Surface soil moisture (SM) is essential for existence of biotic lifeform and geophysical processes. However, with increasing global warming due to climatic changes, its spatiotemporal evolution is uncertain and largely unknown. In this study we detected long-term (40 years; 1981-2020) SM patterns of global vegetated areas through spatial timeseries clustering using the state-of-the-art ERA5-Land dataset. In addition, we also analyzed long-term patterns of precipitation (P), evapotranspiration (bare soil evaporation (BSe) and vegetation transpiration (VT)), and normalized difference vegetation index (NDVI). Our results indicate that surface SM (0-7 cm depth) of about 48 % and 9 % of the global vegetated area is showing drying and wetting pattern over the past 40 years, respectively. The detected soil drying, and wetting patterns were largely consistent across different soil depth, with 90 % and 80 % pattern similarity of surface soil layer with 2^nd soil layer (7-28 cm) and 3^rd soil layer (28-100 cm), respectively. About 80 % of areas with drying soil pattern also showed increasing evapotranspiration and/or decreasing precipitation. Specifically, decreasing P, increasing BSe and VT pattern were detected for 11 % of the soil drying pattern area. Similarly, increasing BSe and VT pattern, only decreasing P and only increasing VT pattern were detected for 17 %, 25 % and 12 % of soil drying areas, respectively. Both decreasing precipitation and increasing evapotranspiration patterns showed about 40 % similarity with decreasing soil moisture patterns. Across different landcover types, broadleaved forests, and cropland areas showed largest drying pattern. Under the future global warming scenario, the global soil water is expected to decrease as evapotranspiration would increase with inconsistent trend of global precipitation change. Our findings are of utmost importance for global soil water resource conservation and management.

In preparation for the NISAR mission soil moisture algorithm, this study performs the validation of the European Centre for Medium-Range Weather Forecast (ECMWF) ERA5-Land volumetric soil water (soil moisture) layer product with in-situ measurements from the Soil Moisture Active Passive (SMAP) core validation sites (CVS) for 2015 – 2021. The ERA5-Land soil water layer was also compared against the SMAP-enhanced radiometer soil moisture product (gridded at 9 km) at a global extent. For comparison between the ERA5-Land and the SMAP soil moisture products, a matching temporal dataset was generated from ERA5-Land based on the acquisition time of SMAP for each 9 km grid. In comparison with the CVS in-situ measurement, the ERA5-Land data exhibits an overall ubRMSE of about ~0.05 m ³ /m ³ but has high wet bias over most of the sites except for sites in Australia, Denmark, and Argentina. The global comparison of the ERA5-Land soil moisture with the SMAP 9 km gridded product shows an overall wet bias with a high correlation in the tropical and temperate regions. The lowest bias was observed over the desert region but has poor correlation as it doesn’t have enough soil moisture variability. Poor correlation with high bias and high RMSD observed over dense vegetated and tundra regions is possibly due to the inferior performance of the SMAP soil moisture product, which has not been validated for these areas.

The very high anomalous precipitation (more than 500 mm) and release of water from reservoirs located in an upper catchment resulted in severe flood inundation situation in downslope regions of Kerala State in August 2018. In the present study, the satellite-based precipitation was analyzed on a daily to weekly basis during the flood duration in Kerala. The Tropical Rainfall Measuring Mission (TRMM) based precipitation pattern indicated very high rainfall (more than 60 mm day^–1) in the central parts of Kerala on 14 and 15August 2018. The climate research unit (CRU) based long-term mean cumulative precipitation (1901-2017) indicated high (300-400 mm) to very high (more than 500 mm) anomalous precipitation in parts of Kerala during August 2018 leading to severe flood inundation in major parts of the state. The Sentinel-1A Synthetic Aperture Radar (SAR) based inundation mapping exhibited flooding of 1100.4 km² area (5.80% of total area) during August 2018, which severely affected Alappuzha, Thrissur, and Kottayam districts. Cropland (218.69 km²), settlement (115.23 km²) and shrubland (79.46 km²) were the most affected land-use/land-cover in the region. The study warrants the restoration of ecological stability and augmentation of resilience to flooding hazard primarily in the upper catchments (the Western Ghats).