Editors’ Vox is a blog from AGU’s Publications Department.
Evapotranspiration is a scientific measurement representing the combined sum of evaporation from the soil (or water) surface to the atmosphere and transpiration from plants, where liquid water inside the plant tissue vaporizes and enters the atmosphere, predominately through stomata. This topic cuts across many disciplines and is important to understand as crops are subjected to increasing environmental stress and management practices.
A new article in Reviews of Geophysics explores the effects of changing environments, abiotic stresses, and management practices on cropland evapotranspiration. Here, we asked the lead author to give an overview of evapotranspiration, how scientists measure it, and what questions remain.
Why is it important to study cropland evapotranspiration?
As a key component of water balance in agricultural systems, evapotranspiration represents the ultimate consumption of agricultural water resources.
Evapotranspiration (ETa) is intricately linked to crop physiological activities and closely coupled with carbon cycle processes. As a key component of water balance in agricultural systems, evapotranspiration represents the ultimate consumption of agricultural water resources. Moreover, variation of regional cropland evapotranspiration reflects the changes of the regional agro–ecological environment. The varying vegetation cover and irrigation methods in cropland will lead to differences in mass and energy exchanges between the surface and the atmosphere, which in turn further affect the local climate and atmospheric circulation. Therefore, accurate evapotranspiration information is important for the development of irrigation systems, establishment of crop planting zones, implementation of regional water–saving agriculture practices, efficient assessment of water resources, and effective development, management, and allocation of water resources, among others.
What sets your review paper apart from previous reviews on this subject?
Given the significance of evapotranspiration, there are numerous reviews covering this subject. The varying perspectives concerning evapotranspiration have been recently reviewed, such as the role of evapotranspiration in the global, terrestrial, and local water cycles; the modeling, climatology, and climatic variability of global terrestrial evapotranspiration; best practices for measuring evapotranspiration; evapotranspiration partitioning methods; land-scale evapotranspiration from a boundary-layer meteorology perspective; spatiotemporal patterns of global evapotranspiration variations and their relations with vegetation greening. However, there is a gap in covering issues related to cropland evapotranspiration, which exhibits high variability due to its fast response to numerous factors.
There is a need to re-examine the primary factors influencing cropland evapotranspiration given the proliferation of long-term manipulation experiments, advancements in estimation models, and exponential growth in new and improved measuring methods at multiple spatial and temporal scales. In our new review, the focus is on factors encompassing key changing environments, abiotic stresses, and management practices that impact cropland evapotranspiration, along with their quantification methods.
What different methods are used for measuring evapotranspiration?
Evapotranspiration can be measured by using several methods such as plant physiology, hydrological, micro-meteorological, and remote sensing methods for different spatial and temporal scales. The leaf and plant scale transpiration can be measured by (potometer) portable photosynthesis system and sap flow method, respectively. The plot and field scale evapotranspiration can be determined by water balance, weighting lysimeter, sap flow plus micro-lysimeter, Bowen-ratio energy balance, eddy covariance, residual in the energy balance, surface renewal, and (microwave) scintillometer method. For regional scale evapotranspiration, remote sensing energy balance and remote sensing using vegetation indices are common methods.
What factors do scientists consider when deciding to use one method over another?
When selecting methods to measure evapotranspiration, scientists prioritize a balance between spatial-temporal requirements, accuracy, and practicality. The choice often hinges on the scale of study: small-scale methods, such as weighting lysimeters or eddy covariance methods, provide high-resolution field-level data but lack regional coverage, whereas satellite-based remote sensing methods offer broader spatial insights at the cost of finer temporal or spatial resolution. Accuracy demands must also align with resource constraints: high-precision tools, like weight lysimeters and eddy covariance, require high financial investment, technical expertise, and maintenance, while low-cost methods, such as the water balance method, introduce large error. Environmental context further guides decisions, such as, uniform vegetation may favor Bowen-ratio energy balance and eddy covariance systems.
What are the main factors that affect cropland evapotranspiration?
Cropland evapotranspiration is affected by the meteorological conditions (e.g. radiation, air temperature, relative humidity, wind speed), changing environments (e.g. elevated carbon dioxide concentration (e[CO2]), elevated ozone concentration (e[O3]), global warming), various abiotic stresses (e.g. water, salinity, heat stresses, waterlogging), management practices (e.g. planting density, mulching, irrigation method, fertilizer application, control of diseases and pests, soil management), underlaying surface (e.g. geography, soil types), and crop–specific factors (e.g. crop type, variety, and development stages). The effect of meteorological conditions on evapotranspiration can be surrogated to a reference evapotranspiration. Therefore, in this review, the focus is on the impacts of key changing environments (e[CO2], e[O3], and global warming), abiotic stresses (water, salinity, and heat), and management practices (planting density, mulching, irrigation method, and nitrogen application) on cropland evapotranspiration.
What major conclusions have been drawn about these factors?
There is general agreement that e[O3], water and salinity stresses, and adopting drip irrigation all lead to lower total growing–season evapotranspiration for almost all crops. However, total growing–season evapotranspiration in response to e[CO2], warming, heat stress, planting density, and nitrogen application were inconsistent across studies.
The impacts of e[CO2] and e[O3], water and salinity stresses on total growing-season evapotranspiration are mainly through stomatal conductance, the ability of soil to conduct water to roots, development of roots and leaf area, microclimate, and possibly phenology. The effect of warming on total growing–season evapotranspiration can be largely explained by both variations in ambient growing–season mean temperature and growing duration. Total growing-season evapotranspiration in response to heat stress (or mulching and appropriate nitrogen supplement) is a compromise between reduced (or enhanced) transpiration and increased (or decreased) evaporation, along with possibly a shortened growth period. Differences in evapotranspiration under varying planting densities can be explained by the direct and indirect effects of leaf area on the constitutive terms of evapotranspiration. The variation of total growing–season evapotranspiration under drip irrigation compared to conventional irrigation was affected by smaller soil wetting area, shortened growing season, less energy partitioning to evapotranspiration, and changes in crop characteristics and microclimate.
What are some of the remaining questions where additional modeling, data, or research efforts are needed?
- The influence of elevated ozone concentration on stomatal conductance can be represented using an adjusted version of the Jarvis function. However, there has been little effort to integrate this response into the Penman-Monteith model, which is used to estimate evapotranspiration.
- Many controlled manipulation experiments are underreported varying types of warming on crop evapotranspiration. Water balance method, the residual in the energy balance method, sap flow plus micro–lysimeters, or even weighting lysimeters can be used to observe cropland evapotranspiration under several warming scenarios.
- There are few studies on evapotranspiration responses to heat stress, and most are based on pot experiments in phytotrons or artificial climate chambers. Obtaining larger–scale data of evapotranspiration under heat stress is beneficial to understand heat stresses on evapotranspiration.
- Models for describing effects of elevated CO2 and ozone concentration on evapotranspiration using a modified Priestley–Taylor and crop coefficient models are rarely reported. More efforts are needed to develop and test these two models.
- In practice, cropland evapotranspiration is jointly affected by multiple factors. The impact of multiple factors on cropland evapotranspiration is a complex and multifaceted phenomenon that requires long–term consideration of many environmental stressors and their interactions.
—Rangjian Qiu ([email protected], 
0000-0003-0534-0496), Wuhan University, China
Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox.
