What Does Seasonal Evapotranspiration Mean For Arkansas Irrigation

Evapotranspiration (ET) is the combined water loss from soil evaporation and plant transpiration. Seasonal evapotranspiration is the cumulative ET that occurs over a crop’s growing season. For Arkansas irrigators, understanding seasonal ET is central to budgeting water, sizing systems, scheduling irrigations, and maintaining crop yields while protecting limited water supplies. This article explains what seasonal ET means in practical terms for Arkansas agriculture, how to calculate irrigation requirements from seasonal ET, and what management steps growers can take to translate ET numbers into efficient, on-farm decisions.

What seasonal evapotranspiration actually represents

Seasonal ET is the sum of daily crop water use over the period when water must be supplied to meet crop demand. It is the product of two components:

• reference evapotranspiration (ETo): the water loss from a well-watered reference surface (usually grass); and
• crop coefficient (Kc): a dimensionless multiplier that scales ETo to the actual crop covering that field and at a particular developmental stage.

Daily crop water use (ETc) = ETo x Kc. Seasonal ETc is the sum of daily ETc over the established growing season (planting to physiological maturity or harvest).
Seasonal ET abstracts away daily weather swings and gives you the total depth of water the crop is expected to remove from the soil (plus evaporation losses) during the season. That number is the starting point for irrigation planning: it helps estimate net irrigation need, informs storage and pumping decisions, and guides allocation of scarce water resources.

How seasonal ET varies across Arkansas

Arkansas has a humid subtropical climate with regional differences driven by elevation, proximity to the Mississippi River and Gulf of Mexico, and local soils. Seasonal ET in Arkansas is not a single number; it depends on:

• geographic location (Delta, West Gulf Coastal Plain, Ozarks), which affects temperature, humidity, and wind;
• crop type and variety (corn, soy, cotton, rice, wheat, pasture); and
• management practices (flooded rice vs. aerobic upland cropping, cover crops, mulches).

Typical seasonal ETc values for common Arkansas crops fall in broad ranges because weather and management vary. As a practical reference:

• Upland row crops (corn, soybean, cotton): seasonal ETc often ranges from about 18 to 30 inches during the active growing season, depending on the season length and heat.
• Rice (flooded systems): effective seasonal water use can exceed upland crops because of additional evaporation from standing water; seasonal water demands for rice systems commonly exceed 25 inches and can be substantially higher depending on management.
• Pastures and forages: seasonal use is variable but often within the 15 to 28 inch range.

These ranges are illustrative; growers should rely on local ETo and crop coefficients for precise budgeting.

Why seasonal ET matters for irrigation planning in Arkansas

Seasonal ET is the backbone of a water balance for a field. Knowing seasonal ET lets you:

• calculate the net irrigation requirement (seasonal ET minus effective rainfall);
• size on-farm storage (ponds, tanks) and pumping capacity to meet peak demands;
• schedule irrigations to avoid yield loss during critical growth stages (flowering, grain fill);
• estimate energy costs by converting irrigation depth to gallons pumped and horsepower required;
• evaluate long-term water sustainability for groundwater and surface sources.

Ignoring seasonal ET increases the chance of under- or over-irrigating. Under-irrigation during sensitive periods lowers yield; over-irrigation wastes water, energy, and can mobilize nutrients.

How to calculate irrigation requirement from seasonal ET (step-by-step)

1. Obtain a reliable reference ETo time series for your site or the nearest weather station for the season of interest.
2. Select crop coefficients (Kc) for each growth stage of the crop you are growing. Use stage-specific Kc rather than a single average if possible.
3. Compute daily ETc = ETo x Kc and sum daily values to get seasonal ETc.
4. Subtract effective rainfall. Effective rainfall is the portion of rainfall that is stored in the root zone and available to the crop; it is lower for heavy storm runoff or irrigated furrows and higher for steady light rains.
5. Adjust for soil water contributions (capillary rise from shallow groundwater) or losses (deep percolation when soils are saturated).
6. Divide the net irrigation requirement by the field application efficiency to compute gross irrigation need. Application efficiency accounts for conveyance and distribution losses (for example, 0.75 for many well-managed pivots, lower for surface systems).

Example (illustrative numbers):

• Seasonal ETo summed = 24.0 inches.
• Average crop coefficient across season = 1.0 (resulting in ETc = 24.0 inches).
• Effective rainfall during season = 8.0 inches.
• Net irrigation required = 24.0 – 8.0 = 16.0 inches.
• Field application efficiency = 0.75.
• Gross irrigation required = 16.0 / 0.75 = 21.3 inches.

