Training Notes by Prof. Bancy Mati
What is a water storage pond?
Excavated ponds and pans are small reservoirs, about 1 m to 3 m deep, usually dug of-stream with raised and compacted banks all around. They are constructed to collect and store runoff water from various surfaces including from hillsides, roads, rocky areas and open rangeland. The difference is that pans receive their water wholly from surface runoff while ponds are constructed where there is some ground water contribution or a high water table. The capacity of pans and ponds can range from 500 to 5,000 m3. Structures whose reservoir capacity is less than 500 m3 are called tanks, while those exceeding 5,000 m3 are called dams.
Pans and ponds are generally built close to settlements, and are located on grazing lands rather than farmlands, since the latter is more valuable and the former more compact. They are excavated on gentle sloping lands; less than 2% or 1:50; this simplifies construction and minimizes erosion. The catchment land above must be gently sloping as well. Catchment size should not be too large or too small, and should be grassed to help in trapping silt. When properly designed and with good sedimentation basins, the water collected can be used for livestock watering or to supplement the irrigation of crops. In this chapter, we adopt the term pond to be inclusive of both pans and ponds unless otherwise stated.