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Hydrology and water management(EVAPORATION & INFILTRATION)...


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Hydrology and water management(EVAPORATION & INFILTRATION)...

  1. 1. LECTURE: 2 SARHAD UNIVERSITY PESHAWAR CIVIL ENGINEERING (6th SEMESTER) Page 1 HYDROLOGY AND WATER MANAGEMENT EVAPORATION & INFILTRATION Losses from Precipitation:  The goal of a hydrologic study is usually to obtain runoff from a catchment due to a storm or storms.  There are losses from storm-water due to evaporation, evapotranspiration, infiltration, interception and depression storage, etc.  Of which, evaporation and infiltration are considered as major losses. These two are discussed here. EVAPORATION:  Water loss due to vaporization is called evaporation. OR  Evaporation is the process by which water is converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere.  It is measured in depth units. RELATED TERMS WITH EVAPORATION: Vaporization:  It is a physical process in which liquid changes to gaseous state without boiling from free surfaces of water bodies and wet soil masses and returns to atmosphere due to high temperature effects. Transpiration:  The process by which water is returned to the atmosphere by evaporation from the surface of the leaves after movement from the soil through the root and stem system of the plant, is called transpiration. Evapotranspiration:  It is combined evaporation and transpiration. The total water loss from soil or free water surface and from trees and plants is termed as evapotranspiration. Potential Evapotranspiration:  It is water evaporated and transpired by plants when it is available in abundance.  Quantity of available water affects the rate and magnitude of evapotranspiration. FACTORS AFFECTING EVAPORATION: Temperature: Evaporation is highly dependent upon temperature. Higher the temperature, greater will be the evaporation.
  2. 2. LECTURE: 2 SARHAD UNIVERSITY PESHAWAR CIVIL ENGINEERING (6th SEMESTER) Page 2 Surface area of water body: Evaporation takes place from free surface of water body. Larger the area exposed to atmosphere, greater will be evaporation. Humidity: Greater the humidity, lesser will be the evaporation. Wind speed: Higher the wind speed more will be evaporation. Solar radiation: Evaporation increases with increasing solar radiation. Atmospheric Pressure: Evaporation decreases with increasing atmospheric pressure. Depth of Water: Greater the depth of water, lesser will be the evaporation. ESTIMATION OF EVAPORATION:  Direct measurement using pans.  Indirect measurement or theoretical methods. Theoretical methods are of the following types:  Water Budget method  Energy Budget method  Mass Transfer method Measuring Evaporation by Pans:  The estimation is done on the basis of evaporation from a pan.  Pan is filled to a standard level, represented by a hook at the end of a rod / wire at some standard time in the morning.  Next day at the same time, water is added so that the water level comes to the same standard level represented by hook. This water added indicates the evaporation of that day.  In case there is rain during that time, its account must be taken by adding mathematically the amount of precipitation to water added; the total represents the evaporation of that day.  In case, precipitation is more than evaporation, then water from pan is to be extracted. Evaporation Pans:  Pans are the most widely used Instrument for measuring Precipitation.  There are three types of pans:  Sunken pans  Above ground or surface pans  Floating pans
  3. 3. LECTURE: 2 SARHAD UNIVERSITY PESHAWAR CIVIL ENGINEERING (6th SEMESTER) Page 3 Sunken Pan  This is installed inside the earth surface.  This pan tends to eliminate objectionable boundary effects such as radiation on the side walls and heat exchange between the atmosphere and the pan.  The results obtained from this pan are more accurate than those obtained from other pans. Sunken pans are of three types • Bureau of Plant Industry ( B. P. I. ) Pan: This pan is circular in shape. It is 1.80 m in diameter and 0.60 m deep. It is sunken into the ground to a depth of 0.55 m. • Colorado Pan: It is square in shape. Having a size of 0.90m x 0.90m and 0.45m deep. These are American pans, with 5 cm of rim projecting from the ground. The water is maintained two inch below the rim. • Young’s Pan: It is also circular in shape, about 0.60 m in diameter and 0.90 m deep covered with a 6 mm wire mesh screen. Other conditions are same as that of Colorado pan. The Colorado Sunken pan Above-Ground or Surface Pan  The surface pan which has found international recognition is U.S. Weather Bureau class-A pan.  This is made of galvanized iron (G.I.) sheet.  The pan is placed on a wooden frame to allow free access of air at the bottom.  The principle advantage of the surface pan is the easy operation and maintenance.  However more evaporation takes place as compared to the sunken and floating pan but this type of pans are most widely used all over the world.
