Agricultural droughts, caused by long periods with little to no rain or heat waves, are generally defined as soil moisture deficits that are severe enough to negatively impact vegetation growth. A new study by van Hateren et al. (2021) shows that there is ambiguity between soil moisture drought and their impacts on vegetation.
The increasing availability of remotely sensed data leads to promising possibilities to use these data for real-time drought monitoring over large scales. In this case, surface soil moisture from the ESA CCI data set and MODIS NDVI vegetation greenness data were compared to study the synchrony between soil moisture and vegetation during major European meteorological droughts in the past two decades. It was expected that these two variables behave in an asynchronous manner, as increased incoming solar radiation during meteorological drought periods can enhance evapotranspiration in humid climates (Teuling et al, 2013), often leading to positive anomalies in vegetation indices, relative to the mean conditions, as illustrated below.
Results showed that asynchronies and discrepancies occurred between the surface soil moisture and vegetation droughts. A clear delay is visible between the onset of soil moisture drought and vegetation drought, with correlations generally peaking at the end of the growing season. At lower latitudes, correlations peaked earlier in the season, likely due to an earlier onset of water limited conditions. In certain cases, the vegetation showed a positive anomaly, even during soil moisture drought events. As a result, using the term agricultural drought instead of soil moisture or vegetation drought, could lead to the misclassification of drought events and false drought alarms. To avoid confusion, soil moisture and vegetation drought should thus be considered separately.
Further reading
van Hateren, T. C.; M. Chini; P. Matgen & A. J. Teuling (2021), Ambiguous agricultural drought: Characterising soil moisture and vegetation droughts in Europe from Earth observation. Remote Sensing, 13(10), 1990, doi:10.3390/rs13101990.
Teuling, A. J.; A. F. Van Loon; S. I. Seneviratne; I. Lehner; M. Aubinet; B. Heinesch; C. Bernhofer; T. Grünwald; H. Prasse & U. Spank (2013), Evapotranspiration amplifies European summer drought. Geophysical Research Letters, 40(10), 2071–2075, doi:10.1002/grl.50495.