getting around PET?

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Jen Kern

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Dec 5, 2020, 1:07:59 PM12/5/20
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Hi all,

Is there any way to not have to plug values in for E? I don't have good data for this, and can't it be estimated based on temperature? 

Thanks,
Jen 

Willem Vervoort

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Dec 5, 2020, 9:36:42 PM12/5/20
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Hi Jen,
You can use daily maximum temperature as a substitute for Epot. All hydromad models include a parameter "e". If you use Epot this parameter should be 1 (or close to 1), if you use maxT your estimate would be 0.15 (but it can also be a calibratable parameter). I have so far mostly used daily maximum Temperature.

I think the original publication is this one:
Dye, P.J., Croke, B.F.W., 2003. Evaluation of streamflow predictions by the IHACRES rainfall-runoff model in two South African catchments. Environmental Modelling & Software, 18(8-9): 705-712. 
But it is also mentioned in this paper:
Croke, B.F.W., Jakeman, A.J., 2004. A catchment moisture deficit module for the IHACRES rainfall-runoff model. Environmental Modelling & Software, 19(1): 1-5. 
And I used it similarly in this paper
Vervoort, R.W., Miechels, S.F., van Ogtrop, F.F., Guillaume, J.H.A., 2014. Remotely sensed evapotranspiration to calibrate a lumped conceptual model: Pitfalls and opportunities. Journal of Hydrology, 519, Part D(0): 3223-3236. DOI:http://dx.doi.org/10.1016/j.jhydrol.2014.10.034

Alternatively you could use a Hargreaves equation or other ET potential equation to calculate ETpot separately.

Willem

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Jen Kern

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Dec 6, 2020, 8:45:48 AM12/6/20
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Hi Willem,

Thanks so much for your reply. 

Just for clarification, I am using HBV. Can I use the exact values of maxT in place of E? Or, is what you are saying (and in the articles), to use .15 as the constant multiplier of maxT to derive Epot? 

Thanks again,
Jen 

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Willem Vervoort

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Dec 6, 2020, 4:48:07 PM12/6/20
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Hi Jen,
Hmm, HBV, I would have to ask Joseph and Alex to comment. Alex has been including HBV, but he is currently travelling to Finland. 
You can use 0.15*maxT as a starting point. Basically the 0.15 scales the temperature to "mm H2O", but if you have subzero temperatures, this will become tricky, you probably will have to set Epot = 0 at this point.

The alternative is to use an equation like Hargreaves or Blaney-Criddke, see page 4.18 in the Handbook of Hydrology (Maidment, 1993) or any good Hydrology textbook

Hope this helps
Willem

Alex

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Dec 7, 2020, 2:28:53 AM12/7/20
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Hi Jen,

I have just noticed I didn't include the `etmult` input option that is available in the other hydromad models. I'll look at changing this soon so it is consistent with the rest of the package. Otherwise Willem has already outlined that you can multiply maxT to derive PET yourself. Subzero temperatures could be dealt with a simple ifelse.

Alex

Guido Wyseure

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Dec 7, 2020, 2:45:22 AM12/7/20
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Dear all,

In attach a Hargreaves-Samani function which is meant for ZOO timeseries. The Julian day you easily get by"jday" from the Lubridate package.

Only Tmin and Tmax is needed along with the latitude. Humidity in not too arid climates is estimated from Tmindew which then becomes equal to Tdew in the morning. The Hargreaves-Samani gives also a good estimate of the solar radiation.

Using Tmax only,  I do not recommend as a surrogate for ET and does not make any sense. 

Kind regards,

Guido Wyseure



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