I would say that the pond evaporation is very similar to pan-evaporation (multiplied by an empirical factor). Also, some definitions of potential evaporation can be used to calculate pond evaporation, like the one defined by Penman.

I think it depends on irrigation management.

Dear Knoche,

It seems to be logical that evapotranspiration should be limited by stomatal resistance. But, in case of a reference crop, which is not short of water, and the leaf surface area is more than lake surface area. So, there is possibility that reference Evapo-Transpiration could be more.

I would expect, if any colleague has some practical experience/measurements in this regard.

The question can also be regarding how to calculate evaporation and transpiration together for the rice crop, i.e. when the crop fields are with standing water, just like many farmers keep standing water in rice fields in Pakistan.

Pan evaporation (Ep) is the largest value, followed by lake evaporation (EL) then (in most cases) reference evapotranspiration (ET).

Ep increased above EL due to the effect of the pan metal sides.

Normally, EL=0.75*Ep

ET is also less than Ep, but it depends on the pan location and the green surrounding width and fetch; ET=kp * Ep, where kp is the pan coefficient=0.35 to 0.85, which increases with the decrease of wind speed, and with the increase of the relative humidity.

I' ve sketched a diagram to show these relationships.

Best regards...

----

References:

http://www.fao.org/docrep/x0490e/x0490e08.htm#pan evaporation method

http://www-das.uwyo.edu/~geerts/cwx/notes/chap04/eoep.html

http://en.wikipedia.org/wiki/Pan_evaporation

In your diagram, shouldn't the humidity and wind speed triangles be the other way around?

@Anurag Mishra

No sir, the figure is correct, please refer to Table 5 Chapter 4 in the FAO 56

the link is below...

many thanks for your contribution

http://www.fao.org/docrep/x0490e/x0490e08.htm#pan

Dear Elnesr

I also have the same opinion as of Mishra. In your earlier answer, you rightly said that coefficient will increase with increase in wind speed. But, your wind speed triangle shows a decreasing value for itself to the right, whereas coefficient increases in this direction.

regards

Here is a chart I drawn from Table 5-Chapter 4, FAO 56.

The mistake was in the first comment not the drawing,

Now the first comment is correct.

Many thanks for all.

Interesting discussion!

The Pan Evaporation is normally higher than Lake evaporation during summer, especially in arid and windy area. There were some good studies by WAPDA consultants during seventies (see Revised Action Plan - RAP reports).

The Crop Et is measured higher for crops with higher Leaf Area Index, like sugarcane and Banana.

I have developed some tools for extracting pan evaporation data and other relevant data needed to use the Penman-Montieth Equation for the US at least--here's a day of the water year plot for Iowa, USA of 4 pans and the Penman Montieth Et using ~25 years of data. The source data come primarily from NCDC which does have world-wide data, although the solar radiation comes from a different database. Anyway, the reference ET is just a bit less than the pan measurements consistent with the other explanations. I think the crop coefficient for open water can exceed 1 in tropical areas accoding to Chapter 6, Table 12 in the FAO documentation (http://www.fao.org/docrep/x0490e/x0490e0b.htm#tabulated%20kc%20values).

Thanks David for providing useful explanation and Kc charts. These basic things sometimes become difficult for practitioners to handle!

For example, Kc values locally adopted could be very high like more than 1.35 for rice, vegetables and sugarcane in Sindh Pakistan. Recently, similar Kc factors are used for Bt cotton, for a longer period. Unless directly validated, these values may contain influences from other factors, like over-irrigation and high groundwater table. There may also be biases from the sub-surface lateral flows. I will highly appreciate, if there are field references to the crop transpiration component in highly arid area, temperature around 48C and strong wind.

An explanation about the groundwater. Because of high salinity of aquifer, farmers use large quantities of fresh water and adopt frequent irrigation, which protects the rootzone by building a freshwater layer.

In general case Lake evaporation indeed...

Dear Zaigham Habib,

With reference to your first answer,I would say that Pan Evaporation is perhaps always more than lake evaporation.

According to your 2nd answer, locally developed/adopted Kc values could be more than 1.35, especially for rice, which represents evaporation from standing water and transpiration from plant leaf area. Now, Kc being more than 1.0, does this mean, standing water in the rice fields is causing higher contribution than transpiration from the rice plants?

From here, I would conclude that reference evapotranspiration is always less than lake evaporation.

The real background to the question was that plants are acting as water pumps, and therefore, might be reference crop evapotranspiration (being not short of water) is more than lake evaporation.

Grateful to all for the rich discussion

The value of Kc equal to 1.35 means rice crop potential Et is 35% higher than the reference grass potential Et for the period. FAO values are up to 1.2 for Kc and 1.15 for mid-season crop transpiration coefficients. In my opinion potential ET can be that higher. The studies I mentioned used standard procedures, also carried out farm level water-balances for major crops. The soil moisture levels remain high in some cases (60% to 80%). However, there are stress factors in the areas having low water allocation and following warabandi schedule. Which must be considered. In reality, it is not possible to have standard Kc values. There is an influence of irrigation practices, as somebody mentioned. We can only have standard Et0, representing the climatic variables.

Pan evaporation is considered as a “model” for calculating pond ( or lake )evaporation. Theoretically, both values should be close to each other for the same climatic condition and wind pattern. However, in most cases pan evaporation is higher than pond evaporation. Both of pan evaporation and pond evaporation are higher than crop reference evapotranspiration due to stomatal resistence . The following quite famous empirical relationship is relating: crop reference evapotranspiration to pan evaporation : ET=kp * Ep, where kp is the pan coefficient which is ranging from 0.35 to 0.85, depending on the climatic conditions and wind speed and pattern of the area in concern.

