Curbing Soil Erosion
An Urgent Need

By Khilendra Basnyat

SOIL, which is a product of biochemical weathering of rocks, is everywhere around us. It is indispensable for the survival of human beings.

In fact, forest vegetation and soil are interrelated. Therefore, any change in forest vegetation may be followed by changes in soil conditions and these changes may have increased sediment loads downstream or on-site consequences such as reduced growth rates in subsequent rotation or cycles.

Since the growing demand for forest products leads to deforestation, the soil is left unprotected. Rainfall rain-off increases leaving inadequate water to percolate downward in order to re-charge aquifers. The increased runoff carries topsoil with it, decreasing land fertility and silting rivers and reservoirs. When firewood becomes scarce, villagers use cow-dung and crop residues as fuel, thereby depriving their fields of necessary nutrients.

When a forest is cleared, the soil becomes loose. Consequently, there is soil erosion.

In reality, soil erosion is a two-phase process comprising the detachment of individual particles from the soil and their transport by erosive agents such as running water and wind. When adequate energy is not available to transport particles a third phase occurs.

Rain-splash is considered the most detaching agent. Due to rain drops striking a bare social surface, soil particles may be thrown through the air over distant places. Continuous exposure to intense rainstorms weakens the soil.

Alternate wetting and drying but also by freezing frost actions does not only break up the soil. In addition, agricultural activities and the trampling of the people and livestock disturb soil. Running water and wind also detach soil particles.

In tropical and semi-sand areas, soil erosion has been considered a hazard allied with agriculture. Its importance has become evident in areas devoted to forestry, transport and recreation.

The impact of erosion is realised in the areas where top soil is removed by water and wind, the subsoil and bedrock exported and the land entrenched by guilies. Also, it is felt in the areas down valleys or downward where the ground is covered with sand.

The increasing population has forced great deforestation for cultivation, for fuel and for settlement. Consequently, the land is over exploited, leading to soil erosion.

The four main rivers of Nepal with over six thousand tributaries carry about two hundred and forty million cubic metres of fertile soil annually into the Bay of Bengal.

The topsoil flushing down into India and Bangladesh is our precious export. As fertile soil slips away, the productive capacity of the hills further declines.

Some of our terraces in the hills have been expertly managed and are relatively stable. Others continue to be cultivated despite their decreasing fertility and still others have reached the point of no return.

Actually, the increase of soil erosion due to combination of interrelated factors, especially population growth, is one of the chief environmental threats in the hills.

In recent times, the incidence of flooding of rivers coming down from the hills has been increasing. It is reported that some of the riverbeds in the tarai are rising from six inches to one foot every year. This not only guarantees a great incidence of floods from even normal volumes of water during the monsoon but also causes the rivers to meander after destroying farms in their courses and poses a great menance to our irrigation structures already completed or at present under construction.

As a result of population growth, cultivation has already been extended on marginal lands and steep slopes, pushing far beyond the physical limits of the land at the cost of green vegetation and forests. The outcomes are landslides and soil erosion. What is more, the sediments washed away from the hills and mountains are deposited at the riverbeds of the plains, making the adjoining fertile land more vulnerable to flood.

Since soil fertility enhances agriculture productivity, decline in fertility affects the whole agrarian system. Our agricultural output has begun to decrease from the decline in soil fertility.

As agriculture is the mainstay of our economy, contributing about forty-two per cent to the gross domestic product and employing over eighty-one per cent of the population, the maintenance of soil fertility deserves high priority.

Most cultivators in the Middle Mountains, who sustained by less than a quarter of hectare of land, added their income by raising livestock. In this zone, forests have deteriorated because of the clearing of forests for cultivation, uncontrolled grazing and the gathering of wood for fuel.

Deforestation on hills leads to serious soil erosion, which further reduces the ability of the land to support an increasing population, and to a decrease in perennial water supplies, thereby reducing the available irrigable land.

In fact, soil depth gets thinner as one goes from south to north. It was discovered that the Siwaliks and mid-hills are more susceptible to erosion. The former has erodible soil whereas the latter is rich in sandy loam with low to high production areas.

There are many degraded areas in watersheds. Soil loss ranges from five to two hundred metric tons per hectare per annum depending on the land use and landforms.

Deforestation along road construction routes also contributed to the loss of topsoil, landslides and adequate water runoff.

The Community Forestry Programme has not only created greenery in the once barren areas but also has reduced the soil erosion to some extent. However, unless the people are educated about the environment and mobilised for conservation such efforts cannot be sustainable.

In reality, in order to grow fruits and tea in hills, there is sufficient rainfall so that very few areas need irrigation. For vegetable cultivation, the valley in hills and also hill slopes are suitable. One can irrigate by drop or sprinkle irrigation for vegetable and fruit cultivation on hills slope. It will help stop soil erosion and landslide.

A casual inspection shows that many areas marked as dense forests have become totally denuded and have suffered either extensive soil-leaching or landslide.

The eight kilometre forest belt (Charkose Jhadi) between the Bhabar range where the hills ended and the plains in the south left preserved during the Rana regime as a filter zone, is now converted into cultivated areas. This has caused further soil erosion. Hence, it has become an urgent need to take stringent steps to curb soil erosion as soon as possible.

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