Evaluation of Satellite and Reanalysis Soil Moisture Products over Southwest China Using Ground-Based Measurements

Long-term global satellite and reanalysis soil moisture products have been available for several years. In this study, in situ soil moisture measurements from 2008 to 2012 over Southwest China are used to evaluate the accuracy of four satellite-based products and one reanalysis soil moisture product. These products are the Advanced Microwave Scanning Radiometer for the Earth observing system (AMSR-E), the Advanced Scatterometer (ASCAT), the Soil Moisture and Ocean Salinity (SMOS), the European Space Agency’s Climate Change Initiative soil moisture (CCI SM), and the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim). The evaluation of soil moisture absolute values and anomalies shows that all the products can capture the temporal dynamics of in situ soil moisture well. For AMSR-E and SMOS, larger errors occur, which are likely due to the severe effects of radio frequency interference (RFI) over the test region. In general, the ERA-Interim (R = 0.782, ubRMSD = 0.035 m/m) and CCI SM (R = 0.723, ubRMSD = 0.046 m/m) perform the best compared to the other products. The accuracy levels obtained are comparable to validation results from other regions. Therefore, local hydrological applications and water resource management will benefit from the long-term ERA-Interim and CCI SM soil moisture products. OPEN ACCESS Remote Sens. 2015, 7 15730


Introduction
Soil moisture (SM) plays an important role in the interactions between the atmosphere and the land surface, and has been identified as an essential climate variable by the Global Climate Observing System (GCOS) [1].Soil moisture connects the energy and water fluxes due to its control on the partitioning of surface net energy into latent and sensible heat fluxes, as well as the partitioning of precipitation into infiltration and runoff [2,3].Therefore, the knowledge of soil moisture dynamics is important for climate change research, flood and drought monitoring, weather forecasting, as well as water resource management [4,5].
Satellite remote sensing has been recognized as a powerful tool to retrieve soil moisture due to the improvements of sensor technologies and retrieval algorithms [6] using either active [7] or passive microwave [8] remote sensing observations.Until now, several global satellite-based soil moisture products have been released.These products include the Advanced Microwave Scanning Radiometer E for the Earth observing system (AMSR-E) [9], the Advanced Scatterometer (ASCAT) [10,11], the Soil Moisture and Ocean Salinity (SMOS) [12], and the European Space Agency's Climate Change Initiative (ESA CCI) soil moisture product [13][14][15].These satellite-based soil moisture products provide an unprecedented opportunity to investigate the interactions between land and atmosphere and their effects on climate change.However, comprehensive evaluations of these soil moisture products are required before using them.Many evaluation studies have been conducted either based on in situ measurements or model simulations.Most of these studies are performed either on a regional scale, for example in Europe [16][17][18][19], Australia [20,21], Africa [22,23], and the United States [24,25], or at the global scale [15,26,27].However, few studies were conducted in China so far.In order to utilize the satellite-based soil moisture products for practical applications in China, it is necessary to validate them against in situ measurements in China.
The objective of this research is to evaluate four satellite-based soil moisture products and one reanalysis product with in situ measurements over Yunnan, China (Figure 1).The validation activity is of great importance, because this region is very sensitive and vulnerable to climate change due to its unique geophysical position and climate.Over the last three decades, frequent droughts and floods occurred in this region, with the most extreme drought occurring from winter 2009 to spring 2010 [28,29].According to the Ministry of Civil Affairs, the drought resulted in a loss of $2.5-billion due to agricultural damage, and about 9.65 million people have had a shortage of drinking water [30].Therefore, accurate long-term satellite-based soil moisture products are urgently required to assess drought conditions and the links between increasing temperature, soil moisture variations, and vegetation productivity.The following section briefly introduces the study area, in situ measurements, and soil moisture products.The evaluation methods are described in Section 3. Results of the evaluation are then presented and discussed in Section 4. Finally, conclusions are summarized in Section 5.

