I do not have direct
experience with terra rossa. We don't have that kind of soil here. I have seen it on holidays though. I remember it being slippery when first wetted. I found out the hard way. For land restoration you want to bring in as little material and equipment as possible.
For you purpose, i would check whether destroying the permeability would work. This is the opposite of what permies ussually try to accomplish. In my garden soil - i have this ironrich soil horizon with 30% or more clay. Undisturbed it has a resonable permeability. However, if i dig down to this level and if spend some time standing on this horizon, then the permeability is reduced drastically.
I have this ongoing project to remove exotic conquerors like bamboo and yucca from my
yard as well as removing bricks and such from my soil. In short i digg holes in the ground. I have this spot where i want to build a wadi (
https://nl.wikipedia.org/wiki/Wadi_(infiltratievoorziening) (there is an english word - but it does not spring to mind). I want it to infiltrate excess water in the soil. In some places this really does not work - i have several smallisch ponds 'hanging' at different depths in my soil at less then 2 feet from each other. I ascribe the observation to the fact that i have worked a different depth in this soil horizon. In effect i destroyed the permeability that i want in the long term for the short time gain of eliminating bricks and bamboo from my yard.
Before you ask - handdrilling has a similar effect - the clay in the layer i want to use 'smears' the sides of the drill hole - thus reducing the permeability.
I suspect you can use this effect to get what you want - a soilhorizon that has strongly reduced permeability. So why not test this? Take some terra rossa at the water depth you want and check what it's permeability after wetting and kneading it. Then you take a transparant plastic tube. Fill it to about halfway with course sand. Add a layer of wetted and well kneaded terra rossa. There should be a tight contact between the tube and the kneaded terra rossa. After that pour water on top. If the kneaded layer holds the coarse sand dry you are in business.
'Kneading' with a digger works. I used to have this done to some extent when in an soil cleanup operation i wanted to catch some liquid pollutant - f.e. oil. That might otherwise wash out. In a pinch you might use a cement mixer and water to knead your terra rossa.
The thicker the kneaded layer-the better it will hold water. Think about how deep you want you pond to be.
In your setting (Israel ? i guess), your pond may be vulnerable to plants rooting towards the water. Another problem might be drought. The terra rossa clay i have seen, cracks when drying. If your pond dries, it might crack and overgrow with grass and/or reeds - both bad ideas if want to keep that impermeable layer going untill you can catch more water.
If you use a cementmixer you might add 'smectite-type' clay if you have that locally OR you might add commercially bought bentonite. This type of clay 'swells' when wet and thus closes up 'leaks' in your pond. That mix is kind of a 'self sealing pondliner'.
You definitely want to do some research before spending time and money. Consult
local builders, local potters, .... and observe local construction sites to check if these ideas might work.
https://en.wikipedia.org/wiki/Clay_minerals
https://de.wikipedia.org/wiki/Smektitgruppe
https://en.wikipedia.org/wiki/Montmorillonite
You can of course try for an organic approach but boar and swine make ponds this way where it is convenient for them. In Israel that is probably a nono?
When digging out Terra Rossa you might consider, using it to increase your rain catchment area. It sounds as if might have not enough input of water during a dry summer. If you are building nearby - filtered runoff from the building might be a good extra watersource. Be carefull about saline buildup in your ponds soil and water.