Evaluation of the MODIS collections 5 and 6 for change analysis of vegetation and land surface temperature dynamics in North and South America
Introduction
Satellites provide the ability to observe large spatial areas over long periods, enabling researchers the opportunity to reveal both abrupt and subtle changes in the vegetated land surface (de Beurs et al., 2015, de Beurs et al., 2018, Fan and Liu, 2016). To allow such analysis, it is important to carefully analyze and compare image time series so that the behaviors of the sensors and the algorithms that generate products can be well understood. With a better understanding of our data sources, researchers can be more confident that no false changes or trends are being reported, for example, due to operational error or degradation of the sensors (Wang et al., 2012, Zhang and Roy, 2016). There is now an abundance of freely available data from multiple remote sensing platforms, enabling comparative studies to analyze the consistency and accuracy of these data. It is important to assess the stability across senor platforms to create long standing archives (Fan and Liu, 2016) but as operational life rises (Belward and Skoien, 2015) it will become increasingly essential to understand how degradation over time impacts results and how degradation can be separated from other observed changes. The Moderate Resolution Imaging Spectroradiometer (MODIS), launched first on board the Terra spacecraft in December 1999 and later on board Aqua in May 2002, forms the basis for several clearly documented and freely available products at various spatial and temporal resolutions. The periodically renewed product “collections”, which result from algorithmic adjustments and improvements, provide strong argument for using the MODIS products.
Over the years several improvements have been made to the MODIS products from one collection to the next. After changes are approved for each new collection, the entire MODIS archive is reprocessed to ensure that users have access to the most consistent and accurate data possible. The latest MODIS collection (C6) provides several algorithm improvements and calibration adjustments that correct for sensor degradation, theoretically making the products more accurate. For example, one important aspect that affects all products based on MODIS Terra data is the correction for sensor degradation. There has been noticeable degradation in both the Terra and Aqua MODIS sensors, but previous research has shown there was substantially greater impact on the Terra sensor (Lyapustin et al., 2014). The sensors are launched with onboard equipment, such as a solar diffuser and a solar diffuser stability monitor to perform periodic calibration, but this equipment can also experience operational degradation (Wang et al., 2012, Xiong et al., 2001). Two documented events are reported to have had significant impact on Terra’s degradation. First, during a pre-launch thermal vacuum test, a portion of the door paint came off the nadir aperture door and coated parts of the optics and scanning mirror. The paint was cleaned but lasting residue or damage to protective coating appears to have impacted performance (Lyapustin et al., 2014). Second, in May 2003, the solar door was permanently opened, and the solar door screen closed. The fixed position of this equipment has caused the solar door plate to degrade at a quicker rate, decreasing the reflectivity (Lyapustin et al., 2014). The implication of decreasing reflectivity is a reduced capability to track sensor response over time (Lyapustin et al., 2014). These events lead us to expect significant differences between the data collected from the MODIS sensors aboard Terra and Aqua.
The collection 6 adjustments were implemented not only to correct sensor degradation impacts, but also to introduce several improvements in the vegetation index (VI) retrieval algorithms (Didan et al., 2015). With these improvements, we expect only minor differences between the Terra and Aqua products, but significantly different results between C5 and C6 (Zhang et al., 2017). Other papers have explored the differences between trends in VI data based on collections 5 and 6. For example, Detsch et al (2016) applied the Seasonal Kendall trend test to compare Terra and Aqua MODIS VI data from C5 and C6 for the period 2003–2010 in a study area surrounding the Kilimanjaro region of Tanzania. They found that at a seasonal scale, products created from Terra and Aqua in C5 and C6 compared well with each other. However, throughout the length of the time series, the negative impacts created by the degradation of MODIS Terra were noticeable; the NDVI collected from Terra in C5 displayed more browning and less intense greening in comparison to the NDVI product collected from Aqua. The NDVI collected from Terra in C6 now displays more greening compared to the Aqua product. The presence of more greening trends in collection 6 points to the calibration changes that were made to compensate for degradation in the Terra MODIS.
