VODCA v2: An updated long-term vegetation optical depth dataset for ecosystem monitoring

Abstract

Vegetation optical depth (VOD), derived from space-borne microwave radiometers, is a parameter that quantifies the attenuation of surface microwave emissions by the overlaying vegetation. VOD depends on several factors, such as the water content and density of the vegetation, and the specifications of the satellites and wavelengths used. VOD has been used in various applications such as phenology analysis, drought and biomass monitoring, and the estimation of the likelihood of fire occurrence, leaf moisture, and gross primary productivity. Most of these applications require consistent long-term measurements, while single sensor timeseries are typically too short.

To bridge this gap, the global, long-term Vegetation Optical Depth Climate Archive (VODCA)[1] combines VOD retrievals from multiple passive microwave sensors spanning from 1987 to 2019, derived through the Land Parameter Retrieval Model (LPRM)[2]. VODCA harmonises these retrievals from various satellites and periods for differences in microwave frequencies, measurement incidence angles, orbit characteristics, radiometric quality, and spatial footprints. VODCA v1 provides separate VOD products in different spectral bands, namely the Ku-band (period 1987–2017), X-band (1997–2018), and C-band (2002–2018). Despite the relatively short time since its publication, VODCA v1 has already been taken up by many researchers and climate reports as an indicator of vegetation condition [3].

Here, we introduce a new and improved version of the VODCA dataset. VODCA v2 includes a multi-frequency product called VODCA CXKu, obtained by merging the C-, X-, and Ku-band observations. This product, which spans over 30 years of observations (1987-2022), is suitable for canopy dynamics monitoring and, due to the merging process, exhibits less random error than the individual frequency datasets. VODCA v2 also includes an L-band product obtained by merging LPRM-derived VOD from SMOS (Soil Moisture and Ocean Salinity) and SMAP (Soil Moisture Active Passive) missions, covering the period 2010-2022. We explore the properties of the new products in comparison to independent vegetation datasets, and present new insights in ecosystem dynamics facilitated by VODCA.