Ways of Measuring Evaporation from Lake Discussion
I’m working on a environmental science exercise and need an explanation and answer to help me learn.
1. What is evaporation? Describe different ways of measuring evaporation from a lake.
2. Explain how salinity and the depth of a water body affect the rate of evaporation.
3. Explain the convective process. Contrast it with an orographic lift.
4. Explain the frontal lifting of air masses.
5. Explain the mechanical turbulence.
6. Explain the three major plant groups with respect to three water requirements.
7. What is transpiration? What are the factors affecting transpiration? How do phreatophytes adapt to dry climates to reduce water loss from transpiration?
Tags: mechanical turbulence frontal lifting of air masses orographic lift
What is Evaporation and How it Occurs?
Before rainfall reaches the outlet of a basin as runoff, certain demands of the catchment such as interception, depression storage and infiltration have to be met. Besides these, evaporation and transpiration processes transfer water to the atmosphere as water vapour. Evaporation from water bodies and the soil mass together with transpiration from vegetation is called evapotranspiration (ET). That portion of Precipitation which is not available as surface runoff is termed as “loss”.
Evaporation is the process in which a liquid changes to the gaseous state as the free surface, below its boiling point, through the transfer of energy. Evaporation is a cooling process- the latent heat of vapourisation (~585 cal/g of evaporated water) must be provided by the water body. Rate of evaporation depends on
Vapour pressures at the water surface and the air above
Wind speed – Incident solar radiation
Atmospheric pressure – Quality of water
Air and water temperatures
Size of the water body
Vapour pressure – Rate of evaporation is proportional to the difference between the saturation vapour pressure (SVP) at the water temperature clip_image002 and the actual vapour pressure in the air (clip_image004)clip_image006
This equation is called Dalton’s Law of Evaporation. Evaporation occurs till clip_image008. If clip_image010 condensation takes place. Temperature – Rate of evaporation increases with an increase in water temperature. Although there is an increase in the rate of evaporation with increase in air temperature, a high correlation does not exist between. For the same mean monthly temperature, evaporation from a lake may be different in different months. Wind – Wind helps to remove the evaporated water vapour from the zone of evaporation, thereby creating greater scope for evaporation. Rate of evaporation increases with increase in wind velocity up to some limit (critical wind speed) and thereafter any further increase in wind velocity does not have any effect on the evaporation rates. This critical wind speed value is a function of the size of the water surface (large water bodies – high wind speeds) Atmospheric Pressure – Other factors remaining the same, a decrease in atmospheric pressure (as in high altitude areas) increases the evaporation rate Soluble salts – When a solute is dissolved in water, the vapour pressure of the solution is less than that of pure water and hence it causes reduction in the rate of evaporation. The percentage reduction in the evaporation rate approximately corresponds to the percentage increase in specific gravity Under identical conditions evaporation from sea water is about 2-3% less than that from fresh water
Estimation / Measurement of Evaporation
This is done by the following methods
Using empirical equations
By analytical methods
Types of Evaporators
These are pans containing water which are exposed to the atmosphere. Loss of water by evaporation from these pans are measured at regular intervals (daily). Meteorological data such as humidity, wind velocity, air and water temperatures, and precipitation are also measured and noted along with evaporation.
(1) USWB Class A Evaporation Pan
A pan of diameter 1210mm and depth 255mm
Depth of water is maintained between 18 and 20cm
The pan is made of unpainted GI sheet
The pan is placed on a wooden platform of height 15cm above ground level to allow free air circulation below the pan
Evaporation is measured by measuring the depth of water in a stilling well with a hook gauge