regions.

Researches are conducted to quantify the amount of near-surface vapor flow in the semi-arid Loess Plateau of China and the central California of USA. Quantitative leaf water absorption and desorption functions were derived and tested based on laboratory experiments. Results show that plant leaves absorb and release water at different speeds depending on species and varieties. The “ideal” native plants in the dry climates can quickly absorb water and slowly release it. This water-holding capacity of a plant is characterized by the plant’s water retention curves.

Field studies are conducted to measure the dynamic water movements from the soil surface to ten meters below the surface in an attempt to quantify the maximum depths of water extraction due to different vegetation types and mulching measures at the surface. Results show that condensation is usually formed on soil surface membranes during the daily hours when the temperature gradients are inverted toward the soil surface. The soil temperature becomes stable at 13 Degree Celsius below the 4-meter depth in the Loess Plateau of China thus vapor flow is not likely deriving from deeper layers. However, the liquid flow may move in and out depending on water potential gradients and hydraulic conductivity of the layers. The near-surface vapor flow can be effectively intercepted by various mulching measures including gravel-and-sand cover, plant residue and plastic membranes. New studies are attempted to quantify the role of vapor flow for the survival of giant sequoias in the southern Sierra Nevada Mountains of California.