Watershed at the first sight is a definite geographical area which receives drained out rain water. Watershed is a big surface drainage system. The magnitude of the watershed is determined by the elevation of point of drainage: lower the drainage point, the greater will be the area of watershed and the higher the elevation of drainage point, the smal1er will be the watershed area. Obviously drainage point at sea level will comprise maximum watershed area. Depending on the area of watershed it may be classified into micro, mini, medi, macro and mega watersheds. A micro watershed is less than 100 ha, whereas the macro watershed may be thousands of hectares.
The total area of watershed may be consisted of various topographical features such as lakes, tanks, marshes, rivers, streams, plain lands, undulated land, hills, mountains, rocks, cultivated land, fallow land, grazing land, degraded land, uncultivable land, forests and residential areas.
In the area approach topographical features form the major basis of planning of a watershed. Generally, topography of watershed varies from perfectly leveled land to 90 degree or 100% slopes. Depending on the percentage of area under a specific topographical designation, we may call an area hilly undulated, highly, medium or moderately sloppy, level able or naturally leveled land. Hence the watershed management practices will vary with degree and nature of slopes of various topographical features in the same watershed.
The geographical approach is necessary in preserving a watershed from degradation or to restore a degraded one. This approach may be adopted even to watersheds uninhabited by people. The main purpose of the watershed management will be to conserve the soil which result in the conservation of water and the enhancement of the regeneration of the vegetative cover. This approach is specifically useful to restore degraded uninhabited lands and deserting or desert areas.
A watershed has not only an area but it has other dimensions such as height and depth with reference to the surface. It has a space or air volume vertically above the watershed area. Similarly, it has a depth dimension comprising of the earth volume vertically below the watershed area.
The sky is the limit of the air volume above the watershed. The air volume or space receives moisture and retains it in the form of relative humidity. Relative humidity varies with temperature and pressure. Higher the temperature and pressure greater will be vapor content in the air. The rainfall in a place is influenced by the relative humidity, pressure and temperature of the air volume above it. The movement of air over the water changes constantly. The speed of the movement may vary from zero to several hundred kilometers per hour.
Temperature influences the watershed. A temperature varying between 15 to 35 degree C will be optimum range for watershed. Above or below this temperature plays a negative effect on the plants and animals in the watershed.
Air volume consisting of 79% nitrogen, 20% oxygen, 0.4% carbon dioxide and other gases (0.6%) is the optimum composition for a watershed. Any change in this composition can affect the watershed directly or indirectly. In many areas air pollution has become a major concern.
In the absolute sense the center of the earth is the limit of the depth of any watershed. However from practical point of view the water table or up to bed rock is considered as the depth of watershed. The volume of earth consists of minerals in the form of rocks and soil, moisture, hibernating animals (frogs, scorpions, snakes, lizards) plant roots and micro-organisms. The parent material of rocks and soil will have bearing on the watershed. Accordingly, one finds too much calcium, mica, iron, phosphorus, sulphur, manganese, sodium etc. more or less in the soil.
The soil is the actual crust of the earth and varies from few centimeters to a meter or two in depth. The main feature of this soil is that it contains organic matter in the form of humus or living organisms. The organisms may be micro as well as macro. The top soil contains more organic matter while the bottom soilless.
Rocks, stones, gravels, coarse sand, fine sand, silt and clay are the mineral matters in the macro form. At the elemental form, practically all the known elements are present in the soil. Soil is like a sponge and there are air spaces between soil particles. These are called pore spaces which may be bigger (macro pores) or small (micro pores). Strictly speaking pore space is the total surface area of all the soil particles. These pore spaces harbor moisture, humus and micro organisms. The soil depends on the texture and structure of the soil particles. The texture of the soil refers to the size of the soil particles which may be rocks, stones, gravel, coarse sand, fine sand, salt or clay. The structure of the soil refers to the arrangement of the soil particles. Water table is the level of water below the surface of the earth. The water table in a watershed may be very low or very high or fluctuate between these two according to season. It is measured vertically below the surface of the earth.
The present, level, movement and the quantity of water in the soil depends on the various factors such as soil particle size, climate, seasons, level of water table and rainfall. The soil in a watershed receives water from the air volume above it though there may be underground sources from adjacent watersheds.
Physically watershed can be defined as a combined volume of air and soil which receives water and drains out through a common drainage.
