Catchment Areas

Credits: Biovision-Infonet

Run-off from catchment areas

Although regions with low and erratic rainfalls appear to be unsuitable for rainwater harvesting, it has been proved many times that rainwater harvesting is the most viable water supply system in arid and semi-desert regions. Rainwater harvesting in dry regions is viable when the following aspects are considered and applied: 

1. Catchment areas are enlarged to increase the volume of run-off water. 
2.  Storage reservoirs are made large to store more water for longer periods. 
3. Evaporation is minimized by roofing storage reservoirs.
4. Underground water storage in situ in the soil of farmland and sand of riverbeds.  

The following two examples show that rainwater harvesting is viable in regions with little rainfall provided the catchment area is enlarged accordingly:

Example 1 

A roof with a catchment area of 100 square metres (m².) and an annual rainfall of 800 millimetres (mm) can supply 72,000 litres of water (100 m² roof x 800 mm rain = 80,000 litres minus 10% loss = 72,000 litres). 

Example 2 

A roof in semi-desert regions with an annual rainfall of 200 mm has to be 4 times larger to supply an equal volume of 72,000 litres of water, because the rainfall is only 1/4 of the 800 mm rain shown in Example 1. 

To design successful rainwater harvesting systems, it is important to know: 

1) How much rainwater falls on a catchment area. 

2) How much of the rainwater runs off the catchment area. 

If these two figures are not known, the storage capacity of a water reservoir and its spillways cannot be designed properly. The ruins of such improperly designed water projects can be witnessed in most parts of Africa. 

The size of a catchment area is measured in square metres (m²) or in hectares (ha). 1 ha consists of 10,000 m² which is equal to 2.47 acres. An acre is equal to 4,047 m².

Farmers measure their acreage of land by walking 70 paces, each pace with a length of 3 feet, equal to 0.915 metre, around the four sides of a square. 70 walking paces x 0.915 metres are equal to 64.05 metres.

When two sides of the square are multiplied with each other the result is 4,102 m² (64.05 x 64.05), this is close to the actual area of 4,047 m² for 1 acre.

When the size of a catchment area and the volume of rainwater falling on that catchment area have been found, the volume of rainwater that can be harvested is be calculated by multiplying the length with the horizontal width of the roof. For example: Length 20 m x horizontal width 5 m = 100 square metres 

Farmers may admit: “Okay, it has rained but it was only a drizzle”. Nevertheless, a drizzle of 10 mm rain can be sufficient to produce the required volume of water, if the catchment area is sufficiently large.

The relationship between rainfall and catchment area can be explained by the following example: 

A drizzle of 10 mm rain on10,000 m2 (1 hectare = 100 m x 100 m) area of a roof, rock or tarmac road, it produces 100,000 litres of run-off water minus about 20 % loss = 80,000 litres. 

• 10 mm rain x 10,000 sq.m. minus 20% loss = 80,000 litres. 
• If the same drizzle of 10 mm rains falls on a ten times larger catchment area of 100,000 sq.m. (10 hectares = 100 m x 1,000 m,) it produces ten times more water, namely 800,000 litres x 10 = 8,000,000 litres of water = 8 million litres = 8,000 cu.m. 

Therefore, a drizzle of 10 mm rains may be sufficient for harvesting a required volume of water, if the catchment area is large enough. Huge volumes of water can thus be harvested from e.g. roads because they have large and hard surface catchment areas.

For example: a drizzle of 10 mm rain on a road 6 metres wide and 1 km (1,000 metres) long road can supply the following volume of water: A 6 m x 1,000 m road x 10 mm minus 20% loss = 48,000 litres = 48 m³ of water.

When designing a reservoir to hold water from a road catchment it is important that spillways can discharge safely the overflowing water during storms. 

For example: A rain storm of 75 mm falling on a 2 km long road produces about: 75 mm rain x 6 m x 2,000 m road minus 20% = 720,000 litres = 720 m3 of water. 

If the storage capacity, such as a pond is 500 m³, then the spillways must be capable of discharging 220 m³ of water (720 minus 500) in a few minutes or the pond will be damaged or perhaps washed away by the flood of incoming water. 

Catchment of rainwater from roads is potentially the cheapest water source in dryland where there are no sandy riverbeds (luggahs or wadis).

Tarmacked roads produce more run-off water than dirt roads but the water may contain harmful tar components for people and livestock and should therefore be used for irrigation only. 

There are two main types of storage, namely:

1. Storage in reservoirs, such as ponds, earth dams and tanks.
2. Storage in situ, that is in the voids between particles of soil and sand.