Study Area
The Yunnan province is located in Southwest China (Figure 1), covering an area of roughly 394,000 km 2 .Mountainous topography and special karst geomorphology characterize this region, with an elevation gradually decreasing from the northwest to southeast, and an average elevation of 1980 m above sea level [31].This region has a subtropical climate with dry winters and wet summers.The precipitation is spread unevenly over the year and the region, and is largely influenced by the Tibetan plateau, the tropical Indian monsoon and the eastern Asia monsoon.As a result, about half of the precipitation occurs between June and August.The annual precipitation ranges from 600 mm in valleys to 1700 mm in mountainous regions [32,33].Due to its special climate and topographical conditions, this region suffers from frequent droughts.The most severe and sustained drought occurred in 2010.It started in the winter of 2009 and became particularly severe in the spring of 2010, leading to significant ecological and economic damages [34].Numerous streams and small reservoirs dried up, which had a severe impact on the supply of drinking water and the agricultural production [30,35].Therefore, long-term and spatially distributed soil moisture information is required for this region.

Satellite-Based and Reanalysis Products
The satellite products used in this study include four soil moisture products as well as one precipitation product.In addition, one soil moisture reanalysis product is also analyzed.In the following subsections, these datasets are briefly introduced.

AMSR-E
The AMSR-E onboard the Aqua satellite is a passive microwave sensor.The instrument operated between May 2002 and October 2011.It provided measurements at six different frequencies.Data is available for descending (1:30 a.m.) and ascending (1:30 p.m.) orbits [36].Several algorithms have been developed for retrieving soil moisture from the AMSR-E measurements.One of them is the land parameter retrieval model (LPRM) [9].The corresponding soil moisture product has been developed and released by Vrije Universiteit Amsterdam (VUA) together with NASA.The LPRM product provides global soil moisture on a regular 0.25° global grid derived from C-band and X-band observations.The observations from the X-band are used only for soil moisture estimation when the C-band observations are affected by radio frequency interference (RFI).Based on the observations from these two bands at two overpass times, the LPRM product contains four soil moisture products (C-band and X-band soil moisture in both, in descending and ascending modes).In this study, all these products were compared with in situ measurements to investigate their performance using the period from 2008 to 2011.The soil moisture data were masked if the vegetation optical depth (VOD) was higher than 0.8, as suggested by Rebel et al. [37] and Lei et al. [38].

ASCAT
The ASCAT is a scatterometer onboard the Meteorological Operation-A (MetOp-A) satellite, which was launched in October 2006 and scans the Earth's surface in descending (9:30 a.m.) and ascending (9:30 p.m.) orbits [39].It operates in the C-band (5.3 GHz) in vertical polarization with a spatial resolution of 25 or 50 km [40].The soil moisture is derived from the backscatter measurements of ASCAT using a change detection algorithm [11].In this study, the soil moisture product provided by the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) with the version of TU-Wien-WARP 5.5 was used.It provides a relative soil moisture saturation degree (using the top 2 cm of the soil column) with values ranging from 0% (dry) to 100% (wet) at spatial resolution of 25 km.To be consistent with in situ soil moisture measurements and comparable with other soil moisture products, the ASCAT soil moisture was rescaled into volumetric soil moisture (unit: m 3 /m 3 ) using the soil porosity estimated from Land Data Assimilation System (LDAS) data [41].The soil moisture products from 2008 to 2012 in both descending and ascending modes were analyzed in this study.The soil moisture data were masked if the processing flags were not equal to 0 (best quality).

ESA CCI
The ESA CCI soil moisture product is a unique 35-year (1978-2013) long satellite-based soil moisture record produced within the framework of the ESA Climate Change Initiative [42].The CCI soil moisture was generated by merging several passive and active soil moisture products [13,43].The involved soil moisture products are derived from Scanning Multichannel Microwave Radiometer (SMMR), Special Sensor Microwave/Imager (SSM/I), Tropical Rainfall Measuring Missions (TRMM) Microwave Imager (TMI), ASMR-E, scatterometers (SCAT) and ASCAT.The CCI soil moisture has three products, which differ in the data sources used: passive microwave only, active microwave only, and merged passive-active product [15].In this study, the merged passive-active product (version 02.1) is analyzed for the period from 2008 to 2012.The dataset has spatial resolution of 0.25° on a daily basis.

