Hydrology 220: Hydrometry

Hydrological Measurement, Instrumentation and Networks

Mark Horan

Room 341

Hydrometry Course• 4 Lectures 1 Practical• Lecture 1: Basics and Introduction• Lecture 2: Measurement of Rainfall• Lecture 2:Measurement of Interception

and Evaporation• Lecture 3: Measurement of Soil Moisture• Lecture 3: Measurement of Streamflow• Lecture 4: Hydrological Network Design• Lecture 4: Hydrological Network Design

cont.

Hydrologic Cycle

ComponentsPrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementStreamflowStorage-Reservoirs

Introduce basic terminology and concepts of measurement of the hydrological cycle.

• What are the sources of water for a watershed?• Where does all the water go?

Water Balance Equation in a Catchment

P - Q - G - ET - S = 0• P = Precipitation• Q = Stream discharge• G = Groundwater Discharge• ET = Evapo-transpiration• S = Change in Storage

Why Measure

• Predict impacts from prior knowledge or experience

• Infer impacts from evidence collected

• Experimentally investigate impact of certain activities

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]

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.

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

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

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

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

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

Recording of Data

• Paper Charts• Data Loggers• Telemetry.

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”

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.

Recording of Data

• Data Loggers – Problems

• Vandalism due to desirability of batteries

Recording of Data

• Telemetry– Data stored by logger can transferred directly

to a base station via some form of telecommunication

Hydrology 220: Hydrometry

Lecture 2

Measurement of Precipitation

Types of Precipitation

• Rainfall• Hail• Snow and Ice

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

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

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

Image here

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.

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

Two types of standard storage raingauge

Recording Raingauges

• Analogue Devices– Weighing Bucket Rain Gauge– Float Type Rain Gauge

• Digital Devices– Tipping Bucket Rain Gauge– Optical Rain Gauge

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.

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.

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

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

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).

Measured Gauge Accuracy• (Un)avoidable Errors

– Equipment failure– Observer error

• Avoidable Errors– Site

• Aspect - parallel to ground• Obstructions• Height - splashing• Surrounds• Wind•

• Ideally, the gauge should be sited with some shelter, but not over-sheltered.

• Windshields may reduce the loss due to turbulence (eddies) around the gauge

Measured Gauge Accuracy• Common Errors

– Evaporation - 1%– Adhesion - 0.5%– Inclination - 0.5%– Splash +1%– Wind -5-8%

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.

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.

Thiessen

• Thiessen method is a method for areally weighting rainfall through graphical means.

Isohyetal

• Isohyetal method is a method for areally weighting rainfall using contours of equal rainfall (isohyets).

RADAR Measurements

• Weather radar has become an increasingly important tool for estimating the spatial distribution of rainfall

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.

Liebenbergsvlei Hydrometeorological Network

Evaporation

­ Direct measurement from large surfaces (land & water) not possible at present­ can do water budget or use indirect

methods· evaporation pans· lysimeters

Evaporation Pans

• different sections, square, round etc.• different positions, on ground, above ground,

sunken & floating• Most common types in SA

– A-Pan– S-Pan (Symons - British Standard)

• Pan coefficient used to relate measured evaporation to free water surface as measured evap. often far greater than actual– e.g. 0.7 for S-Pan on annual basis

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

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

Evaporation Pans

· Symons pan · Galvanised iron· Square of 1830mm· 610mm deep · Set in ground

· rim 100mm above ground· level free of obstructions· natural vegetation surrounds (not tar etc)· no shadows on pan· fenced, bur not obstructed..– protect from birds & animals - chemical or wire mesh

Lysimeter

• A device to measure the quantity or rate of downward water movement through a block of soil usually undisturbed, or to collect such percolated water for analysis as to quality.

• Defined as:– A small unit of soil on which water balance values can

be obtained.

· Lysimeters account for change in water storage· i.e. Measure actual evapotranspiration

Lysimeters

· Large block of undisturbed soil + vegetation surrounded by watertight container installed in ground

· Weighing base to quantify water movement through soil

· Precipitation controlled and known· E=Ppt - percolation through the lysimeter

Principles of a Lysimeter• A tank filled with soil is

weighted on a scale.– The difference in weight

between the beginning and the end of the day indicates how much water was lost during the day, or,

– how much water the crop used.

• At midnight (or some standard time)– water tank below the

lysimeter is filled with water that can be used for irrigation during the day.

– ie no weight change as a result of irrigation during the day.

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.)

Soil Water• Time Domain Reflectometry (TDR)

· TDR measures the transit time, t, for a pulse to travel between the wave guides; the greater the dielectric constant (Ka) of the surrounding medium, the longer the pulse travels through the guides.

· In a soil system, Ka is predominantly determined by liquid water. Thus, volumetric water content can be correlated to Ka through some calibration equations:

• Advantages and disadvantages· Easy to be automated to make continuous

observations

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

· Relatively expensive

Soil Water• Neutron Probe

· A radioactive source emits fast and slow neutrons

· The fast neutrons collide with elements and slow down

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

· A detector counts the number of slow neutrons returned to the source

· A calibration curve or equation relates neutron count to water content

· Advantages and disadvantages· It measures a sphere of about 30 cm in diameter

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

· Radioactive

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.

Soil Water

• Tensiometer -(continued)

· Advantages and disadvantages– Tough, simple and inexpensive– Limitations: narrow pressure range and slow response

to rapid change in matric potential – If the tubing is relatively long, the gauge readings

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.

Soil Water• Absorbent blocks

– Two electrodes embedded in the gypsum or nylon blocks

measure the electrical conductivity (resistance); · Calibration between conductivity and matric potential/water

content

• Advantages/disadvantages and comments· The absorbent blocks can only be used under conditions

where salts do not affect the calibration curve unduly.

· Blocks can be used under drier conditions than the tensiometers and are more sensitive at lower water content

Streamflow• The flow of water in an open channel (or

discharge) is defined as the volume of water passing a specified point in a given interval of time– expressed in units of volume per time

• Common units– litres per second (l.s-1), – cubic meters per second (m3.s -1).

• Various methods of determination– flow is often estimated by determining the velocity at

which water flows through a given cross-sectional area.

– flow may be routed through a measurement device and measured directly

– may be determined indirectly through use of appropriate measurements and mathematical models

StreamflowVelocity-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.

Streamflow Measurement

­ Current meters used to measure velocity

­ Current meters· shaft rotating

vertically or horizontally

· tail vanes - keep it in stream

• weight - keep cable vertical

• 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

StreamflowDetermination 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”

Rating Curves

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

Rating Curve for a sample watershed

204

206

208

210

212

214

216

218

0 2000 4000 6000 8000 10000 12000 14000 16000

Q total (cfs)

Wat

er

Su

rfac

e E

lev

. (ft

)

Streamflow• Measurement of

Stage

– Graduated staff gauge• side of bridge etc.

– Automatic water level recorders• logged

automatically by logger, or

• chart produced and digitised

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).

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.

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.

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