Border Irrigation Calculator
Calculate border strip irrigation volume from strip dimensions and application depth. Determine inflow rate needed for uniform water distribution.
Crop Water Requirement Calculator
Calculate crop water requirement (CWR) by summing ETc values across growth stages. Estimate total irrigation water needed for your crop season.
in/day
days
days
days
days
Initial Stage⧉
2.50 in
Development Stage⧉
7.00 in
Mid-Season Stage⧉
11.50 in
Late Stage⧉
4.38 in
Total CWR⧉
25.38 in
Over 125 days
Planning notes, formulas, and examples
About the Crop Water Requirement Calculator
The Crop Water Requirement (CWR) represents the total amount of water a crop needs throughout its growing season. It is computed by summing crop evapotranspiration (ETc) values over every growth stage from planting to harvest. Each stage has a unique crop coefficient (Kc) that, when multiplied by reference evapotranspiration (ET₀), yields the daily water demand for that period.
Accurate CWR estimation is essential for irrigation planning, water-rights allocation, and reservoir sizing. Underestimating CWR leads to crop stress and yield loss, while overestimating wastes water and energy. This calculator lets you define up to four growth stages with their duration and Kc values, combine them with your local ET₀, and see total seasonal water demand in inches.
Use it to build a season water budget from stage lengths, Kc values, and local ET₀ before finalizing acreage, pumping plans, or water orders. For the most reliable results, use Kc values from FAO-56 guidelines and ET₀ data from your nearest weather station or state extension service.
When This Page Helps
Knowing your crop's total water requirement lets you size irrigation systems, schedule pumping, and negotiate water allocations. This page helps turn stage-by-stage ET assumptions into one working seasonal number for acreage planning, pumping capacity, and water-right discussions.
How to Use the Inputs
- Enter the reference evapotranspiration (ET₀) for your area in inches per day.
- Enter the duration in days for each growth stage (initial, development, mid-season, late).
- Enter the crop coefficient (Kc) for each growth stage.
- Review the total crop water requirement displayed in inches.
- Compare against your available water supply to check feasibility.
- Adjust Kc values for local variety or management differences.
Formula used
CWR (in) = ∑ (ET₀ × Kc_i × Days_i) for each growth stage i
Where:
ET₀ = Reference evapotranspiration (in/day)
Kc_i = Crop coefficient for stage i
Days_i = Duration of stage i (days)Example Calculation
Result: CWR = 26.38 inches
Initial: 0.25 × 0.4 × 25 = 2.50 in. Development: 0.25 × 0.8 × 35 = 7.00 in. Mid-season: 0.25 × 1.15 × 40 = 11.50 in. Late: 0.25 × 0.7 × 25 = 4.38 in. Total CWR = 2.50 + 7.00 + 11.50 + 4.38 = 25.38 inches over the 125-day season.
Tips & Best Practices
- Use local ET₀ data from a weather station rather than regional averages for accuracy.
- FAO-56 lists Kc values for most major crops; adjust for local conditions.
- Kc during the initial stage is low because the canopy is small.
- In arid climates, CWR will be higher due to elevated ET₀.
- Account for effective rainfall to determine net irrigation requirement.
- Recalculate if you change planting date, as ET₀ varies by month.
Understanding Growth Stage Coefficients
Crops progress through four broadly defined stages: initial establishment, canopy development, mid-season full cover, and late-season senescence. Each stage has a characteristic Kc value reflecting canopy size and stomatal activity. During the initial stage, much of the soil surface is bare and Kc is low. As the canopy grows, Kc increases until it peaks at mid-season.
Converting CWR to Gallons
One acre-inch of water equals roughly 27,154 gallons. Multiply total CWR in inches by your irrigated acreage and by 27,154 to estimate the total volume your pump must deliver over the season.
Adjusting for Effective Rainfall
Not all rainfall enters the root zone. USDA NRCS methods estimate effective rainfall based on total monthly precipitation and monthly CWR. Subtract effective rainfall from CWR to obtain the net irrigation requirement, which is the amount your irrigation system must supply.
Sources & Methodology
Last updated:
Frequently Asked Questions
Crop water requirement is the total volume of water a crop needs from planting to harvest. It equals the sum of daily crop evapotranspiration values across all growth stages and is expressed in inches or millimeters.
Most state extension services and weather networks publish daily or monthly ET₀. USDA, state climatologist offices, and networks like CIMIS (California) or CoAgMet (Colorado) are reliable sources.
Kc values range from about 0.3 during initial growth to 1.0–1.2 at mid-season, then drop to 0.5–0.8 at maturity. FAO Irrigation and Drainage Paper 56 provides standard values for dozens of crops.
No. CWR is the gross crop demand. Subtract effective precipitation to get the net irrigation requirement. Effective rainfall is the portion actually stored in the root zone.
Accuracy depends on the quality of your ET₀ and Kc inputs. With good local data, CWR estimates typically fall within 10–15% of actual field measurements.
Yes, but perennials may not have distinct growth stages like annuals. Use monthly Kc values and 30-day periods instead to approximate seasonal water use.
ET₀ is driven by solar radiation, air temperature, humidity, and wind speed. It is highest in summer and in arid, windy environments. The Penman-Monteith equation is the standard method for calculating it.
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