SMOS
The SMOS satellite, launched in November 2009, is dedicated to soil moisture monitoring with an L-band (1.4 GHz) passive radiometer.It provides global coverage every three days for both descending (6:00 p.m.) and ascending (6:00 a.m.) orbits with a spatial resolution of about 40 km [44].The advantage of the L-band is that it has greater sensing depth of up to 5 cm of the soil.The soil moisture is derived using the L-band microwave emission of the biosphere (L-MEB) model [45,46].In this study, the SMOS level 3 daily soil moisture product was obtained from the Centre Aval de Traitement des Donné es SMOS (CATDS).As is the case for AMSR-E and ASCAT, both descending and ascending soil moisture products were used for analysis from the years from 2009 to 2012.According to Al-Yaari et al. [47], the soil moisture data are masked out if the Data Quality Index (DQX) is larger than 0.06, or the DQX is equal to 0 (filled value), or the RFI is larger than 30%.

ERA-Interim
ERA-Interim is a global atmospheric reanalysis product produced by the ECMWF [48].It is based on a sequential data assimilation scheme.It covers the period from 1979 until the present.The soil moisture data from ERA-Interim are provided at four depths (0-7, 7-28, 28-100, and 100-289 cm) [48].In this study, the daily averaged 0.25° upper layer (0-7 cm) of soil moisture products covering the years from 2008 to 2012 were evaluated.

TRMM Precipitation Data
The TRMM precipitation data was used in this study as an alternative to the station precipitation from in situ measurements, because it has been extensively validated against ground-based measurements around the world, including the Yunnan province [33].Details of the TRMM precipitation dataset can be found in Kummerow et al. [49] and Iguchi et al. [50].The daily 0.25° TRMM precipitation products (3B42 V7) were used in this study for the same period as indicated in the other dataset (2008-2012).

In Situ Soil Moisture Measurements
There are a total of 18 soil moisture stations used in the current study.The geographical locations and details of these stations are presented in Figure 1 and Table 1.The elevations of these stations range from 847 m to 2293 m, and the land cover types are mainly grassland (4 stations), forest (3 stations), cropland (7 stations), and bare soil (4 stations).The soil moisture contents over these stations were measured at three different depths (10 cm, 20 cm and 40 cm) using frequency domain reflectometry (FDR) sensors.Data was obtained from the Yunnan Hydrology and Water Resources Bureau.The measurements at a 10 cm depth are considered to best represent the soil layer accessible by the satellite observations.We therefore use this layer as a reference in this study.

