Evaluation of Crop Evapotranspiration Using Satellite Remote Sensing: The First Study in Thailand Advancing Water Balance Objective through Geo-informatics Technology.

Evaluation of Crop Evapotranspiration Using Satellite Remote Sensing:

The First Study in Thailand Advancing  Water Balance Objective through Geo-informatics Technology.

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The first in-depth research and development of a model for evaluating plant evapotranspiration using satellite technology to efficiently expand water utilization of economic crops using data from the Thai context.

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Evapotranspiration (ET) is a significant variable for evaluating plant water use; it is the main indicator of the amount of water used by plants per day.

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In the past, ET measurement in Thailand had limitations, making conventional measurement difficult, and two main methods were used. The first method is to directly measure in pilot cultivation using specialized measurement tools for calculating the crop coefficient (KC), which is the ratio between crop evapotranspiration (ETc) and reference crop evapotranspiration (ETo) and reference crop water use. However, a limitation is that measurements are taken only at a single point and over limited areas.

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The second method is calculated from meteorological data using weather variables such as temperature, humidity, amount of rainfall, and sunlight, using the standard formula of the Food and Agriculture Organization of the United Nations (FAO) to calculate reference crop evapotranspiration (ETo). It is then multiplied by the crop coefficient (KC) to determine actual evapotranspiration.

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From the limitations mentioned above, the Geo-Informatic and Space Technology Development Agency (Public Organization), or GISTDA, conducts in-depth research on the model for evaluating evapotranspiration using space technology as part of improving the efficiency of evapotranspiration of agricultural crops using space technology and geo-informatics.  GISTDA collaborates with the Office of the National Water Resources (ONWR) and the Royal Irrigation Department (RID), Ministry of Agriculture and Cooperatives.

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The Disaster Management Division, GISTDA, explained that improving the efficiency of evapotranspiration of agricultural crops, especially during drought, must be managed as efficiently as possible by using water appropriately, saving water, and using it with the most value. Therefore, it is necessary to know the actual water requirements of crops during cultivation to avoid using more than necessary.

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In 2025, GISTDA began to apply space technology to evaluating crop evapotranspiration for the first time in Thailand.

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However, the main development of the model for evaluating crop evapotranspiration is the transfer of satellite data for actual use. The system will evaluate evapotranspiration (ET) using satellite imagery from the MODIS, Sentinel-2/3, and Landsat-8/9 satellites, merged with meteorological data and calculated using two standard methods that are accepted as global standards.

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The first method is the Penman–Monteith equation, which is a global standard for calculating reference evapotranspiration (ETo) from weather data. The second method is the SEBAL model (Surface Energy Balance Algorithm for Land) for evaluating actual evapotranspiration from the surface energy balance directly from satellite imagery, suitable for analyzing large areas at the river basin level. Both methods are combined with dynamic Kc based on NDVI/LAI for evaluating actual evapotranspiration (ETa) daily at a spatial resolution of 500 meters, as well as evaluating Water Use Efficiency (WUE).

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The Disaster Management Division, GISTDA, explained that satellite technology can overcome previous limitations, although international satellites had previously been used to evaluate ET. However, most evaluations have been conducted at the continental or overall scale and do not have sufficient resolution for agricultural cultivation. The research team will design a system using data from four satellites together to increase data frequency and coverage. For example, the MODIS satellite system can detect land surface temperature, as temperature is the main factor for calculating ET. Landsat-8/9 and Sentinel-2/3 satellites will support the measurement of other factors, especially Sentinel-3, which plays a significant role in measuring surface energy. This satellite orbit passes over  Thailand every day, providing continuous data.

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Furthermore, during the first phase of the pilot project in 2025, the model was used in in-depth research on two main economic crops, representing short-duration crops and a fruit tree: rice and durian, respectively, to identify significant data and factors for efficient water management.

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Rice is a short-duration economic crop with a growth period of approximately 110–120 days. Satellite data show that the water requirements of rice vary across different growth stages. During the first two weeks after sowing, rice requires only sufficient soil moisture for seed germination. Excessive water can cause the plants to rot. However, once tillering begins, water demand increases to the point where standing water is required. Conversely, during the booting and heading stages, the crop does not require standing water, allowing for "alternate wetting and drying" management, and the field must be allowed to dry out completely before harvest. This information enables the Royal Irrigation Department to adjust water distribution plans to align with the crop's actual growth stages, rather than maintaining continuous flooding throughout the season.

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Durian is a high-value fruit. Studies found that most durian farmers water their trees every day because they are concerned about damage. However, analysis of data from the past 20 years provides a different perspective. The practice known as water withholding (or Kak Sok) involves a period of approximately two months prior to flowering, during which farmers cease irrigation to induce stress in the durian trees, thereby triggering them to bloom. Satellite data clearly reflect the resulting decrease in evapotranspiration during this phase. Once the withholding period ends and flowering begins, irrigation is resumed at normal levels to support fruit development.

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These data contribute to establishing a baseline for water use for different types of crops at different times, in order to identify the minimum water requirement that does not cause plant damage and the maximum amount that prevents water from overflowing and being wasted. If water management is carried out based on this baseline, it can be ensured that crops will grow efficiently while maximizing water savings.

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However, the reliability of the satellite data has been validated against actual measurements. The research team calibrated the ET values derived from the MODIS satellite system using data from an Eddy Covariance flux monitoring station located in a rice field in Chai Nat Province, covering the entire cropping period and all crop growth stages. The calibration results indicated that the satellite-derived values aligned well with ground-based data, showing a low root mean square error (RMSE) of approximately 0.56 millimeters per day.

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Calibration using ground-based monitoring equipment has confirmed the potential of satellite data to monitor crop water use at the field level and has improved accuracy prior to nationwide implementation. The system currently covers eight crops — rice, maize, cassava, sugarcane, mango, coconut, durian, and longan — and has undergone real-world testing in pilot areas both within irrigation zones (e.g., Rayong, Yala, and Narathiwat) and outside them (e.g., Surat Thani and Chumphon), aiming to reduce disparities in water allocation between the two types of areas.

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This knowledge has been developed into a spatial platform called Crops Drought, or Water Monitoring to Support Crops, for supporting crops and evaluating drought in agricultural areas at the cultivation level. It can be accessed at https://cropsdrought.gistda.or.th and through the Check Drought mobile application, available on both Android and iOS.

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Furthermore, the research teams from GISTDA, ONWR, and the Ministry of Agriculture and Cooperatives plan to further develop this knowledge through in-depth research to develop a model for crop water evapotranspiration for both types of crops, adapted to the Thai context. The teams also plan to obtain patents and intellectual property rights to demonstrate the capabilities of the Thai research teams in utilizing satellite technology for drought analysis and assessment that meets international standards.

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However, the success in evaluating evapotranspiration using satellite technology has addressed the water demand dimension only. The next step is a greater challenge: collaboration with ONWR and the Royal Irrigation Department to obtain data for processing water supply management, such as water from reservoirs, dams, and natural water sources.

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The goal of the research team is to match demand and supply data from geoinformatics technology to establish a water balance system for Thailand.

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