Q1 Compare & contrast Evaporation pans and lysimeters Flashcards
1
Q
Compare and contrast the use of evaporation pans and lysimeters for measuring evaporation
A
1) What is evaporation?
- liquid to gas (solar energy and water)
- Importance – amount of water and available energy (seasons)
- 3 types
- evapotranspiration
2) The significance of Evaporation
- only loss in water balance – quantity
- direct impact on water reaching stream and infiltration
- precipitation more significant
- Water quality – impurities left behind (Dead sea)
3) Measuring Evaporation
fraught with difficulty
indirect – water balance equations
4 )Evaporation Pans
What is it?
- circular pan – 1.21m dia and 255mm deep - impervious material
- E=ΔS – P
draw
Pros and Cons
- problems relating measurement of real losses from a large reservoir
- simple and inexpensive
- UK – unreliable – hard to control data
Actual evaporation problems
- measurement of Eo
- Eo much higher than Et (majority of catchment is land)
- Rarely used for catchment but useful for bodies of water
Open water Evaporation Problems
- ‘edge effect’
- empirical coefficients
- sides and water absorb radiation (smaller problem / paint white)
- arid regions – pans attract birds and animals (mesh – reduction)
Conclusion
- inaccurate
- long records – identify long trends
5) Lysimeters
What is it?
- fundamental difference filled with soil and vegetation
- Et rather than Eo (Et= E=ΔS – P – Q)
- Percolation
Pros and Cons
- take into account change in water storage (improved Et) most accurate
- never possible to recreate soil and plant life
- care with soil matrix
- accuracy dependant on sensitivity of weighing mechanism
- edge effect less due to vegetation
- trees are expensive and impossible to calculate water balance of a whole forest
- not evaporation that is being measured in a lysimeters – it is almost everything else in the water balance equation, with an assumption being made that whatever is left must be caused by evaporation. One result of this is that any errors in measurement of precipitation and/or percolation will transfer and possibly magnify into errors of evaporation measurement.
6) Conclusion
- limited area of both limits accuracy
- important in arid climates or seasons from tall vegetation
- Lysimeters much more accurate Et
- Evaporation pans inexpensive and long records.
