Assessment of Onion Evapotranspiration and Crop Coefficients in Water-Limited Environments

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Agriculture in semi-arid regions is under increasing pressure due to climate change, population growth, and competing demands for water.

In Ethiopia’s Melkasa region, onion farming plays a vital role in local livelihoods, providing food security and income for many families.

However, inefficient irrigation practices and water scarcity have made it challenging for farmers to sustain productivity.

To address this issue, a groundbreaking study published in Heliyon (2025) focused on quantifying the water needs of the Nafis onion variety, a fast-maturing and high-yield crop widely grown in the region.

The Importance of Water-Efficient Agriculture

Ethiopia’s semi-arid regions, such as Melkasa, experience hot temperatures ranging from 14°C to 29°C and receive an average annual rainfall of only 845.5 mm.

These conditions make water a precious resource, especially for onion farming, which is a key agricultural activity in the area. Unfortunately, farmers often lack accurate, localized data on crop-specific water requirements, leading to inefficient irrigation practices.

Overwatering or underwatering can harm crop yields and waste valuable resources.

The study, conducted by researchers at Adama Science and Technology University, aimed to address this gap by measuring the water needs of Nafis onions. Specifically, the researchers focused on two key metrics:

evapotranspiration (ETc), which represents the total water lost through soil evaporation and plant transpiration, and the crop coefficient (Kc), which compares crop water use to reference evapotranspiration (ETo).

By developing a predictive model for irrigation scheduling, the study provides farmers with practical tools to optimize water use and improve yields.

How the Study Was Conducted

The research was carried out over two growing seasons (2023 and 2024) at the Melkasa Agricultural Research Centre (MARC).

The team used weighable lysimeters, specialized devices that measure water loss from crops by weighing the soil and plant system.

These lysimeters were filled with clay loam soil, which has a bulk density of 1.2 g/cm³ and moisture levels of 26.25% at the wilting point and 38.88% at field capacity.

To mimic real farming conditions, onions were also planted in a 10-meter by 6-meter field surrounding the lysimeters.

The Nafis onion seedlings were transplanted onto ridges spaced 60 cm apart, with 20 cm between each plant. Fertilizers were applied at rates of 100 kg/ha urea and 200 kg/ha DAP to support growth.

The onions were monitored through four growth stages: initial (15 days), developmental (30 days), mid-season (40 days), and late-season (25 days). Irrigation was applied whenever soil moisture depletion reached 30% of field capacity, ensuring optimal growing conditions.

To calculate evapotranspiration (ETc), the researchers used a water balance equation that accounted for irrigation, rainfall, drainage, and changes in soil moisture.

Reference evapotranspiration (ETo) was estimated using the CROPWAT 8.0 model, which relies on daily weather data such as temperature, humidity, wind speed, and solar radiation. The crop coefficient (Kc) was then derived by dividing ETc by ETo.

Key Findings: Water Requirements and Crop Coefficients

The study revealed that Nafis onions require a total of 460.27 mm of water per growing season, which lasts 110 days. Water usage varied significantly across growth stages.

During the initial stage, daily water use averaged 3.12 mm, increasing to 4.19 mm in the developmental stage and peaking at 4.81 mm during the mid-season stage when bulb formation occurs.

In the late-season stage, water use declined to 3.84 mm per day as the plants matured. The crop coefficient (Kc) also showed clear trends across growth stages.

It started at 0.68 during the initial stage, rose to 0.89 in the developmental stage, peaked at 1.03 in the mid-season stage, and declined to 0.86 in the late-season stage.

These values align closely with global benchmarks, such as the FAO-56 guidelines, but are tailored to the specific conditions of Melkasa.

Why These Findings Matter

The study’s results have significant implications for farmers, policymakers, and researchers. For farmers, the data provides a clear roadmap for precision irrigation scheduling.

By adjusting water application based on growth-stage-specific Kc values, farmers can reduce water use by 10–15%, conserving scarce resources while maintaining or even improving yields.

For example, during the initial stage, irrigation can be reduced to around 3.12 mm per day, while the mid-season stage requires up to 4.81 mm per day to support bulb formation.

For policymakers, the findings highlight the importance of investing in localized agricultural research and farmer education.

By equipping farmers with tools like the polynomial equation, governments can promote sustainable water use and enhance food security in water-scarce regions.

Researchers can also build on this work by scaling up lysimeter technology to study other crops, such as maize or sugarcane, and integrating remote sensing data to create regional evapotranspiration maps.

These advancements could further improve irrigation efficiency and support climate adaptation efforts.

Comparison with Global Studies

The study compared its Kc values with those from previous research conducted in different regions. For instance, the initial-stage Kc of 0.68 in Melkasa was higher than the 0.40 reported in Egypt, likely due to Ethiopia’s warmer climate and faster crop establishment.

Similarly, the mid-season Kc of 1.03 closely matched the FAO-56 recommendation of 1.05, validating the study’s methodology.

These comparisons underscore the importance of developing site-specific crop coefficients. While global guidelines provide a useful starting point, local factors such as soil type, climate, and farming practices can significantly influence water requirements.

Practical Recommendations for Farmers

Farmers in semi-arid regions like Melkasa can benefit greatly from adopting the study’s recommendations. Here are some practical steps:

Monitor Soil Moisture: Regularly check soil moisture levels to determine when irrigation is needed. This can be done using simple tools or by observing plant health.

Adjust Irrigation by Growth Stage: Use the Kc values to tailor water application. For example, reduce irrigation during the initial stage and increase it during the mid-season stage.

Adopt Water-Saving Techniques: Techniques like drip irrigation or mulching can further enhance water efficiency.

Use Predictive Models: The polynomial equation can help farmers plan irrigation schedules without relying on complex equipment.

Conclusion

This study equips Ethiopian farmers with vital data for sustainable onion farming: Nafis onions require 460.27 mm of water per season, with crop coefficients (0.68–1.03) guiding precise irrigation.

The polynomial model aids climate-resilient planning. Policymakers must prioritize localized research and farmer training to combat water scarcity. Collaboration among stakeholders can turn challenges into innovative solutions for arid-region agriculture.