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The Vital Importance of Soil and Water Conservation In the Tropics.

Monday, May 14, 2007

Famines in Ethiopia, Sudan and Rwanda. Floods in Bangladesh, Brazil and China. Mudslides in Central America, the Philippines, Peru and California. Toxic leachate from mine dumps poisoning water tables. Effluent from garbage dumps, landfills, feedlots and farms polluting the groundwater, rivers and lakes. Decreased groundwater recharge leading to shortages of potable well water. Continued unabated runoff from croplands; new infrastructure developments such as roads, parking lots, construction sites and airports, all these can impact on silting up rivers, lakes, polluting water supplies and eventually, silting up essential harbours.

Few resource problems are so important, so threatening and yet so little recognized as the disappearance of our soil and the pollution of our water. Each year billions of tonnes of soil are washed out to the sea or carried away with the wind, and little is done about it. It is seldom discussed in the media and virtually ignored by our politicians. Soil is a finite resource, we depend on it for our very existence. Natural processes to replace this soil loss, would take centuries, if not millennia. The disappearance of soil threatens to undermine agriculture and limit our ability to feed ourselves. This is especially true as it effects tiny ocean islands in the tropics.

There is no way we can replace such soil losses. But we do have some options to control this loss.

Because sheet erosion is little understood as a problem, few, including most farmers, planners or governments give it the attention it deserves. Europe, the continent least affected by erosion is estimated to be losing one billion tonnes of soil a year. Asia, the worst affected, could be losing 25 billion tonnes per year. The United States, where soil conservation is Law, loses well over one billion tonnes a year, equivalent to more than 300,000 hectares of crop growing potential.

In India where over 5 billion tonnes of soil is being eroded annually, their soil conservation service estimated that 30% is being lost to the sea, 10% is deposited in reservoirs as silt and 60% being displaced from one location to another. Practically the same distribution pattern is in evidence round the world. Worse still, 30 – 40% of the available rainfall, essential for crop production and ground water recharge, is also being lost to the drainage network with this uncontrolled runoff.

Even in the temperate zones where rainfall is more moderate and evenly distributed, erosion is still out of control and visible on a daily basis. Muddy drains, streams and rivers carrying tons of soil out to sea; even the dust on vehicles is air-borne clay.

Throughout the world today, Man has lost control of the watershed’s hydrology, hundreds of years ago, plants that once managed the hydrology of the landscape are no longer there, we need to reinstall the ‘wetting cycle’ that was there before, and the only way we can do this effectively is with a system of sustainable biological hedges.

Water is recyclable, soil is not.

Erosion is caused by the action of water and wind after the soil has been disturbed by man or the hooves of animals. Water erosion is a most devastating sequential process of ‘sheet erosion – landslides – mudslides - slips - rill erosion’- and ultimately, gulley erosion’.

Sheet Erosion
Sheet erosion is the incipient form of worse to come, it is the beginning of the whole process of land degradation and leads to the most damaging forms of erosion, mainly because it is not recognized and therefore seldom controlled. At first it is not spectacular, and is generally looked upon as surface water ‘carrying a bit of mud’ and considered not worth controlling.

It is only when sheet erosion develops in to rills, then gullies that it captures the attention of the authorities and some action may have to be taken to control the situation.

However, triggered by rainfall, sheet erosion accounts for the loss of billions of tons of soil every year. The energy of a single large rainstorm is enough to “splash” over 200 metric tons of soil into the air on a single hectare of bare and loose soil. Individual particles can be splashed more than 0.5 m in height and 1.5 m sideways. Particles of soil are knocked loose and then carried away by the runoff. This runoff further strips unprotected areas of their valuable topsoil and nutrients and becomes the muddy water that ends up in drains, streams, rivers and finally, the sea.

As the name implies – SHEET -- erosion covers the whole area the rain is falling over and moves a thin layer of soil down the slope in a sheet of water before it is broken up in to rills and gullies. But it is this initial uncontrolled sheet of runoff that is the basis of the whole problem of erosion anywhere in the world and in its initial state, it is the easiest form of erosion to control.

In extreme cases, the excessive uncontrolled runoff from a high intensity rain storm, can form a sheet of water 4 – 500mm deep over the whole area, which eventually ends up as a fast flowing steam tipping down a slope like a waterfall, causing slips, catastrophic mudslides, and landslides capable of wiping out whole villages and causing great loss of life, and this from an area where there seemed to be no previous threat.

The aim of every farmer, government planner, or Aid agency should be to prevent ‘sheet erosion’. Erosion occurs when there is enough energy in the flowing water to cause it, if we want to control erosion, we have got to deprive the runoff of that energy. By preventing sheet erosion we are preventing the other damaging forms of erosion.

Prevention of Sheet Erosion

Constructed earthen embankments, or contour bunds, have slowed erosion throughout the world since the 1930s. But this method of soil conservation creates an unnatural system of drainage and is no longer considered appropriate for small holders in the tropics. The contour banks convey the runoff sideways to a waterway or the drainage network. But the bank is made of the same soil it is supposed to protect, and because of its construction the sides of the bank are steeper than the surrounding soil and over time the bank will erode and "melt" away.

When this happens it will have to be replaced at great cost to the farmer. Moreover, to collect sufficient soil to repair or replace the collapsed bank needs the added costs of bulldozers or hired labour. Worse still the constructed system of conservation is designed to run the ‘water’ off the land and in to the drainage network – the last thing a rainfed farmer wants and the worst possible scenario for protecting the watershed. Delivering all the runoff to the drainage network as ‘fast as possible’ causes massive flooding.

Absorption banks, unlike contour or diversion banks, are constructed and designed to hold runoff. But they are an unnatural system that ‘ponds’ runoff in low patches for little benefit to anything. They have limited length and usually cannot exceed 500m or they risk being breached by the water they are holding. In heavy tropical storms they are often breached causing more damage than if they had never been constructed in the first place.

The most efficient system of control in our ‘toolkit’ of preventative measures is the use of a natural biological system in the tropics or subtropics, namely Vetiver hedges. By planting a single line of vetiver slips across the slope in an upper catchment area, a dense sustainable hedge can be established which will do exactly what is needed to control runoff – it will spread out the runoff, taking the energy out of it while it slowly filters through the hedge leaving behind the soil it was carrying to form a natural terrace, but never concentrating in to a dangerous stream of runoff.

Vetiver hedges planted across the slope can run for kilometres, there is no restriction on their length as they don’t convey water they slowly filter it along the entire length. This can have a major impact on the replenishing of under-ground aquifers, as the hedges can cover a vast area and hold back runoff to a distance 20m behind the hedge.

The hedges can be planted across rills and gullies and, once established, will fill these depressions with filtered silt transforming the shape of the land in to something more manageable and conserving precious soil and water.

The benefits of the vetiver system are too great to elaborate on here but are fully explained on our website

Rill erosion if not controlled leads to gully erosion, these can be controlled with vetiver hedges, but the impact of these is nowhere near as dangerous as the impact of uncontrolled sheet erosion (or sheet runoff) from an episode of high intensity rain in the upper catchment. Properly laid out and once established, the Vetiver System of hedges can control all categories of erosion in the tropics and subtropics.

J.C.Greenfield. May 12. 07.


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