Fundamentals of Soil Science (1+1)

Soil Water Movement

Soil Water Movement

• Water is highly dynamic component in soil system. It moves in all the three phases viz. solid, liquid and vapour.
• In a flooded or saturated soil, soil water moves in liquid phase, while in a partially dry or unsaturated soil, it moves in both liquid and vapour phases.
• Movement in solid phase, commonly occurring in the frozen soil, takes place close to clay surface.
• Movement of water within the soil influences water supply to roots and also contributes to underground water table.
• Water movement in the soil occurs in three distinct ways namely saturated flow, unsaturated flow and vapour movement.

Saturated flow:

• This flow occurs when the soil pores are completely filled with water. Water in liquid form flows through water filled macro-pores under the influence of gravity.
• It begins with infiltration, which is water movement into soil when rain or irrigation water is on the soil surface. When the soil profile is wetted, the movement of more water flowing through the wetted soil is termed percolation.

Unsaturated flow:

• In this type of flow, water moves in thin films surrounding soil particles under the influence of surface tension (matric forces) that are much stronger than gravity.
• Even though the driving force is usually greater than for saturated flow, the resistance to flow is enormous. Water will flow toward a lower (more negative) potential regardless of direction.

Vapour movement: In this water vapour moves through air filled pore spaces under the influence of vapour pressure gradient.

Comparison of three types of water movement in soil:

S.No.	Particulars	Saturated flow	Unsaturated flow	Vapour movement
1	Major driving force	Gravitational	Matric potential difference	Vapour pressure gradient
2	Water form	Liquid	Liquid	Vapours
3	Major direction of flow	Downward	All directions	All directions
4	Pore space used	All pores	Micropores	All empty pores
5	Rate of flow	Fast (1-100 cm/day)	Slow (< 1 cm/day)	-