It is quite clear that ETo is smaller than lake evaporation due to so many factors

- crop factor and the force of plant to prevent evaporation while in the lakes water molecular is free to go into atmosphere.

Interesting discussion.

Pan evaporation as well as lake evaporation is a function of meteorological parameters. Here, we are considering the rate alone, in mm/day. Due to known factors, such as material used in pan manufacturing, location of pan etc, it is found that pan evaporations are more than the lake evaporation. Since, it is very difficult to measure the lake evaporation, we use pan evaporation multiplied with a pan coefficient. Thus, pan evaporation as such is more than the lake evaporation.

Reference crop evapotranspiration ETo is also function of meteorological variables. Many methods are available to estimate it. Kindly refer papers by prof. V. P. Singh and FAO publications. If we use pan evaporation to estimate ETo, then we multiply pan evaporation with appropriate pan coefficient which may be function of RH, wind speed, (published literatures are available). In this case, the ETo is less than pan evaporation. But, we cannot say whether is is always more than or always less than lake evaporation. Because for lake evaporation we normally use a pan coefficient of 0.75 for class A pan, but for ETo we use a pan coefficient varying between 0.5 to 0.85.

The crop evapotranspiration ETc is estimated by multiplying the ETo with crop coefficient Kc. For most of the crops the Kc is less than one, except for water loving plants like rice. As exactly pointed out by many researchers that it is due to standing water evaporation and transpiration from crop ( this also only during mid vegetative stage). In this case, if we use average pan coefficient to estimate ETo, then the ETc will be more or less equal to the pan evaporation. (With in 10% deviation).

good discussion. There is no clear answer in comparing lake evaporation and reference ET, as the latter depends upon the crop (and growing stage), as mentioned by Vinayakram and others on this thread. However, lake evaporation in most cases exceeds ACTUAL crop evapotranspiration (unless there is plentiful irrigation to avoid any water limitation). Exceptions to this are seen in wetlands with tree islands, where the actual ET in tree islands is inferred to be greater than the surrounding flooded wetland, as seen by the lower daytime groundwater table under tree islands that is caused by tree water uptake. Hence to focus on your question on water requirements for crops and aquaculture, usually one can expect higher evaporative losses from ponds over crops, unless the crops are very well watered, high transpiring species like rice and sugarcane, or agroforestry.

Re: plants acting as water pumps, to complicate matters (!), there's usually breezier conditions over a lake than over a farm, on account of the land-water body temperature dynamics; this can whisk away the boundary layer and thereby promote evaporation, while the less breezier conditions over a farm can build up a boundary layer thereby increasing humidity, decreasing the vapor pressure deficit and thereby acting as a resistance to ET. The best way to approach this comparison is to actually carry out some pan evaporation, lake level and lysimeter (if crop) measurements.

How about ET without crops, from bare soil?

ET from bare soil depends upon, depth to watertable and soil type. However, it will always be less than lake evaporation.

I have done a research on this you may read the following link to get some more

https://www.researchgate.net/publication/255909976_CONJUNCTIVE_USE_OF_SURFACE_AND_GROUNDWATER_TO_IMPROVE_FOOD_PRODUCTIVITY_IN_A_RESTRICTED_AREA_Degree_of_Doctor_of_Philosophy

Thesis CONJUNCTIVE USE OF SURFACE AND GROUNDWATER TO IMPROVE FOOD P...

For answer to this question we should attention to Evaporation surface to judgement about subject.

In a farm, for example, leaves have extended surface and evaporation is high and canopy level from soil to top effects on ET.

E from a water surface is not as simple as implied by much of the above. If the water is clear, the radiation from the sun passes through it, leaving it relatively unheated. If the water is cloudy (silt, whatever) the energy from the sun is absorbed, the water warms and E increases. E from a free water surface can thus be more or less than Eto

Dear Perry

Thank you, your answer has a scientific logic behind. I agree that in certain cases ETo can be larger than free water surface evaporation. Are there any experimental evidences available in the literature?

Pan evaporation is highest in most cases, but not all. Plants provide a larger surface area then a pan, which could cause ETcrop to exceed Epan. Also an open pan provides an oasis affect which causes evaporative cooling due to heat advection. There are many factors that influence this, so it would be best to do some more research specific to the area, crop type, and soil. Realistically, the best assumption is that ETpan>ETcrop since this is the case almost all the time.

The lake evaporation is higher than reference crop evapotranspiration. Furthermore,

The lake evaporation is about 60-70% of reference crop evapotranspiration.

@ Ahmet Ertek any reference for this statement? 

The lake evaporation is about 60-70% of reference crop evapotranspiration

The lake evaporation is lower than reference crop evapotranspiration.  Furthermore,

The lake evaporation is about  60-70% of reference crop evapotranspiration.

Because, depth of  lake water is more than class-A pan or soil water.

Hi, my friend. In my recent paper, I discussed the lake evaporation, reference E and potential evaporation. I think that lake evaporation and reference evaporaiton can both represent ET0 in a region, and usually the former is smaller than the latter, but with little difference, according to my study in a hyper-arid region.

For more information, plz check the paper attached. I hope it could help you a little.

Kind regards.

Liu Xiao

Article Lake Evaporation in a Hyper-Arid Environment, Northwest of C...

Ahmet Ertek · 18.17 · T.C. Süleyman Demirel Üniversitesi

The lake evaporation is lower than reference crop evapotranspiration.  Furthermore,

The lake evaporation is about  60-70% of reference crop evapotranspiration.

Because, depth of  lake water is more than class-A pan or soil water.

Based on lysimeter data (work in progress), water loss from wetland/graminoid vegetation is higher than open water evaporation in a humid subtropical region. Now, that is actual ET and not reference ET, but just wanted to point out this possibility.