Dears,
I am computing actual evapotranspirations of a plain area composed of irrigated farmland, shrubland, forest and grazing land in semi-arid area of Ethiopia. Part of this work, the actual evapotranspiration results are partly comparable with the annual rainfalls in the area. Is there any possibility at which the annual actual evapotranspiration ≥ the annual rainfall in such environment? In fact, it is quite obvious that when water is artificially added into the agricultural farms in the dry season, the cumulative actual evapotranspiration could be higher than the seasonal rainfall in the hydrologic year.
But, I need a wider argument for my work in this regard. Thus, would I get your reflection please?
With best regards,
Hailemariam
Dear Hailemariam,
As you said, if PET is really high, which is true for arid areas, and if there no water limitation from the site, then that means AET can be larger than P. But, this is possible only if the source of moisture is not only precipitation (P), and the site is also receiving water inputs from diversion (irrigation) and high ground water accumulations, as you said. However, this is small (site) scale phenomenon. For watershed or basin scale analysis, the first principle is Budyko curve stating P is equal to AET at annual scale. If P > AET, then that mean the input is larger than the output, which is not possible. The spatial scale matters! Please refer to my papers, and others cited there, around this topic.
Dear Dr. Wuletawu,
I thank you for your scientific reflection. For sure your papers are in my mind, and I will refer them for better scientific argument.
(I am not a #Dr# )
With bets regards,
Hailemariam
Actual evapotranspiration depends on the water availability at the field, whether it comes from rainfall or irrigation. it is also mentioned you it is semi-irrigated.
There is a possibility of annual actual evapotranspiration ≥ the annual rainfall
Dear Chandrasekhar,
I thank you for your reflection.
With best regards,
Hailemariam
In irrigated fields, ETa may exceed annual rainfall since there is contribution of irrigation water (from groundwater - pumping and/or surface water - river diversion or water from reservoirs) to the water balance equation. In such case, irrigation water application tends to transform the ET process from water-limited to energy-limited and can result in significant increase of ETa especially in arid and semiarid areas where energy is not a limiting factor. Reading of the article below might help:
Article Evaluating actual evapotranspiration and impacts of groundwa...
Dear Ayele,
I appreciate for coherent reflection and suggestion.
With best regards,
Hailemariam
An oasis is a good example of a place where evapotranspiration exceeds rainfall.
This illustrates that is is not just irrigation using rivers and ground water that can cause that phenomenon. It is also natural irrigation from artesian wells and the escape of water from river beds, fed from distant glaciers, irrigating the nearby land.
Dear Alastair,
I thank you so much for your this justification.
With best regards,
Hailemariam
Dear Dr Hailemariam, evapotranspiration is an challenging topic, for several important matters. One of them, is that unless you measure it (actual evapotranspiration, AET) under controlled conditions (ie, at a large scale of 1:1 that is, in a laboratory; or in, let us say, in a smaller scale (1:20,000) under one crop field boundaries) it is troublesome to calculate. However, at the end of the road it will only be that: a calculation, not a measured factor. Another important concept, that you should be aware of is the groundwater components in your area; you indicate there is rainfall and forest, have you considered a groundwater input in your conceptual model?
Do have from Mexico saludos,jjoel carrillor
Dear Jose,
Thank you for your concern.
The model (Budget Model) I use soil profile, climate file, crop file and climate data. Hence, i did not use groundwater input model.
With best regards,
Hailemariam
Dear Hailemariam,
as it was said in the previous answers, yes AET can be > PR if the income of water in the area is not just by precipitation. Is it possible that you could make an estimation of the annual amount of water which is used in irrigation? This could give a clue to your issue. Another thing is that you mention that you are computing AET. Could you tell us how are you are performing this computation?
Best Regards
Dear Fernando,
I thank you for your reflection.
Yes, I computed AET of the area using Budget Model. Soil profile, climate file, crop file and climate data are the input subroutines of the Budget model.
With best regards,
Hailemariam
Dear Hailemariam,
so as I understood you are not actually measuring AET, you are computing from tabulated Kc and computed soil moisture and ET0. Is this so? If yes, and you are not including any estimation of the total of irrigation in your soil moisture budget model, I cannot see how could it be possible that AET>PR . Probably there is some mistake in your budget model.
Sorry if I misunderstood something...
Cheers!
Dear Fernando,
Thanks once again,
You are correct!
In fact, I found comparable results of AET and Rainfall for some days. My question was to get a deeper insights on the relationships between these variables.
Thank you for your constructive comments!
With best regards,
Hailemariam
Dear Hailemariam
Turk published in 1954 a useful formula relating annual actual
evapotranspiration to annual rainfall. He indicated that the formula
applies to most climatic regions. The formula indicates that the annual
actual evapotranspiration is always less than annual rainfall as long as
irrigation is not used. In my opinion, irrigation causes an increase in
actual evapotranspiration especially during dry spells. Hence, rainfall
distribution during the year has probably equal importance to that of annual rainfall total.
Regards
Dear Mohammad,
I thank you for your critical suggestion.
With best regards,
Hailemariam
Dear Dr Hailemariam, Thanks for the following up... Just a couple of remarks.
- It should be remembered that in any region of the world, the presence of groundwater stands for about 97% of the available water; that is, 3% is represented for surface water. The lack of inclusion of groundwater in any due calculation could become critical unless you prove that the groundwater level does not suggest discharge conditions to be present, that is, to be adding some unseen yield to the surface water input.
- when modelling of any sort is carried out regarding to calculate runoff, infiltration, evapotranspiration, etc, using monthly (or .... yearly) quantities, a important question needs to be answered ... let us argue that the monthly precipitation was 30 mm, then when you apply your model, how would the results represent the real conditions, that could have been represented by two different simple cases... Case 1.- rainfall was one mm a day; and Case 2, rainfall occurred on 3 days only. Would the calculation give a reasonable answer?.... or the answer would be the same?... Is this not more questionable when using yearly quantities???
Do get from Mexico saludos,jjoel carrillor
Dear Jose,
Still my appreciation goes to you. Thanks for the continuous concern. The points you raised are crucial to me now and to the future to come.
Indeed, to circumvent such issue of scale, we have run the model on daily basis, monthly, yearly!
With bets regards,
Hailemariam
Dear Dr Hailemariam, thanks to indicate your view, you are most welcome, saludos,jjoel carrillor
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