Exp01.pptx

Engineering

1. Experiment #01 To estimate the evaporation by using standard evaporation pan and Colorado sunken pan Hydrology and Water R esource Management Lab

2. Objective To estimate the evaporation by using Standard evaporation pan or Class A Pan and Colorado Sunken pan. To understand the real time evaporation from lakes, streams, and any water body because evaporation is the important factor in hydrological cycle. Evaporation . Evaporation is  the physical process by which water changes from a liquid phase to a gaseous phase or vapor below the boiling point. This is due to gaining of energy from sun and depends on different factors. It is a day night process. It is a net process because water vapor is constantly moving back and forth from the water surface.

3. Factors affecting evaporation Temperature: Evaporation is highly dependent upon temperature. Higher the temperature, greater will be the evaporation. • 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

4. 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. Factors affecting evaporation

5. Evaporation Estimation Evaporation can be estimated by different methods but we only discussed two of them in detail Evaporation can be estimated by: Indirect or theoretical method Water Budget Approach , Energy Budget Approach, and Mass Transfer Approach – Direct measurement using pans , The pan is the most widely used evaporation instrument. There are three types of pans . • Sunken pans • Above ground or surface pans • Floating pans.

6. Standard evaporation pan Evaporation through Standard pan The Evaporation pan is circular in shape, 120.7 cm in diameter and 25 cm deep. It is made of galvanized iron (22 gauges) or Monel metal (0.8 mm). The pan is mounted on a wooden open frame platform which is 15 cm above ground level in order to provide good circulation mechanism of wind . The pan must be level. . It is filled with water to 5 cm below the rim, and the water level should not be allowed to drop to more than 7.5 cm below the rim. . Pans should be protected by fences to keep animals from drinking. Pan readings are taken daily in the early morning at the same time that precipitation is measured. Measurements are made in a stilling well that is situated in the pan near one edge. The stilling well is a metal cylinder of about 10 cm in diameter and some 20 cm deep with a small hole at the bottom . Evaporation measurement should be taken in the stilling well accurately by vernier gauge. Important point= NICKEL AND COPPER ALLOY (MONEL)

7. Evaporation through Sunken pan Sunken pans are of three types. Bureau of Plant Industry ( B. P. I. ) Pan This pan is circular in shape. These are 1.80 m in diameter and 0.60 m deep. These are sunken into the ground to a depth of 0.55 m and 0.05 m above the ground i.e. total depth is 0.60 m. 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. Colorado Pan

8. Colorado Sunken pan Colorado Sunken pan The Colorado sunken pan is 92 cm, square in shape and 46 cm deep , made of 3 mm thick iron, placed in the ground with the rim 5 cm (2 in) above the soil level. The pan is painted with black tar paint. Black tar is anti-corrosive. The water level is maintained at or slightly below ground level, i.e., 5-7.5 cm below the rim . It have cylindrical stilling well and measurement of evaporation is taken with help of vernier point gauge . Measurements are taken similarly to those for the pan. Siting and environment requirements are also similar to those for the Class A pan. This type of pan is mostly resemblance to our real water bodies like lakes and seas. . The disadvantage is that maintenance is more difficult and leaks are not visible

9. Procedure (Standard Pan) The pan is installed in the field. The pan is filled with a known quantity of water (the surface area of the pan is known and the water depth is measured). The water is allowed to evaporate during a certain period of time (usually 24 hours). For example, each morning at 7 o’clock a measurement is taken. The rainfall, if any, is measured simultaneously. After 24 hours, the remaining quantity of water (i.e. water depth) is measured. The amount of evaporation per unit time(the difference between the two measured water depths) is calculated; this is the pan evaporation: E pan (in mm/24 hours ). Unit is mm/24hrs.

10. Draw back of standard pan The pans does not give the exact measure of large reservoir with following drawbacks They differ in heat storing capacity Heat transferred from sides and bottom Height of rim in evaporation pan affects the wind action over the surface The rim height also casts a shadow of variable magnitude over surface of water Heat transfer characteristics of pan material is different from that of the reservoir. Due to these reasons a correction factor is required

11. Pan Coefficient Pan Coefficient The correction factor is given as Lake Evaporation = Cp x Pan evaporation Pan coefficient values are given as Sr. No Type of Pan Average Value Range Class A Land type 0.70 0.60‐0.80 ( Modified Class A ) 0.80 0.65‐1.1 Coloroda Sunken Pan 0.78 0.75‐0.86 USGS Floating Pan 0.80 0.70‐0.82

12. Evapotranspiration Transpiration  is a biological process in which the water molecules are lost in the form of water vapours from the aerial parts of the plants. This natural phenomenon is carried in all green plants, green crops and grass. It is a day time process. Evapotranspiration Evapotranspiration  ( ET ) is a term used to refer to the combined processes by which water moves from the earth’s surface into the  atmosphere . It covers both water  evaporation  (movement of water to the air directly from soil,  and water bodies) and  transpiration  (movement of water from the soil, through roots and bodies of vegetation, and then into the air ). We can also estimate evapotranspiration in the term of standard evaporation pan value by introducing pan coefficient which depends on different factors . T he water in the pan and the grass do not react in exactly the same way to the climate.

