Water vapour transport in a soil column in the presence of an osmotic gradient

In arid and semi-arid regions because of water access and soil salinity problems extending of agriculture to a new area has been faced with restriction. Knowledge of the transport process of moisture and energy in subsurface soil is vital to understanding the environmental and economic impact of agricultural practices in these areas. This study was conducted to: (a) assess the relative importance of water vapour movement under moisture, temperature and osmotic gradients on moisture and energy budget in the presence of salts, (b) evaluate the temperature differential method to estimate evaporation in 30 minute interval. The experiment was conducted in a sandy soil column that was buried in the field and irrigated with a pore volume of 50 g/L NaCl solution. Three reference air-dried sandy soil cylinders were buried beside the column. Water content, temperature and electrical conductivity of soil water in depths of 1, 5 and 10 cm of the column as well as surface temperature of soil column and air-dried soil cylinders; and temperature of 5 cm depth in air-dried cylinders were monitored. The result showed that nearly 96% of vapour transfer was due to temperature gradient. Although the osmotic effect on vapour movement was less than 3%, nevertheless still was more than moisture gradient effect in current work. The contribution of the water vapour flux to the total moisture flux was 5%. The heat transported by vapour flux was significant and accounted for 45% of total heat flux in 1-5 cm depth and up to 30% at 5-10 cm depth. The observed difference between estimated cumulative evaporations using the differential method and energy balance equation was less than 5%.