I have simulated the actual evapotranspirations (ETact) of my project for two years. Budget model was used to simulate ETact. However, when I compare ETact with reference evapotranspiration (ETo) values, three daily ETact values (out of two years) are higher than their corresponding ETo values.
Are there any possibilities to have such results ( such as the presence of ponded water over weeks, or other water bodies)? What could be the sources of such errors? If so, are there any possible ways to avoid these observations?
Hello,
At first, what algorithm did you used to calculate ETact ?
Far before going through "physical" explanations, the very first question is whether your calculations are correct (the actual code matches the physical algorithm) ? If yes, then are your ETact and ETo really comparable (matching in space and time resolution) ? If yes, then what is the accuracy of your in situ data and of the forcing data of your algorithm ?
If you can answer these questions and still can't explain your problem in terms of "numerical bias", then we can start thinking about "physically based" interpretation.
Let us know
Dearest,
I thank you for explanation.
I have simulated the actual evapotranspirations (ETact) of my project for two years. Budget model (A soil water and salt balance model) was used to simulate ETact. As the ETo values are input to get the ETact, they match both in time and space.
However, when I compare ETact with reference evapotranspiration (ETo) values, three daily ETact values (out of two years) are higher than their corresponding ETo values.
With best regards,
Hailemariam
Dear Heilemariam, Jerome is completely right. I would add only a brief comment. The actual evapotranspiration can be of course higher than the reference one as the reference one is defined for given conditions and crop (height, albedo, surface resistance etc.) and the actual conditions are usually different which can result in higher actual evapotranspiration. It is clear when you look for example at the values of crop coefficient which is used to calculate ETact from ET0.
Dear Vaclav David,
Thank you for sharing your experience.
With best regards,
Hailemariam
Dear Hailemariam,
I totally share the idea of @Jerome Colin. We could have huge uncertainties due to various model input errors and paramaterization. You can look at this paper of mine how parameterisation could affect AET (http://www.sciencedirect.com/science/article/pii/S0309170816303505). If we are sure that this is not the case, then we can think the physical processes.
The reference evaporation is a representation of the environmental demand for evapotranspiration and represents the evapotranspiration rate of a short green crop (grass), completely shading the ground, of uniform height and with adequate water status in the soil profile (https://en.wikipedia.org/wiki/Evapotranspiration). in simple term, reference evapotranspiration is the potential evapotranspiration for a specific designed land use and physical properties (i.e. most of the time theoretical grass with albedo 0.23, a crop height of 12 cm and a fixed bulk surface resistance of 70 m/s. ). If your study are is different from this, then the reference is not the same as the potential evapotranspiration. If you compare the actual ET with the reference ET of some hypothetical surface, then it could be possible to have AET greater than reference ET.
Dear Dr. Wuletawu,
I thank you for your scientific explanation.
With best regards,
Hailemariam
ETcrop (ETactual) is calculated from ETref (grass) using a crop coefficient. This is a lookup table that gives a multiplier for each part of the growth season since plant emergence. One multiplies the number of the crop coefficient by the ETref . This equals ETcrop (ETA, ETactual). ETref X Crop Coefficient for that day = ETref. ETref is the amount of water to be applied to maintain plant health and minimize stress. Unfortunately, I have found the Crop Coefficient curves to be poorly generated in many cases. Improvements are needed in the curve generation procedure.
In additon, a maturity model based on heat units must be added to the ET model to further the accuracy. In a cold year, the growth will be retarded so the water curve must also be retarded. This model is known as Accumulated Hourly Growth Units or Accumulated Daily Growth Units. I am in the process of developing such a system to generate and utilize crop coefficient curves.
Dear Jeff Tucker,
I thank you for your explaination.
With best regards,
Hailemariam