How Do Evapotranspiration-Based Schedules Improve North Carolina Irrigation

Evapotranspiration-based (ET-based) irrigation scheduling uses measured or modeled water losses from soil and crops to make irrigation decisions. In North Carolina, with its wide range of climates and soils, transitioning from calendar- or intuition-based watering to ET-based scheduling produces measurable water savings, better crop health, lower input costs, and stronger compliance with drought restrictions. This article explains the science behind ET scheduling, describes practical implementation steps for North Carolina conditions, gives concrete calculation examples, and provides actionable recommendations for growers, turf managers, and landscape professionals.

What is evapotranspiration and why it matters

Evapotranspiration (ET) is the combined process of evaporation from soil and plant surfaces plus transpiration through plant stomata. Reference evapotranspiration (ETo) is a standardized measurement of atmospheric demand for water from a reference surface (often a well-watered grass or alfalfa). Crop evapotranspiration (ETc) = ETo x Kc, where Kc is a crop coefficient that adjusts ETo to reflect the water use of a specific crop and its growth stage.
ET-based irrigation focuses on replacing the crop water use that has occurred since the last effective rainfall or irrigation event, rather than irrigating on a fixed calendar schedule. That adjustment to actual demand reduces overwatering in cool, cloudy periods and increases irrigation when hot, dry conditions escalate demand.

Why ET scheduling is particularly useful in North Carolina

North Carolina spans coastal plain, piedmont, and mountain regions with distinct rainfall distribution, temperatures, and soils. Key local considerations include:

• Coastal plain soils tend to be sandy with low water holding capacity, requiring more frequent, smaller irrigations.
• Piedmont soils include more clay and silt, holding more plant-available water but responding slowly to irrigation.
• Mountain valleys have more variable microclimates with rapid swings in ETo.

ET-based scheduling adapts to these differences because it uses measured weather or on-site sensor data rather than a universal schedule. Benefits for North Carolina include:

• Reduced irrigation during frequent summer thunderstorms when rainfall meets much of crop demand.
• Quick response to extended hot spells in summer when ETo rises and water demand increases.
• Stronger drought management and compliance with local restrictions through objective, verifiable records.
• Improved nutrient use efficiency and reduced leaching in sandy soils by reducing unnecessary irrigation.

Components of an ET-based system

An effective ET-based scheduling system has several elements working together:

• Reference ET data source (local weather station, regional network, or on-site weather station).
• Crop coefficients (Kc) for the specific crop or turf, adjusted for growth stage.
• Soil water holding capacity and root zone depth to calculate available water.
• Management allowed depletion (MAD) percentage that defines when to irrigate.
• Irrigation system performance metrics (precipitation rate, application efficiency or distribution uniformity).
• Decision-support tools or controller integration (ET-based controllers, software, or manual calculations).
• Optional soil moisture sensors for verification and fine-tuning.

Practical steps to implement ET-based scheduling in North Carolina

1. Establish a reliable ETo source.
2. Use a local weather station network or install an on-site weather station calibrated to the ASCE Penman-Monteith reference method if precision is required.
3. Select crop coefficients and growth-stage adjustments.
4. Obtain Kc values appropriate for the crop or turf. Example ranges: turfgrass Kc 0.8-1.05 (depending on species and season); vegetables 0.6-1.2; row crops like corn start low (~0.3) and peak near 1.05-1.15.
5. Determine soil water parameters.
6. Calculate plant available water (PAW) in the root zone: PAW (inches) = root depth (inches) x soil available water per inch (inches water per inch soil). Sandy soils often have lower PAW than loam or clay.
7. Choose a management allowed depletion (MAD).
8. For high-value crops or turf, MAD often ranges 30-40% of PAW. For drought-tolerant field crops it can be 50% or higher.
9. Compute irrigation need after each day or interval.
10. Daily ETc = ETo x Kc.
11. Cumulative depletion since last full refill = sum of ETc minus effective rainfall.
12. Irrigation required (inches) = cumulative depletion / system efficiency (accounting for runoff and distribution uniformity).
13. Convert inches to run-time.
14. Use each irrigation zone nozzle precipitation rate to calculate minutes required to deliver the needed depth.
15. Monitor and adjust.
16. Use soil moisture sensors and periodic probe checks to validate that root zone moisture remains within MAD limits.
17. Adjust Kc for local cultivar behavior and seasonal canopy changes.

