Robustness of critical soil moisture to curve-fitting methods and its variability with soil depth, soil texture, and climatic conditions: insights from lysimeter data in Germany

Actual evapotranspiration (ET$_a$) is a vital terrestrial ecosystem process that links water, energy, and carbon cycles. ET$_a$ can be limited by either energy or water availability. The transition between water- and energy-limited regimes is related to soil moisture and is often characterized as a threshold, denoted as critical soil moisture threshold (θ$_{crit}$). However, the determination of θ$_{crit}$ is subject to uncertainties due to the different methods used to evaluate the relationship between ET$_a$ and soil moisture (SM), such as SM depths, definitions of ET$_a$ and curve fitting functions. Typically, surface SM is used to identify θ$_{crit}$ as it is easily accessible and assumed to represent root zone SM status. Weighable lysimeter technology provides a unique opportunity to assess the role of root zone SM on the transition between water and energy limited ET$_a$. It is widely regarded as the gold standard for measuring in-situ ET, and at the same time allows for in-situ SM measurements at different depths. In this study, we estimated θ$_{crit}$ using in situ SM measurements at 10 cm depth and root zone SM by vertically integrating in situ SM (0–60 cm) observations. ... mehrIn addition, we applied three different definitions of relative evapotranspiration (evaporative fraction, the ratio of ET$_a$ to grass reference evapotranspiration and the ratio of actual ET$_a$ to calculated potential (vapotranspiration) as well as two different fitting curves to investigate the sensitivities of θ$_{crit}$. We found robust θ$_{crit}$ estimates across different definitions and fitting curve methods, but the estimates were significantly higher for root zone than for surface θ$_{crit}$. Our results also highlight the high correlation (0.83) between root zone and surface θ$_{crit}$. However, the relation between both values is not unique since it depends on the actual moisture profile and plant root system and, herewith, on the soil type and previous weather conditions. We further observed that both surface and root zone θ$_{crit}$ decreased with increasing sand fraction. Under changing climatic conditions but with identical soil and ecosystem types, both surface and root zone θ$_{crit}$ decreased with increasing aridity. Additionally, we found that using the midpoint between field capacity and wilting point provides a reliable range of root zone θ$_{crit}$ for a given soil texture.