Abstract
Soil moisture is a critical indicator for climate change and agricultural drought, but its measurement is challenging due to large variability with land cover, soil type, time, space and depth. Satellite estimates of soil moisture are highly desirable and have become more widely available over the past decade. This study investigates and compares the performance of four surface soil moisture satellite datasets over Canada, namely, Soil Moisture and Ocean Salinity Level 3 (SMOS L3), versions 3.3 and 4.2 of European Space Agency Climate Change Initiative (ESA CCI) soil moisture product and a recent product called SMOS-INRA-CESBIO (SMOS-IC) that contains corrections designed to reduce several known sources of uncertainty in SMOS L3. These datasets were evaluated against in situ networks located in mostly agricultural regions of Canada for the period 2012 to 2014. Two statistical comparison methods were used, namely, metrics for mean soil moisture and median of metrics. The results suggest that, while both methods show similar comparisons for regional networks, over large networks, the median of metrics method is more representative of the overall correlation and variability and is therefore a more appropriate method for evaluating the performance of satellite products. Overall, the SMOS products have higher daily temporal correlations, but larger biases, against in situ soil moisture than the ESA CCI products, with SMOS-IC having higher correlations and smaller variability than SMOS L3. The SMOS products capture daily wetting and drying events better than the ESA CCI products, with the SMOS products capturing at least 75% of observed drying as compared to 55% for the ESA CCI products. Overall, for periods during which there are sufficient observations, both SMOS products are more suitable for agricultural applications over Canada than the ESA CCI products, even though SMOS-IC is able to capture soil moisture variability more accurately than SMOS L3.
Highlights
Surface soil moisture (SM) is a key component of the Earth system which can impact weather through its influence on evaporation and surface energy fluxes [1], with a lack of SM associated with drought occurrence [2] and an excess related to flooding [3,4]
Two versions of the European Space Agency Climate Change Initiative (ESA Climate Change Initiative (CCI)) SM product, v3.3 (CCI3) and v4.2 (CCI4), Soil Moisture and Ocean Salinity (SMOS)-Level 3 (L3) (L3) and a more recent SMOS product known as SMOS-IC (IC), were evaluated against regional (Ontario and Manitoba) and provincial (Alberta) in situ daily SM monitoring networks over important agricultural regions of Canada
We found that SMOS products generally show higher temporal correlations with in situ measurements as compared to European Space Agency (ESA) CCI, irrespective of the soil texture or location
Summary
Surface soil moisture (SM) is a key component of the Earth system which can impact weather through its influence on evaporation and surface energy fluxes [1], with a lack of SM associated with drought occurrence [2] and an excess related to flooding [3,4]. Ground-based monitoring stations provide accurate point-scale SM estimates but they usually lack the spatial coverage to capture SM conditions over larger regions and these estimates may not be representative of soil conditions even within the vicinity of these stations [9,10,11]. Another common approach is to use microwave remote sensing of SM, which is promising due to the almost linear relationship between the microwave radiance emitted or reflected by the surface soil portion and the soil-water mixing ratio [12]. Even though microwave remote sensing of SM can be challenging due to reasons related to topography [13], the presence of snow and ice [12], human-induced radio frequency interference (RFI) [14] and vegetation water [15], it provides a means to overcome the spatial limitations posed by in situ observations [16,17]
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