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Hydrological Measurements

30 important questions on Hydrological Measurements

What is a rain / precipitation gauge?

A vessel open to the air in which the collected water substance is periodically measured or continuously recorded.

How does a standard gauge / cylindrical container work?

A straight-sided cylinder in which precipitation depth is periodically measured by a dipstick or by weighing the calibrated vessel (typically used for hourly or daily precipitation totals).

How does a weighing gauges work?

It collects precipitation in a vessel on a scale and continuously record the accumulated weight.

How does tipping-bucket gauges work?

It collects precipitation in one of a pair of small-capacity vessels that are balanced on a fulcrum. When one vessel is filled, it tips and empties and records the time of this event and the other vessel is brought into position for filling.

How does float gauges work?

It collects precipitation in a vessel containing a float that is connected to an analog or digital recording device. Precision depends on the float and recording mechanism.

What are two disadvantages of weighing gauges?

1. The sensitivity is reduced as weight increases.
2. Expensive.

How does optical gauges work?

It measures precipitation rate as proportional to the continuously recorded disturbance of a light beam emitted by a sensor. The intensity variation due to raindrops is proportional to rainfall rate.

How does acoustical gauges work?

It records the underwater sound made by raindrops falling into a body of water. Because the sound signatures produced by different sizes of drops are unique, the sound spectra can be used to infer rainfall rates.

How does radar work to determine precipitation?

When radar observes a raining atmosphere, it measures a quantity called reflectivity that can be converted to rain rate.

Where does the relation between radar reflectivity and rainfall rate depends on? (5 things)

1. Diameter of the rainfall drops.
2. Concentration and size distribution of drops.
3. Pattern of vertical wind velocity.
4. Presence of hail, snow or melting snow.
5. Characteristics of the radar system.

Why is radar useful in combination with standard rain gauge data?

To obtain spatial information.

Name six errors that can occur in the measurement of precipitation.

1. Catch errors
2. Splash
3. Evaporation
4. Low intensity / fog
4. Instrument failure (continuous gauges)
6. Observer errors (manual read gauges)

What are measurement issues related to wind? (4 things)

1. An increase in wind is a decrease in catch (more lift force).
2. Rain gauges act as an obstacle to wind flow which can lead to turbulence and an increase in wind speed.
3. Particles that would have entered the gauges tend to be deflected and carried further downwind.
4. The wind reduced the catch of smaller drops.

Name eight areal interpolation of precipitation techniques.

1. Thiessen polygons
2. Inverse distance
3. Kriging
4. Two-axis method
5. Hypsometric method (takes elevation into account)
6. Different smoothing functions
7. TOP-kriging (takes nested information into account)
8. PRISM model

What is a variogram?

A dimensional quantity that increases with separation, the spatial-correlation coefficient is a dimensionless quantity that decreases with separation.

How to measure pan-evaporation?

Epan = P - (Vbegin - Vend)

How does a lysimeter work?

It measures the loss of water by mass balance from a surface layer usually containing soil and vegetation.

How does Eddy covariance work?

It measures vertical movement of air and concentration of gasses.

What does is mean in Eddy covariance if fluctuations of vertical velocity (u) negatively correlate with specific humidity (q)?

There is downward transport of vapor (condensation).

What does volumetric gauging of streams work?

It involved diverting stream flow into a container with a known volume and measuring the time it takes for the volume to fill.

How does the float method work in a stream?

Throw the float in the river and measure the time that the float takes to to travel a certain distance.

How does a propeller current meter work?

Magnetic switch closure generates pulses when propeller rotates, pulses counted by a counter unit.

How does a electromagnetic current meter work?

Electrodes is probes detect voltages generated by flowing water.

What is a disadvantage of the electromagnetic current meter?

Expensive

What are two advantages of the electromagnetic current meter?

1. Low in maintenance.
2. Can be used to measure velocity in vegetated channels.

How does an acoustic Doppler current profiler (ADCP) work?

It sends away a pulse sound of known frequency. This is reflected on solid particles. If the particle moves away from the instrument the pulse gets lower. The time it takes for a pulse to return is a measure for the location of reflection.

How does dilution gauges work?

A tracer is introduced into a flow at an upstream location and measuring the rate of arrival of the tracer at a downstream. location.

Where are dilution gauges often used?

In small, highly turbulent streams with rough, irregular channels.

Name three ways to measure stage.

1. Ruler staff gauge.
2. Float and counterweight system.
3. Pressure.

Four things to know about flumes (goten):

1. They are used in relatively flat water courses.
2. No different in height is required.
3. They are self cleaning and can be used in sediment laden rivers.
4. They have an unique rating curve due to acceleration in the narrow part of the flume.

The question on the page originate from the summary of the following study material:

Catchment Response Analysys

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