This document provides an overview of a hydrometry course covering measurement of various components of the hydrological cycle including precipitation, evaporation, soil moisture, and streamflow. It discusses different types of instruments used to measure these variables such as raingauges, evaporation pans, neutron probes, tensiometers, and current meters. Measurement methods include manual observation as well as automated data recording using devices like data loggers. The goal is to introduce concepts and terminology for quantifying water in the hydrological cycle.

1. Hydrology 220: Hydrometry Hydrological Measurement, Instrumentation and Networks Mark Horan Room 341

3. Lecture 1: Basics and Introduction

4. Lecture 2: Measurement of Rainfall

5. Lecture 2:Measurement of Interception and Evaporation

6. Lecture 3: Measurement of Soil Moisture

7. Lecture 3: Measurement of Streamflow

8. Lecture 4: Hydrological Network Design

12. Why Measure Predict impacts from prior knowledge or experience Infer impacts from evidence collected Experimentally investigate impact of certain activities

13. Observations and variables Variable characteristic measured for each sampling unit, e.g. [P] more than one variable can be measured for each sampling unit, e.g. [P], [T], Wind etc. Observations value of a variable for each sampling unit e.g. [P]

14. Types of variables Continuous: can take any value between fixed limits length, weight, concentration etc. Discrete or categorical: can only take certain, usually integer, values counts, presence/absence, alive/dead etc.

15. Types of variables Ranked: not measured but ranked (often subjectively) by their magnitude e.g. degree of damage from none to high Attributes: qualitative variables with no magnitude scale e.g. position

16. Derived variables Ratios: relation between two variables expressed as single value, e.g. C / N ratio Rates: change in variable per unit time, e.g. m3.s-1 Others: species diversity, indices of health/integrity

17. Statistics and parameters Sample statistics estimate population parameters Central (middle) value: mean, median, mode Spread (variability) of values: variance, standard deviation Standardised spread: coefficient of variation

18. Accuracy and precision Accuracy: closeness of measurements to true value Precision: closeness of measurements to each other High precision usually means high accuracy unless measuring device is biased Focus on precision

19. Sources of uncertainty Measurement error: difference between two measurements due to measuring device, human error etc. Sampling error: difference between two measurements due to natural variability Need for replicate measurements

20. Recording of Data Paper Charts Data Loggers Telemetry.

21. Recording of Data Paper Charts Simplest method Chart moved by spring or electronically driven clock past pen Pen moves with weight/float etc Two Types Drum - rotates Strip - moves past pen Charts then “digitised”

23. Recording of Data Data Loggers A data logger is a computer that records and stores data from sensors both analog (voltage) and digital(counts). The data logger can also be used as a controller to turn on and off electrical The data logger requires a program to tell it what to do. Preloaded computer chip that already has the program in it or create the program Data can then be accessed by a computer to monitor current conditions or download stored data.

25. Recording of Data Data Loggers Problems Vandalism due to desirability of batteries

26. Recording of Data Telemetry Data stored by logger can transferred directly to a base station via some form of telecommunication

27. Hydrology 220: Hydrometry Lecture 2 Measurement of Precipitation

28. Types of Precipitation Rainfall Hail Snow and Ice

29. Measurement of Precipitation magnitude, intensity, location, patterns of precipitation quantity of precipitation as well as, the spatial and temporal distributions of the precipitation have considerable effects on the hydrologic response. Measurement by Raingauge RADAR Satellite

30. Raingauges The purpose of a rain gauge is to measure the amount of rainfall at a single point Measure What? Depth of water on a flat surface Depth is assumed to be same as surrounds

31. Raingauges With What? Container of varying dimensions and heights SA Standard 127mm diameter (5 inches) 1.2 m height above ground (4 feet) Requirements Sharp edge Rim falls away vertically Prevent splashing Narrow neck prevents evaporation

32. Image here

33. Raingauges Non-recording and recording rain gauges A non-recording rain gauge is typically a catchment device calibrated to provide visual observation of rainfall amounts. Recording gauges are equipped with paper charts and/or data logger equipment.

