Let’s harvest water! 2 - Food and Agriculture Organization Harvesting... · A rock catchment is...
1
I n the arid lands of Africa, a lot of rainwater is lost as it falls and flows away. We can harvest a great deal of this water and store it to use when we need it. The 2 main ways of harvesting rainwater are collecting it from roofs and collecting surface runoff—the water that flows off land when it rains. Harvesting surface runoff There are many ways to harvest rainwater on the land. Building a runoff harvesting system is a community project. The community must decide who will own and operate the water source, what method to use and who will build it. A rock catchment is another method for harvesting surface runoff. It can be made wherever there are large rock outcrops. When rainwater falls on rock most runs off the rock. The rock catchment system catches and stores runoff water. Vegetation growing on the rock is cleared and cracks are filled up to stop water sinking in. Gutters made of flat stones and cement surround the catchment area. The gutters catch the runoff water and channel it to the tank or dam.They must have an upward slope (gradient) of at least 3 cm for every 100 cm. The rock surface from which water is collected is the catchment area. The size of the area, the slope and the average rainfall are used to work out the expected volume of runoff water. This sum decides the size of the reservoir*. The reservoir—water storage place—can be a tank or a dam. A dam, built across a natural dip or hollow in the rock, is better for large catchment areas. A closed brick or ferrocement tank is smaller but prevents water being lost through evaporation. An aqueduct is a channel that leads water to a lower level across a gap. If a tank is used, an aqueduct leads water from the gutters to the tank. The catchment area and reservoir must be cleaned and cracks repaired every season. Make the water pan in a natural dent or depression or in a small valley where there is heavy soil, like clay—sandy soil won’t trap water. One way to collect surface runoff is to build a water pan, a pond 400–1,000 m³ dug in the ground to collect runoff from fields, roads or luggas (dongas). A small one can be made bigger if needed. Silt traps in the inlet channel catch silt (soil and other matter) carried in the runoff water so less gets to the pan. water from inlet water to pan silt water Most of the soil dug out of the pan is used to make the wall or embankment. The wall and bottom are lined with clay or thick plastic sheeting. To make the wall strong the soil is compacted (pressed down), stones are put on the sides and grasses and shrubs are planted on the wall. A fence around the pan keeps livestock out and keeps small children safe. Water flows from the pan to a trough in a pipe laid through the wall. Troughs can be made from old tyres or drums cut in half. Trees reduce evaporation by screening the pan from hot, dry winds and providing shade to keep the water cool. A shallow channel to direct water to the pan. It must start at the highest point, in this case where the land meets the road. Plants growing here help to trap soil. A low wall to carry overflow to the sides of the pan. Some of the soil dug out of the pan is used to make spillways on either side of the inflow channel. Let’s harvest water! A school roofwater harvesting system. Water from the roof is channelled through gutters to a storage tank. Have you noticed the puddles that form next to roads after even a brief shower of rain? That is surface runoff from the road. Keeping water clean Even when we have a source of clean water, we need to keep it clean when collecting and storing it. Always use a container • that is clean inside and out Do not touch the • water with dirty hands Do not let things fall • into the water Keep the storage • container covered If the container does • not have a tap, use a clean ladle or scoop for taking water from it A rock catchment in Kajiado collects enough water to feed a dam and storage tanks. 1 2 These are 2 commin methods. More information Free handbooks on various harvesting methods can be downloaded from www.waterforaridlands.com. The Pace Project (www.paceproject.net) and www.rainwaterharvesting.org have lots of information on ways to harvest water.
Transcript of Let’s harvest water! 2 - Food and Agriculture Organization Harvesting... · A rock catchment is...
In the arid lands of Africa, a lot of rainwater is lost as it falls and flows away. We can harvest a great deal of this water and store it to use when we need it. The 2 main ways of harvesting rainwater are collecting it from
roofs and collecting surface runoff—the water that flows off land when it rains.
Harvesting surface runoffThere are many ways to harvest rainwater on the land. Building a runoff harvesting system is a community project. The community must decide who will own and operate the water source, what method to use and who will build it.
A rock catchment is another method for harvesting surface runoff. It can be made wherever there are large rock outcrops. When rainwater falls on rock most runs off the rock. The rock catchment system catches and stores runoff water.
Vegetation growing on the rock is cleared and cracks are filled up to stop water sinking in.
Gutters made of flat stones and cement surround the catchment area. The gutters catch the runoff water and channel it to the tank or dam.They must have an upward slope (gradient) of at least 3 cm for every 100 cm.
The rock surface from which water is collected is the catchment area. The size of the area, the slope and the average rainfall are used to work out the expected volume of runoff water. This sum decides the size of the reservoir*.
The reservoir—water storage place—can be a tank or a dam. A dam, built across a natural dip or hollow in the rock, is better for large catchment areas. A closed brick or ferrocement tank is smaller but prevents water being lost through evaporation.
An aqueduct is a channel that leads water to a lower level across a gap. If a tank is used, an aqueduct leads water from the gutters to the tank.
The catchment area and reservoir must be cleaned and cracks repaired every season.
Make the water pan in a natural dent or depression or in a small valley where there is heavy soil, like clay—sandy soil won’t trap water.
One way to collect surface runoff is to build a water pan, a pond 400–1,000 m³ dug in the ground to collect runoff from fields, roads or luggas (dongas). A small one can be made bigger if needed.
Silt traps in the inlet channel catch silt (soil and other matter) carried in the runoff water so less gets to the pan.
water from inlet water to
pan
silt
water
Most of the soil dug out of the pan is used to make the wall or embankment. The wall and bottom are lined with clay or thick plastic sheeting. To make the wall strong the soil is compacted (pressed down), stones are put on the sides and grasses and shrubs are planted on the wall.
A fence around the pan keeps livestock out and keeps small children safe.
Water flows from the pan to a trough in a pipe laid through the wall. Troughs can be made from old tyres or drums cut in half.
Trees reduce evaporation by screening the pan from hot, dry winds and providing shade to keep the water cool.
A shallow channel to direct water to the pan. It must start at the highest point, in this case where the land meets the road. Plants growing here help to trap soil.
A low wall to carry overflow to the sides of the pan. Some of the soil dug out of the pan is used to make spillways on either side of the inflow channel.
Let’s harvest water!
A school roofwater harvesting system. Water from the roof is channelled through gutters to a storage tank.
Have you noticed the puddles that form next to roads after even a brief shower of rain? That is surface runoff from the road.
Keeping water cleanEven when we have a source of clean water, we need to keep it clean when collecting and storing it.
Always use a container •that is clean inside and outDo not touch the •water with dirty handsDo not let things fall •into the waterKeep the storage •container coveredIf the container does •not have a tap, use a clean ladle or scoop for taking water from it
A rock catchment in Kajiado collects enough water to feed a dam and storage tanks.
1
2
These are 2 commin methods.
More informationFree handbooks on various harvesting methods can be downloaded from www.waterforaridlands.com. The Pace Project (www.paceproject.net) and www.rainwaterharvesting.org have lots of information on ways to harvest water.
