# Evapotranspiration in irrigation: how to use ETo and Kc to estimate how much to irrigate Source page: [https://perinirrigacao.com.br/en/blog/evapotranspiracao-na-irrigacao/](https://perinirrigacao.com.br/en/blog/evapotranspiracao-na-irrigacao/) Automatically translated from Portuguese. Our projects, service information and technical references relate to Brazil. Contact us to discuss your location and requirements. [Read the Portuguese original](https://perinirrigacao.com.br/blog/evapotranspiracao-na-irrigacao/) Planning Evapotranspiration helps to estimate crop consumption. See how to use ETo, Kc, rain and soil water to assess how much and when to irrigate. By Team Perin Irrigação Published in October 28, 2026 (2026-10-28) 9 min read ![Drip system in the Perin Irrigação collection](https://perinirrigacao.com.br/assets/perin/responsive/service-drip-640-055633cbf116.webp) Photograph from the Perin Irrigação collection. ## Before you start Understand evapotranspiration, ETo and Kc and see how the water balance helps to estimate the depth and irrigation time, with a hypothetical example. **Summary:** evapotranspiration helps estimate the water that crops lose to the atmosphere. To decide how much to irrigate, this consumption needs to be balanced with rain and water stored in the soil. Understand the function of ETo and Kc, the precautions with localized irrigation and the irrigation depth path to operating time, with a hypothetical example. ## What is evapotranspiration and why does it define how much to irrigate Evapotranspiration (ET) brings together the evaporation of water from the soil and the transpiration of plants. It helps monitor the water demand of the cultivated area, but it is not an order to turn on irrigation nor an isolated measure of the depth that must be applied. The soil stores water between one rain and another. Part of the rain can replenish this stock; another part may drain or drain beyond the roots. It is the balance of these inputs and outputs, monitored on the field, that guides the replacement. The necessary irrigation may be less than the ET of the period or even zero. The EPAMIG circular highlights two management decisions: **when** and **how much** irrigate. ET contributes to both, along with soil, root and system conditions. ## Reference evapotranspiration (ETo): what the climate demands A **reference evapotranspiration (ETo)** is an estimate of the atmospheric demand for a standardized surface. In the FAO methodology, the reference is a hypothetical grass 0.12 m high and without water limitation. ETo is not the measured consumption of the farm crop. The FAO Penman-Monteith method uses information on temperature, humidity, radiation and wind, with adjustments for location and height of measurements. The data quality and calculation time scale need to be compatible. In daily management, an isolated value or one from an unrepresentative source can lead to a bad decision. ### Where to get ETo in practice - **Own station:** can represent the area well when its installation, maintenance and data quality are adequate. Having the station does not eliminate checking sensors and calculations. - **INMET network:** offers observed weather data. The automatic station service reports hourly availability for the last 90 days. This data still needs verification, processing and ETo calculation; Proximity alone does not guarantee representation. - **Class A Tank:** allows estimating ETo based on tank evaporation and a Kp coefficient appropriate to installation and exposure conditions. - **Limited data:** there are methods and procedures for dealing with missing variables. The choice must consider validation for local conditions and uncertainty of the result. Before using a number on the controller, confirm the season, period, unit and method adopted. ## Crop coefficient (Kc): reference for your crop O **crop coefficient (Kc)** relates crop evapotranspiration to ETo. In the single coefficient approach and under standard conditions: **ETc = Kc × ETo** ETc is expressed, for example, in mm/day. Kc values ​​vary throughout development and need to correspond to the crop, soil cover and conditions considered in the reference used. This standard calculation assumes adequate agronomic and water availability conditions. Under water stress, estimating actual ET requires adjustments; the FAO treats this effect with the coefficient Ks. It should not be confused with a free discount to reduce irrigation. Tabled values ​​and models need to be evaluated by those who define management and checked on the ground. ## Drip and microsprinkler: adjustment for the wet area Drip and micro-spray wet part of the surface. The distribution of the canopy, the vegetation between the rows and the frequency of wetting also influence evaporation and transpiration. EPAMIG describes the use of a **reduction coefficient for localized irrigation (Kl)** in your approach. This does not mean multiplying any Kc by a fixed factor. Other procedures already incorporate coverage and wetness in the coefficients; applying an additional reduction may count the same effect twice. The project must identify the methodology, the origin of the coefficients and the validity conditions. The percentage of wetted area is not automatically the percentage of ET reduction. As the crop changes, the suitability of the model needs to be reviewed. ## From evapotranspiration to irrigation time The calculation only becomes management when considering the water balance and the application capacity of the system. **1. Net irrigation depth.** It is the replacement defined for the root zone. Its estimate considers ET, effective rainfall, storage and other relevant inputs and outputs. It is not enough to subtract the total rainfall from a daily ET number without evaluating what was available to the plants. **2. Gross irrigation depth.** It is the amount that the system must apply to achieve the planned replacement, according to the efficiency criteria adopted. Application efficiency and uniformity are different concepts. One [drip uniformity test](https://perinirrigacao.com.br/en/blog/teste-de-uniformidade-gotejamento/) evaluates the distribution of flows; it does not alone measure water losses or determine actual efficiency. The calculation must use a coherent methodology, without adding corrections that represent the same effect. **3. Irrigation interval and operating time.