Evaluation of a Global Soil Moisture Product from Finer Spatial Resolution SAR Data and Ground Measurements at Irish Sites

Abstract

Abstract:In the framework of the European Space Agency Climate Change Initiative, a global, almost daily, soil moisture (SM) product is being developed from passive and active satellite microwave sensors, at a coarse spatial resolution. This study contributes to its validation by using finer spatial resolution ASAR Wide Swath and in situ soil moisture data taken over three sites in Ireland, from 2007 to 2009. This is the first time a comparison has been carried out between three sets of independent observations from different sensors at very different spatial resolutions for such a long time series. Furthermore, the SM spatial distribution has been investigated at the ASAR scale within each Essential Climate Variable (ECV) pixel, without adopting any particular model or using a densely distributed network of in situ stations. This approach facilitated an understanding of the extent to which geophysical factors, such as soil texture, terrain composition and altitude, affect the retrieved ECV SM product values in temperate grasslands. Temporal and spatial variability analysis provided high levels of correlation (p < 0.025) and low errors between the three datasets, leading to confidence in the new ECV SM global product, despite limitations in its ability to track the driest and wettest conditions.

Statistical Modeling of Soil Moisture, Integrating Satellite Remote-Sensing (SAR) and Ground-Based Data

“…Yet, despite its broad importance, field-scale soil moisture data are often not available or closest neighbor values are used when modeling hydrological and biochemical processes or when calibrating regional-scale predictions generated by complex agroecosystem models. This is, in part, due to constraints and limitations in acquiring and assembling such data over large regions and across sufficient time-periods; the acquisition process is not only costly, but labour intensive, and has high variability when upscaled from the field, to landscape, up to the regional-scale [4][5][6]. Instead of relying on direct soil moisture information validated against remote-sensing data, auxiliary predictions are often substituted based on indirect, interpolative or extrapolative assumptions that may not be statistical accurate, nor readily verifiable.…”

Intercomparison of Soil Moisture Retrievals From In Situ, ASAR, and ECV SM Data Sets Over Different European Sites

“…For example, Zeng et al (2015) found the ECV SM product to be highly related to in-situ data from two different soil moisture networks at the Tibetan Plateau, with the highest R values (0.70 -0.85) and smallest ubRMSD values (0.034 -0.042m 3 m -3 ) compared to six other satellite derived soil moisture products (AMSR-E (NASA product), AMSR-E (JAXA product), AMSR-E (LPRM product), AMSR-2, ASCAT, and SMOS). Similarly, Pratola et al (2014) found strong correlations (R = 0.72 -0.88) and associated low ubRMSD values (0.05 -0.06) between ECV SM and SAR-derived soil moisture values across three grassland sites in Ireland.…”

“…The ECV SM product has been validated across different regions using in-situ, model and SAR-derived soil moisture datasets in previous studies (e.g. Albergel et al, 2013b;Loew et al, 2013;Dorigo et al, 2015;Pratola et al, 2014;Zeng et al, 2015) where good agreement between the datasets was generally found. For example, Zeng et al (2015) found the ECV SM product to be highly related to in-situ data from two different soil moisture networks at the Tibetan Plateau, with the highest R values (0.70 -0.85) and smallest ubRMSD values (0.034 -0.042m 3 m -3 ) compared to six other satellite derived soil moisture products (AMSR-E (NASA product), AMSR-E (JAXA product), AMSR-E (LPRM product), AMSR-2, ASCAT, and SMOS).…”

“…The network also measured precipitation and soil temperature and has been used predominantly for modelling N 2 O fluxes from agricultural grasslands, but has also been used for the validation of soil moisture products (e.g. Pratola et al, 2014). The sites have a temperate maritime climate with annual precipitation of 900-1200mm and an annual average temperature of 10°C.…”

“…The Advanced Synthetic Aperture Radar (ASAR) instrument onboard the ENVISAT satellite was capable of providing global measurements at 1km and 150m spatial resolution every four to seven days, depending on the acquisition plan (Desnos et al, 2000). Although there are certain technical limitations in retrieving surface soil moisture from SAR data, significant progress has been made in recent years to the point that SAR data could be used not only as another validation source for coarse-scale soil moisture products but also for applications which require finer spatial resolution soil moisture data such as hydrological or runoff modelling (Dostálová et al, 2014;Pratola et al, 2014). Furthermore, the regular temporal coverage and higher spatial resolution of current C-band sensors such as Sentinel-1 will provide greater opportunities to characterise surface soil moisture within the large areas covered by coarse-scale product cells and help strengthen the understanding of such products.…”