Inverting surface soil moisture information from satellite altimetry over arid and semi-arid regions

Abstract

Monitoring surface soil moisture (SSM) variability is essential for understanding hydrological processes, vegetation growth, and interactions between land and atmosphere. Due to sparse distribution of in-situ soil moisture networks, over the last two decades, several active and passive radar satellite missions have been launched to provide information that can be used to estimate surface conditions and subsequently soil moisture content of the upper few cm soil layers. Some recent studies reported the potential of satellite altimeter backscatter to estimate SSM, especially in arid and semi-arid regions. They also pointed out some difficulties of such technique including: (i) the noisy behavior of the backscatter estimations mainly caused by surface water in the radar footprint , (ii) the assumptions for converting altimetry backscatter to SSM, and (iii) the need for interpolating between the tracks. In this study, we introduce a new inversion framework to retrieve soil moisture information from along-track altimetry measurements. First, 20 Hz along-track nadir radar backscatter is estimated by post-processing waveforms from Jason-2 (Ku-and C-Band during 2008-2014) and Envisat (Ku-and S-Band during 2002-2008). This provides backscatter measurements every ∼300 m along-track within every ∼10 days from Jason, and every ∼35 days from Envisat observations. Empirical orthogonal base-functions (EOFs) are then derived from soil moisture simulations of a hydrological model, and used as constraints within the inversion. Finally, along-track altimetry reconstructed surface soil moisture (ARSSM) storage is inverted by fitting these EOFs to the altimeter backscatter. The framework is tested in arid and semi-arid Western Australia, for which a high resolution hydrological model (the Australian Water Resource Assessment, AWRA

Impact of Surface Soil Moisture Variations on Radar Altimetry Echoes at Ku and Ka Bands in Semi-Arid Areas

“…Following the study conducted in Australia by the authors of [20], who demonstrated the effects of soil moisture and soil roughness on the nadir C-and Ku-band backscattering coefficients, recent studies demonstrated the capabilities of spaceborne altimeters for estimating SSM in semi-arid regions such as in west-Africa [21][22][23][24] and in Australia [25]. Indeed, thanks to their nadir-looking capability, altimeters minimize the attenuation by the vegetation layer and a relationship can be directly found with the moisture content of the underlying soil.…”

Improved remotely sensed satellite products for studying Lake Victoria's water storage changes

“…A number of literature have studied Lake Victoria using various satellite remotely sensed products, e.g., satellite radar altimetry to observe the lake's water level variations (e.g., Awange et al, 2013b;Uebbing et al, 2015;Sichangi and Makokha, 2017) and their importance for flood monitoring (Birkett et al, 1999), the Gravity Recovery and Climate Experiment (GRACE) for studying the lake's total water storage (TWS) changes (e.g., Awange et al, 2008Awange et al, , 2014Hassan and Jin, 2004), satellite precipitation data for studying the lake's rainfall (e.g., Kizza et al, 2009;Awange et al, 2013b), a combination of both ground-based and remotely sensed observations for studying the lake's water balance (e.g., Yin and Nicholson, 1998;Swenson and Wahr, 2009). Despite this plethora of studies, a precise study of the hydrological processes of Lake Victoria using merged and improved coherent datasets from multiple sources, what would also benefit other inland lake waters the world over, is still missing.…”

The significance of soil properties to the estimation of soil moisture from C-band synthetic aperture radar

“…To achieve higher spatial resolution with microwave systems requires active radar, which exploits the relationship between the radar backscatter coefficient and soil moisture (Oh et al, 1992). Non-imaging radar systems include scatterometers (Bartalis et al, 2007) and radar altimeters (Uebbing et al, 2017). Synthetic aperture radar (SAR) is an active microwave imaging technology with high spatial resolution (10-50m).…”