In ASAL areas where it may rain once, all roof catchment may be collected and stored.
Tank size thus = Annual rainfall x roof area x run-off co-efficient

eg. Take Kamulu Area:
Annual rainfall (90% probability) - 500mm/year
Roof area - let's say 10m x 20m = 200m sq
Run-off coeff for mabati roof - 0.9 (it means you get 90% on the rain falling on the roof)

Tank size = 500/1000 x 200 x 0.9 = 90 m cubic

The most critical parameter in ASAL areas is to have storage that will span the drought period. Further, 90% probability rainfall for the area (not average rainfall)should be used. For the most economical design, always consult a water(civil) engineer esp if your only water source is rain.

Trying to figure out how best to use rain water that flows to the shamba after rains. My thinking is that I could use it when it dries up, but how best do I store it? Anyone?

Any advice where to buy PVC gutter system?

Asante sana. Will call them.

Are there special filters used to keep out twigs,dust etc.. from entering the water tank i,e if the water is for domestic use?

Yes. They are called first flush diverters.

http://www.rainharvest.c...first-flush-diverter.asp

try

+254 722 894121 very calm and responsible .

start with getting gutter mesh it filters out the big leaves and insects.



then try the first flush system which catches the first liters of rain before directing to your tank, you need to maintain it though.

try the wet system instead of going from the roof directly to the tank try bringing the downpipe to the ground run it across the ground then up to your tank I find this better because it acts as a first flush system and does not bring sediments into the tank.

You guys are complicating a very simple issue with unnecessary madoido mob.
If you have an underground tank, have a wet system as delineated by Kayhara above, install inspection tees that come above ground at every corner, link all the pipes together and then connect them into the underground tank. At the lowest point of the system, install a very short inspection tee. When it rains, this will be your first flush point. Leaves and debris are hardly an issue for me but BIRD POOP is, and like it or not some of it will dissolve into your system upende usipende. About 1-2 minutes of first flushing will sort out of the bulk of this problem (I know hard to do in a water-scarce region like dustbowl) but necessary. The underground tank itself then serves as your SECOND FLUSH system. All the tiny solid particles that make it into the tank will settle at the bottom when all is said and done. CLEAN YOUR TANK at least twice a year to remove this stuff. It is made up mostly of HARMLESS things like clay/cement (from roof if made of tile) and bits of sand. From there, if you are smart, you will not waste your time building a hideous looking (and expensive) water tower high above your house. Build an ATTIC TANK of at least 1500-2000 litres (can fit very comfortably in the average maisonette roof). No unsightly concrete towers, no hefty expense, just a good clean hidden tank within your attic. This is you THIRD FLUSH system, because any further debris will settle at the bottom of it too. Clean it at least FOUR TIMES a year. I prefer to clean with bleach and vinegar alternately over time. I also have a second separate pump for my solar water heater on the roof. The tank holds 100L which is more than enough for a good weekend dip in the hot tub upstairs. I use tankless water heaters downstairs (the beautiful ones that rest on the wall, not those ugly cheap instant showers) wherever there is a showerhead. I do this of course to conserve water as baths "eat" too much water. The tank pressure is more than perfect for them. I dump the requisite amount of bleach (you can find how much you need to use for your capacity of tank from various sources online) into the underground tank as soon as the rain dies down and adjust according to how full the tank is. This sorts 95% of any organism/algae/contamination etc problem. Most importantly, I ONLY use this water for bathing, washing dishes and other household tasks. NOT for drinking. For drinking and cooking I further filter and clean the water and store in a water dispenser in the kitchen.

NIMESEMA!