A total sum of 8246 kilometres of palaeo-channels was mapped using both SMTVI and spectral decomposition techniques (Figure 6
). The palaeo-rivers and channels were delineated by hand using a vector-based GIS desktop platform as testing of edge/line detection algorithms and automatic processing did not provide results that were reliable enough. The resulting map of palaeo-rivers in the area cannot in any way be taken as definitive, as multiple palaeo-rivers may remain undetected. This is particularly true of the southernmost sector of the study area (south of the main Ghaggar-Hakra channel) as defined by the results of the NDVSI analysis (Figure 4
). The results of the two methods adopted will now be described and analysed.
SMTVI resulted in the clear visualisation of a range of formerly reported palaeo-rivers, but also multiple previously undetected ones. This approach only failed to yield results in the southern sector of the area under study, in particular below the main Ghaggar-Hakra channel where dune morphologies dominate the landscape. In this area, the results of the NDVSI analysis (Figure 4
) indicate a very low seasonal variability of vegetation.
A total of 5034 km of palaeo-rivers have been mapped through SMTVI, and these were predominantly in the northern sector of the study area (Figure 6
). The results of this technique show that the traces of palaeo-rivers form a more complex network than previously suspected. Previous studies in this area have been successful in detecting a number of these palaeo-rivers (Figure 7
). For example, the results obtained by Yashpal et al. [22
] and van Dijk et al. [40
] are partly coincident with our own even if, in the Yashpal et al. [22
] case, some features have been joined to form continuous channels for which we have not found evidence. In several instances, palaeo-rivers detected by these authors seem to be coincident with large modern irrigation channels, which is unsurprising given the early date and small scale at which the analysis was conducted. SMTVI has also been unable to find traces of the old channel of the Sutlej and the old channel of the Yamuna 1, as described by Yashpal et al. [22
]. All features detected in the northern sector of the study area by van Dijk et al. [40
] have been identified, as have most of those detected in the west sector, but we have also detected many others not documented by van Dijk et al. [40
]. Bhadra et al’s [39
] hypothesised location of palaeo-rivers in the area do not coincide with the results of our study nor with any other previous analyses under consideration. In general, it can be said the SMTVI provided a more consistent approach for the medium resolution detection of palaeo-rivers across a very large area displaying higher values of seasonal vegetation variability.
The visibility of the palaeo-rivers in SMTVI (Figure 3
) is a reflection of the interplay between past fluvial geomorphology and current multi-year vegetation dynamics. In this regard, a more detailed analysis of the characteristics of the detected features can help the understanding of past environmental conditions. Monsoonal rains imply a strong rise of soil moisture and surface vegetation. However, palaeo-rivers in SMTVI are not detected through higher vegetation values but, on the contrary, as linear groupings of low value pixels, that is, as features where the average vegetation values during the last thirty years have been significantly lower than in their surroundings. The very low terrain slope (average of 0.37%) and the high volume of rainfall during the winter and summer rain periods, which merge and flow down the foothills of the Himalayas, combine to create a large depositional floodplain. The rivers that have been documented form a parallel drainage pattern and present a highly sinuous and meandering morphology in which multiple meanders and meander scars are visible. The low value pixels that define their trace seem to correspond to two types of features: (1) large natural levees typically formed after multiple flooding episodes [49
] - these are usually elevated with respect to the floodplain, as confirmed by SRTM 30 m data, and accumulate thicker and coarser sediment [50
]; and (2) coarser channel deposits and alluvial deposits accumulated in the inside of river bends. Both proximal areas of levees, inner river bends and channel bedload tend to concentrate coarser sediments [51
], which might have resulted in less fertile soils with higher water subsurface infiltration and lateral water runoff in the case of the levees. Water would have been concentrated and retained by the finer material and flatter topography present at the backswamp areas of the floodplain, where bright pixels reflect a correlation between seasonal rainfall and healthier vegetation. The contrasting vegetation response between the coarser deposits close to the river channel and their surrounding floodplain account for the clear palaeo-river visibility in this area. The presence of ribbon-like deposits and their attendant levees is consistent with the existence of rivers with a high suspended sediment fraction forming a rapidly aggrading system where avulsive channels are common [51
] (p. 6).
Several relevant results for the reconstruction of the hydrologic history of the northern sector of the study area have been obtained through the use of seasonal vegetation mapping: (1) the confirmation of a major palaeo-course of the later Sutlej river, which contributed to the Ghaggar-Hakra system, though when and for how long remains unknown (top right corner of the lower image in Figure 3
); (2) the migration of this same major watercourse from the Ghaggar-Hakra catchment to that of the Sutlej, which would have significantly reduced the amount of water available in the Ghaggar-Hakra’s lower course; and, perhaps most significantly, (3) the multiplication of the palaeo-rivers known in the area, which indicates that as a whole, the region has an extremely complex fluvial history, which will have had important and as yet poorly resolved consequences for water availability and thus also for past human habitation and land-use. SMTVI also allowed study of the morphology of the palaeo-rivers and documentation of multiple avulsion episodes, with consequences for the human habitation and use of the area through which these flowed.
The use of vegetation indices, however, did not offer any indication of the location of palaeo-rivers in the southeastern sector of the study area. This area is characterised by arid conditions and a reduced rainfall and, as NDVSI (Figure 4
) clearly shows, seasonal variability does not produce significant changes in vegetation. Vegetation indices are, therefore, not considered a consistently reliable approach for the detection of palaeo-rivers in this area.
