Surveying Lake Titicaca

In October 2024, officers from the Category “A” Specialization Programme in Hydrography for Naval Officers of the Peruvian Navy conducted their complex multidisciplinary field project at Lake Titicaca at Puno in Peru, which is the highest navigable lake in the world (3,800 metres above sea level). The purpose of this survey was to address issues in the lacustrine environment due to hydrological processes that have occurred in recent years in this water body.

For example, it aimed to evaluate the vertical reference level used for sounding reduction and the creation of backscatter mosaics by optimizing resources to achieve a characterization of the lake-bed using a Norbit iWMBS (0.9° x 1.9°) 400kHz multibeam echosounder integrated with Applanix Wavemaster II OEM GNSS/INS and acquisition software Norbit WBMS GUI v11.3.2 and DCT v3.2.1. The post-processing was done with QPS maritime geomatics software solutions Qimera v2.6.2 (multibeam processing) and FMGeocoder Toolbox v7.11.1 (backscatter processing), and maps were developed with QGIS v3.34.15 Prizren.

Vertical reference level employed

The vertical reference level is a persistent issue in aquatic areas during hydrographic surveys, and even more so in a navigable water body such as Lake Titicaca, which presented an anomaly of -1.23 metres relative to its historical level according to the National Meteorology and Hydrology Service (SENAMHI) in 2023, reaching levels not recorded since 1999. This trend towards historically low levels is due to changes in the usual rain patterns that directly feed the lake, as well as the tributary basins and aquifers that contribute to the lake’s water balance (see Figure 1). SENAMHI indicates that the lake’s water level is at critical levels lower than those of 2023 due to a rainfall deficit, with certain regions experiencing periods of up to 40 days without precipitation and higher daytime temperatures, which cause an increase in the evaporation rate in the Titicaca hydrographic region. Data was obtained from the Hydrological and Meteorological Station within the Bay of Puno and analysed by SENAMHI.

The vertical reference level of Lake Titicaca is 3,809.93 metres above sea level. This level has been established as the hydrographic zero of the lake since 1955. Due to the issues presented in the HIDRONAV-6525 chart ‘Puno Bay’, a comparative analysis was conducted between the 2008 bathymetric survey and the multibeam survey carried out by the officers from the Category “A” Specialization Programme in Hydrography for Naval Officers of the Peruvian Navy, using the same vertical reference level marks. During the 2024 survey, the water level was at 3,807.80 metres above sea level, more than two metres below the historical level.

Multibeam data for bottom characterization

Previously, bottom characterization was carried out by means of physical sediment samples at homogeneously distributed sampling points determined according to the area extent, which was time-consuming and costly. However, the methodology recommended by the International Hydrographic Organization (IHO) through its standard S-44, sixth edition, suggests that bottom characterization be performed by a combination of complementary methods. These include inference methods, such as backscatter, derived from the processing of data obtained by multibeam bathymetry, and physical methods, which involve the direct collection of bottom samples using a grab, allowing a detailed analysis of the granulometry and composition of the sediments in the study area. The different types of bottom classified by granulometry scatter sound in various ways, providing information on their roughness and hardness. The integration of backscatter and bathymetry data using multibeam echosounders therefore provides a detailed picture of the bottom.

The backscatter acquisition parameters were adjusted due to the lake environment conditions, which differ from those of the marine environment. The backscatter intensity (dB) presented in Figure 3 corresponds to the fundamental echo level from the lake bottom, obtained without any calibration procedures. The processing was performed using QPS FMGT, which provides tools for normalizing backscatter imagery. Initially, the backscatter mosaic was generated with a resolution of 0.3 metres, and greyscale values were adjusted according to reflectivity. Secondly, angular range analysis (ARA) was conducted to characterize the data, and thirdly a beam pattern correction was applied. Finally, 14 sampling points were selected in areas with the highest reflectivity variation, where physical sediment samples were collected. The backscatter values were correlated and adjusted in the mosaiced data conforming to the physical sediment samples obtained with a Van Veen grab sampler, inferring their granulometry and providing key information for their analysis and description. This procedure is recommended for backscatter analysis for future studies.

To ensure a highly accurate product, the survey was conducted according to the standards of a special order hydrographic survey. This approach was particularly relevant in the peripheral areas of the acoustic beam, where ambient noise and beam scattering introduce higher uncertainties.

Comparative analysis of multibeam bathymetry

The comparison between the data obtained in the present survey and that from the 2008 nautical chart showed differences of up to three metres. It should be noted that this comparison was made in the depth ranges of three to ten metres, ten to 20 metres and above 20 metres. These discrepancies highlight the need for more frequent updates of the lacustrine basin cartography and underscore the importance of considering water-level variability in future hydrological analysis. The observed differences between historical and current data and their impact on navigational safety are notable. Furthermore, this considerable variation in lake depths suggests that the vertical reference level used in the 2008 nautical chart, given that more than 16 years have passed and recent records show lower-than-usual levels, should be evaluated to establish a new reference level for sounding reduction. This would provide greater safety for navigators, in line with the current average levels of the lake.

The analysis of the obtained results also revealed patterns of circular depressions in certain areas of the lake-bed, whose genesis could be associated with aquaculture activities in the region, as shown in Figure 4. These findings are important for understanding the dynamics of the bottom of Lake Titicaca.

Analysis of backscatter mosaics

The physical sediment samples collected at the 14 sampling points were analysed in the chemical laboratory of the Directorate of Hydrography and Navigation and the results obtained were correlated with the reflectivity values ​​represented in greyscale, based on the decibels backscattered from the lake bottom. This correlation allowed the sediments to be classified into the following categories: silty sand (-71dB to -41dB), fine sand (-41dB to -28dB) and medium sand (-28dB to -11dB). This analysis facilitated the reclassification of the raster image generated from the backscatter mosaic, achieving a comprehensive characterization of the lake bottom throughout the study area. The results of this reclassification are displayed in Figure 5.

Conclusion

The water level of Lake Titicaca has shown significant depth variations in recent years, which are primarily attributed to decreased precipitation in the region and the tributary rivers that feed the lake. This issue has been demonstrated through comparisons made using an optimized methodology to leverage state-of-the-art multibeam technology – in which derived backscatter data was key – revealing significantly lower depths than historically recorded. This methodology allows for a more accurate and detailed representation of the study area’s bottom, significantly reducing associated costs and time. The analysis leads to the proposal to update the lake’s reference level for sounding reduction, also known as the hydrographic chart vertical datum, with the aim of providing greater safety for lake navigators. Additionally, the lake-bed depressions, which were found below the aquaculture stations, presented a case study for using geophysical survey methods to determine their origin. The results of a bottom characterization therefore provide valuable insights and contribute to the understanding of the lake bottom composition. Public and private geosciences research institutions are interested in being part of this kind of hydrographic activity to share knowledge and complement their work.

The authors would like to thank the head of the hydrography department of the Peruvian Directorate of Hydrography and Navigation, Commander Rodrigo Torres Santa Maria, without whose support and management this complex multidisciplinary fieldwork would not have been possible.