Meteorological data: Get average annual rainfall, intensity and spatial distribution
Geological data: Nature of rocks and soil
Hydrogeological data: Depth of groundwater table and receptiveness of aquifer
User water profile: Total water requirements, proportion already available, source of availability
Objectives and uses
Legislation and incentives
Catchments: Types, area and location of catchments
From the site plan or from actual measurements, find out the different types of catchments that are available for harvesting rainwater, the dimensions of each of these types of catchments and the location.
Types of catchments: From the site plan, mark out the different types of catchments that are available for water harvesting. The type of catchment such as paved or upaved and roof or other paved ground surfaces will determine the collection efficiency and quality.
Area of catchment: The amount of rainfall that you can collect depends directly on the area of the catchment - the larger the surface area, the more the water. Regardless of the shape of the roof, the area for calculation of rainfall collection would be the area equivalent to the area under the roof.
Thus, area of catchment (C) = length (l) x breadth (b)
In the case of catchments at ground level, the area can be found out from the measurement given on the site plan. For irregular dimensions of ground level catchments such as a winding driveway, break it into measurable shapes such as rectangles or triangles and measure.
Rainwater runoff from the following catchments should be avoided:
Tar felted roofs: Source of biological and heavy metal contamination
Asbestos sheets: Weathered and leached fibres of asbestos – highly toxic
Chemically treated roofs: Eg. Chemicals used for water proofing, will have high concentration of heavy metals like lead.
Water from kitchen and bathrooms should not be used for harvesting.
Any other contaminated runoff (industrial or domestic waste water) should not be used for harvesting.
There are four types of rainfall information that you need:
The annual average rainfall: Will give an overall picture of the total amount of water that can be collected.
The pattern of rainfall over different months: Will tell you when the rainfall is available – is it available most of the year or only during a certain part of the year.
Number of rainy days: Will give an indication to decide whether to store the rainwater or to recharge it. If most of the rainfall comes only in a short span of time, then it is better to recharge the aquifer.
The peak rainfall intensity: Will give an indication to design the size of the storage or recharge structure. The sizing will based on how much water will need to be stored or recharged during the most intense spell of rain.
Geological and hydrogeological data
For systems where the harvested rainwater will be used to recharge the aquifer, selection of site is important. Information must be collected on the following
Poor or well sorted sand or gravel, fine sand, silt, loam, layered or unweathered clay
Fractured or massive rocks, sandstone, limestone
Confined or unconfined, perched, thickness of aquifer
Depth of water table
Shallow or deep water table zones
Soil: The soil must have properties that will allow the easily water to move downward. Sand, sandy loam and loamy sand soils have high infiltration rates even when they have high moisture content and low runoff. Infiltration and water movement will be greater in materials, which have greater porosity and permeability such as sands, gravel or fractured rock.
Aquifer: The aquifer must not be at shallow depths and should be at least 8-10 metres below the ground level. The aquifer should be unconfined and must have good hydraulic conductivity as well as transmissivity so that the water that is recharged is quickly spread horizontally to prevent a water mound forming below the surface.
Geology: In rocky areas, you need information about the nature of rocks, whether they are fissured or weathered and their capacity to hold and release water.
Type of soil/rock
Porous and permeable
Clay lenses interbedded with sand
Clay, shale, mudstone
Porous but practically impervious
Basalt, granite and quartzite
Neither porous nor permeable
The size of the water harvesting is determined by two factors – how much is needed and how much is available.
What is the total water currently used
What is the per capita water demand
What is the water demand during the driest period
Objectives and uses
Primary water source: Find out what is the source of supply for drinking and cooking, for gardening and for other purposes. Then determine which portion of this you want to replace with rainwater.
Dependence on municipal & groundwater sources: If there is no municipal water supply and you are purchasing water from tankers then the objective may be to recharge the aquifer or store the water so that you can reduce your dependence on tanker supply and reduce the water bill.
Number of existing & abandoned borewells
Quality of groundwater
Hours of pumping
Legislation and incentives
Today many state governments and city municipalities have passed laws that make it mandatory for existing or new buildings to have rainwater harvesting systems. You must find out the details of these laws in order to make sure you are compliant on all counts. At the same time, there are also many incentives to motivate people to take up rainwater harvesting and you must find out about what incentives are available in your city.