COS 267-5 - The importance of soil water retention in regulating ecosystem drought stress responses in land process models

Abstract: The functioning of terrestrial ecosystems depends on water transport along the soil-plant-atmosphere continuum. Considering the projected increases in drought frequency and severity in many regions of the world, improving our understanding of the biological and biogeophysical controls on the soil-plant-atmosphere continuum has become a research priority in recent years. Many Earth System Models have overhauled the representation of plant hydraulic function to improve simulated ecosystem drought responses. However, less investment has gone into examining modeled soil water retention (SWR). Most land surface models use pedotransfer functions to describe SWR curves. Pedotransfer functions relate soil texture class to SWR curves and are broadly applied but are known to be inaccurate because they fail to account for rock content, clumping, compaction, and other processes. In this study, we examined the impact of using directly measured SWR curves versus those calculated from pedotransfer functions on model simulations of ecosystem drought stress responses.

SWR curves, soil texture, plant hydraulic vulnerability curves, and other plant traits were measured at four conifer-dominated forests in the western US that span a range of soil and species composition. These sites are part of various eddy-covariance networks and have rich data records including meteorology, soil water content, and total ecosystem gross primary productivity and evapotranspiration. Simulations were performed with the Soil-Plant-Atmosphere (SPA) ecosystem model, parameterized with measured plant traits and forced with local meteorology and radiation data and prescribed soil moisture. The model uses SWR curves on three soil horizons to relate measured soil water content to soil water potential, which determines plant-available water. We first prescribed SWR curves calculated from soil texture class using Rawls et al. (1982) and van Genuchten et al. (1980) pedotransfer functions. We next implemented SWR curves measured from intact soil cores using the ceramic plate methodology. We then compared simulations using the calculated versus measured SWR curves. The discrepancy between measured SWR curves and pedotransfer functions varied among sites, but generally, we found that the calculated SWR curves led to higher plant available water which reduced the severity of drought stress compared to the measured SWR curves. Accurately capturing soil water retention relations is highly influential on ecosystem water cycling, which illustrates the importance of soil water relations in the soil-plant-atmosphere continuum. This work highlights the need to examine how soil hydraulic properties are represented in land surface models to improve simulations of water cycling, particularly when making future projections.