As has been confirmed in Ethiopia the Vetiver System has a great capacity to recharge groundwater – how does this work? Why does Vetiver do this better than other species?
Vetiver enables significant ground water recharge because:
(1) the stiff dense Vetiver hedgerow when planted on the contour spreads out rainfall runoff behind the hedgerow before allowing the runoff to proceed downslope. Often the hedgerow creates a back pond of 2 meters or more upslope from the hedge.
(2) Because the hedgerow spreads out water, the water when passing through the hedgerow moves evenly to the downslope area thus enhancing soil infiltration over the whole downslope area.
(3) Vetiver’s unique, dense and deep root system will penetrate deep and through plow and other hardpans, thus opening the way for rainfall infiltration to the groundwater table. Because the crown of the Vetiver plant is so dense rainfall runoff moves very slowly through the hedgerow thus allowing time for the infiltration process
Other grasses cannot compete with these characteristics of Vetiver, in particular its hedge density and rooting characteristics. Addition most are badly effected by drought, fire and over grazing, whereas Vetiver is not.
NOTE: good inter-hedgerow cultivation practices, such as minimum tillage further enhances runoff reduction.
Research results mostly show that Vetiver hedgerows are superior to comparative biological systems. Note that these experiments are carried out on standard runoff plots that do not reflect the reality of microcatchments where rainfall is most often concentrated into defined flow channels of high velocity flows. In addition many of the experiments are one or perhaps at the most two year trials, and do not take account of vetiver’s phenomenal ability to hedge up and become denser over time.
Engineered terraces and other conservation structures generally divert precious water off the land into waterways that more than often become gullies. They do not enhance rain water infiltration as they do not have the capacity to break trough hardpans or create improved infiltration. Additionally they take up a lot of land and harbor weeds and rodents, and are costly to maintain.
Further Vetiver hedgerows can mitigate catastrophic rainfall events as was shown so well in Honduras when Hurricane Mitch hit that country. Farms with vetiver hedgerows were fully protected (Texas A&M).
Dear All,
ReplyDeleteThank you veru much to Dick for this execelent and very
useful informations.With the reference to Honduras and the Hurican Mitch made by Dick in his information, I like to indicate that there is a high need to translate this infomation into Spanish! Hopefully we can get the help of Maria Teresa Rodrigues en Lima - Peru with this translation!
Greetings from a 2 days work-shop on Vetiver System in the buffer-zone of the Natural Protected Area "Tabaconas-Namballe" in the extrem North of Peru, close to the border-line to Ecuador,
Joe Boehnert.
True that vetiver is a great tool in controlling erosion and enhancing groundwater recharge, but it must be considered also that vetiver hedges can worsen conditions in landslide-prone soils. Encouraging water infiltration implies a remarkable increase in weight and the formation of a lubricated layer at the interface with the impermeable subsurface layer where sliding originates - this must be carefully pondered
ReplyDeleteIn theory you are probably correct, but in practice this does not seem to happen. We have virtually no reports of slipping vetiver slopes on slip prone areas. This may be for two reasons: (1) on shallow profiles the roots penetrate the underlying weathering material and thus pin the soil down, and (2) vetiver, contrary to its ability to improve infiltration, it also acts as a huge pump that when planted in close lines, removes water from the profile, thus relieving pore water pressures.
ReplyDeleteHere is a quote from Vetiver System Applications - Technical Reference Manual (Part 3 para3.5):
"Vegetation cover on sloping lands increases water infiltration. Concerns have been raised that the extra water will increase pore water pressure in the soil and lead to slope instability. However, field observations actually show improvements. First, planted on contour lines or modified patterns of lines that trap and spread runoff water on the slope, vetiver’s extensive root system and flow though effect distributes surplus water more evenly and gradually and helps prevent localized accumulation.
Second, the likely increase in infiltration is offset by a higher and gradual rate of soil water depletion by the grass. Research in soil moisture competition in crops in Australia (Dalton et al, 1996) shows that, under low rainfall conditions, this depletion would reduce soil moisture up to 1.5m (4.5’) from the hedges. This increases water infiltration in that zone, leading to the reduction of runoff water and erosion rate. From a geotechnical perspective, these conditions help maintain slope stability. On steep (30-60º) slopes, the space between rows at 1m (3’) VI (Vertical Interval) is very close. Therefore, moisture depletion would be greater and further improve the slope stabilisation process. However, to reduce this potentially harmful effect of vetiver on steep slopes in very high rainfall areas, as a precautionary measure, vetiver hedges could be planted on a gradient of about 0.5% as in graded contour terraces to divert the extra water to stable drainage outlets (Hengchaovanich, 1998)."
This practice of planting vetiver hedgerows on a gradient has proven successful on very steep and large slopes in the Congo DR. Massive slides on the Ho Chi Minh Highway were successfully prevented by using the Vetiver System. Slides due to hurricane size rainfall events in Madagascar and El Salvador have been prevented through the application of VS.
This comment is from Van Tan Tran in Vietnam. He has worked a lot with VS. He is Vice director of Vietnam's Institute of geoscience
ReplyDelete"In theory he is not fully correct either as he doesn't count all the factors:
- When reaching the ground surface, precipitation splits into 3-4 parts: run-off on the surface; infiltration and evaporation (and where there is a vegetation cover, evapotranspiration). With dense vetiver cover that evapotranspiration is huge.
- He firmly assumes there is an impermeable layer immediately below the Vetiver root system.
- He forgets that lubricated layer is now binded.
- And last but not least, and very important, you are absolutely correct about the distributed pore pressure which quickly dissipates, against the localized build up which geotechnical engineers fear most.
Of course there is no 100% guarantee that there will not be any more landslide on Vetiver planted slopes but that may find other reasons than the grass. E.g. in case the failure surface is much deeper than the root can reach, or the plant is not mature enough etc. But incidents are much, much less than without vetiver.
And we have discussed many times that landslides can be of two types: 1). sudden, huge volume and deep failure surface where Vetiver can not help; and 2). gradual, small and shallow where Vetiver is of great help. And very often that 2) will gradually cause 1) i.e. without a vegetation cover a slope will degrade itself (by surface erosion, gully development, soil disintegration and weakening etc.). So by preventing 2) one can eventually minimize 1)."