* A large number of small reservoirs designed to hold water have high seepage rates (up to 24mm/day), so this is important to know for design purposes. However, seepage is often disregarded in design calculations as it is difficult to quantify. A field method to determine seepage rates in the bottom of reservoirs has been developed which can be used to assist in design. While in general it may be better to design for extra seepage loss in pond volume, seepage can still be reduced by:
# Covering the pond base with clay soil and compacting it with vehicles or animals. Addition of powdered anthills or lime is said to make this lining more robust.
# Large open reservoirs have been lined in the past with natural or artificial liners, but it is expensive and the lining material is prone to damage by cattle and ultraviolet light, not to mention when de-silting desilting is required.
====Maintenance====
Siltation is probably the greatest risks for failure with ponds and dams. The idea is to keep silt out in order to reduce the need for subsequent de-siltingdesilting, and to have de-silting desilting mechanisms and institutional arrangements that actually work.
# Keeping a good cover of indigenous grasses in the run-off area seems to prevent silt build-up. Kambiti Farm in Kitui District provides a good example of previously degraded land being managed and where open dams did not silt up due to pasture management. Contour lines with trees or grasses in the runoff area also work.
# If the inflow channel is defined, silt traps can be tried out to reduce silt load as is done with Charco dams in Tanzania. In this case, stones laid across the channel form mini dams and perennial vegetation can be grown between these mini dams to reduce flow velocity of water, thereby encouraging silt deposition.
# De-silting will most probably need to be carried out at some stage. There may be more sustainable ways of doing this compared to the usual approach used in the recovery stage of DCM, where this process is often paid for by NGOs and where there is a lack of community will to contribute. While animals seem to be a good option for effective de-siltingdesilting, food-for-work or cash-for-work incentives are commonly still needed to entice communities to improve their own ponds. It is better to train only a few animals for de-silting desilting work to save damaging the equipment, but farmers tend not to want to use their animals to work on someone else’s land. This lack of ownership in communal projects is a recurring thread of failure in WASH projects, and should require new and innovative ways to engender ownership and management of facilities. An institutionally-resilient way to de-silt ponds may be to promote ponds on private land, where one landowner has a vested interest to maintain and de-silt desilt the pond, thus reducing the need for NGO intervention in the longer run.
====Other considerations====