The impact of drought on vegetation does not increases proportionally with decreasing soil moisture content. The so-called critical soil moisture content below which evapotranspiration reduction and vegetation impact rapidly increase is a key parameter, but knowledge on the critical soil moisture is limited due to a lack of soil moisture and flux observations during extreme drought.
The 2018 drought hit hardest in the part of Europe surrounding The Netherlands (see map below). As a result, the soil moisture networks of Twenthe and the Raam were in a prime location to study how extreme drought is reflected in soil moisture dynamics. The networks each consist of multiple soil moisture profiles across a larger region, which allows to average out some inherent small-scale spatial variability in soil moisture, but also allows for a study of soil moisture dynamics and root water uptake at different depths in the root zone.
Because no flux observations were available near the soil moisture networks to study the relation between soil moisture and evapotranspiration, remotely sensed information on vegetation productivity was chosen instead for its continuous cover and high temporal and spatial resolution. Whereas solar-induced chlorophyll fluorescence (SIF) is most directly related to vegetation productivity, its usefulness for high-resolution studies is limited. Instead, soil moisture was linked to temporal variations in near-infrared reflectance of terrestrial vegetation (NIRv), which was shown to be a good proxy for SIF (Badgley et al., 2017), but which can be derived at high spatial and temporal resolution.
The analysis revealed a clear onset of vegetation stress when soil moisture levels started dropping below 15 Vol.% at the surface. The deeper soil moisture observations, however, also revealed that the largest anomalies developed later in time deeper in the profile, likely reflecting a relatively higher root water uptake at depth to compensate for increased water stress near the surface. As a result, the inferred critical moisture content strongly depends on the depth over which the soil moisture observations are averaged (see Figure above). The research shows that valuable knowledge can be obtained by integrating in situ and satellite observations during drought, but that challenges remain in the identification of model parameters reflecting water stress and root water uptake during drought.
Further reading
Buitink et al. (2020), Anatomy of the 2018 agricultural drought in the Netherlands using in situ soil moisture and satellite vegetation indices. Hydrol. Earth Syst. Sci., 24, 6021–6031, https://doi.org/10.5194/hess-24-6021-2020.
Badgley, G., Field, C. B., and Berry, J. A.: Canopy near-infrared reflectance and terrestrial photosynthesis, Sci. Adv., 3, e1602244, https://doi.org/10.1126/sciadv.1602244, 2017.
Teuling, A. J.; R. Uijlenhoet; F. Hupet & P. A. Troch (2006), Impact of plant water uptake strategy on soil moisture and evapotranspiration dynamics during drydown. Geophysical Research Letters, 33(3), L03401, doi:10.1029/2005GL025019.