Irrigation Basics: Water Budgeting and Measuring How Much You Apply

Why “water budgeting” matters

Water budgeting is the practice of matching how much water plants use (demand) with how much your irrigation system applies (supply). Instead of relying on intuition, you estimate a weekly target, measure your system’s actual output, convert that output into “inches of water” over a known area, and then set runtimes per zone. The goal is a repeatable method you can verify with simple measurements.

Key terms you will use

• ET (evapotranspiration): water lost from soil and plant surfaces. For scheduling, many growers start with reference ET (ETo) from a local weather source and adjust for crop type.
• Inches of water: a depth spread evenly over an area (e.g., 1 inch over a bed). This makes it easy to compare demand vs. applied water.
• Application rate: how fast your system applies water, often in inches per hour (sprinklers) or gallons per hour (drip).
• Zone: one valve-controlled area that runs at a time. Each zone needs its own measurement and runtime.

Step 1: Estimate weekly plant water demand

You can estimate demand using (A) local ETo guidance or (B) simplified rules-of-thumb. Either way, you’ll adjust for heat and wind and then translate the estimate into a weekly “inches needed” target.

A) Using local ETo (reference ET) + a crop factor

Many weather services publish daily or weekly ETo. To estimate plant water use:

Plant water use (in/week) ≈ ETo (in/week) × Crop factor

Crop factor (sometimes called Kc) is a practical multiplier that reflects canopy size and crop type. For a simplified approach:

• 0.3–0.5: small seedlings, sparse canopy, early season
• 0.6–0.8: most vegetables mid-season, moderate canopy
• 0.8–1.0: dense canopy, peak season, well-watered turf-like cover

Heat and wind adjustment: if you’re in a hot spell and/or persistent wind, increase the weekly target by 10–30% (start with +15% for noticeable heat/wind; +25% for extreme conditions).

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B) Rules-of-thumb when ETo isn’t available

If you don’t have ETo, start with a baseline weekly depth and adjust:

• Mild weather: ~0.7–1.0 in/week for many established vegetables
• Hot weather: ~1.0–1.5 in/week
• Very hot + windy: ~1.5–2.0 in/week

Then refine by observation and measurement (catch-cans, flow checks). The point is not perfection on day one—it’s replacing guessing with a measurable target you can improve.

Step 2: Measure how much your system actually applies

Different delivery methods require different measurements. Do not assume labeled flow rates are accurate; pressure, elevation, filter condition, and wear can change output.

2A) Measure gallons per minute (GPM) at a tap or hose bib

This is a quick way to understand your available flow and to sanity-check zone design.

1. Get a bucket with a known volume (e.g., 5 gallons).
2. Open the tap fully to the setting you normally use.
3. Time how long it takes to fill the bucket using a stopwatch.
4. Compute flow:

GPM = Bucket gallons ÷ Fill time (minutes)

Example: 5-gallon bucket fills in 40 seconds (0.667 minutes). GPM = 5 ÷ 0.667 ≈ 7.5 GPM.

2B) Sprinklers: run a catch-can test to get inches per hour

A catch-can test measures real application rate and distribution. Use identical straight-sided containers (tuna cans, short jars, rain gauges).

1. Place 8–12 catch-cans evenly across the irrigated area (include edges and near the sprinkler).
2. Run the zone for a measured time (e.g., 20 minutes).
3. Measure water depth in each can with a ruler (or pour into a measuring cup if cans are identical and you’ve calibrated volume-to-depth).
4. Average the depths (inches).
5. Convert to inches per hour:

Inches per hour (in/hr) = Average catch depth (in) ÷ Run time (hours)

Distribution check (quick): if some cans have much less than others, you may need nozzle/spacing/pressure fixes before runtimes will be reliable.

2C) Drip: verify emitter flow rates (GPH) and line totals

Drip output is often expressed as gallons per hour (GPH) per emitter. Real flow depends heavily on pressure and clogging.

Spot-check a few emitters:

1. Place a measuring cup under an emitter (or disconnect and capture flow).
2. Run for a known time (e.g., 5 minutes).
3. Measure collected volume and convert to GPH:

GPH = (Collected gallons ÷ Minutes) × 60

Convert cups to gallons: 16 cups = 1 gallon.

Total zone flow (drip): multiply average emitter GPH by number of emitters (or use manufacturer spacing for dripline and multiply by length, then verify with a flow test at the supply if possible).

Step 3: Convert gallons applied into inches over an area

To compare “how much you applied” to “inches needed,” you need a conversion between gallons and inches of water over a known area.