Evaluation Strategies
In order to evaluate the performances of the satellite-based and reanalysis soil moisture products, they were first compared directly with the in situ soil moisture.Established skill scores, like the correlation coefficient (R), the BIAS, the root mean square difference (RMSD), and the unbiased RMSD (ubRMSD) were used to quantify the differences between various soil moisture products and measured soil moisture [51,52].In addition, the comparisons for soil moisture anomalies were also performed, because the seasonal variations of soil moisture may influence the correlation statistics [53].The soil moisture anomalies were computed similar to Dorigo et al. [15], using the equation below to avoid the seasonal effects on correlations.The anomaly calculation is based on a sliding window of five weeks.The difference from the mean is calculated if there are at least five measurements available during this time window.
where SM(t) is the soil moisture at day t, the overbar indicates the temporal mean for the five weeks period.Based on the above equation, the soil moisture anomalies of in situ soil moisture, satellite-based, and reanalysis products were calculated.From direct comparisons with in situ soil moisture, it is found that the AMSR-E X-band has better performance than the C-band with lower RMSD and ubRMSD values.It implies that the C-band observations might be influenced by the RFI in this region.Regardless of the frequency band, ascending and descending soil moisture products also have different accuracies due to the diurnal variations of the land surface conditions.On one hand, overpass times at night and in the early morning are more suitable for soil moisture retrieval from passive microwave satellites, because the isothermal conditions at night can minimize the errors associated with surface temperature [54].On the other hand, the soil emission is influenced by the diurnal variation of the water in the vegetation canopy [55].The attenuation of soil emission caused by water in the vegetation canopy is minimized during the daytime due to loss of water through evapotranspiration [56].Therefore, both daytime and nighttime retrievals have certain advantages.For AMSR-E, many studies such as Brocca et al. [17] have found that ascending soil moisture is more reliable than descending soil moisture.However, in other studies such as Griesfeller et al. [57], descending was found to be better than ascending.This demonstrates that the performance of different AMSR-E soil moisture products depends on the surface condition of the study area.In this study, it is found that ascending AMSR-E performs better than descending AMSR-E, which might be due to the heavily vegetated characteristics of the study area.Similar results have been reported by Lei et al. [38], who found that the advantage of the nighttime overpass is reduced as the vegetation cover increases and the AMSR-E ascending retrieval is superior over heavily vegetated areas in the United States.Therefore, the descending and ascending soil moisture products should be evaluated before applying them in different regional studies.Compared to AMSR-E, the differences between ascending and descending retrievals are smaller for SMOS.It may be due to the smaller soil moisture temperature differences between ascending/descending overpass times of SMOS than that of AMSR-E.The Faraday rotation effect in the ionosphere is also expected to be minimized at SMOS ascending overpass time [38].Although SMOS ascending performs slightly better than descending, the sample days of SMOS ascending soil moisture are less than descending soil moisture in the current study.Therefore, the SMOS descending soil moisture is used for the following analyses.Compared to AMSR-E and SMOS, the ASCAT presents quite similar performance for ascending and descending soil moisture.This is partly because the scatterometer-based ASCAT does not require accurate estimates of surface temperature [38].It is worth noting that the ASCAT descending soil moisture has slightly better correlation values than ascending soil moisture in the current study area.The statistics of the above comparisons are provided in Appendix Figure A1.In the following analyses, only the best performed AMSR-E ascending X band (AMSR-E_A_X), ASCAT descending (ASCAT_D), and SMOS descending (SMOS_D) are considered in comparisons with in situ measurements.

Temporal Evolution of Soil Moisture and Precipitation
Figure 2 shows the time series patterns of all the soil moisture products, in situ measurements and rainfall for the average values over all stations.All the products except for AMSR-E and SMOS correspond well with rainfall, with the soil moisture increasing during rainfall events and decreasing after rainfall events.Additionally, the temporal dynamics of in situ soil moisture are captured well by CCI SM and ERA-Interim.It can also be seen that CCI SM and ERA-Interim, which are in line with in situ measurements, capture the spring droughts in 2010.Compared to in situ soil moisture, the ASCAT has higher seasonal variability.The poor performance of AMSR-E and SMOS might be due to the effects of RFI over this region, because both AMSR-E and SMOS are reported to correlate well with groundbased measurements over other regions, such as America, West Africa, and Europe [23,25,26].

Validation of the Soil Moisture against in Situ Measurements
To quantitatively evaluate these soil moisture products, Figure 3 shows the comparison statistics between different soil moisture products and in situ measured soil moisture at each station.In general, the ASCAT, CCI SM, and ERA-Interim have similar R values.However, in terms of RMSD and ubRMSD, the best performance was found with the ERA-Interim, followed by CCI SM and ASCAT.It can also be seen that the accuracies of AMSR-E and SMOS are much worse than the expected accuracy of 0.06 m 3 • m −3 for AMSR-E and 0.04 m 3 • m −3 for SMOS.In theory, the SMOS should provide reliable soil moisture product due to the use of L-band measurements.As stated before, the poor performance found here might be caused by the effects of RFI.Similar results have been reported by Cho et al. [58] and Zeng et al. [59].In addition to the validation at each individual station, the comparisons between averaged in situ soil moisture over all stations and corresponding averaged soil moisture products were also conducted.This comparison method is expected to reduce the uncertainties introduced by scale differences between the in situ point and satellite pixel [60].Table 2 presents the statistical scores of the comparison results at the network scale.Compared to the results at station scale, the comparisons at the network scale generally show the same tendency but better scores.The ERA-Interim outperforms other soil moisture products with lowest RMSD and ubRMSD.The above results give an overview of the evaluation of soil moisture absolute values.To remove the influences of the seasonal cycle in the comparison, the soil moisture anomalies were calculated based on Equation (1). Figure 4 shows the anomaly time series of the soil moisture and rainfall products.The CCI SM, ASCAT, and ERA-Interim generally represent the variations of in situ measurements.As expected, AMSR-E and SMOS have large variations compared to in situ soil moisture.Together with the analysis of the absolute soil moisture time series, these results suggest that in situ soil moisture temporal variations can be well represented by the satellite-based soil moisture and reanalysis datasets.

