Metop ASCAT soil moisture trends: Mitigating the effects of long-term land cover changes

Abstract

The Satellite Application Facility on Support to Operational Hydrology and Water Management (H SAF) is providing surface soil moisture data record products based on a change detection technique applied to the Advanced Scatterometer (ASCAT) on-board the series of Metop satellites. At the moment two of the three Metop satellites are still operational (Metop-B and Metop-C), while the first satellite (Metop-A), launched in 2007, completed its mission in November 2021. Thus, the latest ASCAT surface soil moisture data record product covers a period of more than 15 years (2007-2022).

First analysis of long-term trends in the H SAF ASCAT surface soil moisture data record product have indicated strong anomalies for specific regions around the globe. Trend similarities have been found compared to other data sets such as soil moisture information provided by the ERA5 land surface model. However, certain soil moisture anomaly pattern did not match spatially or in their trend direction. It has been observed that land cover changes contribute to the overall ASCAT backscatter signal with a noticeable impact on the retrieved soil moisture information especially over longer time periods (>10 years). Most notably are areas with slowly changing ground conditions such as growing cities or regions suffering deforestation.

In this study we want to present a new method to mitigate the effects of long-term land cover changes based on a regular re-calibration of the dry and wet backscatter reference. It is important to address and remove this non-climatic effects from the surface soil moisture data record products to correctly detect and monitor climate extremes.