Dear Hasanthi;

Pan readings are taken daily in the early morning at the same time that precipitation is measured. Measurements are made in a stilling well that is situated in the pan near one edge. The stilling well is a metal cylinder of about 10 cm in diameter and some 20 cm deep with a small hole at the bottom. (FAO irrigation and drainage no.56)

To calculate the evaporation it is necessary to measure the rainfall and the water level in the pan at the same time.

Normal Rainfall Situation:

1. Read the raingauge every day at 8:00 or 9:00 O’ clock. If the raingauge is empty no entry need to be made against the date [i.e in column (L)]. [see figure (4) and figure (6)].

2. If the reading is 0.1 mm (i.e below the first graduation in the measure) enter the word “trace” in column (L) [see figure (4) and figure (6)] . Note: Days in which rainfall is less than 0.1 mm are not considered as rain days.

3. If the amount measured is due entirely to frost, fog or dew please write the word “frost”, “fog” or “dew” beside the entry of the amount. Note: although an amount of 0.2 mm or more may be entered these particular days are not counted as rain days.

4. If there is an error in the raingauge reading due to “raingauge overflow”, “raingauge leaking” or “funnel blocked by hail” enter the amount and enter the reason for suspecting the reading in the “remarks” section.

Unusual Heavy Rainfall:

1. In addition to the routine check of rainfall at 8:00 or 9:00 O’clock, it is valuable to provide information of the actual time when heavy rainfall occurs.

2. If heavy rainfall has been falling for some hours a reading of the raingauge at the time is considered a valuable information.

3. A reading of the raingauge at the end of a thunderstorm also provides valuable information. (attached file)

So, in simple words:

We have a sign in the pan to determine the evaporation. If there was a rainfall from last read, so we should do as follow:

1. if the water in the pan is below the sign, and we added 'e' value of water, and also the precipitation is 'p', for total evaporation 'E' we have: E=e+p;

2. If the water in the pan is exactly at the sign, which mean the is no change, so evaporation is equal to precipitation, so: E=p;

3. In case that precipitation occurs, and the water is above the sign, we should reduce the water up to the sign, which could be shown as 'e', so the total evaporation is: E=p-e;

So, generally if we show added water in positive or reduced water in negative, then total evaporation could be shown as: E=e+p;

Hope it helps.

Dear Hasanthi Wijesekara,

* The equation you quoted is normally used to estimate the crop evapotranspiration (ETc)

where, Kp = pan coefficient (varies according to the type of pan used. Typically Kp = 0.8 for Class A evaporation pan), and Ep = pan evaporation.

* But, really to estimate the evapotranspiration, this equation misses the cultural coefficient (Kc).

* If you want to estimate the evaporation of a water body (E), in this case, the equation becomes E = Kp * Ep (2)

* If there is a rainfall, the equation (2) becomes E = Kp * Ep + P (3)

where, P = precipitation.

Attention, if the water in the pan is above the sign, the value of (Ep) must be negative (Ep < 0).

Best regards.

Initial reading +rainfall -last reading

Thank you all for helping me.

During rainy day,

If water level in the stilling well is above pointer: the the water need to be removed and quantified until sharp point touches the water surface.

Evaporation = rainfall-removed quantity of water

If water level in the stilling well is below pointer:

Evaporation = rainfall + added quantity