The Food and Agriculture Organization of the United Nations (FAO) have launched the first Remote Sensing (RS) Determination of Evapotranspiration (ET) webinar series that aims to update the participant’s knowledge regarding RS ET determination, the adopted algorithms and approaches, and the latest advances. As well as increase the capacity of key water professionals in the assessment of RS ET uncertainty and their awareness on the strengths and limitations for the range of RS ET models and databases available.

In agricultural, and other vegetated lands, Evapotranspiration (ET) nearly always represents the largest share of water outflow, identified as the ‘consumptive’ fraction of water used within the system. Several methods for ET determination have been developed over time, generally at field scales, that include: soil-moisture depletion methods; weighing lysimeters; the Penman Monteith equation; the Bowen-ratio/energy balance; the eddy covariance; and the Large Aperture Scintillometer. However, these methods are generally complex, expensive, focused on one field, and are therefore mostly confined to academic and research environments. Scaling them up to larger areas becomes prohibitive due to cost. Thanks to the progress and advances in availability of moderate resolution earth observation over recent years and evolution of sophisticated algorithms, determination of the spatial distribution of ET is now possible.

In fact, there are several well-established RS-based algorithms for the determination of ET, of course, each method has specific advantages and limitations, spatial and temporal resolutions, strengths and weaknesses, uncertainties and best-fitting domains of application. Confronted with this quite ample and diversified offer of RS ET data, users ask different questions including the method of conducting a qualitative error analysis of RS-ET determinations for the various algorithms/models; the level of uncertainty of the ET data provided by the various sources and how to deal with it; testing RS ET with field ET measurements; and which ET data sources best fit one’s purpose. This said, FAO in consultations with various countries’ stakeholders in the Region, partners, and major experts in this domain, has elaborated a plan of webinars addressing these various questions.

The webinar series will come in four modules, where the main objectives are multiple, including:

1. updating and upgrading the knowledge regarding RS ET determination, the most common and adopted algorithms and approaches and the latest advances;
2. increasing the capacity of key water professionals in the assessment of RS ET uncertainty and related acceptable limits for their field of application;
3. building awareness on the strengths, limitations and fits-for-purpose for the range of RS ET models and databases available;
4. providing the participants with a complete and advanced set of documentation on the various topics treated during the webinar series.

The webinar series started with the first session under Module one “Setting the Scene” which provided a background on the rationale, objectives, speakers, audience, format and expectations of the webinars’ series, followed by a brief review of concepts and basic algorithms of RS ET determinations, raising  doubts, questions to and uncertainties. In addition to that, a summary of the various satellites and sensors used for ET determination, along with their spectral, spatial and temporal resolutions have also been discussed in the first webinar. The session took place on 24 March 2021 with over 230 participants from all over the world, including key water professionals, ministry representatives and project stakeholders.

This webinar series is implemented within the framework of the Water Scarcity Initiative of the FAO Regional Office for Near East and North Africa, and with the support of the regional project ‘Implementing the water efficiency/productivity and water sustainability in NENA countries’, funded by the Swedish International Development and Cooperation Agency.

For more information on the webinar series and to register to upcoming sessions under this series, please visit the Remote Sensing Determination of Evapotranspiration webinar series page here.