How to decrease Evaporation

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epelde.ane@gmail.com
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Joined: Thu Jan 01, 1970 00:00

How to decrease Evaporation

Postby epelde.ane@gmail.com » Mon Mar 11, 2013 15:55

Hii!

Making a water balance, I realize that the evapotranspiration fluxes are too high for my catchment (about 85%, when they should be around 65%). The keywords I think (correct me please if I´m wrong) are related to the amount of water is going to be lost thought this proccess are LIMIT_EVAP_WATER_VELOCITY, LIMIT_EVAP_HEAD and IGNORE_WATER_COLUMN_EVAP. Then there is the keyword HEAD_LIMIT, which is the value for maximum head at which evaporation can occur and which default value is -100. And what about the evaporation velocity? Which is the keyword to set its velocity? I couldnt find it in the code..

I see that evaporation is calculated after the transpiration so both these parameters as well as the ones related to vegetation as Kc, LAI and so.. are going to have big impact on the results.

So, can anyone advice me please where to start with this so that I adjust the evapotranspiration of the model? :roll:

Thanks a lot!!

Jauch
Site Admin
Posts: 98
Joined: Thu Jan 01, 1970 00:00

Re: How to decrease Evaporation

Postby Jauch » Tue Mar 12, 2013 11:44

Hi :)

Let's do some clarifications regarding these keywords.

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1. IGNORE_WATER_COLUMN_ON_EVAP

This keyword is used in Porous Media module. It's a logical keyword and its default value is 0 (OFF).

The STANDARD behavior for porous media is to evaporate water from the first layer of soil ONLY if there is no water column at the surface. This water column can be formed, for example, if it rains more than the soil is capable of infiltrate. In this case, the excess water will be stored at the surface.

When you activate this keyword setting it to 1 (ON), you are telling to the porous media module to IGNORE the water at the surface, so, to evaporate water from the first layer of the soil even if there is water at the surface.

I usually turn on this keyword (set its value to 1) when I turn off the surface water evaporation on BASIN (EVAP_METHOD = 3 or EVAP_FROM_WATER_COLUMN = 0).

2. LIMIT_EVAP_HEAD and HEAD_LIMIT

LIMIT_EVAP_HEAD is a logical keyword and its default value is 0 (OFF). If this keyword is set to 0, HEAD_LIMIT will not be read.
HEAD_LIMIT is a numerical keyword, with a default value of -100.0. If LIMIT_EVAP_HEAD is set to 1 (ON), you can provide this keyword and a different value to it.

HEAD_LIMIT is a HEAD value at which the evaporation will cease.

3. LIMIT_EVAP_WATER_VEL

This is a logical keyword and its default value is 0 (OFF). When this keyword is ON it will limit the evaporation to the conductivity on the cell. So, in this case, evaporation will never ceases completely, but will decrease as the cell water conductivity decreases.
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As you can see, there isn't a keyword to define evaporation velocity, because the velocity will depend on the available energy and water. The only thing you can do is to limit it's velocity due to the water content (automatically), or define a point (head limit) at which there is so little water left that is not possible to evaporate it anymore.

Usually, if you have very water demanding vegetation, like agricultural crops, for example, the transpiration will be much higher than the evaporation. On bare soil or vegetation in its initial stages of growth, where the LAI is low, the evaporation will tend to be more important. Also, the weather conditions will have some impact on evapotranspiration. On summer the evtp will be higher (usually) than in the winter.

So, the first thing to do is to look to your results and see if this 85% of evapotranspiration is more transpiration or more evaporation. If it's more transpiration (as I suspect), than change the evaporation related keywords will have little impact on the total amount of evapotranspiration.

In this case, changes in Kc and LAI will probably have much more impact. You can also try to change the FEDDES parameters for the vegetations that you are using. Maybe they are too "loose", allowing the plants to transpirate under conditions (low or high head) that they usually would not transpirate so much.

Another aspect that you should observe is the amount of water in soil.

Maybe you are infiltrating too much water in soil, and this could be the cause of this higher percentage of evapotranspiration.

You have to look to your river data, to see if the water flow is not being underestimated by the model. In this case, you have to make changes that will increase the RunOff and Ground Water Flow to the rivers, in order to correct the basin water balance and this way have a more correct distribution of the water lost in the basin (infiltration, river flow and evapotranspiration).


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