Swales are made for dry soils they have a wider base than a ditch that has been dug to drain water off the land so as to increase the area in which the water is in contact with the ground and so increase the water that will seep in to the ground. In wet areas you have ditches to drain off the water.
Trees all behave differently
willows and poplars, are poplars called cotton woods in north America,? take up a lot of water ,other trees don’t take up so much, so it depends on your tree.
Some trees have pores that will carry air down to water logged roots or even sticking out knees with pores on them that take oxygen down to water logged roots, mangroves and such.
What about swamp cypresses they are pretty trees, they make a exciting atmosphere to a landscape, they probably take up a lot of water and they are American. We have one in the Madrid park of the Retiro.
Some trees bring up and evaporate a lot of water and others less, dry country trees are made to loose less water than willows say.
Willows will kill willows planted at their side, in a garden
say, were there is not much water, it is only along a stream you can have a line of them.
I have a pot of papyrus. a sort of reed from Egypt , on my balcony and in summer i give it enormous quantities of water. I have the pot in a bowl and fill up the bowl with water. The papyrus don’t look sweaty they have a heard shine and look dry but they don’t half loose water, Maybe its all taken up to allow them to grow, they grow like crazy as long as they have a lot of water. So some plants do evaporate off or lose a lot more water than others. Poplars grow like crazy too.
Cattails loose a lot of water too. They can be used to take arsenic out of water which they will absorb, plants can block out things they don’t want to absorb so itis not all plants that will take up and store things like arsenic.. Some soils contain arsenic and so the ground water is poisonous.
The trouble with cattails for taking arsenic out of water in a dry place, the place i have heard of where they have a lot of arsenic in the water is parts of India, is that the cattails evaporo.-transpire off so much of the water that they can't afford to lose in this dry country. Maybe you need them in a green house where the water they sweated off would condense on the roof and return to the ground.
Rocks and soils usually come in layers, with some permeable layers that water can sink in to and go through, so the land does not get water logged, like sand and some more impermeable layers that the water cannot traverse and so sits on top of.
Wet clay is pretty impermeable, just try making a pottery bowl and filling it full of water before the clay bowl dries out, it will hold the water . If you get vegetable matter in clay roots, dead roots and bits of plant material, it could help the water sink through the soil and into other and maybe more porous layers of the earths crust.
Tree roots might widen cracks in impervious rocks like granite, making it easier for water to drain into the ground.
People put drains under their fields if they live in very wet places and have undulating feilds with drains under the trough of the wave of the undulations to catch water and take it off to ditches.
I suppose the whole body of the tree holds water but i don’t think they hold much around their roots.
I have a photo of the roots of an oak in a quarry and they have made their way through the rock and made what looks like earth in the space they have created in the rock a crack like that might hold water, the soil that has developed in the crack would.
It has been proved recently that trees do hydraulic redistribution in the ground. When the ground is very dry the superficial roots and trees and bushes have a lot of roots that run horizontally just below the ground lose water instead of picking it up and then the tap roots supply them with water . They have put instruments on roots that measure water flow to discover this and prove it . The flow of liquids reverses in a shower
then the shallow roots
take up their usual function of absorbing water again and the water is passed down to the tap roots. So they do store water to a certain extent in the soil. Look up hydraulic redistribution trees in google. The article I have on it, that I printed from a google article, has been shorten as I last saw it and is harder to understand.
I took a photo of willow roots, when a bit of the stream bed fell downstream, because they made such a tight mesh that it looked like they would mat up anything, I don't know what for Trees by rivers are meant to keep the river flowing, I don’t know why. They shade it but they also transpire, do evaporatranspiration, so they lose water as well as stopping loss by shading rivers, so it would need a lot of experiment to find out how that all came out. Whether the shade that reduces the evaporation of the water in the river saves more or less water more than the tree loses transpiring.
that the land around rivers is wooded does increases the amount of rivers around. They say that if you cut down the trees the rivers dry up. Wangari Mathai is one of the people
who talks of this So does the Spanish writer Jesius Charco
Mediteranean trees shut off their stomata at midday in summer so as to prevent too much water loss in the heat of the day. Agri rose macaskie.