This gross irrigation depth is the volume the irrigation system must apply to meet crop demand given the assumed efficiency.

Soil, root zone, and irrigation timing considerations

Seasonal ET tells you how much water the crop will use, but irrigation events must be sized to root-zone storage and soil available water. Key practical parameters:

• rooting depth: corn and soybean commonly exploit the upper 2 to 3 feet of soil; cotton also reaches deep but varies by variety; rice roots are shallower in flooded systems.
• plant available water (PAW): the inches of water a soil holds per foot that plants can use. Typical approximate PAW values: sandy soils ~0.5 to 1.0 inch per foot; loams ~1.2 to 1.8 inch per foot; clays ~1.8 to 2.2 inch per foot. Measure or use local soil survey values.
• management allowable depletion (MAD): the fraction of PAW you allow to be depleted before irrigating. Many annual row crops use MAD in the 30-60% range, with lower MAD for sensitive stages.

Use these numbers to compute how much water to apply when you irrigate. For example, if a root zone is 2.5 feet with PAW of 1.5 inch/ft, total PAW = 3.75 inches. If you set MAD at 50%, maximum allowable depletion = 1.875 inches; you would typically irrigate to refill the zone, applying slightly more than 1.875 inches to replace depletion and accommodate losses.

Practical irrigation strategies for Arkansas farms

• Schedule by crop stage: prioritize irrigation during reproductive stages (flowering, grain fill) when yield response per inch is highest.
• Combine ET-based scheduling with soil moisture sensing: use tensiometers, capacitance probes, or gravimetric checks to confirm soil water status rather than relying on ET alone.
• Use proper Kc values: adjust crop coefficients for local varieties and management. Mulch, residue, and canopy cover reduce soil evaporation and lower Kc during early growth.
• Adopt conservation practices: variable rate irrigation, deficit irrigation during tolerant stages, tailwater recovery for furrow systems, and alternate wetting and drying in rice can reduce seasonal water demand without sacrificing yield.
• Improve application efficiency: maintain equipment, use matched precipitation rates to soil infiltration rates, check nozzle uniformity on pivots, and minimize conveyance losses.
• Plan storage and pumping for peak seasonal demands: seasonal ET tells you total volume, but irrigation system capacity must handle peak daily or weekly demands during hot dry spells.

Measuring and monitoring ET and related data in Arkansas

Reliable inputs improve ET-based decisions. Practical monitoring tools include:

• on-farm weather stations to generate local ETo estimates;
• soil moisture sensors for direct measurement of soil water;
• extension and university ETo records or models when on-farm data are not available;
• satellite-based ET products for field-scale estimates when local weather data are sparse;
• regular field checks and a simple water accounting log that tallies rainfall, irrigation applied, and estimated ETc.

Combine datasets: use weather-based ET estimates for planning and soil measurements for timing.

Policy, water access, and sustainability implications

Seasonal ET and resulting irrigation requirements inform more than just on-farm decisions. Aggregated irrigation demand determines regional water withdrawal needs and affects aquifer levels and stream flows. For Arkansas, where irrigated agriculture is a major water user, improving water use efficiency based on accurate ET budgeting supports long-term agricultural viability and reduces conflicts over water allocation.
Incentives and programs that support better measurement (weather stations, soil sensors), infrastructure (tailwater recovery, efficient pivot systems), and training on ET-based scheduling will have high returns in water savings and sustained yields.

Key takeaways and action checklist

• Know your ETo: install or access a reliable local weather station to generate reference ET for your fields.
• Use stage-specific Kc values: do not rely solely on a single seasonal coefficient.
• Convert seasonal ETc to irrigation need by subtracting effective rainfall and dividing by field application efficiency.
• Size irrigations to refill the root zone based on soil PAW and chosen MAD; use soil moisture sensors to verify.
• Prioritize water during critical crop stages and consider deficit irrigation only where yield response is acceptable.
• Invest in efficiency: maintain equipment, reduce conveyance losses, and consider tailwater capture or AWD for rice.
• Keep a season-long water budget log to compare actual applied water to ET-based estimates and learn year to year.

Seasonal evapotranspiration is not an abstract research metric; it is the central planning number that translates weather, crop physiology, and soil into inches of water that must be managed. For Arkansas growers, applying the ET framework–local ETo measurement, crop coefficients, soil water accounting, and efficiency adjustments–turns seasonal ET into practical irrigation schedules, reduced risk, and better stewardship of water resources.