  4. 4. LECTURE: 2 SARHAD UNIVERSITY PESHAWAR CIVIL ENGINEERING (6th SEMESTER) Page 4  Certain correction factors are applied to every pan. Floating Pans  As the name indicates the floating pans float over the surface of lake.  Evaporation from a floating pan is approximately the same as that directly from the lake, because the atmospheric conditions around the pan are the same as those for the lake.  Lake evaporation measured by such pans is free from the effects of the atmosphere. MEASUREMENT OF EVAPOTRANSPIRATION: The instrument most commonly used for the measurement of evapotranspiration is known as Lysimeter.
  5. 5. LECTURE: 2 SARHAD UNIVERSITY PESHAWAR CIVIL ENGINEERING (6th SEMESTER) Page 5 • The lysimeter consists of a small tank filled with soil and having the same vegetation cover as that of the adjacent area. • It contains a drain and all facilities for measuring the quantity of water entering and leaving the tank. • The vegetation in the tank is either watered from lower side of the tank by maintaining a constant water table or from above. INFILTRATION: Infiltration is the process by which precipitation water is abstracted by seeping into the soil below the land surface.  It is also taken as water lost due to absorption of water by the ground surface.  The infiltrated water may move horizontally, vertically or in both the directions.  The horizontal movement of water is called Interflow. The interflow joins streams, lakes and rivers.  The vertical movement of water from deeper layers of soil is called percolation. The percolated water moves as Groundwater flow.  Infiltration is measured in terms of depth of water lost in unit time.  This is necessary as amount of infiltration varies with time.  At a particular instant, infiltrated depth of water divided by time is called Instantaneous Infiltration Rate.  If total infiltrated water depth is divided by total time, it is known as Average Infiltration Rate. FACTORS AFFECTING INFILTRATION: Infiltration is a complex process and depends on many factors like;  Condition of ground  Presence of vegetation  Type of soil  Storm characteristics  Temperature and  Properties of water. MEASUREMENT OF INFILTRATION: Infiltration rate can be measured by simultaneous measurements of rainfall and runoff. Infiltration may be determined by:  Using Equations  Infiltration Indices
  6. 6. LECTURE: 2 SARHAD UNIVERSITY PESHAWAR CIVIL ENGINEERING (6th SEMESTER) Page 6 MEASUREMENT OF INFILTRATION USING EQUATIONS:  Various researchers proposed formulae to calculate Rate of Infiltration for a given soil.  One of these formulae which is most commonly used is ‘Horton’s Equation’ as given below: f = fc + (fo-fc)e-kt Where, o f = infiltration rate at any time ‘t’ o fo = Initial infiltration rate o fc = final infiltration rate o k = a constant having units of 1/t  Integration of above equation gives total infiltration as given below.  Total Infiltration ‘F’ = [ (fo-fc) / k ] + fct  Figure shows typical plot of Horton’s Equation.  The area under the curve gives total amount of infiltration.  Usage of Horton’s Equation requires three parameters, the initial Infiltration rate, the final infiltration rate and value of constant. MEASUREMENT OF INFILTRATION USING INDICES: Although rate of infiltration varies with time, for practical purposes, it is assumed constant. Two types of infiltration indices are used:  Ф Index  W Index
  7. 7. LECTURE: 2 SARHAD UNIVERSITY PESHAWAR CIVIL ENGINEERING (6th SEMESTER) Page 7 Ф Index  It is mean infiltration rate for a particular storm.  Ф index can be found if rainfall occurrence with respect to time is known.  It is assumed that total depth of rain minus ø gives depth of runoff.  Ф value is found by trial and error procedure.  A value for ø is assumed and subtracted from total rainfall for each time interval.  The sum of values after subtraction should be equal to total direct runoff.  That value of Ф which gives this value is taken as Ф index. W Index  W index gives the average rate of infiltration.  It can be found by subtracting the sum of total direct runoff, interception and depression storage from the total rainfall.  If, F = Total Infiltration o T = Duration of Storm P = Depth of Precipitation R = Runoff Depth I = Interception and depression storage, then  F = P - R – I And ‘W’ index = F / T = (P – R – I) / T For I = 0, ‘W’ Index = F Index