Credits: Biovision-Infonet
Almost every type of storage reservoir loses some of its stored water to evaporation, seepage or leakage.
Evaporation losses
Open water reservoirs, such as; tanks without roofs, ponds, earth dams and rock catchment dams, lose water due to evaporation. In hot and windy climates the evaporation rate may be over 3 mm per day which is equal to losing a depth of 0.9 metre of water in a month.
Water tanks and rock catchments should therefore be roofed but that is not feasible for ponds and earth dams.
Rainwater stored in the voids between the sand particles of riverbeds is the most economical water storage because up to 350 litres (35%) of water can be extracted from 1 cubic metre of coarse sand while only about 10% of water can be extracted from fine structured sand because its voids are smaller.
In hot regions without sandy riverbeds, rainwater can be stored in situ between the voids between soil particles, e.g. downstream of ponds and earth dams or in terraced land or in seasonal macro and micro irrigation structures as described in “Water Storage”.
Seepage losses
In addition to evaporation loses, water stored in ponds and earth dams also lose water to seepage through the floor of these reservoirs.
Fortunately, some of the seepage loses from ponds and dams can be utilized by sinking a shallow well into a seepage area.
However, the combined losses of seepage and evaporation during long dry seasons usually result in the ponds and earth dams drying up except for very large earth dams.
The seepage loses through the floor of ponds and earth dams can be partly sealed by either:
1. Covering the floor with clayey soil or cow dung followed by cattle driven over the floor for compaction.
2. Waiting for several rainy seasons to deposit layers of silt. Seepage losses from sand in riverbeds can be avoided if proposed construction sites with boulders and fractured rocks are rejected.
Leakage losses
Loss of water from leaking water tanks, water pipes, water taps should be prevented by using either bitumen paste or cement mortar.