Recharge Pit & Refill the Water Table

A recharge pit is a dug structure that captures rooftop or yard runoff and lets it soak into the ground to refill the water table, instead of letting that water run off and escape. Filled with boulder, gravel and sand and fed through a filter chamber, it turns each storm's runoff into stored groundwater you can draw on later.

Runoff (m³) = catchment area × rainfall × runoff coefficient. You size the pit to hold the runoff from the heaviest design storm, and pit area = volume ÷ depth. The tool chains these so you enter the catchment, the design rainfall, the runoff coefficient and the depth, and it returns the pit volume, area and side length to dig.

It's the fraction of rain falling on a surface that actually runs off rather than soaking in or evaporating. A hard roof sheds almost everything — about 0.85 — while paved yards are a bit lower and bare soil lower still. Multiplying rainfall by this coefficient gives the runoff the pit must capture from the catchment.

Size the pit for the heaviest storm you expect to capture, not the average shower — that's the peak the pit has to hold so water doesn't overflow and be lost. Use a high one-event rainfall figure typical of an intense local downpour; the bigger the design storm, the larger the pit volume the tool returns.

Depth is a trade-off: a deeper pit needs less surface area for the same volume and reaches further toward permeable layers, but is harder to dig and de-silt. Set the depth you can practically build and maintain, and the tool gives the matching pit area and side length to hold the design runoff.

Fill a recharge pit in layers — coarse boulders at the bottom, gravel in the middle and sand near the top — so water filters as it percolates and silt is trapped before it clogs the soil. Lead the runoff in through a desilting or filter chamber, and clean that chamber and the top layer before each monsoon.

Keep the pit well away from building foundations so soaking water doesn't undermine them, and away from septic tanks, soak-aways and any contamination source that could pollute the groundwater. Place it where rooftop or yard runoff can be channelled to it easily, on ground that drains rather than waterlogs.

Annual recharge depends on how many storms the pit captures over the season — far more than a single storm's volume. The tool estimates the yearly recharge from your inputs so you can see the cumulative groundwater benefit, not just the one-event capacity, when deciding whether the pit is worth building.

Yes — enter the catchment area and the runoff coefficient that matches the surface (high for roofs, lower for yards or fields), and the tool sizes the pit for that catchment. A bigger or harder catchment produces more runoff and needs a larger pit; the calculation adapts to whatever you feed it.

They're solid planning figures. Real recharge depends on soil permeability, the water table depth, how often the pit is de-silted and the actual storm pattern, so add margin and maintain the pit. De-silt before each monsoon, and treat the outputs as a design starting point to confirm against local hydrogeology.