34. Non-recording Raingauges Measure with calibrated flask or dipstick Flask usually tapered to allow accuracy if little rain In SA - manual daily observation at 08h00 Storage gauges in remote areas Evaporation losses high Prevention by oil film small exposed surface area poor ventilation low internal temperature

35. Two types of standard storage raingauge

36. Recording Raingauges Analogue Devices Weighing Bucket Rain Gauge Float Type Rain Gauge Digital Devices Tipping Bucket Rain Gauge Optical Rain Gauge

37. Analogue Recording Raingauges Weighing Bucket Rain Gauge Standard instrument used to quantify rainfall. Spring scale beneath the collecting bucket platform that is calibrated to mark the rainfall depth on a paper chart. The chart is rotated by a spring-driven or electric clock at speeds of 1 revolution in 6, 9, 12, 24, or 192 hours. The rain gauge chart is a record of the accumulated of rainfall for the selected time interval.

38. Analogue Recording Raingauges Float Type Rain Gauge Standard instrument used to quantify rainfall. Float within collecting bucket rises with level Vertical movement marked by pen and shows rainfall depth on a paper chart. The chart is rotated by a spring-driven or electric clock at speeds of 1 revolution in 6, 9, 12, 24, or 192 hours. The rain gauge chart is a record of the accumulated of rainfall for the selected time interval.

39. Analogue Recording Raingauges Float Type Rain Gauge with Siphon Standard instrument used to quantify rainfall. Usually with Float Type Rain Gauges System siphons itself at a certain level (typically 25mm) Empties container completely Stores siphoned water in separate (total) container Total container as check Pen returns to bottom line Problems 15 seconds to siphon Freezes Digitising

41. Digital Recording Raingauges Tipping Bucket Rain Gauge Two containers on balance beam form a “tipping bucket” Rain fills one container until it threshold weight reached Bucket then tips over, emptying collected water into total container and continues to collect rainfall in other container Magnet generates electric pulse which is recorded Problems Evaporation from buckets Discontinuous record in light rain Susceptible to freezing

46. Digital Recording Raingauges Optical Rain Gauge (ORG) The ORG is mounted on a small pole The ORG sends a beam of light (which you cannot see) from one of its ends to a detector at the other end. When raindrops fall, they break the beam. The rain rate is measured by the ORG by measuring how often the beam is broken. The rain rate can be used to calculate the total amount of rain that has fallen in any given period ORG measures the rate of rainfall in millimeters per hour (mm/hr).

48. Measured Gauge Accuracy (Un)avoidable Errors Equipment failure Observer error Avoidable Errors Site Aspect - parallel to ground Obstructions Height - splashing Surrounds Wind

50. Measured Gauge Accuracy Common Errors Evaporation - 1% Adhesion - 0.5% Inclination - 0.5% Splash +1% Wind -5-8%

51. Measured Gauge Accuracy Two problems arise in quantifying precipitation input to a given land area: how to measure precipitation at one or more points in space how to extrapolate these point measurements to determine the total amount of water delivered to a particular land area.

52. Rainfall Surfaces If precipitation gauge data is used, then the MAP's are usually calculated by a weighting scheme. A gauge (or set of gauges) has influence over an area and the amount of rain having been recorded at a particular gauge (or set of gauges) is assigned to an area. Thiessen method and the isohyetal method are two of the more popular methods.

55. RADAR Measurements Raindrops in the atmosphere and the characteristics of the reflected signal(Z) can be related to rainfall rates (R). Most common is Marshall-Palmer relationship Radar is far from an absolutely accurate measurement method Provides detailed information on the time and space distribution of rain and can be particularly valuable for heavy rainfall.

58. Liebenbergsvlei Hydrometeorological Network

60. can do water budget or use indirect methods

61. evaporation pans

64. Evaporation Pans US Class A-Pan Standard instrument used to measure evaporation. Diameter = 1210 mm depth = 255 mm Usually set on 150 mm high base allows circulation of air Must be level Water level maintained 50mm below rim Measure with point gauge & still well graduated cylinder staff

66. Example of A-Pan Setup "US class A" pan is used to measure the rate of evaporation. A hook gauge is used to measure the water level inside the pan and A cup anemometer is placed beside the pan to measure the surface wind movement over it

68. Galvanised iron

69. Square of 1830mm

70. 610mm deep

71. Set in ground

72. rim 100mm above ground

73. level free of obstructions

74. natural vegetation surrounds (not tar etc)

75. no shadows on pan

76. fenced, bur not obstructed..protect from birds & animals - chemical or wire mesh

80. Weighing base to quantify water movement through soil

81. Precipitation controlled and known

84. Soil Water Soil-Water Content amount of water in the soil (volumetric and gravimetric) - quantative Soil-Water Potential the availability of the water to plants (largely qualitative) Methods of soil water content measurement include direct measurement by gravimetric methods (oven or microwave drying) indirect measurements by neutron probes, capacitance probes, time domain reflectometry (TDR), tensiometers, etc.)