** The interval between irrigations depends on the water supply that can be consumed without exceeding the management limit. Soil, roots, demand and geometry of the wetted volume come into this assessment. The time results from the planned gross volume divided by the available flow, also checking infiltration, pressure and water distribution. ### Hypothetical example This example only shows the account sequence. None of the factors represent a recommendation for a crop or property: - One-day ETo: 5.0 mm; - Hypothetical Kc: 0.75; - hypothetical location factor: 0.60, in a methodology that still requires this adjustment; - reference area per plant: 6 m × 4 m = 24 m²; - balance hypothesis: fully replace the adjusted ET of that day, without effective rain or other contributions; - hypothetical application efficiency: 90%, as distinct from CUD or emission uniformity; - total flow assumed in operation: four 4 L/h emitters, totaling 16 L/h per plant. The accounts look like this: - ET estimated with the adopted adjustment = 5.0 × 0.75 × 0.60 = **2.25 mm/day**; - net volume = 2.25 × 24 = **54 L per plant per day**, as 1 mm over 1 m² is equivalent to 1 liter; - gross volume = 54 ÷ 0.90 = **60 L per plant per day**; - time = 60 ÷ 16 = **3.75h**, or **3 hr 45 min**. The factor 0.60 does not mean that 60% wetted area should generate this result. The 90% was also not measured in a real installation. If Kc already includes coverage adjustment, the additional factor may be inappropriate. Before operating, the timing needs to be compatible with soil, roots, flow rates and system limits. ## Where evapotranspiration management often fails - **Unrepresentative data:** use ETo without checking origin, unit, period and quality. - **Coefficient out of context:** copy Kc or apply Kl without checking the methodology. - **Incomplete balance:** ignore actual rainfall and water already stored in the soil. - **Presumed efficiency:** confuse good uniformity with absence of losses. - **Lack of conference:** keep calculating for weeks without observing your response in the field. Readings from [tensiometer or soil moisture sensor](https://perinirrigacao.com.br/en/blog/tensiometro-ou-sensor-de-umidade-do-solo/) help in this conference, as long as they are interpreted for the ground and the installation. The stage of crop also matters; see the specific context of [coffee irrigation in flowering](https://perinirrigacao.com.br/en/blog/irrigacao-do-cafe-na-florada/). ## FAQ ### What is evapotranspiration, in a nutshell? It is the water that leaves the cultivated area through evaporation and transpiration. It is included in the balance used to decide irrigation, along with rainfall and water available in the soil. ### What is the difference between ETo and ETc? ETo represents a reference surface. ETc represents the crop under the conditions defined by the model; in the standard single-coefficient approach, it is calculated by Kc × ETo. ### Can I manage irrigation with just the Class A tank? The tank can provide an estimate of ETo with suitable Kp. The decision also requires crop data, water balance and soil monitoring. Your reading alone does not provide the irrigation time. ### Do I need a weather station on the farm? Not always. External data can be useful when it represents conditions in the area. In projects of [automation and fertigation](https://perinirrigacao.com.br/en/automacao-fertirrigacao/), integration must include checking data and operational limits, without transforming ETo into an isolated automatic command. ## Conclusion Evapotranspiration helps justify the decision to irrigate. The result is more useful when the origin of the data, the coefficients and the balance are clear and checked in the field. In [localized irrigation](https://perinirrigacao.com.br/en/irrigacao-localizada/), it is also worth checking whether coverage and wetness have already been considered in the model. To review irrigation times for your area, staff at [Perin Irrigação](https://perinirrigacao.com.br/en/sistemas-de-irrigacao/) can help evaluate the case. Send municipality, crop, approximate area and water source via WhatsApp [(14) 99824-5219](https://wa.me/5514998245219) or by email contato@perinirrigacao.com.br. If there is, also gather the design and operation records. ## Sources consulted Original research on 10/28/2026, with technical review on this date. References support concepts; do not validate the hypothetical factors in the example for a real area. - [ALLEN, R. G.; PEREIRA, L. S.; RAES, D.; SMITH, M. *Crop evapotranspiration: guidelines for computing crop water requirements*. FAO Irrigation and Drainage Paper 56, 1998. Chapters 1, 2, 4, 5, 8 and 9.](https://www.fao.org/4/x0490e/x0490e00.htm) - [SIMÃO, F. R.; REIS, J. B. R. S.; OLIVEIRA, P.M. *Irrigation management: intelligent use of water during dry periods*. EPAMIG, Technical Circular 210, 2015.](https://livrariaepamig.com.br/wp-content/uploads/2023/03/CT-210.pdf) - [COELHO, E. F.; OLIVEIRA, A. S.; MAGALHÃES, A. F. J. *Irrigation and fertigation in citrus*. Embrapa, Technical Circular 38, 2000. Historical reference on management and balance in citrus.](https://www.infoteca.cnptia.embrapa.br/infoteca/bitstream/doc/639851/1/IrrigacaoefertilizacaoEugenioCoelhoCircularTecnica382000.pdf) - INMET. *Automatic stations*. Service consulted on 10/28/2026. [https://portal.inmet.gov.br/servicos/estações-automáticas](https://portal.inmet.gov.br/servicos/esta%C3%A7%C3%B5es-autom%C3%A1ticas). - [USDA/NRCS. *National Engineering Handbook*, Part 650, Chapter 15: Irrigation, 2021. Sections on application efficiency and uniformity.](https://directives.nrcs.usda.gov/sites/default/files2/1712930921/31803.pdf) ## From knowledge to your project. In agricultural production or green areas, the project starts with the conditions of your area and the water supply. Tell Perin what you need to irrigate. [Request quote](https://perinirrigacao.com.br/en/#contato)[Chat on WhatsApp](https://wa.me/5514998245219?text=Hello%21+I+want+to+talk+about+an+irrigation+project.)