It is known that Vetiver hedgerows increase ground water and allow streams to flow continuosly even during summers. But I am not satisfied with the explanations on how vetiver does it. The main explanation is that the thick overgrowth of vetiver forms a bund like structure that reduces the turbulent flow of water and allows the silt to settle down and form a barrier behind which the mass of water is allowed to collect and percolate into the ground. The dynamic, growing bund is unlike a constructed rock or cement bund in that it continues to grow over the silt and is held firmly into the ground by the vetivers dense deep roots.
Secondly, the vetiver roots can break the hard pan soil layers and increase infiltration.
These two mechanisms are not enough to explain the recharge. I feel the impact of the vetiver is more underground through its roots rather than because of its thick overgrowth. The vertical, dense , deep vetiver roots form an underground system. How exactly this root system is to be modeled is not clear to me. I feel that the vetiver hedge root layer can be modeled as a ditch for surface water flow and as a subsurface barrier for subsurface water flow. That is, the vertical hydraulic conductivity along a vetiver hedgerow is increased and the horizonatl conductivity is reduced. The vetiver root system introduces anisotropic hydraulic conductivities.
Otherwise, it is difficult to explain the vetiver hedgerow. Suppose we say that the root system increases the conductivity isotropically. Then the root region of the vetiver hedgerow can be modeled as a ditch. A ditch will increase surface flows into the ground during the rains but afterwards it will easily allow the subsurface water to flow out of the upper regions into the ditch.
If the vetiverhedgerow root system is behaving as a subsurface barrier, then it can explain how the upper ground is able to retain water longer. But then it cannot explain how the vetiver overgrowth is able to calm the rushing waters and allow the silt to settle and form a living overground dam. There appears to be a sucking in of surface water at the vetiver hedgerow to explain the calming of the waters and the formation of a growing deposit.
These are tentative ideas to explain the observed behaviour of vetiver hedgerows and only thought experiments have been done. I may be entirely off track.
Can an expert help please?