Village-Scale Water Catchments in Nepal
an excerpt by Chris Evans — Designed Visions, Wales
Background
The hills and mountains of Nepal contain thousands of very distinct watershed areas that are characterized by steep slopes between both smooth and rugged high ridges, and valley bottoms with alluvial plains of varying width depending on their height in the watershed. Soil on the relatively young Himalayan slopes is extremely susceptible to erosion, especially during the monsoon, and nutrients are typically easily leached. The valley bottoms act as a natural conduit for this leached fertility leading to productive flat land where crop productivity is generally high. Meanwhile, on the slopes there is a constant struggle for farmers to hold on to valuable fertility, and labour inputs are high in attempts to replace lost nutrients. This is mainly done through animal manure produced after trees have been cut for fodder which is carried often long distances from ever denuding forests and fed to livestock such as cows and buffalo, together with leaf litter used as bedding. The resultant compost is then carried to the fields.
The problems of erosion of young soil on steep slopes are exacerbated by the monsoon when 80% to 90% of the country’s rainfall (annual average 1,600mm) falls in just four months between June and September. There is usually a small amount of rain in winter that falls as snow in high elevations (above 2,000m), followed by a long dry season from February until the monsoon breaks again in June.
In the monsoon, the soil becomes quickly saturated with water. In farmed or grazed areas with less permanent vegetation, there is less storage capacity in soil and biomass, and this is where soils are more quickly eroded and leached. Runoff from these slopes is soil-laden, turning rivers turbid and causing siltation of riverbeds and plains lower down in the watershed. Well-forested watersheds have a much greater capacity to catch and store water, and runoff from these areas is much clearer.
Villages in the hills and mountains are often situated in keyline areas, where the slope changes from roughly concave to convex, and permanent springs are the clean water sources. Higher up slopes less permanent springs are found that issue during the monsoon but that dry up when the rains stop. The figure above describes limiting factors and opportunities offered by such landscapes.
Storage of Water
In more arid or evenly rainfall-distributed regions of the world, water storage in dams and ponds is a key productive feature of water-related design. The conditions in steep sloped monsoon/dry season regions are peculiar, however, because of the huge volume of water in relatively short periods. Here, a need is actually to divert water run-off from potentially erodable or leachable soils into water courses as the soil becomes saturated. Water storage structures such as dams, ponds or tanks become filled within days of the monsoon starting, and have little further use until the monsoon ends when they will quickly become exhausted and are not able to be recharged during the dry season. While there are benefits of this it is less likely that they offset the capital and labour costs involved in construction. There is the additional risk of water bodies created on sloping land actually causing erosion due to their weight and over-saturation of soil. Sites would need to be carefully selected where this risk is minimal, and small-scale systems prioritized.
Therefore the key strategy for collection and storage of water is in the biomass and soil: maintaining a permanent canopy and/or cover to the soil, with plenty of biomass to soak up precipitation — three inches of mulch can absorb an inch of water. Methods to do this, such as agro-forestry, mulching and green manures are described below. Thus the soil is covered, overland run off down the slope is slowed down, and an added benefit is that there is lots of organic matter to feed the soil life, made up of macro- and microorganisms such as earthworms, bacteria and fungi.
Conservation and Use of Water
In the hills and mountains of western Nepal the Himalayan Permaculture Centre (HPC) has developed some innovative and integrated ways of working with water conservation and use in its work.
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The settled hills and mountains of Nepal offer tremendous potential for diverse annual and perennial cropping systems. This potential can be reached through sustainable management of water resources in the catchments of the plethora of rivers that run through the country. Starting with conservation and regeneration of existing resources of forest, soil, water and biodiversity, many techniques exist that can help to increase soil moisture to the benefit of crop diversification and ultimate improvement in productivity. Permaculture design can help to integrate these methods within traditional cultural and social systems to create a flourishing economy as well as domestic self-reliance.
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