Validation of the Soil Moisture Anomalies
Figure 5 presents the bar plots of the comparison of soil moisture anomalies at each station.Similar to the results from the comparison of absolute soil moisture, the CCI SM, ASCAT, and ERA-Interim have similar performance in terms of R values.As expected, the R values are smaller than that of absolute comparison due to the removal of seasonal cycles.Similar results have been reported by Albergel et al. [16] and Brocca et al. [17].However, the RMSD, ubRMSD, and BIAS values of anomaly comparisons are smaller than that of the absolute comparisons.The ERA-Interim has the smallest RMSD, ubRMSD, and BIAS.Similar performance was found with CCI SM, followed by ASCAT.The AMSR-E and SMOS have the worst performance.
The statistic scores of the comparison of soil moisture anomalies at a network scale are summarized in Table 3. Due to the decreasing of uncertainties of scale difference, the scores of network scale are improved compared to that of the station scale.In comparison to the scores of absolute soil moisture (Table 2), the correlations of all the products worsen, while RMSD, ubRMSD, and BISA get smaller.In general, the ERA-Interim has the smallest RMSD and ubRMSD, which indicates that the ERA-Interim agrees best with in situ soil moisture.The performance of CCI SM is comparable with the ERA-Interim soil moisture.Furthermore, the seasonal analysis was conducted to identify any seasonal variations of the dataset accuracies.The study period is separated into four seasons September/October/November (autumn), December/January/February (winter), March/April/May (spring), and June/July/August (summer) respectively.It can be seen from Figure 6 that all products have small variations over different seasons in terms of R, RMSD, and ubRMSD values.This indicates that the accuracy of these soil moisture products is stable in different seasons.
In addition, the influence of land use on the statistical scores was also examined.The stations are divided into four land use groups: bare soil (4), cropland (7), forest (3), and grassland (4).The results were shown in Figure 7.In general, different land use categories have similar performance in terms of statistical scores for all the products except SMOS.SMOS has very low R value over cropland.These results suggest that the performance of soil moisture products is generally insensitive to land use.

Discussion
This study examined the ability of several soil moisture products to represent the surface soil moisture conditions over Yunnan, China.The results highlight the performance of ERA-Interim and CCI SM.From the time series analysis of both absolute values and anomalies, it is found that ERA-Interim and CCI SM can capture the variations of in situ soil moisture and respond well to the rainfall events.From quantitative comparison with in situ measurements, the ERA-Interim has the lowest RMSD and ubRMSD.The CCI SM has comparable performance to ERA-Interim.The accuracy levels are also similar to that reported by validation studies over other regions [59,61,62].Additionally, the seasons and land use have little effect on the performance of these products.This indicates that both ERA-Interim and CCI SM have the potential to benefit the local hydrological applications such as drought monitoring, and water planning and management.
The unfavorable results of AMSR-E and SMOS are very likely due to the affects of RFI.The RFI disturbs the natural microwave emission observed by SMOS, leading to the unreliable estimation of soil moisture.The impacts of RFI on SMOS and AMSR-E have been found in Western Europe, Northeast Asia, and Middle East [58,63].The present study further indicates the strong impacts of RFI on SMOS and AMSR-E in Southwest China.
Although most of the soil moisture can represent the surface soil moisture, there are still several issues that need to be addressed in future studies: (I) the scale mismatch between satellite pixel and in situ point; (II) the mismatch between the satellite penetration depth and in situ measurement depth; (III) the inaccuracies of the input data (such as land surface temperature and soil texture) for the soil moisture retrieval; and (IV) the improvement of the retrieval algorithms.
In addition to the improvement of soil moisture retrieval accuracy, there is also a need to improve the spatial resolution of the soil moisture product.The current global soil moisture products normally have spatial resolutions of tens of kilometers.However, many regional hydrological applications require a spatial resolution of 1-10 km [64,65].The optical/thermal infrared (TIR) sensors can provide complementary information of soil moisture at higher spatial resolutions.Therefore, the synergistic use of microwave and optical/thermal data to estimate soil moisture at high spatial resolution should be another direction in future study.