A global comparison of collection 5 and 6 VIs focused only on trends in the annual maximum vegetation index (Zhang et al., 2017). They noted sensor degradation in both Terra and Aqua: during the time span of 2003–2015, Aqua experienced an increase in NDVI of 0.03% year−1 and an increase in EVI of 0.11% year−1 from C5 to C6, respectively. We also expect to find at least some minor differences in MODIS land surface temperature data (LST) between C5 and C6. There are few comparative studies covering the LST products except those of (Duan et al., 2017, Duan et al., 2018, Duan et al., 2019), which describe the algorithm improvements for bare soil regions.
In our analysis, we investigate four MODIS products (MOD13C1, MCD43C4, MCD43A4 and MOD11C2) to determine the extent that changes made to C6 impact the overall trend results. We compare vegetation index products—the NDVI and EVI available from MOD13C1 with the NDVI and EVI calculated from MCD43C4 and MCD43A4. We also explore changes in LST from MOD11C2 to see if the changes in trends evident in the VIs are also apparent in a different product from the same sensor. Our study differs from previous studies (Detsch et al., 2016, Zhang et al., 2017) in that we focus less on the difference between Terra and Aqua and more on the comparison between C5 and C6. We also apply a nonparametric trend analysis that is less sensitive than simple linear regression to outliers, seasonality, and autocorrelation (de Beurs and Henebry, 2004). Unlike many related studies, we look beyond the regional scale and calculate the trend results for the entire Western Hemisphere (excluding Greenland because of its extensive ice cover). We have previously demonstrated that the use of more than a single index time series can significantly improve trend interpretation and attribution, and we have advocated for the use of complementary suites of multiple indicators as the new standard approach for change analysis (de Beurs et al., 2015). Here we will follow this approach and not only investigate the changes in the vegetation indices by themselves, but also link the vegetation changes with warming and cooling of the land surface as observed by the land surface temperature data. Combining the results from different products observed at different regions of the electromagnetic spectrum—but linked through the biogeophysical processes of surface energy balance—allows us to portray change with more confidence than when relying on vegetation index data alone.
Section snippets
C5 and C6 data for comparison
The focus of this study relies on trend results derived from vegetation index data (VI) and land surface temperature data (LST). VIs are spectral transformations of two or more bands designed to enhance the contribution of vegetation properties. Although many vegetation indices exist, two of the most widely used are the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI), and these are the primary datasets analyzed in this study.
We use four different products
Results
An overview of all general trends for all products can be found in Table 3. We will discuss the differences in the individual products and the different collections below.
Trends in MODIS vegetation indices in collections 5 and 6
As early as 2012, Wang et al. (2012) reported on the impact of sensor degradation on MODIS collection 5 NDVI time series from Terra. That paper demonstrated a nearly threefold difference in the percentage of negative trends derived from Terra compared to those derived from Aqua for the period 2002–2010 (17.4% vs. 6.7%). Here we have presented a thorough and careful analysis of the consistency between MODIS collections C5 and C6 for the western hemisphere for three different products: a combined
Conclusions
There have been few papers discussing the discrepancy between C5 and C6 vegetation index data (Lyapustin et al., 2014, Zhang et al., 2017), and the major trend discordances between the two collections have received limited attention. As a result, even in 2018, there are still papers published based solely using C5 NDVI data (e.g., Fang et al., 2018), while others do not specify the collection of the data used (e.g., Browning et al., 2018, Murthy and Bagchi, 2018).
The aim of this study was to
Acknowledgements
This research was supported, in part, by the NASA Science of Terra & Aqua project NNX14AJ32G entitled Change in our MIDST: Detection and analysis of land surface dynamics in North and South America using multiple sensor datastreams, and the Center for Global Change and Earth Observations at Michigan State University. All MODIS data products used in this study were retrieved from the online Data Pool, courtesy of the NASA Land Processes Distributed Active Archive Center (LP DAAC), USGS/Earth
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