If ewand ea are the saturated vapour pressures of the water surface and air respectively, the Dalton’s law for evaporation EL in unit time is given by EL =
The rate of evaporation is proportional to the difference between the saturation vapour pressure at the water temperature, ew and the actual vapour pressure in the air, ea . Thus,
where,EL = rate of evaporation (mm/day) C = a constant
ew and ea are in mm of mercury
The average pan coefficient for the standard US Weather Bureau class A pan is
A canal is 80 km long and has an average surface width of 15 m. If the evaporation measured in a class A pan is 0.5 cm/day, the volume of water evaporated in a month of 30 days is (in m3) : -
Volume of water evaporated in a day
Pan constant for class A pan = 0.7
∴ Volume of water evaporated in a month
The ISI standard pan evaporimeter is
An ISI standard pan evaporimeter has about 14% less evaporation than a US class A pan.
The chemical that is found to be most suitable as water evaporation inhibitor is
Wind speed is measured with
Evapotranspiration is confined to
Transpiration is essentially confined to day light hours and rate of transpiration depends upon the growth period of the plant. Evaporation on the other hand, continues all through the day and night although the rates are different.
A part of rain may be caught by vegetation and subsequently evaporated. The volume of water so caught is called interception. Interception loss is solely due to evaporation and does not include transpiration, through fall or stemflow.
The rainfall on three successive 6-h periods are 1.3, 4.6 and 3.1 cm. If the initial loss is 0.7 cm and the surface runoff resulting from this storm is 3 cm, the φ-index for the storm is
If for a given basin in a given period, P - precipitation, E = evapotranspiration R = total runoff and ΔS - increase in the storage of water in the basin, the hydrological water budget equation states