This document discusses various methods for computing average rainfall over a basin, including the arithmetic mean method, Thiessen polygon method, and isohyetal method. It also covers precipitation intensity, mass curves, and depth-area-duration curves. The arithmetic mean method calculates the average rainfall as the mean of measurements from rain gauges. The Thiessen polygon method weights each gauge measurement by the area it represents. The isohyetal method involves drawing lines of equal rainfall on a map to determine rainfall distribution.

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3. Precipitation intensity

4. Mass curve

6. Thiessen Polygon Method

7. Distance weighting

8. Isohyetal Method

10. If P1, P2, P3, … Pn etc are the precipitation or rainfall values measured at ‘n’ gauge stations, then

12. Basin area is flat

13. Stations are distributed uniformly

14. Rainfall is distributed uniformly

16. Find average precipitation

18. The area is assessed by drawing polygons

19. It is also called Weighted Mean Method

20. Suitable for large areas

22. Connect all gauging stations with dotted lines

25. Calculate average precipitation

27. An isohyetal map shows contours of equal rainfall on the ground

28. Gives more accurate picture of rainfall distribution

30. Indicate points of rain gauges

31. Write rainfall value at gauge points

32. Draw isohyets

33. Measure area enclosed or b/w every two isohyets

35. The weight given to the precipitation is inversely proportional to the square of the distance between centroid of basin and gauge point

38. Float type and weight type gauges produce record in form of mass curve

41. Quantity of precipitation as well as time duration of the storm is required

43. Also known as D-A-D analysis

45. Draw isohyetal map of the area

46. Isohyetal method is used to calculate the average precipitation

47. For construction of D-A-D curves the storm is divided in to some time increments

50. The accumulated rainfalls at each station for four 6-h increment periods are given in the table

51. To provide area weightings to the gauge values, Thiessen polygons are drawn around the rainfall stations over the isohyetal pattern

53. 38 24 hour Precipitation (mm) Area sq.km

55. Then estimate area between isohyets

56. Calculate average rain fall over the area

57. Then draw results on the graph

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59. 40 Precipitation (mm) 24 hour 18 hour 12 hour 6 hour Area sq.km

61. If the spread of storm is 60 sq.km

62. Then an estimate of the average rainfall can be done by above curves

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64. 42 Precipitation (mm) 24 hour 18 hour 12 hour 6 hour Area sq.km

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