* A limitation of EPDM is its size they typically arrive in sheets ranging from 5’ x 10’ to a 50’ x 100’ roll.
====Roof constructionLimiting evaporation====
Sub-surface tanks are usually small enough that it is viable to have a roof to limit evaporation (and improve water quality if possible, which means less algae build-up). Shading can reduce evaporation by around 30%. Placing local bush or grass materials on a frame of wires doesn’t seem to work well because they get blown off, and also still let light in, which creates algae growth. Corrugated iron roof on wooden frame works well but is expensive (about $20 per m2 in Somaliland). In addition, if the tank is not fenced, animals walking on the roof can damage it. The challenge is to make a roof that is cost-effective for small-scale farmers – one idea is to investigate income-generating roofs since that can help pay for the structure (e.g. passion fruit). Excavated water cellars by their nature have small area roofs.
Deeply dug tanks mean less evaporation, but will provide more water quantity to last longer into the dry season. Perhaps a rule of thumb should be that depth should be greater than the maximum PET rate for the area in question. For example, the average length and width of berkeds from projects in Somaliland were 11.4 and 6.3 metres, while average water-holding depth was 2.9 metres, whereas PET rates ranged from 1.75 – 2.25 metres per year. Problem: deeper tanks could mean more investment.
Another method to reduce evaporation and at the same time improve water quality is to use a lined, sand-filled tank. It appears that plastic pond liners are in general more tolerant to earth tremors than solid lining like concrete – in some situations when the rains might cause swelling of the surrounding ground which might move the existing wall in a similar way, the plastic lining might in fact still be functional. In such a case it might be good to try out the following method which has been tried in Botswana in a lined rectangular tank:
* Use a plastic lining to create an impermeable layer on top of the existing lining. Protect the lining with a sand layer both on the floor of the berked before the lining is laid (evens out floor, protects against sharp objects, dried clay fragments etc), and also on top of the plastic after it is laid (to protect from flotsam and when people walk on it).
* Create an abstraction point.
* Fill the remaining volume with sand.
* In case of cracked linings, the following could be tried to salvage the tanks:
##If the crack is only at the base, covering the tank base with clay and compacting it might work. Addition of powdered anthills or lime is said to make this lining more robust. If the cracks are also found in the walls, then rehabilitation or an alternative lining might be a solution. Taking the example of berkeds in Somaliland, many remain unused due to previous poor workmanship, yet rehabilitation is expensive ($8 per m3), requires skill and is not always successful. In some cases it is also not even possible to rehabilitate – some cracked berkeds can be rehabilitated if the original walls were made solidly enough, but otherwise there are many berkeds that can never be rehabilitated. In such cases, plastic linings might be worth trying.
==Costs==