A vetiver hedgerow is a biological “weir”

What is so special about a vetiver hedgerow?  Why does a vetiver hedgerow spread water so well?

In recent comments on vetiver and bananas it was observed that vetiver enhances moisture availability to adjoining plants.  Why?
Let me try to explain.  On any slope, however flat or steep, rain water eventually collects in small rills (channels) that join bigger rills; these bigger rills often become gullies. Whatever the size, rain water becomes concentrated, and that concentrated flow results in increased velocity, erosion and loss of rain water that can be afforded to be lost. Because of this water concentration much of the area between the rills has less water than it would otherwise have had.
As soon as a vetiver hedgerow is established across the slope of the land these rills – channels – are blocked off and the water backs up behind the hedgerow and moves laterally, depositing silt behind the hedgerow until the land is perfectly level. (see top photo showing young hedge, rills and initial entrapment of sediment). The leading edge of this level land is the hedge which becomes a biological weir allowing water to move through it evenly along its total length. The lower photo shows a mature hedge that has backed up large amounts of water over its entire length. In effect it behaves just like a river weir that when perfectly horizontal assures an even flow of water through out its length.
As a result excess water moves slowly and evenly through the whole length of the hedgerow that results in excellent and evenly grown crops.
Subsequently the water passes through the hedgerow to a lower level and repeats the process.
Vetiver performs this filtering function better than other plants because few plants can match the density of vetiver stems nor its atypical growth patten of regenerating new stems from the center of the plant. Most other clump grasses do not regenerate from the center and end up disintegrating into small clumps.
In addition to enhancing even water flow through the hedgerow, and the consequential spreading of water across the field that improves soil moisture and benefits plant growth, vetiver, because of its deep root system, breaks through hard pans thus allowing better infiltration for ground water recharge. There is plenty of field evidence that shows improved spring and ethemeral stream flows, enhanced wetlands and higher levels in wells when associated with vetiver grass hedgerows.
Vetiver hedges have been shown to reduce rainfall runoff by as much as 70%.  This is a huge amount of water that would otherwise have been lost.  In many parts of Africa and India rainfall hardly exceeds 600 mm of rain.  Under these dry conditions most rainfall comes in a few intense events, and most of that is lost to the land and ends up in downstream flooding.   Vetiver hedgerows will cause the retention of enough rainfall that makes the difference between a successful or failed crop.  I remember asking a vetiver using farmer in India during a year of extreme drought and crop failure whether he had experienced drought conditions, his response was “what drought?”.  This is the power of the Vetiver System.
Dick Grimshaw