1. Calculated Net Evaporation from
Reservoirs in the Chi-Mun River Basins, Thailand
GISWR Term Project 2005
by Nat Marjang

2. Study Area
Large reservoirs in the Chi-Mun River Basins, in the northeast Thailand
The basins cover about 23% of the nation area (120,000 sq.km.)

3. Objective
To calculate monthly net evaporation over the large reservoirs in the study area (one of the input data for HEC-3)

4. Methodology Enet = 0.7Epan – (1 – C)R
Monthly net evaporations are calculated from:
Enet = 0.7Epan – (1 – C)R
Enet = Net reservoir evaporation (mm)
Epan = Pan evaporation (mm)
C = Runoff coefficient
R = Rainfall (mm)

5. CR = CR[out] + CR[reservoir boundary with no water]
Why this formula?
Nam Phrom River Basin
Pre-Project Condition
Present/Future Condition
Natural Flow
(C*R)
CR = CR[out] + CR[reservoir boundary with no water]
HEC-3 Reservoir System Analysis for Conservation

6. = R – 0.7Epan + CR[res.] – CR[res.]
Why this formula?
Consider the Reservoir Area
CW = 1.0
Net Flow = CW*R – 0.7*Epan
Depth
= R – 0.7Epan + CR[res.] – CR[res.]
= {CR[res.]} – {0.7Epan – R + CR[res.]}
= {CR[res.]} – {0.7Epan – (1 – C)R}
= Natural Flow – Net Evaporation

7. Climate Station with monthly evaporation data
10 stations inside the area
13 stations around the area

8. Rainfall stations with monthly rainfall data
421 stations inside the area
89 stations around the area

9. Evaporation for April by using ArcGIS function
Analysis
Evaporation for April by using ArcGIS function
(Kriging Method)
198.9
Error 4.5%
172.2
Error 7.1%
Units: mm

10. Evaporation for April by using Surfer program
(Kriging Method) 200 m Grid size
208.0
Error 0.1%
161.9
Error 0.2%
Units: mm

11. Evaporation for April

12. Evaporation for May

13. Evaporation for June

14. Evaporation for July

15. Evaporation for August

16. Evaporation for Setptember

17. Evaporation for October

18. Evaporation for November

19. Evaporation for December

20. Evaporation for January

21. Evaporation for February

22. Evaporation for March

23. Annual Evaporation

24. Rainfall for April

25. Rainfall for May

26. Rainfall for June

27. Rainfall for July

28. Rainfall for August

29. Rainfall for September

30. Rainfall for October

31. Rainfall for November

32. Rainfall for December

33. Rainfall for January

34. Rainfall for February

35. Rainfall for March

36. Annual Rainfall

37. Result
Enet = 0.7Epan – (1 – C)R

38. Result
Enet = 0.7Epan – (1 – C)R

39. Enet = 0.7Epan – (1 – C)R

40. Question ?