The management practices of a watershed should be adopted according to the humidity, temperature, air movement, air composition, soil depth, parent material, texture and structure of soil paI1icles, organic matter content, pore space, and water table.
Thus, in an area with high humidity and rainfall watershed management will mainly consists in conservation of soil and drainage of excess of water without damage to the watershed. For this, properly designed and laid out drainages are necessary. In a dry and arid area the emphasis will be on moisture conservation. More of mechanical and biological measures are resorted to this. Areas with high wind velocity, soil loss through wind has to be minimized. For this windbreaks and vegetative covers are established. In an area where the sun- shine is too hot, the key aspect in watershed management will be to provide vegetative cover to the lanc.1. If the soil is too shallow (less than one feet deep) the primary concern in the watershed management is to increase depth of the soil by repeated ploughings and harrowings. In sandy and sandy loam soil watershed management will be focused towards incorporation of the organic matter into the soil. If it is clay soil, sand and organic materials may be added to make it loamy. In area where soil particles are gravelly, addition of silt or clay and organic matter should be emphasized. In places where the water table is high, soil drainage becomes the key water management practice.
Watershed is a complex of chemical elements and their reactions. The elements are present in the watershed volume in solid, liquid and gaseous forms. These chemical elements, in the presence of each other, and under the influence of physical factors such as moisture, pressure, heat and light are in constant changes (different chemical reactions) are taking place in different forms (solid, liquid and gases). Depending on those various factors cel1ain chemical reactions may be favoured more than the others and consequently the chemical nature of the soil will be affected by them. For example under highly hot climate watershed areas having high water table can encourage high rate of evaporation leaving the salt on the surface of the soil. Depending on the type of salts the soil may turn out to be saline, alkaline or sodic. But if the rainfall is high and leaching rate is higher the soil becomes acidic. The anion and cation exchange capacity and the pH of the soil depends on the various types of inorganic com- pounds present in the soil.
Innumerable inorganic reactions are constantly taking place in the watershed volume. Hence watershed can be de- fined as a volume of inorganic materials which receives and drains out water through a common drainage.
Watershed management should take into consideration the chemical nature of mineral material of the soil. Whether acidic, alkaline, sodic, calcareousness or saline. In areas with high acidic soil, the watershed management practices should focus on besides other things, correction of acidic soils. Similarly, in areas of saline or alkaline soils corrective measures are introduced along with other watershed management practices. In the same way calcarious and gypsiferous soils are also treated with corrective measures in the watershed management system.
A watershed receives rain water and drains out and again receives and again drains it out. Such a process in the water- shed is called water cycle. In water cycle, water moves from ocean and other water bodies into air volume where it condenses to form rain which moves through and over the surface of the earth volume. The frequency of water cycle through the soil volume of watershed varies from continuous to a very short duration of less than an hour. It is through the process of water cycle that water is made available to every non- living and living things. But in order to make water available sufficiently to every thing all the time, a watershed should have a continuous water cycle irrespective of the frequency of the rainfall. The aim of the watershed management is to make the process of drainage slow enough to maintain the continuity of water cycle and establish a balance between the quantity of water received and drained during any given period of time. In other words the period of drainage of certain quantity of water received is lengthened to such an extent that it overlaps the period of reception of the same quantity of water into the watershed. The main source of water in watershed is rain or precipitation. However, water may be received both by the rain as well as from underground sources such as springs located adjacent to watersheds.
The inorganic reactions in the watershed are only a basis for much more complex actions and reactions at the organic level. In fact, the inorganic actions and reactions are part of the organic chemical reactions. Inorganic reactions with water is the beginning of organic reactions. Inorganic reactions can take place without water, whereas all organic reactions essentially require water. Water, thus form a necessary link between organic and inorganic reactions. Through these organic reactions chemical substances such as carbohydrates, proteins, vitamins, enzymes and hormones are produced. The most common and basic organic reaction in the nature is photosynthesis.
Concluding remarks
Watershed is not only a eco-geographical but also a socio-economic and political unit. People in this unit depend on it primarily for their existence and secondarily for their development with social justice and equity. In a watershed development approach, both these aspects should be taken into consideration. Existence without development is meaningless and development without existence is impossible. The ultimate aim of watershed management is development with justice. To attain such an aim we need to have an integrated approach in the watershed development.
Dr. K.T. Chandy
(Agronomist & Retired Professor
Environment and Natural Resource Management
Xavier Institute of Management, Bhubaneswar)
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