When to Start and Stop Irrigation in Kansas Growing Seasons

Kansas spans a wide gradient of climate, soils, and cropping systems. Deciding when to start and stop irrigation depends on region, crop, soil water holding capacity, current weather and forecast, and long-term water supply considerations. This article synthesizes principles and provides concrete, actionable guidance for Kansas producers and managers so irrigation timing maximizes yield, conserves water, and reduces risk.

Kansas climate and soil context

Kansas climate ranges from humid continental in the east to semi-arid in the west. Annual precipitation can exceed 35 inches in the far east but commonly falls to 15 inches or less in western counties. Growing season rainfall distribution is highly variable year to year, and June through August are the months of highest crop water demand.
Soils influence how frequently and how much you must irrigate. Typical root zone effective water holding capacity (plant available water, PAW) varies by texture:

• Sandy soils: PAW roughly 0.06 to 0.10 inches of water per inch of soil depth.
• Loams: PAW roughly 0.12 to 0.18 inches/inch.
• Clay soils: PAW roughly 0.15 to 0.20 inches/inch.

Rooting depth differs by crop and management: corn commonly explores 36 inches or more, soybeans 24-36 inches, winter wheat 18-30 inches. Effective water available to the crop is PAW times root zone depth.

Regional differences and practical consequences

Western Kansas: limited rainfall and more evaporative demand mean irrigation often begins earlier in the season and is the dominant source of crop water. Start planning irrigation at planting or soon after if soil moisture is shallow.
Central Kansas: variable — some seasons natural rainfall meets early needs while mid-season irrigations are needed during hot dry periods.
Eastern Kansas: more rain, fewer irrigations in average years. Irrigation often is used to supplement during extended dry spells or to protect sensitive reproductive stages.

Principles of irrigation timing

Irrigation timing is best driven by two complementary approaches: soil moisture status and crop water demand (ET). Use both together for robust decisions.

Soil moisture thresholds and allowable depletion

Irrigate to refill the root zone before crop stress causes irreversible yield loss. The most common operational rule is to apply water when root-zone depletion reaches a crop- and soil-specific allowable depletion fraction of total available water (TAW).
Key guidance:

• Determine TAW = PAW (inches/inch) x effective root zone depth (inches).
• Choose an allowable depletion fraction (AD). Typical choices:
• High-value or very sensitive crops (vegetables, turf, seedlings): AD = 0.30 to 0.35 (refill earlier).
• Many annual field crops (corn, soybean) under normal conditions: AD = 0.40 to 0.50.
• Drought-tolerant crops or deficit strategies (grain sorghum, managed deficit irrigation): AD = 0.50 to 0.60 or more, recognizing yield tradeoffs.

Example: Corn with 36 in root zone and loam PAW 0.14 in/in -> TAW = 36 x 0.14 = 5.0 inches. If AD = 0.50, irrigate when 2.5 inches are depleted; refill to near field capacity (apply ~2.5 inches plus some allowance for system inefficiencies).
During critical reproductive periods (corn tassel-silk, soybean R1-R4, wheat heading and grain fill), reduce AD (refill earlier) because yield response per unit water is high.

Using ET and weather forecasts

Reference evapotranspiration (ETo) combined with crop coefficients (Kc) gives crop evapotranspiration (ETc = ETo x Kc). In Kansas, mid-summer daily ETc often ranges from roughly 0.15 to 0.35 inches per day depending on region and heat. Use local ETo estimates, adjust for effective rainfall, and schedule irrigations to replace the accumulated ET losses up to the allowable depletion.
Practical tips:

• Track 7- to 10-day cumulative ETc minus effective rainfall to estimate depletion since last refill.
• Short-term forecasts (hot, windy spells) should trigger earlier irrigations to avoid acute stress.
• Avoid applying the full seasonal deficit in a single irrigation if it would exceed system capacity or cause runoff; instead split water into close events that refill root zone without losses.

Crop-specific timing guidance for Kansas

Different crops have different critical periods and rooting behavior. Below are practical, field-ready timing rules.

Corn (grain)

• Start: In humid areas, you may delay irrigation until V3-V6 if soil moisture after planting meets germination and early root expansion needs. In central to western Kansas, begin monitoring early and plan the first irrigation before rapid vegetative growth if soil depletion approaches 40-50% of TAW.
• Critical window: Tassel to early grain fill (R1 to R3) is most sensitive. Maintain soil moisture at higher levels (AD 30-40%) during pollination.
• Stop: Irrigation can be reduced or stopped once kernels reach physiological maturity (black layer) or when grain moisture management for harvest requires drying. Practically, many producers stop after dent stage if grain moisture and harvest logistics permit.

