What it shows
Households asking how big a rainwater tank should be often get rule-of-thumb answers. A daily water-balance simulation using 20 years of local rainfall, a roof area and the family demand shows what percentage of demand a tank of a given size can meet. The project builds the model in Python and gives a sizing chart for a real house.
The science
Daily inflow equals rainfall × roof area × runoff coefficient (0.8 to 0.9 for a tiled or concrete roof) minus the first flush. Each day the tank storage S = min(capacity, S + inflow − demand), and if demand exceeds the stored water, the shortfall is counted. Reliability is the fraction of demand supplied or of days with no shortfall. The tank size gives a curve of diminishing returns, so past a certain size, extra capacity gives little extra supply.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A laptop with Python 3, pandas, numpy and matplotlib
- Daily rainfall data for a nearby station for 20 years (open CSV from a public source)
- A tape measure to find the roof area of a house, with the owner’s permission
- A survey form for the water demand of the family
- A report template
Safety
For this project: Take permission before measuring a roof, and an adult must do any roof access. Use the ground plan area and not climb up. Use only anonymous demand data.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
11 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Measure the roof plan area of the house from the ground and make a sketch, or use a satellite map to find the area.
Ask the family how many litres they would use from the tank daily for toilets, washing and gardens, and write it down as an average.
Load the rainfall data and check missing days, with a count of how many there are.
Write a function that calculates the daily inflow with the runoff coefficient and subtracts a first flush of 1 mm.
Write the daily water-balance loop to update the tank storage with overflow and shortfall.
Run the model for tank sizes of 1, 2, 5, 10, 20 and 30 cubic metres.
Calculate the percentage of annual demand met and the number of dry days for each size.
Plot reliability against tank size and find the knee of the curve.
Run a sensitivity test with a drought year and a 20 percent smaller roof.
Compare the cost of the tank per cubic metre with the water supply tanker cost saved.
Write the report with the recommended size, a payback period and the limits of the method.
What you should see
The chart shows that the supply fraction rises rapidly for small tanks and flattens for large ones, so the best size is the knee. Because most rain comes in a few months, a tank cannot supply the whole dry season.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the runoff coefficient?
The fraction of rainfall that is collected from the roof after losses.
2Why does a bigger tank give diminishing returns?
Most rain falls in a short monsoon period, and beyond a certain size the tank is rarely full or the demand is already met.
3Why simulate daily rather than use annual totals?
Storage depends on the timing of rain and use, which annual totals hide.
4What is first flush?
The first part of the rainfall that carries roof dirt and is diverted away.
Take it further
Add a recharge pit to receive the overflow and estimate the volume of water that goes to groundwater each year.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Mini and Major Project Guide
For diploma and BTech students: choose, plan, build, document and present