There is always humidity in the soil, everywhere, on 3 meters deep. However, the available quantity of the humidity differs. So when the roots of the trees penetrate, they find a certain quantity of humidity. This quantity depends on 4 things:

  1. Annual precipitation
  2. Annual penetration after precipitation
  3. Annual evaporation
  4. Tree cover

I give eight examples.

   1.Horizontal clay soil with a hard pan (this is a thin crust layer of minerals, difficult to penetrate for water)

  • Annual precipitation 300 mm
  • Annual penetration after precipitation 50 mm
  • Annual evaporation 40 mm

Net input = 10 mm. Each 1 mm ingress = 1 liter of water per m2.

   2. Horizontal sandy soil without a hard pan

  • Annual precipitation 300 mm
  • Annual penetration after precipitation 300 mm
  • Annual evaporation 40 mm

Net input = 260 mm

   3. Horizontal clay soil with a hard pan covered with trees

  • Annual precipitation 400 mm (the annual precipitation rises when you have a tree cover on sufficient scaled surface, so 10 ha doesn’t rise the precipitation, 100.000 ha does)
  • Annual penetration after precipitation 250 mm
  • Annual evaporation 40 mm
  • Use of trees 160 mm

Net input = 50 mm

   4. Horizontal sandy soil covered with trees

  • Annual precipitation 400 mm
  • Annual penetration after precipitation 400 mm
  • Annual evaporation 40 mm
  • Use of trees 160 mm

Net input = 200 mm

    5. 25⁰ Inclined clay soil with a hard pan

  • Annual precipitation 300 mm
  • Annual penetration after precipitation 10 mm
  • Annual evaporation 10 mm

Net input = 0 mm

   6. 25⁰ Inclined sandy soil without a hard pan

  • Annual precipitation 300 mm
  • Annual penetration after precipitation 150 mm
  • Annual evaporation 40 mm

Net input = 110 mm

   7. 25⁰ Inclined clay soil with a hard pan covered with trees

  • Annual precipitation 400 mm
  • Annual penetration after precipitation 200 mm
  • Annual evaporation 40 mm
  • Use of trees 160 mm

Net input = 0 mm

   8. 25⁰ Inclined sandy soil covered with trees

  • Annual precipitation 400 mm
  • Annual penetration after precipitation 400 mm
  • Annual evaporation 40 mm
  • Use of trees 160 mm

Net input = 200 mm

You see that the factors of water penetration are 1) precipitation, 2)soil type, 3) inclination of surface and 4) tree cover.

As you understand now, only in cases of example 5 and 7 you have a situation where the available quantity of water on 3 meters deep will almost be zero. However, there is also water rising up, through the capillary system of soil. So there is always a certain quantity of water, no matter how dry it is. Of course, in such cases where hardly no water is available, you have to plant the tree that is capable to survive such circumstances, e.g. Prosopis.