## Eto Penman monteith vs Eto Hargreaves

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elizabeth.ramirez.z
Posts: 8
Joined: Sat Oct 26, 2019 4:03 am
Location: Chile

### Eto Penman monteith vs Eto Hargreaves

Eto Hargreaves and Eto Penman Monthei

Hello, I write because I had a doubt.
I have been using hydrus 1d and to estimate the eto, use the part of hydrus that allows you to estimate the eto according to Penman monteith. Use hydrus in centimeters and per hour.

When I enter the data it asks me to calculate the eto, radiation input, tmax, t min, relative humidity and wind speed with their respective units.
Then when I check the evapotranspiration outlet, the model tells me that the total is 720 mm. But the value does not seem very logical, since I have an accumulated rainfall in the place of 360 mm.

To see if there is any error, I use the estimation of eto by Hargreaves, and the eto that appears in meteo.out is 280mm, which seems to me much more logical with the rainfall that is in place.

I am sure that you enter the weather variables with the corresponding units. Is it possible that they give so different the two estimates of eto by different methods?
Attachments
1.PNG (37.82 KiB) Viewed 1750 times

elizabeth.ramirez.z
Posts: 8
Joined: Sat Oct 26, 2019 4:03 am
Location: Chile

### Re: Eto Penman monteith vs Eto Hargreaves

Dear Jirka, could this be the problem?
Attachments
2.PNG (99.78 KiB) Viewed 1745 times

Jirka
Posts: 4982
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Eto Penman monteith vs Eto Hargreaves

The problem listed above has been fixed a decade ago.

My experience is that these two models give very similar results if correct input is used. See an example for a Nevada desert.
Hargreaves.png (26.35 KiB) Viewed 1737 times
Note that ETo is reference ET, not actual ET, and thus can be larger than precipitation (and almost always is for arid and semi-arid regions). In the example above you can see reference ET calculated by these two models and actual ET measured on lysimeters.

J.

crhansen
Posts: 25
Joined: Thu Dec 03, 2015 4:21 pm

### Re: Eto Penman monteith vs Eto Hargreaves

Provided you are in an arid or semi-arid environment, an annual P and PET of 360 mm and 720 mm, respectively, is not surprising. At my location in Western Canada, the annual P is 420 mm, and the annual PET is about 750 mm, which are similar to your values. What does look suspicious to me is your solar radiation data. If the units are MJ/m2, and if you add up your values from hour 10 to 14, you get a total solar radiation of 186.5 MJ/m2. From the data I have , which is from Canada and Australia, the global solar radiation should be roughly in the range between 15 to 35 MJ/m2/day. I don't know your specific location, but I doubt very much the solar radiation could be as high as 186.5 MJ/m2/day (* and especially since this value only cover 5 hours). Are you sure you are working in the correct solar radiation units ??

Jirka
Posts: 4982
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Eto Penman monteith vs Eto Hargreaves

I have no idea how you got these solar radiation values from my posting above (where I only show potential evapotranspiration fluxes calculated by either PM or Hargreaves equations). J.

crhansen
Posts: 25
Joined: Thu Dec 03, 2015 4:21 pm

### Re: Eto Penman monteith vs Eto Hargreaves

Hi Jirka,

I should have made it clear I was addressing Elizabeth Ramirez. I was not referring to your post. I was simply referring to the solar radiation values that are shown in the attachment provided by Elizabeth. The values look far too high, if the units are MJ/m2.

Colin

Jirka
Posts: 4982
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Eto Penman monteith vs Eto Hargreaves

Thanks for clarifying. I have been really confused by your post. J.