4.2.1. Dataset Characteristics
The inundation maps generated by Hess et al. from JERS-1 SAR [9
] and Chapman et al. from ALOS PALSAR [12
] were used for a basin-wide comparison. Although the three datasets cover different time periods and use different class definitions, this presents an interesting opportunity to compare the classifications also at sub-basin scale across the 25 BL2 catchments defined by Venticinque et al. [26
To accommodate a simple comparison between the datasets, the detailed and more numerous classes present in the JERS-1 classification by Hess et al. were combined to form the four main classes used in this study (i.e., IV, SV, OW and NF). As this study did not include macrophytes and did not distinguish between different vegetation types, as was carried out for the JERS-1 study; the macrophytes class in the JERS-1 study was therefore incorporated into the submerged vegetation class (SV), while the inundated shrub, woodland and forest classes were all combined into the inundated vegetation (IV) class.
Classes in the PALSAR-1 derived dataset by Chapman et al. included “occasionally flooded” and open water, and these were combined with the inundated vegetation (IV) and open water (OW) classes, respectively, for the maximum inundation extent. Similarly, occasionally flooded and occasionally open water classes defined in the PALSAR-1 dataset were associated with the non-flooded class (NF) in the minimum extent dataset.
It should be noted that the inundation figures for the JERS-1, PALSAR and PALSAR-2 products are not directly comparable, as each product represents a slightly different definition of inundation. The maps by Hess et al. are based on “snapshots” of the (September–December) 1995 low water and (May-Aug) 1996 high water extents, while the Chapman et al. product depicts the high water and low water averages during the 2006–2010 time period. The figures for this study used for this comparison, in turn represent the three-year combined maximum and minimum values.
As flood extents can vary significantly between years and between sub-basins across the Amazon, relative river gauge data from the ten ANA stations (in Table 1
) are also provided in Table 4
and Table 5
. For this study the stage values represent the water levels measured during actual PALSAR-2 acquisition dates, while for the Hess and Chapman maps (for which the exact SAR acquisition dates are not provided) the stage values indicate the station maximum and minimum readings for the respective acquisition periods, and thus only provide an approximate indication of the river levels during the JERS-1 and PALSAR acquisitions.
below shows the high water/maximum estimates for the three products. The total area of the Amazon Basin used in this study is 5,954,414 km2
, corresponding to the sum of the 25 BL2 sub-basins by Venticinque et al. [26
An initial observation is that both this study and Chapman have mapped fewer inundated environments (IV, SV and OW) than that by Hess, despite the fact that maximum river heights measured during the PALSAR and PALSAR-2 observations along most gauging stations are similar or higher than the corresponding values during the JERS-1 observation period. The fractions of total flooding mapped by this study and Chapman et al. amount to 9.0% and 9.5%, respectively; compare that to 10.7% by Hess et al. The discrepancy is to the largest extent caused by the significantly different estimates in the (enormous) Madeira basin (#11), where the difference in total flooded area estimated by Chapman et al. and this study on one hand, and that by Hess et al. on the other, amounts to about 100,000 km2.
Excluding the Madeira basin (as well as the Amazon Delta (basin #2) which is only partially covered in the JERS-1 product), we note much closer correspondence between the three products, with total maximum flooded area (including IV, SV and OW) estimated at 414,000 km2, 441,000 km2 and 423,000 km2 for this study, Chapman et al. and Hess el al., respectively.
On a sub-basin level, we note that the Amazon/Solimões floodplain (Basin #3) has a greater percentage of inundated vegetation mapped by PALSAR-2 compared to the other datasets. This correlates well with the river stage data for Purus, Amazon and Solimões stations, where the maximum for 2015 is the highest among the four years gathered between PALSAR-2 and JERS-1. The station maximums among the three stations are also close in value with that of PALSAR-2, and the dates of the station maximum and PALSAR-2 acquisition dates for its maximum across the three years for Purus, Solimões and Amazon coincide very well. Basin #3 has a 10–20% lower inundation percentage for 2006–2011 compared to the other two decades.
The large Xingu basin (#25) shows very low (<1.5%) inundated vegetation regions for this study; while twice and three times greater by Chapman and by Hess, respectively Comparing this to the Xingu stage data, the maximum levels are lower for the sensors compared to the station’s values; and 2015 shows a very low stage level for the PALSAR-2 acquired date compared to the other years, but that is not reflected in the estimated inundation percentage in Table 4
At the Negro station, the maximum levels for all sensors show a very close correspondence to the station level maximum. The maximum stage values for JERS-1 and the three years in this study are within 1 m of one another. The percentages of inundation in the Negro basin (#17) for the three years in this study range between 7.9 and 8.4%, and their similar inundation percentages correlate well with the similar stage values across the three years. The inundation percentage for JERS-1 is 11.0%, which is slightly higher than that from this study, despite the stage levels between the years being similar to one another.
shows the low water/minimum estimates for the three products. We note an overall close correspondence between the PALSAR and PALSAR-2 classifications. This study’s “minimum extent” constitutes the lowest total flooded area estimate (1% or 57,000 km2
), even though the PALSAR-2 datatakes did not manage to target the annual (2014–2017) minimum gauges very accurately. The Chapman study estimates the “average low water” at 1.6% (93,000 km2
), mainly due to high estimates for Amazon/Solimões floodplain (and two of its small tributaries, #13 and #18). The JERS-1 “1995 low water” classification estimates total flooded areas at 4.8%.