6. EVAPORATION MEASURMENTS

7. EVAPORATION MEASURMENTS

9. Water Budget Method
This method depends on drawing up a balance of all the water entering and leaving a particular lake, pool, or basin. All quantities in units of volume (m3) or depth (mm) over a reference area. EL= P + (VIS – VOS ) + (VIg – VOg ) – TL - ∆S Where: EL : Daily lake evaporation P : Daily precipitation VIS : Daily surface inflow into the lake VOS : Daily surface outflow from the lake

10. Water Budget Method
Where: Vig : Daily groundwater inflow Vog : Daily seepage outflow TL : Daily transpiration loss ∆S : Increase in a lake storage in a day

15. Assignment 3
1- Estimate the evaporation rate (in mm/day) from an open water surface based on the energy balance method. The net radiation is 1000 W/m2 and air temperature is 200C. Assume no sensible heat or ground heat flux. The water density is 1000kg/m3.

16. Assignment 3
2- A reservoir had an average area f 20 km2 during June In that month the mean rate of inflow = 10 m3 /s, outflow = 15 m3 /s,, monthly rainfall = 10 cm and change in storage = 16 million m3. assume the seepage losses to be 1.8 cm, estimate the evaporation in that month.

17. Pan Method
Previous evaporation methods are analytical methods. Although they provide better results, they involve parameters that are difficult to assess or expansive to obtain. Empirical equation can give approximate value of the correct order or magnitude. Pan measurements find general acceptance for practical applications.

18. Pan Method
VE= A x Epm x Cp Where : VE: water lost in evaporation monthly (m3) A : Average reservoir area during month (m2) Epm : pan eavaporation loss in meters in a month (m) Cp : Pan coefficient Epm = EL (mm/day) x No. of days in a month x 10-3

19. Ex2. Compute the daily evaporation from a Class A pan if the amounts of water required to bring the level to the fixed point are as follows: If the pan coefficient is 0.70, what is the lake evaporation for the 5-day period for a lake with a 250 m2 surface area?

20. Ex3. A class A pan was set up adjacent to a lake
Ex3. A class A pan was set up adjacent to a lake. The depth of water in the pan at the beginning of a certain week was 195 mm. in that week there was a rainfall of 45 mm and 15 mm of water was removed from the pan to keep the water level within the specified depth range. If the depth of the water in the pan at the end of the week was 190 mm, calculate the pan evaporation. If the pan coefficient is 0.70, what is the lake evaporation in that week with a 100 hectares surface area?

21. Homework
in a given month the net radiation is 300 w/m2 and the air temperature is 30 o C. assume a value of zero for sensible heat, ground heat flux. The density of water is 996 kg/m3. estimate the 600 hectares lake evaporation for that month if the pan coefficient is equal to 0.65.

22. Methods to Reduce Evaporation Losses
1- Reduction of surface area since the volume of water lost by evaporation is directly proportional to the surface area of the water body, the reduction of surface area reduces the evaporation losses. Like having a deep reservoir in a place of wider ones.

23. Methods to Reduce Evaporation Losses
2- Mechanical Covers temporary roofs and floating roofs can be adopted. These measures are limited to very small water bodies such as ponds.

24. Methods to Reduce Evaporation Losses
3- Chemical Films this method consists of applying a thin chemical films on the water surface to reduce evaporation. Effect of heavy wind is the only factor affecting the efficiency of chemical films.

30. Evapotranspiration Measurmentrs