|CERES No. 158 March - April 1996 (FAO Ceres, 1996, 50 p.)|
A rainy day can be good weather for households in need of a free supply of water. The origin of all the fresh water the world uses, whether from surface or underground sources, is precipitation. A simple way to collect this precipitation is to rig up pipes to carry rainwater from rooftop gutters to a cistern, but that means making do with whatever water happens to run off the roof. A far better way is to design a more efficient catchment system by modifying the position of the house and construction of the roof.
The first step is to understand something of the physics of rain. Each raindrop has a diameter of 0.5 to 5 millimetres. Each rainfall has a spectrum of drop sizes with most of the larger drops being 2 or 3 mm in diameter. Drops of 0.04 to 0.5 mm are called drizzle, and of 0.001 to 0.04 mm form clouds or fog.
In still air, a drop with a diameter of 0.1 mm falls vertically at about 25 centimetres per second, a 0.5 mm drop at 2 m per second and a 5 mm drop at 9 m per second. The effect of a wind of even a few metres per second is to push the drops sideways so they fall at an angle. The stronger the wind or the smaller the drops, the closer to horizontal will be their path.
When it is raining and there is a strong west wind, it is logical to seek shelter on the east side of the house. By the same logic, it is from the west side that the most rain can be collected.
· Positioning the house. The house should stand free of any trees or taller buildings that would intercept precipitation. Its length should face the wind, and it should have an eaves-trough (rain gutter) at the bottom of the west wall to collect the rain striking and running down the wall.
· Designing the roof. A flat roof is an efficient rain collector only if the rain falls straight down, which is not normally the case. A centre peaked roof is better for collecting rain falling at an angle. Even better would be to eliminate the peak and have a roof with a single slope rising up from the windward side. This, however, might look a little odd so a more reasonable approach is to put a vertical panel along the peak of the roof. Even a small panel 1 m high can significantly increase the amount of rain collected in windy weather.
· Making use of rain shadows. The reason for the increased collection is the existence of a rain shadow behind each vertical part of the house - the west wall, the peak of the roof and the panel on the roof. When the wind blows rain against one side of the vertical wall, the rain-free area on the other side is known as a rain shadow. The wall will collect the rain that would have fallen in the rain-shadow area in the absence of the wall. The rain collected by a house designed to make use of rain shadows can be two or three times as much as the rain collected by a house with a simple flat roof collection system.
· Taking the measurements. The sizes of the rain-shadow areas can be determined by measuring the wind speed and inferring the droplet sizes from the measured rainfall rates. Once the rain-shadow area is determined, it is possible to use a rain gauge to measure the rainfall on the ground and calculate the amount of water that should be collected. Climatological data for the locality will reveal the prevailing wind direction during rainfalls and average wind speed and rainfall rate. Then, informed decisions can be made on how to orient new villages or individual buildings and which buildings are best suited for modification to improve their rainfall collection.
Using the rainshadow concept can improve any effort to collect precipitation. In particular, it can make a critical difference in semi-arid or mountain areas where there is frequent drizzle but little heavy rain and, not only is the total amount of precipitation low, but the drops tend to move toward the horizontal.
- Robert S. Schemenauer is a scientist in the Cloud Physics Research Division of Environment Canada's Atmospheric Environment Service.