Soybean

• Start: Soybeans are relatively tolerant early, but emergence and stand establishment require moist seedbeds. In dry seedbeds in western Kansas, an early refill at planting or quickly after emergence prevents stand loss.
• Critical window: R1 (beginning bloom) through R5 (beginning seed). Maintain AD at about 35-45% during these stages to protect pod set and seed size.
• Stop: After R6-R7 when seed filling is substantially complete, irrigation value declines.

Grain Sorghum (milo)

• Start: Sorghum is drought tolerant early; you can use a more conservative schedule early season and delay first major irrigation. However, ensure adequate moisture for germination.
• Critical window: Flowering and early grain fill. Maintain adequate moisture then; AD can be higher (50% or more) early, reduced to 35-45% around heading.
• Stop: After hard dough stage, yield response to additional water decreases.

Winter Wheat

• Start: Fall seeding often benefits from a soil moisture refill at planting in dry western Kansas. In spring, irrigation begins at green-up or jointing if soil water is low.
• Critical window: Jointing through heading and grain fill. Keep soil moisture adequate during stem elongation and heading.
• Stop: After grain fill and when grain reaches hard dough/maturity.

Alfalfa and forage crops

• Start: Alfalfa should not suffer water stress between cuts. Irrigate to prevent yield reduction and to maintain stand health; AD is commonly 30-40%.
• Cut management: Stop irrigating a few days to a week before harvest for leaf drying on hay; that window depends on weather and cut timing.

Vegetables and high-value horticulture

• Start: Irrigate frequently and shallowly for shallow roots; AD typically 20-30% and intervals may be days rather than weeks.
• Stop: Cease irrigation prior to harvest if it impedes drying or increases disease risk (typically adjust by crop).

Scheduling and practical management

Tools and monitoring

• Soil moisture sensors: capacitance probes, TDR, or gypsum blocks give real-time depletion estimates. Place sensors at representative depths (surface, mid-root zone, lower root zone).
• Tensiometers: work well in finer-textured soils for tension-based scheduling.
• Weather/ET-based systems: provide predictive scheduling and irrigation depth recommendations when calibrated to local conditions.
• Simple checks: probe with a soil auger to observe moisture, pinch test for topsoil, and crop visual stress observations.

Irrigation depth and frequency

• Application depth should match refill required, root zone size, and system efficiency. Typical center pivot sets deliver 0.25 to 1.5 inches per event depending on run time; many operators target 0.5 to 1.0 inches per pass to refill moderate depletions while maintaining uniformity.
• For drip or subsurface systems, frequent small applications are efficient and avoid deep percolation losses.
• Avoid irrigation during the windiest part of the day to reduce evaporation losses and improve uniformity. Night or early morning application can reduce evaporation.

Conservation, risk management, and the Ogallala Aquifer

Water resource limits define long-term viability in western Kansas. Consider:

• Managed deficit irrigation: purposely allow greater depletion during less-sensitive stages and concentrate water during reproductive stages.
• Crop selection and hybrids: choose drought-tolerant hybrids and earlier maturities in chronically short-water years.
• Soil health practices: no-till, residue cover, and organic matter increase infiltration and water holding, reducing irrigation frequency.
• Monitoring and governance: know local water rights, irrigation district rules, and well capacity constraints.

Practical takeaways and a simple decision checklist

• Know your soil PAW and effective root zone depth; compute TAW and set your allowable depletion target based on crop sensitivity.
• Start irrigation early in western Kansas and wherever seedbed moisture is limiting germination. In eastern Kansas, delay until depletion approaches your AD unless weather forecasts predict extended dry periods.
• Protect reproductive periods: reduce allowable depletion and maintain higher moisture during flowering and grain fill for all major crops.
• Use soil sensors plus ET tracking for best results; do not rely on calendar dates alone.
• Typical operational numbers: AD 30-35% for vegetables and high-value crops, 35-45% for corn and soybean around critical periods, 45-60% for drought-tolerant crops or managed deficit strategies.
• Apply irrigation depths that refill the root zone to near field capacity, accounting for system inefficiency and expected rainfall; split large deficits into multiple applications if needed.
• Stop irrigating when crop is physiologically mature or no longer responds economically to water; for corn that is often near black layer, for soybean after R6-R7, and for sorghum after hard dough.
• Prioritize conservation in areas dependent on the Ogallala Aquifer: consider deficit irrigation, rotation to less water-intensive crops, and practices that increase water retention.

Closing summary

Irrigation timing in Kansas requires a balance of soil moisture monitoring, crop growth stage awareness, and regional climate realities. Start irrigation when depletion of plant available water reaches a crop-specific allowable level, and stop when additional water no longer increases yield or when physiological maturity is reached. Protect critical reproductive windows by maintaining higher soil moisture. Combining soil sensors, ET tracking, and practical field checks will produce the best results: higher yields with smarter, more sustainable water use.