WATER PANS FOR RUNOFF WATER HARVESTING

Introduction
A lot of water is lost in arid and semi-arid lands (ASAL) as surface runoff. Harvesting of this runoff and storage of the same into reservoirs such as water pans makes it available for use when required.
What is a water pan?
It is an excavated water storage structure that is square, rectangular or round, used to impound and retain surface runoff from uncultivated grounds, roads or laggas.

Why use water pan?

Simple to construct
Provides water for domestic/livestock use and supplementary irrigation
Simple operation and maintenance needed Prerequisite in water pan construction
Community mobilization through participatory rural appraisal (PRA), for a communal water pan to ensure ownership and guarantee future operation and maintenance. Availability of human labour, draught animals or earth moving machinery depending on size of pan.

Factors to consider when sitting water pans
A site with soils such as clay that retain water.
Avoid sandy soils
A natural depression or small valley to minimize excavation
A road or lagga nearby to act as a source of runoff.
A vegetated catchment to minimize salutation
A standard water pan showing main features

Procedure in water pan construction
Step 1: Site the water pan and mark the embankment, inlet and spillway
Step 2: Excavate the reservoir section and use the soil to build the embankment wall, with side slopes of 1:2.5 for shallow pans to 1:3 for deep pans
Step3: Construct spillway to discharge excess runoff water when the pan is full
Step 4: Construct silt trap(s) along the inlet channel to filter excess sediment load
Step 5: Close off the water pan with live fence to keep off the livestock
Step 6: Provide livestock watering trough off the fenced area.

What is the capacity of a water pan?
The capacity is variable and depends on site conditions and how much one wants to invest. Common ones are 400 to 1,000m 3 A water pan capacity can be increased with time to hold more water.

How do you minimize water losses in a water pan?
Compaction of the embankment fill with drums filled with water or with a roller Lining the bed and walls with clay soil or polythene sheet on soils that are not very good for a pan. Plant trees such as commiphora as euphorbia which can be propagated through cuttings around the water pan.

How do you stabilize the walls of a water pan?
By ensuring proper embankment side slopes and compaction, Planting shrubs and grasses on the embankment wall Placing stones on the embankment sides

Operation and maintenance of a water pan
Repair broken perimeter fence as need arises. Avoid direct entry of livestock into the pan to prevent trampling on bed and walls Where livestock draw off point is not provided, use portable wooden troughs, a drums cut into half or old tyres to water livestock. Clean inlet channel by removing silt every season

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