Flood Irrigation Volume Calculator

About the Flood Irrigation Volume Calculator

Flood irrigation applies water across the soil surface in a controlled sheet or ponded basin. The volume of water needed depends on the area to be irrigated and the target application depth. Expressing this volume in acre-feet and gallons lets you coordinate with water districts, size conveyance structures, and plan pump run times.

One acre-foot equals 325,851 gallons — the volume of water required to cover one acre to a depth of one foot. One acre-inch (1/12 of an acre-foot) equals 27,154 gallons. These conversions are fundamental to irrigation water budgeting.

This calculator converts your irrigated area and depth into volume in acre-feet, acre-inches, and gallons, helping you plan water orders, pump scheduling, and water-rights accounting. Use it to translate field plans into acre-feet and gallons before placing a water order or setting a pump schedule.

When This Page Helps

Water districts deliver and meter water in acre-feet. This page helps you tie a target depth to acre-feet and gallons before the turnout opens so allocation tracking, pump hours, and field timing are all built on the same number.

How to Use the Inputs

1. Enter the irrigated area in acres.
2. Enter the target application depth in inches.
3. Read the total volume in acre-feet.
4. Review the volume in gallons for pump planning.
5. Compare against your water allocation.
6. Adjust depth for efficiency losses if flood irrigating.

Example Calculation

Tips & Best Practices

• Flood irrigation efficiency is typically 40–65%; gross depth must exceed net need.
• Schedule water orders 24–48 hours ahead with your district.
• Level fields to within ±0.1 ft for uniform depth across the basin.
• Surge flow valves improve flood irrigation uniformity.
• Track cumulative seasonal volume against your annual allocation.
• Convert pump GPM and run time to volume: Vol (gal) = GPM × 60 × hours.

Surface Irrigation Methods

Flood irrigation encompasses several methods: basin flooding (ponded), border strips (long narrow strips), and furrow irrigation (water in channels between rows). All share the fundamental volume relationship of area times depth.

Improving Flood Efficiency

Laser-leveling fields dramatically improves uniformity, raising efficiency from 40% to 60–70%. Surge flow irrigation alternates water on and off to the furrow, reducing deep percolation. Poly-pipe with gated outlets provides more uniform inflow than head ditches.

Water Rights and Allocations

In the western U.S., water rights are often expressed in acre-feet per year. Knowing the volume for each irrigation event and tracking cumulative use ensures compliance and avoids penalties for over-pumping or exceeding surface-water diversions.

Frequently Asked Questions

• What is an acre-foot?

  An acre-foot is the volume of water that covers one acre to a depth of one foot. It equals 325,851 gallons or 43,560 cubic feet.

• What is the difference between acre-foot and acre-inch?

  An acre-inch is 1/12 of an acre-foot (27,154 gallons). Irrigation depths are often expressed in inches, while water allocations and reservoir sizing use acre-feet.

• How do I account for efficiency in flood irrigation?

  Divide the net depth needed by the flood efficiency (typically 0.50–0.65) to get the gross depth. Then use the gross depth in this calculator to determine total volume.

• How long does a flood irrigation take?

  Duration depends on supply flow rate: Time (hrs) = Volume (gal) / (Flow GPM × 60). A 40-acre set at 4 inches with a 2,000 GPM supply takes about 36 hours.

• Can I use this for border or basin irrigation?

  Yes. The volume calculation is the same for any surface irrigation method. The difference is in the field layout and how water is distributed, not in the total volume required.

• How do water districts measure delivery?

  Districts use weirs, flumes, or ultrasonic meters at turnouts. Flow is measured in cubic feet per second (CFS) and volume in acre-feet. 1 CFS flowing for 1 day ≈ 2 ac-ft.