Our rainwater tank

I have some experience with collecting rainwater in the tropics. Both for household use and for increasing success of tree plantings in semi-arid conditions.

Collecting rainwater from the roof of the house where it is to be used can be very practical in many areas. When living in the northwest corner of Tanzania 1988-89, we built a rainwater tank with a couple of refinements that worked well.

  1. Division of the tank into top part (to save the potential energy of the water entering the tank which provided a higher pressure {with better water flow} even when the tank contained a small amount of water) and a bottom part to save the overflow from the top as a long-term storage..
  2. Use the top of the tank as one of the "collection roofs". Water tanks are usually covered for many good reasons (prevent algae growth, mosquito breeding, evaporation, limit direct contamination). But this often has resulted in that the rain which happens to land on the tank itself, runs off to no use, while the rain which hits the roof of the house next to it is carefully lead into the tank. The house had a roof area of about 42 m2 so using the top of the tank (>2,5 m2) increased the effective area with 6 % (actually more because we weren't able to put gutters on the whole house perimeter). Capturing the rain on the tank top was done with a small cement mortar ridge along the edge.
Usual rainwater tank rainwater tank w top capture and upper section
On the left is the usual rainwater collection tank. The rain that hits the tank itself runs off the roof. The water that enters the tank goes directly to the bottom. When there´s not much water in the tank, there is no "head" (pressure) to send it to a tap of useful height. On the right is the kind of tank we used. It collects even the rain that falls on the tank itself. The water stays at a high level until the upper tank is filled. This gives enough "head" to supply water even to a shower that is not too high, even when there is little water.

To a Swedish description of our use of this water tank
Till en beskrivning av vår användning av denna vattentank

Planting holes for tree planting

The "Village Forest" tree planting project in Karatu (near Ngorongoro, Tanzania) included som expermental activities. One question was "How can the planting hole influence the survival of the planted trees?" (The number of surviving trees is more important than the the number that are planted.) For example a local Catholic Priest had very good survival of his trees, but each tree had a 1 meter deep and wide hole which is partially filled with compost and manure. These holes needed a lot of work compared to the ordinary 30*30cm holes, but the ordinary holes had a worse result regarding survival of the planted trees.
Microcatchment leads water to plant I saw a description of an interesting idea called "microcatchments" in a book from VITA. Microcatchments are simply small ditches for catching and leading surace water into planting holes.
But there was no suggestion about suitable dimensions to use. I made a spreadsheet which used: slope of the ground, greatest rainfall during a short time, size of planting hole, how fast water sinks in a test hole in the ground. The spreadsheet than gives a suggested height for the catchment wall below the planting hole and how long the two ditches should be. The microcatchment planting holes made according to the suggested measurements worked better than both the standard holes and the big "priest" holes. At the moment I don´t know where that spreadsheet is, but if anyone is interested, I can find it or try to recreate it.

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