Conclusions
Validation of satellite-based soil moisture products is essential at both the regional and global scale.The present study evaluated four satellite-based and one reanalysis soil moisture product over an intensive soil moisture network in Southwest China.The results show that these products can capture well the seasonal variations of in situ soil moisture.Larger deviations were observed in particular for AMSR-E and SMOS products.The SMOS underestimates soil moisture, while AMRS-E overestimates soil moisture.The biases of AMSR-E and SMOS are very likely due to the strong radio frequency interference (RFI) that affects the measurements in this region.Quantitative validations show that ERA-Interim (R = 0.782, ubRMSD = 0.035 m 3 /m 3 ) and CCI SM (R = 0.723, ubRMSD = 0.046 m 3 /m 3 ) outperform the other products.In addition, these soil moisture products were found to provide similar performance across the year and for different landcover types.No systematic differences in soil moisture accuracy could be identified throughout the different seasons.Due to their long-term coverage (1978-2013 for CCI SM, 1979-present for ERA-Interim), the CCI SM and ERA-Interim dataset have the potential to provide valuable information for hydrological applications and water resource management applications in the future.

Figure 1 .
Figure 1.Study area in Southwest China (a) and locations of in situ soil moisture stations (b).

Figure 2 .
Figure 2. Temporal evolution of the averaged in situ measurements, soil moisture products, and precipitation at network scale: in situ mean soil moisture (black solid line) and its inter-quartile range (grey shaded areas); European Space Agency's Climate Change Initiative (CCI), Interim Reanalysis (ERA), Soil Moisture and Ocean Salinity (SMOS), Advanced Microwave Scanning Radiometer for the Earth observing system (AMSR-E)_A_X and Advanced Scatterometer (ASCAT)_D (points with different color); and precipitation (top of each panel).The soil moisture has the unit of m 3 /m 3 (left y-axis), while precipitation has the unit of mm (right y-axis).

Figure 3 .
Figure 3. Bar plots of the comparison results between soil moisture products and in situ measurements at each station.

Figure 4 .
Figure 4. Temporal evolution of the anomalies from averaged in situ measurements, soil moisture products, and precipitation at the network scale: in situ mean soil moisture (black solid line) and its inter-quartile range (grey shaded areas); CCI, ERA, SMOS, AMSR-E_A_X, and ASCAT_D (points with different colors); and precipitation (top of each panel).The soil moisture has the unit of m 3 /m 3 (left y-axis), while precipitation has the unit of mm (right y-axis).

Figure 5 .
Figure 5. Bar plots of the results from anomalies comparison between soil moisture products and in situ soil moisture at each station.

Figure 6 .
Figure 6.Bar plots of the comparison results between soil moisture products and in situ measurements over different seasons.

Figure 7 .
Figure 7. Bar plots of the comparison results between soil moisture products and in situ measurements over different land cover types.

Figure A1 .
Figure A1.Bar plots of the comparison results between in situ measurements and soil moisture products: (a) AMSR-E; (b) ASCAT; (c) SMOS.

Table 1 .
Descriptions of the in situ soil moisture stations used in the study.

Table 2 .
Statistics of comparisons between averaged in situ soil moisture and soil moisture products at network scale.

Table 3 .
Statistics of the anomalies comparison between averaged in situ soil moisture and soil moisture products at network scale.