Unsaturated Matrix Case Study

4.1.2 Lateral infiltration into the unsaturated matrix (qlu)

Lateral infiltration of macropore water into the unsaturated soil matrix takes place strictly over the depth where stored macropore water is in contact with the unsaturated matrix. Absorption is the dominant mechanism at low soil moisture contents. It will be negligible under wet conditions even when there is a large pressure head gradient and for this condition Darcy flow will be dominant. Darcy flow is very small under dry conditions because of very low hydraulic conductivities.

Using one dimensional flow equation combined with Darcy’s equation, Shakya (1995) computed the lateral infiltration as qlu = Sr / 2t-1/2

Where,

Here perched groundwater is defined as the subsurface water that forms a saturated horizon within porous media at an elevation higher than the local or regional groundwater table, dpol is the effective diameter of soil polygon which is nothing but the macropore diameter and is given by

dpol = dp,min + (dp,max - dp,min )*(1-M)

Where, dp,min and dp,max are the minimum and maximum diameter of the macropores, M is the relative macropore density which is the ratio of the static macropore volume to the static macropore volume at surface.

4.1.5 Rapid drainage (qrd)

Rapid drainage to drainage systems can occur via a network of lateral interconnected cracks or via otherwise (nearly) interconnected macropores. Also when macropores are separated by just thin walls of soil matrix they can enhance drainage considerably. It occurs only in the MB domain of macropores.

4.2 Macropore dominated processes

• Retention excess flow – This flow occurs if the soil is already saturated. Main factor affecting this flow is the availability of soil moisture content. Retention excess flow can be calculated as Qre = P – I

Where, P is the rainfall intensity in mm/hr and I is the infiltration capacity in

• Interflow (Shallow Subsurface Flow) - Interflow is type of subsurface flow which comes to the surface after flowing in the top horizon instead of getting into the water table. In this type of flow, the movement of subsurface water is roughly parallel to the ground surface.

• Pipe Flow – Natural soil pipes are cavities greater than 1 mm diameter that are continuous in length such that they can transmit water, sediment and solutes through the soil and bypass the soil matrix. Soil pipes are created by range of processes including animal burrows and turbulent flows in macropores. Pipes may transmit a large proportion of water to the stream in some catchments. The pipe networks can be complex and can provide a rapid connectivity of water, sediment and solutes coupling distant parts of hillslopes with stream channels.

• Subsurface Flow – The runoff movement is principally below the ground surface. This will occur wherever the soil’s infiltration capacity is greater than zero. This is the dominant runoff process in the temperate humid regions where lush vegetation cover has produced a porous soil structures with infiltration capacity of many inches per