مهندسی آبخیزداری میم

The ratio of catchment to command is inversely related to the amount and intensity of rainfall, the impermeability of soil, and the slope of the land on which it falls. Rainfall intensity is particularly important, since intense storms generate the most runoff.

A watershed is an area that drains to a common point. It may be managed for various objectives, depending on local needs, including capturing runoff, minimizing erosion, and reducing nonpoint source pollution. In management of small watersheds, capturing runoff for local use is conceptually equivalent to harvesting water. This brief encompasses all small-scale, local systems for capturing runoff from rainfall.

Water-harvesting systems either concentrate water into a storage reservoir or apply water directly to the soil in the cropped area. Both types of systems can vary in scale from a few square meters benefiting a single household to a few square kilometers serving a larger group of people.

RESERVOIR SYSTEMS

Systems that concentrate water into storage reservoirs can be used for a variety of purposes, including household, irrigation, or livestock consumption. Rooftop water-catchment systems provide domestic water in many places, especially dry areas with inadequate municipal supplies. A rooftop that is 50 square meters can supply an annual average of 50 liters per day with 500 millimeters annual rainfall. This supply can help reduce competition between agricultural and household demands and can free women from the chore of collecting water.

Harvesting water to collect it in village ponds is an ancient system in some places, including southern India. The principle behind this approach is to concentrate water spatially and temporally. In arid and semi-arid areas where rainfall is low and variable, harvesting rainwater allows users to conserve it until enough water is collected to reliably support a crop. The volume of water stored at the end of the rainy season determines how much area can be cultivated. Traditional water-harvesting systems store water in ponds and reservoirs and deliver it by gravity. With the spread of motorized pumps in recent decades, water can now be captured in some places using the same techniques but allowing collected water to percolate into groundwater aquifers. The systems are small in scale, drawing from gullies and microcatchments to recharge groundwater that supplies a number of local wells.

SOIL-MOISTURE STORAGE SYSTEMS

In many traditional water-harvesting systems, runoff water is channeled directly to the cropped area during rainfall and stored in the soil. Where rainfall is unevenly distributed and soils have high water-holding capacity, this system may store water until the end of the rainy season, when a crop is grown under gradually receding moisture. Where soils are sandier and do not retain moisture for a long time, moisture may be channeled spatially to the location where crops or trees can take advantage of it. Farmers in West Africa commonly use such systems, cultivating their dryland crops behind a variety of small earthen barriers designed to capture moisture. These systems can remain productive several months into the dry season when the surrounding land is barren.

Water-harvesting systems vary a great deal in the ratio of catchment to cultivated area. In dry areas with less available water, this ratio can easily reach 20:1. In such water-harvesting systems, cultivation systems must be extensive and located in sparsely populated areas so that command and catchment areas do not interfere with each other. Where rainfall is higher, the ratio declines-sometimes to the point that the entire catchment area lies within a single plot.

ADOPTION AND REPLICABILITY

Constraints to developing improved water-harvesting systems and adapting existing ones for productive use relate to technical feasibility, financial viability, and social organization.

Technical Feasibility

The key technical question for agricultural water-harvesting systems is whether capturing and storing enough water to raise crop yields is feasible. Much research has gone into measuring the water available to plants under different rainfall, soil, and slope conditions and into developing ways to increase runoff, reduce erosion, or improve storage. In field conditions, technical feasibility depends in large part on local agroclimatic conditions. For example, in India the Indo-German Watershed Development Project works only in villages where the terrain is conducive to water harvesting -that is, there is a sloped, uncultivated catchment area and a good opportunity to build storage ponds.

Financial Viability

Even where water harvesting is technically feasible, it may not be cost-effective. A review of African water-harvesting systems found that many of them cost much more than US$1,000 per hectare to install, making adoption prohibitively expensive.

A typical problem in storing water is the need to line catchment or storage areas so that water can be stored until dry periods when it is actually needed. However, using a plastic or concrete lining can raise costs beyond the financial returns. An example of where lining is most likely to be cost-effective is in mountain regions where water enables farmers to grow counterseasonal crops that can be sold in the plains at high prices.

The main constraints to rooftop collection systems are the need for a tile or sheet metal roof that can capture runoff and a storage tank, both of which can be prohibitively expensive in very poor areas. In that case, village-level systems housed on public buildings may be more viable, along with smaller-scale household systems. The key issue with storage tanks is how to make them large enough to store plenty of water while also keeping costs down.

Given growing water scarcity and the high financial, social, and environmental costs of alternatives, governments may find subsidizing water-harvesting systems attractive, especially those for domestic supply. Some countries already subsidize rooftop collection systems.

Social Organization

Water harvesting requires collective action in densely populated areas where more than one person uses catchment and command areas for multiple purposes. The key question in such cases is whether the benefits of water harvesting exceed the costs of coordination among users. Often this can be the most daunting constraint of all, especially where some resource uses are mutually incompatible and any intervention will impair at least one potential use, as in India. Successful water harvesting requires protecting upper reaches of small catchments against erosion that would re-duce water-storage capacity in the lower reaches. Typically, upper catchments are denuded, so protecting them requires limiting their use for grazing and fuel collection. This restriction imposes the greatest costs on landless people who depend the most on these areas and who do not stand to gain directly from water harvesting. Long-term success requires devising institutional approaches to ensure that landless people gain from the water-harvesting intervention.

CONCLUSION

Water harvesting is certain to grow in importance in coming years as policymakers and planners seek cost-effective solutions to water supplies. Many simple systems can easily be put in place to overcome water scarcity, particularly rooftop collection for domestic use. As water becomes increasingly scarce, treatment of storage ponds to reduce leakage may become more financially viable. On the other hand, systems that require cooperative management and strict limits on land use will probably not become any simpler as local economies and land-use systems become increasingly complex.

Water harvesting does not make large dams and groundwater extraction obsolete. Rather, each system has its own advantages and disadvantages, and all systems can complement each other. The major advantage of water harvesting is that it begins to address problems locally, relieving the pressure on large-scale, centralized systems. Integrating water harvesting with other systems, as well as improving the management of demand, will lead to reliable, costeffective water systems.