Example calculation (practical numbers)

Assume a turf zone in the NC piedmont with the following:

• Daily ETo = 0.25 inches/day (typical in hot summer).
• Turf Kc = 1.0.
• Root depth = 6 inches.
• Soil available water = 0.10 inches water per inch soil (typical sandy loam conservatively).
• PAW = 6 x 0.10 = 0.6 inches.
• MAD = 40% – allowable depletion before irrigation = 0.6 x 0.40 = 0.24 inches.
• Irrigation system efficiency = 80% (accounting for distribution uniformity and losses).

If the turf has been dry for three days with no rainfall:

• Cumulative ETc = 3 x (0.25 x 1.0) = 0.75 inches.
• Irrigation required to refill to field capacity (or to refill to MAD target) will be at least cumulative depletion minus any prior irrigation. To refill to field capacity: 0.75 / 0.8 = 0.9375 inches applied.
• If the target is to refill down to zero depletion (preferred practice is to restore to near full), schedule an irrigation of about 0.94 inches. That equals roughly 25,500 gallons per acre (1 inch/acre 27,154 gallons).

This example shows how ET data quickly translates into a precise, measurable irrigation volume.

System performance and adjustments

• Check distribution uniformity (DU or CV). Aim for DU > 65% and correct major nozzle clogging, pressure imbalances, and head spacing issues to increase efficiency.
• Adjust runtime by zone based on actual precipitation rates measured with catch cans.
• In sandy coastal plain soils, reduce cycle length and repeat to avoid deep percolation losses–use cycle-and-soak.
• In heavy clay soils, use slower application rates to avoid runoff.

Integration with technology

ET-based controllers can automatically fetch ETo and adjust run times daily. Soil moisture sensors and telemetry systems can provide redundancy: when sensors show adequate moisture, skip the ET-based irrigation. For many North Carolina operations, combining weather-based ET with soil moisture validation yields the best results–weather sensors handle atmospheric demand while soil sensors verify actual root zone response.

Water savings, yield, and economic outcomes

Studies and practical implementations show ET-based scheduling often reduces irrigation water use by 20-50% compared with fixed schedules, with equal or improved yields and crop quality. Savings come from:

• Avoiding unnecessary watering during cool or rainy periods.
• Applying only what the crop needs, reducing leaching of fertilizers in sandy soils.
• Preventing plant stress episodes that reduce yield or turf quality.

Economic benefits include lower pumping energy, reduced fertilizer loss, extended pump and system life, and potentially higher marketable yield or turf performance. Maintain records of ETo, irrigations applied, precipitation, and yields to quantify benefits.

Best practices specific to North Carolina operators

• Use a local ETo data source when possible; adjust Kc for local varieties and microclimates.
• For small farms and landscapes, use an ET calculator or smartphone app plus a soil moisture sensor for low-cost accuracy.
• Prioritize system audits: repair leaks, optimize pressure, and replace worn nozzles to improve efficiency before fine-tuning schedules.
• During drought alerts, tighten MAD values and increase monitoring frequency; document water use for compliance with local regulations.
• Train irrigation staff on reading ET reports, converting depth to run-time, and interpreting soil sensor data.

Common pitfalls and how to avoid them

• Relying solely on generic Kc values without local calibration. Solution: monitor plant response and adjust Kc empirically.
• Ignoring system uniformity–accurate ET scheduling requires delivery uniformity. Solution: perform DU tests regularly.
• Overlooking effective rainfall. Solution: use a rain gauge and subtract effective rainfall (only the portion that infiltrates the root zone) from cumulative ET.
• Applying too much water in a single event on sandy soils. Solution: use shorter cycles with soak-in pauses.

Conclusion: practical takeaways

• ET-based irrigation aligns water application with actual crop demand, delivering measurable water savings and improved plant health in North Carolina’s diverse climates.
• Implementing ET scheduling requires reliable ETo data, appropriate Kc values, knowledge of soil water holding capacity, and attention to irrigation system efficiency.
• Use a combination of weather-based ET calculations and soil moisture sensors to balance responsiveness and accuracy.
• Conduct periodic system audits, maintain records, and adjust schedules seasonally and by growth stage.
• For most operations, the transition to ET-based scheduling pays back quickly via reduced water use, lower energy costs, and more consistent yields or turf quality.

Adopting ET-based irrigation schedules is a practical, science-based step for North Carolina irrigators who want to conserve water, protect crop quality, and prepare for variable weather patterns and drought management requirements.