Designing flows to resolve human and environmental water needs in a dam-regulated river
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Toggle navigation. Search Results Results 1 -2 of 2. Nagy, K. Asante-Duah, I. The pan is placed over a square wooden platform of mm width and mm height to enable circulation of air underneath the pan. Colorado Sunken Pan This pan, mm square and mm deep is made up of unpainted galvanised iron sheet and buried into the ground within mm of the top Fig.
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The chief advantage of the sunken pan is that radiation and aerodynamic characteristics are similar to those of a lake. However, it has the disadvantages like difficult to detect leaks, extra care is needed to keep the surrounding area free from tall grass, dust etc.
The water level in the pan is kept at the same level as the lake leaving a rim of 75 mm. Diagonal baffles provided in the pan reduce the surging in the pan due to wave action. Its high cost of installation and maintenance together with the difficulty involved in performing measurements are its main disadvantages. In view of the above, the evaporation observed from a pan has to be corrected to get the evaporation from a lake under similar climatic and exposure conditions. Thus a coefficient is introduced as. The WMO recommend the minimum network of evaporimeter stations as below:. Currently India has about pan-evaporimeter stations maintained by the India Meteorological Department.
The water is taken up by the plant-root system and escapes through the leaves. The important factors affecting transpiration are: atmospheric vapour pressure, temperature, wind, light intensity and characteristics of the plant, such as the root and leaf systems. For a given plant, factors that affect the free-water evaporation also affect transpiration.
However, a major difference. Transpiration is essentially confined to daylight hours and the rate of transpiration depends upon the growth periods of the plant. Evaporation, on the other hand, continues all through the day and night although the rates are different.
For a given set of atmospheric conditions, evapotranspiration obviously depends on the availability of water. Potential evapotranspiration no longer critically depends on soil and plant factors but depends essentially on climatic factors.
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Field capacity is the maximum quantity of water that the soil can retain against the force of gravity. Any higher moisture input to a soil at field capacity simply drains away. Permanent wilting Point is the Moisture Content of a soil at which the moisture is no longer available in sufficient quantity to sustain the plants.
At this stage, even though the soil contains some moisture, it will be so held by the soil grains that the roots of the plants are not able to extract it in sufficient quantities to sustain the plants and consequently the plants wilt. The field capacity and permanent wilting point depend upon the soil characteristics. The difference between these two moisture contents is called available water, the moisture available for plant growth. If the water supply to the plant is adequate, soil moisture will be at the field capacity and AET will be equal to PET. As can be expected, when the soil moisture reaches the permanent wilting point, the AET reduces to zero Fig.
For a catchment in a given period of time, the hydrologic budget can be written as. Except in a few specialised studies, all applied studies in hydrology use PET for various estimation purposes. It is generally agreed that PET is a good approximation for lake evaporation. Lysimeters A lysimeter is a special watertight tank containing a block of soil and set in a field of growing plants.
The plants grown in the lysimeter are the same as in the surrounding field. Evapotranspiration is estimated in terms of the amount of water required to maintain constant moisture conditions within the tank measured either volumetrically or gravimetrically through an arrangement made in the lysimeter. Lysimeters should be designed to accurately reproduce the soil conditions, moisture content, type and size of the vegetation of the surrounding area.
They should be so hurried that the soil is at the same level inside and outside the container. Lysimeter studies are time-consuming and expensive.
Measurements are usually confined to precipitation, irrigation input, surface runoff and soil moisture. This method provides fairly reliable results. It is seen that the annual PET ranges from to cm over most parts of the country. Extreme south-east of Tamil Nadu also show high average values greater than cm.
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The variation of monthly PET at stations located in various climatic zones in the country. However, two other processes, though small in magnitude, operate to reduce the water volume available for runoff and thus act as abstractions. This abstraction represents the quantity of storage that must be satisfied before overland runoff begins. The following two sections deal with these two processes briefly.
Before it reaches the ground, a part of it may be caught by the vegetation and subsequently evaporated. The volume of water so caught is called interception. The intercepted precipitation may follow one of the three possible routes:.
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The amount of water intercepted in a given area is extremely difficult to measure. It depends on the species composition density and also on the storm characteristics. Quantitatively, the variation of interception loss with the rainfall magnitude per storm for small storms is as shown in Fig.
It is seen that the interception loss is large for a small rainfall and levels off to a constant value for larger storms. It is found that coniferous trees have more interception loss deciduous ones.
www.hiphopenation.com/mu-plugins/wv/best-free-indian-dating.php Agricultural crops in their growing season also contribute high interception losses. In view of these the interception process has a very significant impact on the ecology of the area related to silvicultural aspects and in the water balance of a region. However, in hydrological studies dealing with floods interception loss is rarely significant and is not separately considered, The common practice is to allow a lump sum value as the initial loss to be deducted from the initial period of the storm. The volume of water trapped in these depressions is called depression storage.
This amount is eventually lost to runoff through processes of infiltration and evaporation and thus form a part of the initial loss. Depression storage depends on a vast number of factors the chief of which are :. This movement of water through the soil surface is known as infiltration and plays a very significant role in the runoff process by affecting the timing, distribution and magnitude of the surface runoff. Infiltration is the flow of water into the ground through the soil surface and the process can be easily understood through a simple analogy.
Consider a small container covered with wire gauze as in Fig. If water is poured over the gauze, a part of it will go tainer and a part overflows. Further, the container can hold only a fixed quantity and when it is full no more flow into the container can take place.
This analogy, though a highly simplified one, underscores two important aspects, viz. Since the infiltered water may contribute to groundwater discharge in addition to increasing the soil moisture, the process can be schematically modelled as in Fig. This figure considers two situations, viz. The actual rate of infiltration f can be expressed as. The infiltration capacity of a soil is high at the beginning of a storm and has an exponential decay as the time elapses. Characteristics of Soil The type of soil, viz.
A loose, permeable, sandy soil will have a larger infiltration capacity than a tight, clayey soil. A soil with good under drainage, i. When the soils occur in layers, the transmission capacity of the layers determine the overall infiltration rate. Also a dry soil can absorb more water than one whose pores are already full.