Land‐Atmosphere Coupling Constrains Increases to Potential Evaporation in a Warming Climate: Implications at Local and Global Scales

• Published in: Earth's future Vol. 11; no. 2
• Main Authors: Kim, Yeonuk, Garcia, Monica, Johnson, Mark S.
• Format: Journal Article
• Language: English
• Published: Bognor Regis John Wiley & Sons, Inc 01-02-2023; Wiley
• Subjects: Arid lands; Arid zones; Atmosphere; Atmospheric models; Climate change; Climate models; Climatic analysis; Climatic conditions; Coupling; Drought; Evaporation; Evaporation rate; evaporative demand; evapotranspiration; General circulation models; Global warming; hydroclimate; Impact analysis; Local climates; Management tools; Modelling; Potential evaporation; Runoff; Trends; Water management; Water resources; Watersheds
• ISSN: 2328-4277; 2328-4277
• DOI: 10.1029/2022EF002886

The magnitude and extent of runoff reduction, drought intensification, and dryland expansion under climate change are unclear and contentious. A primary reason is disagreement between global circulation models and current potential evaporation (PE) models for the upper limit of evaporation under warming climatic conditions. An emerging body of research suggests that current PE models including Penman‐Monteith and Priestley‐Taylor may overestimate future evaporation for non‐water‐stressed conditions. However, they are still widely used for climatic impact analysis although the underlying physical mechanisms for PE projections remain unclear. Here, we show that current PE models diverge from observed non‐water‐stressed evaporation across site (>1,500 flux tower site years), watershed (>10,000 watershed‐years), and global (25 climate models) scales. By not incorporating land‐atmosphere coupling processes, current models overestimate non‐water‐stressed evaporation and its driving factors for warmer and drier conditions. To resolve this, we introduce a land‐atmosphere coupled PE model by extending the Surface Flux Equilibrium theory. The proposed PE model accurately reproduces non‐water‐stressed evaporation across spatiotemporal scales. We find that terrestrial PE will increase at a similar rate to ocean evaporation but much slower than rates suggested by current PE models. This finding suggests that land‐atmosphere coupling moderates continental drying trends. Budyko‐based runoff projections incorporating our PE model are well aligned with those from coupled climate simulations, implying that land‐atmosphere coupling is key to improving predictions of climatic impacts on water resources. Our approach provides a simple and robust way to incorporate coupled land‐atmosphere processes into water management tools.
Plain Language Summary
Water resources are supply side constrained by precipitation, and demand‐side constrained by terrestrial evaporation. Terrestrial evaporation is commonly estimated by reducing its upper limit which is determined by potential evaporation (PE) models. It is important to understand how supply and demand sides of hydroclimate features change with time, particularly for projected future climatic conditions. Conventionally, a warming and drying climate system has been understood to increase PE. However, this demand‐side perspective neglects land‐atmosphere coupling effects. For example, hot dry air is also an indicator of dry soil, implying that increasing demand (e.g., hot dry air) may not be met due to supply constraints (e.g., dry soil). We introduce a land‐atmosphere coupled PE model to better predict the upper limit of evaporation under future climatic conditions. In evaluating the model across site, watershed, and global scales, we report a slower increase in PE in a warming climate compared to studies not incorporating land‐atmosphere coupling, which is significant for water resources planning. Improved representation of PE under future climate conditions is necessary to aid in planning for climate adaptation, including agricultural water management, improvement of drought indices, and other critical societal informational needs.
Key Points
Potential evaporation (PE) models overestimate the upper limit of evaporation for warmer future climates, leading to a hydrologic drying bias
To resolve this, we developed and evaluated a PE model by extending the emerging Surface Flux Equilibrium theory
PE will likely increase slower than previously thought, implying land‐atmosphere coupling limits continental drying