Irrigation scheduling: when and how much to irrigate with ET0, Kc and ETc

An irrigation schedule answers two questions: when to irrigate and how much water to apply. The most widely accepted method today is the one described in FAO Irrigation and Drainage Paper 56: the evaporative demand of the air is expressed as reference evapotranspiration (ET0), the crop's own water use as the crop coefficient (Kc), and their product gives crop evapotranspiration (ETc) (Allen et al., 1998). In Türkiye, the Crop Water Consumption Guide published in 2017 by TAGEM and DSİ follows the same path: using 30 years of daily data from 259 stations of the Turkish State Meteorological Service, it gives ten-day ETc values for 85 crops (TAGEM and DSİ, 2017). This guide explains the logic of the calculation; for emitter layout and sensor choice, see the platform articles on drip irrigation of Çukurova citrus and on soil moisture sensors.

Step 1: Calculating ET0 from weather station data

ET0 is the daily water loss of a well-watered standard grass surface and depends only on the weather. The FAO Penman-Monteith equation needs daily maximum and minimum temperature, relative humidity, wind speed at 2 m, and solar radiation or sunshine hours; the latitude and altitude of the station also enter the calculation. Weather stations usually measure wind at 10 m; the logarithmic correction in FAO-56 converts it to 2 m, and the multiplier for 10 m is about 0.75. If humidity, radiation or wind data are missing, FAO recommends estimating them with the procedures in Chapter 3 of the paper and still using Penman-Monteith. The temperature-only Hargreaves equation is an alternative, but it should be checked against Penman-Monteith results in each new region (Allen et al., 1998).

The table below shows the steps for a hypothetical July day at the location of the Kozan weather station (37.44° N, 123 m). The steps follow the same order as the FAO-56 worked example with daily data (Uccle, Belgium, 6 July; result 3.9 mm/day).

For comparison, the TAGEM-DSİ guide gives a long-term ET0 of 56 mm for the first ten days of July at Kozan (about 5.6 mm per day) and an annual total of 1,236 mm (TAGEM and DSİ, 2017). A hot, clear day coming out above the average is an expected result.

Step 2: From Kc to ETc

FAO-56 gives Kc values for four growth stages (initial, development, mid-season, late season) in a sub-humid climate with minimum relative humidity of about 45% and wind of about 2 m/s. Kc is held constant in the initial and mid-season stages and changes linearly in between. The TAGEM-DSİ guide has adapted the coefficients to the climate and planting dates of each station:

In non-standard climates the mid-season Kc is adjusted as follows: Kc mid = Kc mid (table) + [0.04 (u2 - 2) - 0.004 (RHmin - 45)] (h/3)^0.3, where u2 is wind speed, RHmin minimum relative humidity and h crop height. Dry and windy weather raises Kc. FAO also notes that in humid and sub-humid climates citrus values can be increased by 0.1-0.2 because stomatal control is weaker. Stage lengths for cotton also vary by region: 30, 50, 60 and 55 days (195 in total) for March-May planting in Egypt, Pakistan and California, and 30, 50, 55 and 45 days for April planting in Texas (Allen et al., 1998).

Back to the example day: cotton ETc = 1.16 × 6.3 ≈ 7.3 mm/day and a mature citrus orchard 0.56 × 6.3 ≈ 3.5 mm/day. In the guide's long-term calculation for Kozan, seasonal cotton water use is 763 mm, with a peak ten-day value of 69 mm in the last eleven days of July (about 6.3 mm per day), and annual citrus water use is 741 mm (TAGEM and DSİ, 2017).

Step 3: When? Soil water balance and allowable depletion

Total available water in the root zone is TAW = 1000 × (θFC - θWP) × Zr and readily available water is RAW = p × TAW, where θFC and θWP are the volumetric water contents at field capacity and wilting point and Zr is the rooting depth in metres. p is the fraction the crop can use without stress, the threshold known in practice as management allowable depletion (MAD): FAO-56 gives 0.65 for cotton and 0.50 for citrus, adjusted as p = p (table) + 0.04 × (5 - ETc) when daily ETc differs from 5 mm. Root zone depletion (Dr) is updated daily: today's depletion = yesterday's depletion - (rain - runoff) - irrigation - capillary rise + ETc + deep percolation. The rule is simple: irrigate before or when depletion reaches RAW, and keep the net depth no larger than the depletion to avoid deep percolation (Allen et al., 1998).

On hot days p gets smaller: the crop comes under stress while the soil is still wetter. With drip irrigation the same rule is applied with more frequent, smaller doses. The TAGEM-DSİ guide recommends correcting drip water use for the shaded area percentage: T = ETc × Ps / 85, with Ps taken as 80% for field crops and vegetables, 75% for vineyards and orchards with rows narrower than 4 m, and 70% for wider orchards. If the cotton in the example is drip irrigated, T ≈ 7.3 × 80 / 85 ≈ 6.9 mm/day; with 90% application and 98% conveyance efficiency for surface drip, the gross requirement is about 7.8 mm per day. When calculating net requirement, the guide counts 80% of rainfall as effective (TAGEM and DSİ, 2017).

Practical checklist

1. Get daily data from the nearest agrometeorological station; if wind is measured at 10 m, convert it to 2 m.
2. For Kc, first look up the nearest station in the TAGEM-DSİ guide; otherwise use the FAO-56 value with the climate adjustment.
3. Calculate TAW and RAW once from the texture and rooting depth of the plot.
4. Update the depletion every day, irrigate as it approaches RAW, and limit the net dose to the depletion.
5. Check the calculation against soil moisture readings; if the gap persists, revisit the Kc or rooting depth assumption.

Sources

1. FAO, 1998. Allen, R. G., Pereira, L. S., Raes, D., Smith, M.: Crop evapotranspiration, FAO Irrigation and Drainage Paper 56, Chapter 3: Meteorological data (accessed 29 September 2026)
2. FAO, 1998. Allen, R. G., Pereira, L. S., Raes, D., Smith, M.: Crop evapotranspiration, FAO Irrigation and Drainage Paper 56, Chapter 4: Determination of ETo (accessed 29 September 2026)
3. FAO, 1998. Allen, R. G., Pereira, L. S., Raes, D., Smith, M.: Crop evapotranspiration, FAO Irrigation and Drainage Paper 56, Chapter 6: ETc, single crop coefficient (Tables 11 and 12) (accessed 29 September 2026)
4. FAO, 1998. Allen, R. G., Pereira, L. S., Raes, D., Smith, M.: Crop evapotranspiration, FAO Irrigation and Drainage Paper 56, Chapter 7: ETc, dual crop coefficient (Table 19, soil water characteristics) (accessed 29 September 2026)
5. FAO, 1998. Allen, R. G., Pereira, L. S., Raes, D., Smith, M.: Crop evapotranspiration, FAO Irrigation and Drainage Paper 56, Chapter 8: ETc under soil water stress conditions (Table 22) (accessed 29 September 2026)
6. T.C. Tarım ve Orman Bakanlığı TAGEM ve DSİ Genel Müdürlüğü, 2017. Türkiye'de Sulanan Bitkilerin Bitki Su Tüketimleri (Kozan istasyonu ETo, Kc ve ETc tabloları) (accessed 29 September 2026)