Emergent relation between surface vapor conductance and relative humidity profiles yields evaporation rates from weather data

Abstract

The ability to predict terrestrial evapotranspiration (E) is limited by the complexity of rate-limiting pathways as water moves through the soil, vegetation (roots, xylem, stomata), canopy air space, and the atmospheric boundary layer. The impossibility of specifying the numerous parameters required to model this process in full spatial detail has necessitated spatially upscaled models that depend on effective parameters such as the surface vapor conductance (C surf ). C surf accounts for the biophysical and hydrological effects on diffusion through the soil and vegetation substrate. This approach, however, requires either site-specific calibration of C surf to measured E, or further parameterization based on metrics such as leaf area, senescence state, stomatal conductance, soil texture, soil moisture, and water table depth. Here, we show that this key, rate-limiting, parameter can be estimated from an emergent relationship between the diurnal cycle of the relative humidity profile and E. The relation is that the vertical variance of the relative humidity profile is less than would occur for increased or decreased evaporation rates, suggesting that land-atmosphere feedback processes minimize this variance. It is found to hold over a wide range of climate conditions (arid-humid) and limiting factors (soil moisture, leaf area, energy). With this relation, estimates of E and C surf can be obtained globally from widely available meteorological measurements, many of which have been archived since the early 1900s. In conjunction with precipitation and stream flow, long-term E estimates provide insights and empirical constraints on projected accelerations of the hydrologic cycle.canopy conductance | moisture stress | hydroclimatology I n the simplest terms, evapotranspiration (E) is controlled by the gradient of humidity near the land surface. At the surface, humidity depends strongly on temperature, which itself reflects a balance of radiative heating and cooling, conversion to latent heat, canopy and soil heating, and the turbulent transport of sensible heat into the atmosphere. For a given radiative forcing, the key parameter controlling the relative strength of the turbulent sensible and latent heat fluxes (and thus the magnitude of evapotranspiration) in most land surface models is the surface vapor conductance (C surf ) (in meters per second). The surface vapor conductance accounts for the biophysical (e.g., vegetation structure, leaf area, senescence state, stomatal conductance) and hydrological (e.g., soil texture, soil moisture, water table) status of the land surface.The turbulent fluxes of heat and moisture, however, in turn modify the humidity and temperature of the surface layer, and thus the gradients that drive the fluxes themselves. Given this tight coupling of the surface and boundary layer, one can anticipate statistically meaningful relations to emerge between E and the screen height humidity and temperature, independent of the surface conditions (and thus independent of site-specific model paramete...

Evaluating the complementary relationship for estimating evapotranspiration using the multi-site data across north China

“…A complete description of the stability functions for m and v is presented in Supplement 2. Further, the roughness height for water vapor (z 0v ) is estimated using the so-called kB −1 approach (Crago and Suleiman, 2005;Crago et al, 2010) relating the momentum (z 0m ) and scalar (z 0v ) roughness heights to the roughness Reynolds number R e * as follows (Salvucci and Gentine, 2013):…”

The pattern across the continental United States of evapotranspiration variability associated with water availability

“…Specifically, Salvucci and Gentine (2013) found that the surface conductance to water vapor transport, the key rate-limiting parameter of typical evapotranspiration models, can be estimated as the value that minimizes the vertical variance of the calculated relative humidity profile averaged over the day. The key advantage of this approach is that biophysical and hydrological surface parameters (stomatal conductance, soil texture, soil moisture, etc.)…”

“…The Rigden and Salvucci (2015) evapotranspiration dataset was derived from meteorological data using a method based on the relationship between the diurnal cycle of relative humidity and evapotranspiration (Salvucci and Gentine, 2013)-a method known as "ETRHEQ" (evapotranspiration based on relative humidity at equilibrium). Specifically, Salvucci and Gentine (2013) found that the surface conductance to water vapor transport, the key rate-limiting parameter of typical evapotranspiration models, can be estimated as the value that minimizes the vertical variance of the calculated relative humidity profile averaged over the day.…”

Impact of type of salt and ambient conditions on saline water evaporation from porous media

“…A small increase in ambient temperature modifies soil water evaporation thus influencing salt transport and distribution in soil. Although the effects of atmospheric conditions on pure water evaporation from porous media are relatively wellunderstood [Smits et al, 2012;Haghighi et al, 2013;Aminzadeh and Or, 2013;Salvucci and Gentine, 2013;Ben Neriah et al, 2014;Vanderborght et al, 2017], their effects on saline water evaporation and the associated salt precipitation patterns are surprisingly not very advanced. Such knowledge will become even more important in the near future with the advent of climate change.…”