89. Does not work well in soils with high clay content and/or EC equipment cost is very high

92. The fast neutrons collide with elements and slow down

93. Of all the elements, H (Hydrogen) in water is the most effective in slowing down fast neutrons

94. A detector counts the number of slow neutrons returned to the source

95. A calibration curve or equation relates neutron count to water content

96. Advantages and disadvantages

97. It measures a sphere of about 30 cm in diameter

98. Background H, bulk density, and other chemical components may influence the measuring results

101. Soil Water The water tension reflects the sum of the water holding forces of the soil. Tensiometer - Measures soil matric potential or tension Cylindrical tube, typically PVC, with a porous cup mounted on the end The cup, typically ceramic or teflon, is porous but with fine pores that remain saturated under the water tensions (i.e., capillary-pressure heads) to be measured. The tube is inserted into the soil, ensuring that a close contact is established between the porous cup and the soil. The tube is filled with water and tightly capped. A pressure gauge is used to measure the pressure in the water.

103. TENSIOMETER MEASURING PRINCIPLE All water movements in the soil are directly dependent on the water tension, since water will tend from areas of high potential to those of low potential.

107. Streamflow Velocity-Area Method Estimate flow volume by determining the velocity at which water flows through a given cross-sectional area. Flow = velocity X cross-sectional area or Q = VA Need estimates of channel: cross-sectional area "average" current velocity Final flow estimate accomplished by subdividing the cross-section of the channel, determining the "average" flow for each subdivision, and summing the subdivision flows into a total flow for the channel.

109. Current meters

110. shaft rotating vertically or horizontally

111. tail vanes - keep it in streamweight - keep cable vertical

115. In a deep stream subsection, the average velocity is estimated by the average of velocities measured 20% depth (0.2D) and 80% depth (0.8D). In a shallow stream subsection where measurement at two depths is difficult, the average velocity is determined by measuring velocity at 60% of the depth (0.6D). The flow for each subdivision is determined by multiplying the cross-sectional area of the subdivision by the average flow velocity within the subdivision

116. Streamflow Determination of: Depth or height of the water surface (known as stage) Derivation of a relationship between stage and volume of discharge allows determination of a “rating curve” specific to the section of river i.e. “rated section”

117. Rating Curves Rating curves establish a relationship between depth (stage) and the amount of flow in a channel.

118. Streamflow Measurement of Stage Graduated staff gauge side of bridge etc. Automatic water level recorders logged automatically by logger, or chart produced and digitised

120. Weirs and Flumes Commonly used on small streams and rivers SA rivers small by international standards no navigation issues Rigid, stable structures with closely defined cross-sectional area. Velocity of falling water depends on height of fall acceleration due to gravity - constant Therefore, possible to estimate velocity of water by causing water to fall (over weir or flume) and measure head of water at an appropriate point i.e. Discharge through weir notch is primarily dependent on the head (H), measured from the lowest point of the crest (where the fluid flows over the weir) to the surface of the stream at a distance upstream from the weir plate (where the surface elevation is not affected by the flow over the weir).

122. Stage Height Most common method of measuring the stage of a river is through the use of a stilling well. Stilling wells are located on the bank of a stream or on a bridge pier and are topped by a shelter that holds recorders and other instruments associated with the station. The well is connected to the stream by several intakes such that when the water level changes in the stream, the level simultaneously changes in the well Thus, the water surface in the well is maintained at the same level (stage) as the water surface in the stream.

123. Weirs Two main types: Sharp crested Broad crested V-Notch weir sharp-crested weir used to measure a wide range of flow rates decrease in the flow area will cause a decrease in the head. Therefore, even for small flow rates reasonable heads are developed and accurate results can be obtained.

124. Flumes Flumes include various specially shaped and stabilized channel sections that are used to measure flow. Use of flumes is similar to use of weirs in that flow is related to flow depths at specific points along the flume. Parshall Flume