This document discusses precipitation measurement and analysis in hydrology. It defines various forms of precipitation like rain, snow, hail, etc. Factors influencing precipitation formation like cooling of air and water vapor condensation are explained. Methods of precipitation measurement including non-recording and recording rain gauges are described. Techniques for estimating missing precipitation data using arithmetic mean and normal ratio methods are presented. Sources of errors in measurement and how to estimate average precipitation over a basin are also summarized.

1. NUST Institute of Civil Engineering/Engr Sajjad Ahmad 1

3. Factors influencing precipitation formation

4. Measurement of precipitation

6. Rain

7. Glaze

8. Sleet

9. Snow

10. Snowflake

12. Drop size < 0.5 mm

13. Intensity < 1.0 mm/hr

14. Do not produce runoff

16. Maximum drop size ≈ 6.25 mm

17. Can produce runoff if

18. Rainfall intensity greater than the rate of infiltration

20. Ice coating forms when

21. Drops comes in contact with cold surfaces on the ground

23. Drops freezes while falling through air at subfreezing temperature

25. Form due to sublimation

28. Diameter > 5 mm

29. Formed due to alternate freezing and drying

31. Water vapor condensation and formation of droplets

32. Mechanism for growth of cloud droplets

33. Mechanism for accumulation of moisture

37. These nuclei comes from sea salt or combustion products of sulfurous and nitrous acids

38. NUST Institute of Civil Engineering/Engr Sajjad Ahmad16

40. Droplet size can grow by two means

41. Coalescence of droplets through collisions

42. Bergeron’s effect

44. Due to higher saturation capacity around the droplets, some water evaporates

45. While due to low saturation capacity around the ice crystals vapors condense and ice crystal size increases

47. Therefore continuous supply of moisture is required

48. This process is known as convergence

50. It can be defined as

51. The net horizontal influx of air per unit area

53. Orographic precipitation

54. Cyclonic precipitation

55. Turbulent ascent precipitation

57. In the hot day, the ground surface gets heated, causing the warmer air to lift up

58. the colder air comes to take its place

59. The vertical air currents develop tremendous velocities the vapors condensed and Convective precipitation occurs

61. Hence rise up, causing condensation and precipitation

62. All the precipitation in Himalayan region is because of this nature

64. Cyclonic precipitation may be either frontal or non-frontal cyclonic precipitation

66. A front may be warm front or cold front depending upon whether there is active or passive accent of warm air mass over cold air mass

68. Vertical depth of water on a level surface

69. Expressed in inches, ft, cm etc

71. Precipitation amount per unit time

72. Expressed in inch pr hour or cm per hr etc

74. Stream flow

75. Ground water infiltration

76. Accurate measurement is necessary

78. Non recording rain gauge

79. Recording rain gauge

80. Recording rain gauges also gives the intensity of rain fall

82. Due to smaller diameter 2.5mm rainfall fills 25mm of tube

83. Hence gives more accuracy

86. Weighing type

87. Tipping bucket type

89. Float within collecting bucket rises with level

90. Vertical movement marked by pen and shows rainfall depth on a paper chart

91. The chart is rotated by a spring-driven or electric clock at speeds of 1 revolution in 6, 9, 12, 24, or 192 hours

92. The rain gauge chart is a record of the accumulated of rainfall for the selected time interval

94. It weighs rain or snow which falls into a bucket, set on a platform with a spring or lever balance

95. The increasing weight of the bucket and its contents are recorded on a chart

96. The record shows accumulation of precipitation

98. Rain fills one container until its threshold weight reached

99. Bucket then tips over, emptying collected water into total container and continues to collect rainfall in other container

100. Magnet generates electric pulse which is recorded

103. Dents in collector rim may change its receiving area

104. Instrumental error in gauge or in their recording or measuring arrangements

105. Some rainwater may get lost due to splash from the collector

108. Blowing wind may tilt the rains from vertical which thus brings less rain catch in the gauge

109. Vertical upward air currents may impact upward acceleration to precipitation thus brings less rain catch in gauge

111. No rainfall recorded during tipping of bucket

112. Tipping of bucket may be affected due to rusting or accumulation of dust on pivot

114. inclination - perpendicular to ground

115. Obstructions height

116. Wind

119. Recording rain gauge also gives intensity of rainfall at any time, while non-recording gauge gives only total rainfall for time interval

120. Recording rain gauges can be installed far off places

121. Possibility of human error is obviated

123. Error in recording rainfall due to fault in electrical or mechanical mechanism

125. Estimation of missing precipitation record

126. Simple arithmetic mean method

127. Normal ratio method

128. Consistency of precipitation data or double mass curve analysis

129. Estimation of average precipitation over a basin

131. Index station ≈ surrounding stations around interpolation station whose data is used to calculate the missing rainfall data

133. Here n ≈ number of nearby stations

134. And Pi is the precipitation at ith station

137. Here n ≈ number of nearby stations

138. And Pi is the precipitation at ith station

139. Nx and Ni are the normal annual precipitation value for station x and ith respectively

141. Nx and N1 , N2 and N3 are the normal annual precipitation value for station x and 1,2 and 3rd station respectively