Water Vapour and Humidity Variables Questions and Answers

Q: Which term describes evaporation from a surface with unlimited water?

Potential evapotranspiration is the rate that would occur if water were freely available, and it is determined by the atmospheric demand alone. Actual rates fall below it wherever the supply is limited, and the ratio between the two is a useful index of water stress. The distinction underlies every irrigation scheduling calculation.

Q: Which index compares rainfall with atmospheric demand?

The aridity index divides mean annual precipitation by potential evapotranspiration, and values below 0.05 define hyper-arid regions while values above 0.65 define humid ones. It identifies drylands independently of the absolute rainfall. The standardised precipitation index measures anomalies rather than climate type, which makes it a drought index rather than a classification. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Q: Which quantity does a drought index attempt to capture?

Drought is a relative concept, since a rainfall total that would be normal in a desert would be an extreme drought in a rainforest, so indices measure departures from the local distribution. Consecutive dry days is one simple measure among many. The relativity is why a single global threshold cannot work and why several indices coexist for different purposes. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Q: Which timescale distinguishes meteorological from hydrological drought?

A rainfall deficit is felt first as meteorological drought, then propagates into soil moisture as agricultural drought and finally into rivers, reservoirs and groundwater as hydrological drought. Each stage lags the last, so hydrological drought can persist long after the rain returns. Recognising the sequence explains why reservoir restrictions often continue through a wet autumn, and it is the reason several distinct indices are maintained rather than one.

Q: Which measurement gives soil moisture from space?

The microwave emissivity of soil depends strongly on its water content, and the long wavelengths of L band penetrate vegetation well enough to sample the top few centimetres. The SMOS and SMAP missions were built for the purpose. Surface temperature does carry an indirect signal through the evaporative cooling, which makes the infrared option a genuine if weaker technique.

Q: Which depth do such measurements sample?

Microwave emission originates within roughly the top five centimetres, so the root zone moisture that matters for agriculture must be inferred by combining the observation with a land surface model. Vegetation attenuates but does not block the L band signal. The mismatch between what is measured and what is wanted is the central difficulty of satellite soil moisture work.

Q: Which mission measures changes in total water storage including groundwater?

Measuring tiny variations in the Earth's gravity field reveals changes in the total mass of water stored in a region, including groundwater far beyond the reach of any electromagnetic technique. The GRACE missions revealed the depletion of major aquifers in India and California. The measurement is of change rather than absolute storage, which is an important qualification.

101.

Which term describes evaporation from a surface with unlimited water?

A.Interception loss
B.The Bowen ratio, which measures the partition of the available energy between sensible and latent heat flux
C.Potential evapotranspiration
D.Actual evapotranspiration, which is the quantity that occurs under the conditions prevailing at the site

Answer: C

Potential evapotranspiration is the rate that would occur if water were freely available, and it is determined by the atmospheric demand alone. Actual rates fall below it wherever the supply is limited, and the ratio between the two is a useful index of water stress. The distinction underlies every irrigation scheduling calculation.

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102.

Which index compares rainfall with atmospheric demand?

A.The Bowen ratio, which compares the sensible heat flux with the latent heat flux at the surface directly
B.The Palmer index
C.The standardised precipitation index, which compares the rainfall with its own long term distribution instead
D.The aridity index

Answer: D

The aridity index divides mean annual precipitation by potential evapotranspiration, and values below 0.05 define hyper-arid regions while values above 0.65 define humid ones. It identifies drylands independently of the absolute rainfall. The standardised precipitation index measures anomalies rather than climate type, which makes it a drought index rather than a classification. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Discuss in Forum

103.

Which quantity does a drought index attempt to capture?

A.A deficit relative to the normal conditions of the region
B.The absolute amount of rainfall, so that any region receiving less than a fixed threshold is classed as in drought
C.The soil temperature
D.The number of consecutive days on which no measurable precipitation has been recorded at the station concerned

Answer: A

Drought is a relative concept, since a rainfall total that would be normal in a desert would be an extreme drought in a rainforest, so indices measure departures from the local distribution. Consecutive dry days is one simple measure among many. The relativity is why a single global threshold cannot work and why several indices coexist for different purposes. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Discuss in Forum

104.

Which timescale distinguishes meteorological from hydrological drought?

A.Meteorological drought affects only the atmosphere while hydrological drought affects only the deep groundwater
B.Meteorological drought is a rainfall deficit while hydrological drought is a deficit in rivers and stores
C.Meteorological drought lasts for years while hydrological drought is confined to a single season at most
D.There is no distinction

Answer: B

A rainfall deficit is felt first as meteorological drought, then propagates into soil moisture as agricultural drought and finally into rivers, reservoirs and groundwater as hydrological drought. Each stage lags the last, so hydrological drought can persist long after the rain returns. Recognising the sequence explains why reservoir restrictions often continue through a wet autumn, and it is the reason several distinct indices are maintained rather than one.

Discuss in Forum

105.

Which stage of drought propagation affects crops most directly?

A.Hydrological drought
B.Socioeconomic drought, which describes the effect of the water shortage upon the supply of goods and services
C.Agricultural drought expressed as a soil moisture deficit
D.Meteorological drought, since the crops depend directly upon the rainfall arriving during the growing season

Answer: C

Crops draw water from the root zone, so the relevant deficit is in soil moisture rather than in rainfall directly, and a well timed shower can relieve agricultural drought without ending the meteorological one. The distinction matters for forecasting impacts. Socioeconomic drought is the final stage in the sequence and concerns the consequences rather than the physical deficit.

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106.

Which measurement gives soil moisture from space?

A.Infrared imagery, from which the moisture content is inferred by measuring the temperature of the surface below
B.Visible reflectance
C.Radar altimetry, which measures the elevation of the land surface and therefore the amount of water stored in it
D.Passive microwave sensing at long wavelengths

Answer: D

The microwave emissivity of soil depends strongly on its water content, and the long wavelengths of L band penetrate vegetation well enough to sample the top few centimetres. The SMOS and SMAP missions were built for the purpose. Surface temperature does carry an indirect signal through the evaporative cooling, which makes the infrared option a genuine if weaker technique.

Discuss in Forum

107.

Which depth do such measurements sample?

A.The top few centimetres of the soil
B.The whole of the root zone down to about one metre, which is the layer relevant to the growth of most crops
C.The deep groundwater
D.The surface layer only when the ground is bare, since vegetation prevents the signal from reaching the soil

Answer: A

Microwave emission originates within roughly the top five centimetres, so the root zone moisture that matters for agriculture must be inferred by combining the observation with a land surface model. Vegetation attenuates but does not block the L band signal. The mismatch between what is measured and what is wanted is the central difficulty of satellite soil moisture work.

Discuss in Forum

108.

Which mission measures changes in total water storage including groundwater?

A.The precipitation measurement satellites, from which the storage change may be computed as a residual balance
B.The gravity recovery satellites
C.The soil moisture missions, whose microwave radiometers penetrate deeply enough to sense the water table below
D.Radar altimeters

Answer: B

Measuring tiny variations in the Earth's gravity field reveals changes in the total mass of water stored in a region, including groundwater far beyond the reach of any electromagnetic technique. The GRACE missions revealed the depletion of major aquifers in India and California. The measurement is of change rather than absolute storage, which is an important qualification.

Discuss in Forum

Water Vapour and Humidity Variables Questions and Answers

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Water Vapour and Humidity Variables Questions and Answers

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