Determination of evaporation losses
The measurement of evaporation losses can find out by using three methods pan measurement method, water budget or water balance method and Energy balance method.In these pan measurement method is commonly used.
1.Pan measurement method
A pan of standard dimension is filled with water to a stipulated level.When the pan is exposed to the atmosphere, there will be a change in the water level of the pan.
When the water level is below the stipulated level, the same volume of water needs to add to bring back water level into the stipulated level
That is, when water is below the level, the same volume of water has to be added or when water is above the stipulated level, some amount of water has to be removed from the pan to bring back into the stipulated level.
By knowing the volume of water removed or added, the depth of water removed or added can be calculated by
The depth of water added or removed=Volume of water removed or added/Surface area of the pan
The following three cases are examined for Measurement of Evaporation losses from the pan.The following cases need the measurement of rainfall.
a.When there is no rainfall
The water added or removed in mm=Evaporation losses in mm
b.When there is rainfall
P<E (Precipitation is less than evaporation)
Evaporation loss in mm=Precipitation value(Rainfall in mm)+ Water added in mm
c.When there is rainfall
P>E (Precipitation is more than evaporation )
Evaporation in mm=Precipitation value(Rainfall in mm)+water removed in mm
2.Water budget Method.
According to water budget method, the measurement of evaporation losses can be found by
∑Inflow -∑Outflow=Change in storage+ Evaporation losses
E=(P + Isf + Igf) – (Ost + Ogt+ T) ± Δs (Water budget method)
- Isf =Surface water inflow
- Igf=Ground water inflow
- Ost=Surface water inflow
- Ogt=Ground water inflow
- T=Transpiration losses
- Δs=Change in storage
3.Energy Balance Method
According to Energy Balance Method, the measurement of evaporation losses can find out by
- Hn= Net heat energy received by water surface
- Hb=Black radiation from the water body.
- Ha=Sensible heat transfer from the water surface to air=ΒρwLa E
- Where Β=Bowen ratio= Ha/Ha
- He= Heat energy used up in evaporation=ρwLa E
- La=Latent heat of evaporation
- Hg=Heat flux into the ground.
- Hi=Net heat conducted out of the system by water flow.
- Ha=Heat stored in a water body.
Evaporation losses=Hn-Hg-Hs-Hi/ρwLa(1+Β) (Energy balance method)
Hope you learned this article, how to measure evaporation losses.