4.2. Spectral Decomposition Techniques
A total of 1920 kilometres of palaeo-channels have been detected through the use of spectral decomposition techniques of seasonal multi-temporal data. Not surprisingly, TCTs of the mean Monsoonal image composite values for Greenness and Wetness produced similar results for the northern area to those obtained with the application of vegetation indices. The RGB composite of the different bimensal EVIs was more efficient in detecting palaeo-rivers due to the higher temporal resolution of the data employed, which incorporated differences in seasonal rains. In contrast to the vegetation indices, TCT, however, excelled in the detection of palaeo-rivers in the southern sector of the study area where a large relic water course with several tributaries contributing to the Ghaggar-Hakra system was identified and its trace could be followed for more than 300 km (Figure 8
). This feature was visible through its contrasting low value pixels in the fourth (where it was clearly discernible), sixth and only marginally in the second axis corresponding to greenness. The palaeo-river was noticeably more visible in the TCT of the dry months composite (Figure 8
), slightly less in the TCT of the mean image of all available Landsat 5 imagery and scarcely visible in TCT of the monsoonal composite. This pattern of visibility indicates that, although seasonally variable, the visibility of this palaeo-river is not related to vegetation changes. This palaeo-channel might have been previously documented in part and with large inaccuracies by Yashpal et al. [22
], who considered it a second old channel of the Yamuna River (Figure 7
). Later, Rajani and Rajawat [37
] identified its terminal sector as the Vedic Drishadvati. Recently, Mehdi et al. [39
] again identified this palaeo-channel and reliably traced its course and those of several tributaries covering a total length of approximately 416 km.
Although slightly meandering, the course of this river does not show evidence of levees or aggradation, neither are these features present in the 30 m/cell SRTM data. Instead, its course is marked by a clear erosive channel, which corresponds to a higher slope than that documented from the northern plain. The orientation of this palaeo-river in conjunction to topographic data suggests that it might have been collecting water from different sources. While its northern section could have been sourced from the Yamuna River catchment, as suggested by Yashpal et al. [22
], its southern tributary, which joins the northern channel in Bhadra (Figure 8
b), appears to originate in the Aravalli range. This area of the Aravalli has an important component of carbonates. Dissolved carbonates could have been incorporated in the water and influenced the composition of the sediments transported by the river and deposited along its banks. Sediment rich in calcium carbonate has a clear colour, which could be responsible for the noticeably higher albedo of the fields along the palae-oriver course (Figure 8
). The presence of carbonate, gypsiferous clay and strings of playa lakes are all considered residual elements of palaeo-channels [9
] (p. 49). Although little geological coring has been done in this area, the presence of gypsum and carbonates is well documented in Karsandi palaeo-environmental record [52
], 21 km south of this channel at Nohar. This particular composition of the channel’s filling sediment could also explain the decreased visibility of the palaeo-river during the rainy season, as higher soil moisture would have reduced the contrast of carbonate-rich sediment. SRTM data also show how the southernmost tributary of this river is completely obliterated under the dunes marking the northeastern edge of the Thar Desert (Figure 8
, dunes in bright yellow).
The southeastern sector of the study area between Hisar and the Yamuna (Figure 9
), where NDVSI analysis produced average values for seasonal vegetation variability, also offered interesting results. Figure 9
a shows a strong visibility of palaeo-rivers and channels in the southeastern extreme of the study area only during wet months, where an anastomosing river with multiple meander scars can be appreciated coming from the Yamuna bluff line. TCT of images acquired during the wet months shows similar detection capabilities to that of SMTVI (Figure 9
b). In contrast, the complex network of palaeo-channels west of Hisar, and much closer to the Thar Desert edge, identified in Figure 9
c area was only visible in the TCT produced from images taken during dry months. In many cases, particularly for the spectral decomposition techniques and the southeast sector, the alternation of multi-temporal seasonal means for dry and wet seasons provided complementary identifications as some palaeo-rivers or stretches of these were only visible during dry or wet seasons. The combination of seasonal images was also important because it allowed a clear differentiation between palaeo-rivers and irrigated fields, which formed linear and dendritic patterns (similar to those typical of palaeo-rivers) following water channels. Artificial channel-irrigated fields were particularly visible in the composites of dry seasons as their moisture content boosted their visibility with respect to their surrounding fields. Their identification was an important step to avoid the mapping of false-positive palaeo-rivers. This clearly exemplifies the importance of seasonality for the identification of palaeo-features in those areas where NDVSI shows average values for seasonal vegetation variability.
Seasonal PCA analysis of the mean values of L5 images produced similar, if less clear, results to those of the TCT. The northern network of palaeo-rivers was visible, although with much less detail than that obtained with the seasonal EVI analysis. This indicates the close relationship between the visibility of northern palaeo-rivers and long-term vegetation dynamics, which diffuses with the use of spectral decomposition techniques. The southern palaeo-river network (Figure 8
) was only visible in the dry season PCA, where it displayed high values in principal components 3 and mostly 4, where it was clearly identifiable. It seems therefore that seasonal PCA could provide a good approach to areas with less vegetation. In addition, the visibility of this palaeo-river network in the more marginal axes of TCT and PCA points to the inadequacy of the more conventional RS techniques usually applied in the detection of the ancient rivers in the Indus watershed. The success of the approach adopted by Mehdi et al. [39
] in detecting a channel (Figure 9
) joining the modern town of Hisar and the Indus city of Rakhigarhi is probably due to specific environmental conditions during the acquisition of the single Landsat 7 image from which these authors developed a PCA. Unfortunately, no information about this particular image was provided, and it thus not possible to evaluate their reconstruction.
For those areas with average and low NDVSI values, PCA seasonal analysis provided a complementary source of information to previous TCT and SMTVI data and was useful to increase the number of detected traces while reducing the number of false positives.