Well Water Capacity 

Assessment Manual

This manual guides you through evaluating your current water system’s capacity to support alfalfa farming. Use it to determine if your setup allows for expansion or requires upgrades.

Well Watering Capacity Calculator







🔗 For ET-based regional presets, visit:
California CIMIS ET Zones

Instructions to find local ET data:

  1. Go to cimis.water.ca.gov
  2. Click on “Data” in the top menu, then select “Station Report” or “Daily ET Data”
  3. Select a station nearest your farm (e.g., “Campo” area)
  4. Choose the date range and output format (e.g., Daily or Monthly ET)
  5. Click “Submit” to view or download the evapotranspiration values in inches/day
  6. Use this ET rate with the conversion: ET (in/day) × 27,154 = gallons/acre/day

Step 1: Gather Information on Your Current Water System

  • Well Pump Capacity:
    • Pump Rate (GPM or GPH)
    • Operating Hours per Day
    • Recovery Rate (GPM or GPH)
  • Storage Tank:
    • Tank Volume (gallons)
    • Refill Time (how long to fill from empty)
  • Booster Pump:
    • Flow Rate (GPM)
    • Pump Cycling (on/off frequency)

Step 2: Calculate Daily Water Availability

Formula:
Daily Water Availability = Pump Rate (GPM) × Minutes Operated Per Day + Tank Storage (gallons)

Note: Consider well recovery rate to avoid overuse.

Step 3: Estimate Alfalfa Water Requirements

  • 6–8 acre-feet per year (1 acre-foot = 325,851 gallons)
  • ~1.95–2.6 million gallons/year/acre
  • Peak season: 60–80 gallons per acre per day
  • Example: 10 acres × 80 gallons = 8,000 gallons/day

Step 4: Compare Supply vs. Demand

Item
Value
Well pump flow rateGPM
Operating hours/dayhours
Daily well outputgallons
Tank capacitygallons
Booster pump flowGPM
Alfalfa acresacres
Water needed per daygallons
Surplus or deficit?gallons

Step 5: Evaluate Options

  • If supply > demand: Consider expansion
  • If supply < demand:
    • Add more water storage
    • Reduce irrigated acreage
    • Upgrade or deepen the well
    • Use smart irrigation scheduling and sensors

How to Measure Well Pump Flow Rate

Tank Fill Test

  1. Drain a known amount from the tank (e.g., 500 gallons)
  2. Turn on the well pump and start your timer
  3. Time how long it takes to fill the known volume
  4. Use the formula: GPM = Gallons ÷ Minutes
  5. Example: 500 ÷ 10 = 50 GPM

✔️ Ensure no other water usage during test. Use tank gauge or markers to track gallons.

Alternative Flow Measurement Methods

  • Flow Meter: Digital or mechanical inline meter
  • 5-Gallon Bucket Test:
    • Time to fill 5 gallons
    • GPM = 5 ÷ (Seconds ÷ 60)

How to Measure Booster Pump Flow Rate

Timed Bucket Test

  1. Run pump normally with a full tank
  2. Fill a 5-gallon bucket and time it
  3. Use formula: GPM = Gallons ÷ (Seconds ÷ 60)
  4. Example: 5 ÷ (15 ÷ 60) = 20 GPM

Other Methods

  • Pump Label: Look up model specifications (GPM, PSI, HP)
  • Flow Meter: For real-time and total gallons used
  • Sprinkler Head Estimate: Heads × GPM per head