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How do vetiver hedgerows increase groundwater?

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?

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Comments

1. If no explanation fits an

If no explanation fits an alleged conclusion, then the conclusion must be re-examined. Vetiver helps increase ground water recharge, but how much?

I do not think there is any scientific evidence for your assertion/ belief that Vetiver hedgerows increase ground water to such an extent that the streams flow continuously even during summers.

The main difference between an activist perspective and a scientific perspective is that of quantification. Vetiver increases ground water recharge, and increased GWR can help streams flow even during summers, and diminish floods. But that does not mean Vetiver increases GWR so much as to actually make this happen. There are two reasons activists usually do not go in to quantification.

First, few have the necessary background in sciences. Second, they are driven by some ideology, and have a need to reach a certain conclusion.

Therefore, they are prone to go overboard and jump to romantic conclusions.

Can it be that your faith in Vetiver is derived from websites like vetiver.org and gaia-movement.org ?

Read http://www.cigrjournal.org/index.php/Ejounral/article/view/1267/1123 for a more scientific assessment.

It makes no mention of so much recharge as to make streams continuously flow even during summers, etc.

Chetan Pandit

2. studies on vetiver and water flow/infiltration

Some scientific studies on vetiver and  water flow/infiltration  are:

  1. [Deesaeng et al.] Vetiver potential for increasing groundwater recharge,  (www.vetiver.org/ICV4pdfs/DAS01.pdf) The paper reports average soil water content along 150 cm profile of few crops with and withoutt vetiver hedgerows (Fig 2).  The accumulative runoff from these plots (Fig 4)  are also measured. Thus the effectiveness of vetiver for reducing runoff and increasing soil moisture is quantified in this paper. However, the study does not go into the mechanisms of  this quantitative improvement.
  2. [Metcalfe, Truong and Smith] Hydraulic Characteristics of Vetiver Hedges. in Deep Flows, (prvn.rdpb.go.th/files/icv/4-04t.pdf) is a Vetiver Network Award winning paper that measured the resistance of overground vetiver hedges to water flow and the reduction of effective flow area because of the  above ground  parts of the vetiver hedge. 
  3. [Hussein et al]  Sediment Retention by a Stiff Grass Hedge under Subcritical Flow Conditions,  ( www98.griffith.edu.au/dspace/bitstream/10072/16321/1/44395.pdf) measured the water and sediment depths , the shape of the sediment deposits in the back water region behind a vetiver hedge.
  4. [C. Smeal and P. Truong] at GELITA APA in Queensland, Australia (www.vetiver.com/ICV4pdfs/BA05.pdf ) reported an R&D program on "Effects of vetiver grass on some soil physical properties". They reported that an extensive soil sampling program is being conducted to compare soil physical parameters like water infiltration rates and soil bulk density under vetiver cultivation. I was looking forward to results from this experiment but could not obtain them.

Papers [1-3]  measure the  above ground  effects of vetiver  to surface  water flow.  [1] just measures the runoff and increase in soil moisture but does not offer any models or model parameters. [2] offers a model of reduced flow area. [3] attempts to create a model  taking into account the particle sizes and the dynamics of the back water formation.

I was hoping  that  [4]  would measure the  resistance of  vetiver roots  to subsurface  flow and  how the roots modify infiltration. A model  relating  soil hydraulic conductivity isotropically to the root growth can be plugged into a hydraulic model (parameters from [2, 3] also would be needed for above surface effects) which then would be able to predict the GWR and runoff corresponding to precipitation rates.

The paper quoted [Islam et al.] Vetiver Grass as a Potential Resource for Rural Development in Bangladesh  (http://www.cigrjournal.org/index.php/Ejounral/article/view/1267/1123 ) is a general paper  and does not report any  experimental results  or model unlike  [1- 3].

पृथिवी सस्यशालिनी
the earth be green

3. vetiver hedgerows

Dear Vinod Kumar,

Vetiver , as has been explained is a grass which grows rapidly and due to the typical chracteristic of the root system tends to create a mesh which holds the soil in place. This is a bio soil conservation and stabilizing method.

This is essentially used to stop soil erosion especilly in sandy areas and along hill slopes, instead of stone bunding this grass is planted along the contour and due to the inherent properties it helps in retarding the top soil from being eroded.

Due to the presence of the grass on surface the velocity of the flowing water is also considerably reduced. But this is true for water flowing thru any grass. 

Coming to, the water conservation part of it. 

For addirional recharge to take place, the two most essential factors are;

  • A thick soil cover which is conducive to infiltration
  • Ssecond a longer retention of water on that surface

Looking at Ventiver Hedgerows, beign planted  along the contours does land up having the exact same effect - it reduces soil erosion, and there by promotes accumulaton of soil on surface, and  the reduction of the velocity of water increases the time water is retained on surface, there by it gets the additional time to seep downwards and infiltrate in ot the subsurface.

If the roots go deeper, and create a subsurface dam, even the lateral flow of  subsoil water would be stopped and it would try to move further downward. All this will happen provided there is sufficient rainfall. Secondly, the area where this is planted is a potential recharge area.

But just becaue, something like this has been used in other countries, the plant species need not be introduced in other countries, becasue we really do not know what kind of bilological impact it may have on the environment, it is better to use localy growing  plants exhibiting simalar chracteristic.

But dont expect the streams to start flowing all year round in the hard stony mountain terrains, especially so in the higher elevation areas where the first order stress originate,

This may probably, be  possible in the low lying valley portions, that too after a couple of years, and with a strict control on Groundwater abstraction, because Groundwater does play and important role in the contribution to base flow of streams.

Hopefully, this clears up some doubts regarding the use of Vetevier  etc..

If you are not satisfied with this , please read the extensive research findings on use of Vetiver hedgerows which have been carried out in various countries where it is being used for soil conservation purpose.

Regards

Dr. Anil Lalwani

www.wellwaterworks.com

4. further information

can this help if planted on slope of the land to prevent the erosion and in storm water channel / how will it help?

nesture

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