Gravity is the driving force which promotes drainage from saturated soil. What is the driver for upward movement from a shallow water table into the root zone? How does this help explain soil salinization in some extensive irrigated areas (eg. Indus basin)?
Mainly the capillary force is prominent till depth of 1.5m below surface. As per references I have read about salinity, when ground water table elevates above 1.5m below surface, the salt and solute movement in upward direction starts.
As pointed out by Koptsik, capillary force seems to be only responsible (only under normal condition.)
Have a look at SVAT modelling. it is a long time since i did it, but it should provide some answers.
Besides capillary forces in soil matrix, absorption of water by roots of plants and evaporation from the land surface also contribute for the upward mobility of underground water. Salt water on the otherhand, has a lesser mobility since salts reduce the free energy of water. Eventually, sea water evaporation is slow due to the presence of salts in excess.
When the groundwater table reaches 2m from surface then dryland salinity is happened. The discharge of water helps to upward movement of saline water to the surface. Besides, in case of transient salinity (one type of dryland salinity), in wetter the salt reaches into the root zone when pearched water table created for presence of sodic B horizon. In dry season when water evaporates then salt accumulate into the root zone.
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