Understanding GPM Well Flow Rate

Understanding the technical aspects of your well system helps you make better decisions about maintenance, upgrades, and troubleshooting. Here's what every well owner in Southern California should know.

Key Well Metrics

• Static water level: The depth to water when the pump has been off for at least 24 hours. This tells you how much water is "stored" above your pump. In SoCal wells, this can range from 20 feet to 300+ feet below ground surface.
• Pumping water level: The depth to water while the pump is running. The difference between static and pumping level is called "drawdown." Heavy drawdown may indicate a low-yield well or an oversized pump.
• Well yield (GPM): The sustainable flow rate your well can produce without excessive drawdown. Measured during a flow test (ideally 4+ hours). Typical residential wells in San Diego County produce 3–15 GPM.
• Specific capacity: Yield divided by drawdown (GPM per foot of drawdown). This is the truest measure of a well's productivity and lets you compare wells regardless of pump size.
• Total depth: How deep the well was drilled. This is NOT the same as the depth to water.

Reading a Well Log

Every well drilled in California gets a Well Completion Report (well log) filed with the Department of Water Resources. This document tells you:

• Total depth drilled
• Casing size, material, and depth
• Geologic formations encountered at each depth
• Water-bearing zones identified during drilling
• Static water level at time of drilling
• Yield test results
• Grout seal specifications

You can look up well logs for your property (and nearby properties) at the DWR Well Completion Report database online. This is extremely useful when planning a new well—you can see what neighboring wells found.

Understanding Aquifers in Southern California

SoCal has several types of water-bearing formations:

• Alluvial aquifers: Sand and gravel deposits in valleys and along rivers. Generally high-yield and easier to drill. Common in the San Luis Rey River valley, San Pasqual Valley, and inland valleys.
• Fractured rock (granite): Water moves through cracks and fractures in bedrock. Yields are unpredictable—one well might produce 20 GPM while the neighbor's produces 1 GPM. Common in mountain and backcountry areas.
• Decomposed granite (DG): Weathered granite that acts somewhat like sand, storing water in pore spaces. Common transition zone between alluvial and hard rock.
• Sedimentary formations: Sandstone and other layered formations in some inland areas. Can be productive but may have water quality issues.

Flow Testing

A flow test measures your well's sustainable production rate. There are several types:

• Air lift test: Done during drilling. Uses compressed air to lift water—gives a rough estimate of yield.
• Pump test (4-hour): The standard test for real estate transactions. Measures flow rate and drawdown over 4 hours.
• Extended pump test (24-72 hours): For more accurate yield assessment, especially for wells that may have limited recharge.
• Step-drawdown test: Pumps at progressively higher rates to find the optimal pump setting.

Total Dynamic Head (TDH)

TDH is the total work your pump must do, measured in feet. It determines what size pump you need:

• Vertical lift: Pumping water level to ground surface
• Elevation gain: Ground surface to the highest fixture in the house
• Friction loss: Energy lost to pipe friction (depends on pipe size and length)
• Pressure requirement: Desired pressure at fixtures (50 PSI = 115 feet of head)

Example: A well with a pumping level at 200 feet, 20 feet of elevation gain, 10 feet of friction loss, and 50 PSI requirement has a TDH of 200 + 20 + 10 + 115 = 345 feet. The pump must be rated for this TDH at your desired flow rate.