Core conversion

1 inch of water over 1 square foot = 0.623 gallons

So:

Gallons needed for 1 inch over an area = Area (sq ft) × 0.623

And the reverse:

Inches applied = Gallons applied ÷ (Area (sq ft) × 0.623)

Area reminders

• Rectangle bed: Area = length × width
• Orchard row strip: Area = row length × wetted width (the width you actually water, not the whole field)

Step 4: Determine runtime per zone

Once you know (1) your weekly target in inches and (2) your zone’s application rate, runtime becomes a straightforward calculation.

Sprinkler runtime from catch-can rate

Runtime (hours) = Target depth (in) ÷ Application rate (in/hr)

Then convert hours to minutes.

Drip runtime from total GPH and gallons needed

Runtime (hours) = Gallons needed ÷ Total zone flow (GPH)

If you prefer minutes, multiply hours by 60.

Worked example 1: 4 ft × 20 ft bed (80 sq ft)

Scenario

• Bed size: 4 × 20 ft → Area = 80 sq ft
• Weekly ETo from local source: 1.4 in/week
• Crop factor for mid-season vegetables: 0.7
• Hot/windy week adjustment: +15%
• Irrigation method: overhead sprinklers
• Catch-can test: ran 20 minutes, average catch depth = 0.30 inches

1) Estimate weekly target depth

Base demand = 1.4 × 0.7 = 0.98 in/week

Adjusted for heat/wind = 0.98 × 1.15 ≈ 1.13 in/week

Weekly target ≈ 1.1 inches.

2) Convert catch-can result to application rate

20 minutes = 0.333 hours.

Application rate = 0.30 in ÷ 0.333 hr ≈ 0.90 in/hr

3) Compute weekly runtime for this zone

Runtime = 1.13 in ÷ 0.90 in/hr ≈ 1.26 hr/week

1.26 hr/week × 60 ≈ 76 minutes per week.

4) Optional cross-check in gallons

Gallons for 1 inch over 80 sq ft:

80 × 0.623 ≈ 49.8 gallons per inch

For 1.13 inches:

49.8 × 1.13 ≈ 56 gallons/week

This is useful if you also measure zone flow in GPM and want to confirm the runtime makes sense.

5) Turn weekly runtime into a schedule

If you want to split into 2 irrigations per week:

• ~38 minutes each run (76 ÷ 2)

If you split into 3 runs:

• ~25 minutes each run (76 ÷ 3)

Use your measurements to decide, then verify by repeating the catch-can test after any nozzle/pressure changes.

Worked example 2: Small orchard row (strip irrigation with drip)

Scenario

• Row length: 100 ft
• Wetted strip width (mulched strip you actually irrigate): 6 ft
• Area = 100 × 6 = 600 sq ft
• Weekly target depth (from local guidance): 1.0 in/week (already adjusted for canopy/season)
• Drip system: 10 trees in the row
• Each tree has 2 emitters
• Measured emitter flow: 0.9 GPH each (spot-checked)

1) Convert weekly target inches to gallons

Gallons per inch over 600 sq ft = 600 × 0.623 ≈ 374 gallons

So weekly target ≈ 374 gallons/week.

2) Compute total zone flow

Total emitters = 10 trees × 2 = 20 emitters.

Total flow = 20 × 0.9 GPH = 18 GPH

3) Compute weekly runtime

Runtime = 374 gallons ÷ 18 GPH ≈ 20.8 hours/week

That may look surprisingly long, but remember: drip applies slowly over a strip area. If your wetted width is narrower (or you irrigate only around each tree rather than a 6-ft strip), the area—and gallons needed—drops substantially.

4) Refine the “area” assumption (common orchard adjustment)

If instead you irrigate only a 3-ft-wide strip:

• Area = 100 × 3 = 300 sq ft
• Gallons per inch = 300 × 0.623 ≈ 187 gallons
• Runtime = 187 ÷ 18 ≈ 10.4 hours/week

This illustrates why defining the wetted area correctly is critical for drip budgeting.

5) Turn weekly runtime into practical sets

If you choose 3 irrigations per week with the 3-ft strip assumption:

• 10.4 hr/week ÷ 3 ≈ 3.5 hours per set

If you choose 5 irrigations per week:

• 10.4 ÷ 5 ≈ 2.1 hours per set

After setting a schedule, re-check a few emitters monthly (or after filter cleaning) to confirm flow hasn’t changed.

Quick reference formulas (copy/paste)

Tool checklist for measuring and verifying

• Bucket (known volume, often 5 gallons) for GPM tests
• Stopwatch (phone timer works) for fill times and run times
• Measuring cup (with cup markings) for emitter checks and small volumes
• Pressure gauge (hose-thread type is convenient) to verify operating pressure at the source and near zones
• Catch-cans (8–12 identical containers) and a ruler for sprinkler application rate
• Tape measure for bed/row dimensions and wetted width
• Notebook (or notes app) to record: date, weather, target inches, measured rates, and runtimes per zone