Characterising recent drought events in the context of dry-season trends using current reanalysis and remote-sensing soil moisture products

Abstract

Drought events have multiple adverse impacts on environment, society, and economy. It is thus crucial to monitor and characterise such events. Here, we compare the ability of selected current reanalysis and merged remote-sensing products to represent major seasonal and multi-year drought events of the last two decades globally. We consider the ERA5, and the related ERA5-Land, as well as the MERRA-2 reanalysis products, and the ESA CCI, and the corresponding near-real time C3S remote-sensing soil moisture products (both encompassing an ACTIVE, a PASSIVE and a COMBINED product). In addition, an ESA CCI-based root-zone soil moisture estimate derived from the COMBINED product is used. The considered products offer opportunities for drought monitoring since they are available in near-real time.

We focus on soil moisture (or agroecological) drought and analyse documented events within pre-defined spatial and temporal bounds derived from the scientific literature. Based on standardised daily anomalies of surface and root-zone soil moisture, the drought events are characterised by their magnitude, duration, spatial extent, and severity (i.e., the combination of duration and standardised anomalies below -1.5).

All investigated products capture the investigated drought events. Overall, responses of surface soil moisture are weakest for the ACTIVE remote-sensing products in all metrics. The magnitudes (i.e., the minimum of the standardised anomalies over time) are also reduced in MERRA-2. This is also the case for the spatial extents of most of the remote-sensing products. These differences in drought severity and magnitude for single events are also consistent with inter-product differences in dry-season trends in soil moisture, which are diverse and party contradictory. In the case of MERRA-2, the reanalysis shows regional biases in surface air temperature trends compared to a ground observational product, which suggests that this reanalysis product underestimate drought trends. In the case of the microwave remote sensing products, their lower penetration depth compared to that of the top layer of the involved land surface models, as well as sensing issues of active microwave remote sensing under very dry conditions are likely to explain their partly weaker drought responses. In the root zone (based on the reanalysis products and the ESA CCI root-zone soil moisture estimate), the drought events often show prolonged durations, but weaker magnitudes and smaller spatial extents. Based on the overall observational evidence and the consideration of the respective performance and limitations of the included products, the present analyses suggest a consistent global tendency towards drying during the last two decades in several regions.