13. Evapotranspiration The pan coefficient, K pan, depends on: the type of pan used. the pan environment: if the pan is placed in a fallow or cropped area the climate: the humidity and wind speed. For evaporation pan, the K pan varies between 0.35 and 0.85. Average K pan = 0.70 . If the pan factor is not known the average value could be used. Formula ET о = K pan × E pan ET о : reference crop evapotranspiration K pan: pan coefficient E pan: pan evaporation

14. Class A pan Case A: placed in short green cropped area CASE B: Pan placed in dry fallow area RH mean (%)   Low < 40 Medium 40-70 High > 70   Low < 40 Medium 40-70 High > 70 Wind speed Windward side distance of green crop       Windward side distance of dry fallow       ( ) (m)       (m)       Light 1 .55 .65 .75 1 .7 .8 .85 <2 10 .65 .75 .85 10 .6 .7 .8 100 .7 .8 .85 100 .55 .65 .75 1000 .75 .85 .85 1000 .5 .6 .7 Moderate 1 .5 .6 .65 1 .65 .75 .8 2-5 10 .6 .7 .75 10 .55 .65 .7 100 .65 .75 .8 100 .5 .6 .65 1000 .7 .8 .8 1000 .45 .55 .6 Strong 1 .45 .5 .6 1 .6 .65 .7 5-8 10 .55 .6 .65 10 .5 .55 .65 100 .6 .65 .7 100 .45 .5 .6 1000 .65 .7 .75 1000 .4 .45 .55 Very strong 1 .4 .45 .5 1 .5 .6 .65 >8 10 .45 .55 .6 10 .45 .5 .55 100 .5 .6 .65 100 .4 .45 .5 1000 .55 .6 .65 1000 .35 .4 .45 Class A pan Case A: placed in short green cropped area CASE B: Pan placed in dry fallow area RH mean (%)   Low < 40 Medium 40-70 High > 70   Low < 40 Medium 40-70 High > 70 Wind speed Windward side distance of green crop       Windward side distance of dry fallow       (m)       (m)       Light 1 .55 .65 .75 1 .7 .8 .85 <2 10 .65 .75 .85 10 .6 .7 .8 100 .7 .8 .85 100 .55 .65 .75 1000 .75 .85 .85 1000 .5 .6 .7 Moderate 1 .5 .6 .65 1 .65 .75 .8 2-5 10 .6 .7 .75 10 .55 .65 .7 100 .65 .75 .8 100 .5 .6 .65 1000 .7 .8 .8 1000 .45 .55 .6 Strong 1 .45 .5 .6 1 .6 .65 .7 5-8 10 .55 .6 .65 10 .5 .55 .65 100 .6 .65 .7 100 .45 .5 .6 1000 .65 .7 .75 1000 .4 .45 .55 Very strong 1 .4 .45 .5 1 .5 .6 .65 >8 10 .45 .55 .6 10 .45 .5 .55 100 .5 .6 .65 100 .4 .45 .5 1000 .55 .6 .65 1000 .35 .4 .45 Table 1 : Pan Coefficients ( Kp ) for Standard pan for different pan siting and environment and different levels of mean relative humidity and wind speed.

15. Table 2 Pan Coefficients ( Kp ) for Colorado sunken pan for different pan siting and environment and different levels of mean relative humidity and wind speed. Sunken Colorado Case A: placed in short green cropped area CASE B: Pan placed in dry fallow area RH mean (%)   Low < 40 Medium 40-70 High > 70   Low < 40 Medium 40-70 High > 70 Wind speed Windward side distance of green crop       Windward side distance of dry fallow       ( ) (m)       (m)       Light 1 .75 .75 .8 1 1.1 1.1 1.1 <2 10 1.0 1.0 1.0 10 .85 .85 .85 100 1.1 1.1 1.1 100 .75 .75 .8         1000 .7 .7 .75 Moderate 1 .65 .7 .7 1 .95 .95 .95 2-5 10 .85 .85 .9 10 .75 .75 .75 100 .95 .95 .95 100 .65 .65 .7         1000 .6 .6 .65 Strong 1 .55 .6 .65 1 .8 .8 .8 5-8 10 .75 .75 .75 10 .65 .65 .65 100 .8 .8 .8 100 .55 .6 .65         1000 .5 .55 .6 Very strong 1 .5 .55 .6 1 .7 .75 .75 >8 10 .65 .7 .7 10 .55 .6 .65 100 .7 .75 .75 100 .5 .55 .6         1000 .45 .5 .55                           Sunken Colorado Case A: placed in short green cropped area CASE B: Pan placed in dry fallow area RH mean (%)   Low < 40 Medium 40-70 High > 70   Low < 40 Medium 40-70 High > 70 Wind speed Windward side distance of green crop       Windward side distance of dry fallow       (m)       (m)       Light 1 .75 .75 .8 1 1.1 1.1 1.1 <2 10 1.0 1.0 1.0 10 .85 .85 .85 1.1 1.1 1.1 100 .75 .75 .8         1000 .7 .7 .75 Moderate 1 .65 .7 .7 1 .95 .95 .95 2-5 10 .85 .85 .9 10 .75 .75 .75 .95 .95 .95 100 .65 .65 .7         1000 .6 .6 .65 Strong 1 .55 .6 .65 1 .8 .8 .8 5-8 10 .75 .75 .75 10 .65 .65 .65 .8 .8 .8 100 .55 .6 .65         1000 .5 .55 .6 Very strong 1 .5 .55 .6 1 .7 .75 .75 >8 10 .65 .7 .7 10 .55 .6 .65 .7 .75 .75 100 .5 .55 .6         1000 .45 .5 .55                          

16. Calculations and Observations Sr. No Water depth/level before Evaporation Water depth/level after Evaporation Evaporation mm/24hrs ETо = K pan × E pan   1 20 cm   18cm   2cm/24hrs   ETo =0.7*2   = 1.4 cm/24hrs 2   13+12*0.05=   13.6mm 18+0.05*8   = 18.4mm 4.8mm/24hrs   Eto =.7*4.8=3.36 mm/24hrs            

17. WRITE YOUR OWN COMMENTS