Evapotranspiration (ET) values is most difficult to measure. Even if it is measured for particular site, this is the most difficult variable to measure on a watershed. That is why ET measurements are often indirectly obtained from the water budgets or using equations that predict based on many variables (like Penman Monteith). These still need calibration and validation. That is why calibration is often done using measurements that can be done reasonably well, like stream flows or other measurable variables. Satellite measurements of ET are also not exact measurement but indicators through other variables like reflectance. The best way to calibrate ET is use use stream flow measurement and use specific parameters related to ET during calibration. But the choice of calibration should be based on sensitivity of the parameter and the purpose of calibration (like matching the hydrograph shape or matching base flows, etc) You can choose these in SWAT-CUP and measure efficacy of calibration using the stream flow measurements.

Hello Huha,

Please, see the attached documents.

Dear Shantamoy Guha,

Allow me to start with some basic information.

SWAT features four different methods for evapotranspiration, i.e. Priestley-Taylor method, Penman Monteith method, Hargreaves method, and “read in potential ET values”. The first three are well known and broadly used methods, whilst the fourth provides the option for inseting user-specified values for potential evapotranspiration for the entire basin. These four methods correspond to codes 0, 1, 2, and 3 of the IPET variable in the .bsn file. So, one first has to define the method for the calculation of evapotranspiration.

I, personally, have not tried the “read in potential ET values” (user-specified values) method, but I presume that calibration of evapotranspiration would not make sense and would most probably have no effect in this case. Moreover, in order to use this method you would have to calculate evapotranspiration manually for your study area. This is not impossible, of course, but it is vary laborious, mainly due to the various parameters involved.

As far as the first 3 methods are concerned, there are quite a few parameters available for the calibration of potential and actual evapotranspiration. Nevertheless, you must keep in mind that you have to perform a sensitivity analysis to determine the impact of each parameter for the specific characteristics of your basin (i.e. soil characteristics, meteorological conditions, etc.).

ESCO

The Soil Evaporation Compensation factor (ESCO, .bsn, .hru) controls the range of the soil depth used to meet the soil evaporative demands. The parameter varies between 0.01 and 1 and has a default value of 0.95. A decrease of the ESCO value results in a decrease of streamflow, as more water is extracted from lower levels of the soil layer to meet the evaporative demands.

EPCO

The Plant Evaporation Compensation factor (EPCO, .bsn, .hru) defines the range of soil depth used to meet plant water uptake. The parameter varies between 0.01 and 1 and has a default value of 1. When EPCO approaches 1, deeper soil layers contribute to the plant water uptake, whilst when EPCO approaches 0, plant water uptake demands are met by the top soil layers.

SOL_ALB

The soil albedo (SOL_ALB, .sol) expresses – as a fraction – the ratio of the amount of solar radiation reflected by a body to the amount incident upon it. The easiest way to estimate soil albedo is from tables, as a function of the soil and land cover types of the under study basin.

CANMX

CANMX (.hru) (mm H2O) is the plant canopy, which can significantly affect infiltration, surface runoff and evapotranspiration. The default value is 0.

You could experiment with different values of CANMX, depending on the density of the plant cover and the morphology of the plant species found in your study area. This is not a required parameter.

GW_REVAP

GW_REVAP (.gw) controls the water movement from the shallow aquifer into the overlying unsaturated soil layers and its value varies between 0 and 1. As GW_REVAP approaches 0, movement of water from the shallow aquifer to the unsaturated zone is restricted. As GW_REVAP approaches 1, the rate of evaporation increases up to the rate of potential evapotranspiration, reducing thereby the baseflow.

REVAPMN

REVAPMN (.gw) (mm H2O) is the threshold depth of water in the shallow aquifer for revap (see GW_REVAP) or percolation to the deep aquifer to occur. Movement of water from the shallow aquifer into the overlying unsaturated soil layers (unsaturated zone) is allowed only if the volume of water in the shallow aquifer is equal to or greater than REVAPMN.

It is very important to keep in mind that the calibration of evapotranspiration could affect other processes, as well. For instance, CANMX can significantly affect the water infiltration into the ground and thus, the water volume in the shallow aquifer and baseflow. It can also affect, in a more direct way, the surface runoff. REVAPMN, apart from evapotranspiration, can affect percolation to the deep aquifer (permanent water loss). So, be very careful when calibrating SWAT.

Hope I helped!

Feel free to contact me if you think I can help on something.

Best of luck with you research!!

Kostas

P.S.: Most of the aforementioned are mentioned in the following article (see attached) and thesis, which I believe you will find particularly helpful.

Kaffas K., Hrissanthou V. and Sevastas S., 2018. Modeling Hydromorphological Processes in a Mountainous Basin Using a Composite Mathematical Model and ArcSWAT. Catena, 162: 108-129. doi: https://doi.org/10.1016/j.catena.2017.11.017

Kaffas K., 2017. Development of Mathematical Model for Calculating Continuous Hydrographs and Sediment Graphs in a Basin Due to Rainfall. Ph.D. Dissertation, Democritus University of Thrace, Greece, 153 pp.

It is also very useful to council the SWAT manual and theoretical documentation.

Arnold J.G., Kiniry J.R., Srinivasan R., Williams J.R., Haney E.B. and Neitsch S.L., 2013. SWAT 2012 Input/Output Documentation. Texas Water Resources Institute.

Neitsch S.L., Arnold J.G., Kiniry J.R. and Williams J.R., 2011. Soil and Water Assessment Tool Theoretical Documentation - version 2009. Texas Water Resources Institute TR-406.

Dear all,

. I am using SWAT in different research activities. On this regards, we are currently trying to make future projections for runoff in different climate change scenarios. We use the Swat cup application but some problems arise and we are not able to go ahead with our research. The model works really well with the data but the problem is for future predictions. We would very grateful if you could provide some light on the following questions:

1.- Could you please provide us some information or manual to work with future climate predictions in QSWAT or SWAT CUP? We aim to assess changes in runoff for two different climate scenarios

2.- There is a folder on SWAT CUP where you can find all the data and app (swat.exe) needed to make the model work. However, once we complete the input required data (file.cstcst and file.cio) and run swat.exe nothing happens. We are unable to get an output file. Something wrong with the app (swat.exe) or with the way we put the input data?

3.- Sometimes, we run the swat.exe app and suddenly it is closed.

Thanks a lot for your time and help.

Sincerely

Manuel Lucas

This paper will be useful: Article Hydrologic model predictability improves with spatially expl...

Did you solve the problem? I encounter the same problem, can you give me some suggestions?

Gengxi Zhang Sorry to say, but we were trying using Swat Cup, with gridded Evapotranspiration data from MODIS in Indian region. We did not get success.

Shantamoy Guha Thanks for your reply.

You can now download Global ET observation from the Water Weather Energy Ecosystem Technology and Data websites (https://www.2w2e.com/) which are extracted in SWATCUP format and can be directly applied in SWATCUP for calibration. I think this will solve your problems.

@Ibrahim Hassan Thanks a lot for your reply. I am gonna try to validate the soil moisture using this dataset.

@Shantamoy Guha.

Thank you so much.

Please,can you suggest me what is the error with my project.

I run SWAT model 1985-2001, and try to use for calibration 1985-1996,and my NS VALUE is only 0.53,couldn't be more than this.i tried .

My flow data is monthly ,some of my pcp stations are out side the basin,but near.

For Run off modelling which type of parameters I should use?

thank you!

Tewekel M. Gemechu You can refer to this paper and its supplementary materials.

Gengxi Zhang Thank you!

Gengxi Zhang In my country at all, in my watershed no Snowfall totally!

So, should I use snow parameters or neglect all snow parameters?

Thank you!

Srishti Gaur

Tewekel M. Gemechu Yes. You can neglect snow parameters in this case.

@Gengxi Zhang

Thank you so much.I read your paper ,it is helpful.

My Stream Discharge data is monthly,and in 5800km square there is only one gauge.

May be that can impact the result?

This requires the use of observational data

Dear Shantamoy Guha,

My research objective is same with different watershed. If you cracked this objective, Kindly also update me. An early response is highly appreciated. plz feel free to respond @ 9813389500