Here, we use approximately three hundred ninety-two thousand radar altimeter bursts of Sentinel-3 satellite constellation data (S3A and S3B) collected along six ground tracks (Fig. 1) since 2016 within the Salar de Uyuni boundary to observe how the smoothness of the surface evolves spatially and temporally.
The first-ever field survey in the interior of the Salar de Uyuni occurred from 16-20 February 2024 to check the validity of the interpretation of the radar altimeter observations. The photograph taken from a drone camera on 20 February 2016 at 10:17 UTC, which is exactly coincident with the Sentinel-3 satellite overpass, shows a uniform white disc-like spot that is the mirror image of the sun (Fig. 2). This finding indicates that surface imperfections do not substantially impact its specular reflective characteristics. With an optical tool, we measured zero vertical surface displacement within ±0.5 mm at a water depth of 1.8 cm. According to Fraunhofer's criterion, a water surface can be considered electromagnetically smooth at a radar altimeter wavelength of 2.2 cm if the root-mean-square (rms) of the surface height distribution variations in height is less than 0.69 mm. The measured RCS values were 120 ± 0.3 dBsm over 100 km along the track around the in situ site, which closely aligns with the theoretical maximum expected for specular bursts in the Salar de Uyuni (RCS = 119.76 dBsm; see the 'Methods'). Observations also reveal that the wind decreased from an initial value of 4.5 ms to 3.4 ms (at the time of satellite overpass and optical measurements) and continued to decrease thereafter. During the four days prior to the satellite overpass, at this and other locations at similar water depths, a maximum wind speed of 5 ms was recorded, and similar vertical surface displacements were calculated. The measured values match the expected February wind speed of less than 8 ms, as also reported in Schmidt.
The statistical distribution of RCS values along the six Sentinel-3 tracks within the Salar de Uyuni boundary during the period from 2016 to 2024 is shown in Fig. 3. The red line represents the RCS shape as a continuous function of dBsm, with a mixture of two distributions that peak at 111.72 and 119.5 dBsm. The narrow distribution with a peak of an approximately 119.5 dBsm corresponds to specular wet surfaces, with the satellite crossing water areas that are not disturbed by wind. The RCS values are very high and close to the maximum theoretically expected value for specular echoes in the Salar de Uyuni; therefore, the surface is considered smooth at the radar altimeter wavelength.
The wide distribution of the approximately 111.72 dBsm peak represents nonspecular surfaces. The RCS values vary in the broad range of 74-118 dBsm. They are probably due to extensive free-standing water that is rougher due to the windiest conditions during the wet season. We suggest that the highest values are also related to the surface characteristics during the dry season. Notably, surfaces that have no moisture content (which we call 'dry') can have varying RCS in the Ku-band depending on their material, geometry and smoothness. The Salar de Uyuni during the dry season is composed of a salt crust. It exhibits higher RCS values compared to other dry surfaces e.g. desert made of sand; pavement made of asphalt. With the evaporation of free water, the continuous film of water is replaced by a flat surface of salt that exhibits closely packed cauliflower structures. Although it is quite rough at the subcentimetre scale, the surface appears to be a smooth reflector at a 2 cm wavelength. The dominant effect is a quasi-specular reflection. Similar responses are found in the Utah Desert, which has geologic features comparable to those of the Salar de Uyuni.
Figure 4 shows the 15-day fraction (%) of specular and nonspecular bursts relative to the monthly total, corresponding to the two previously identified RCS value distributions. The analysis reveals that approximately 11% of the total RCS values refer to specular surfaces, whereas the remaining RCS values are for nonspecular surfaces. The surface starts becoming radar smooth at the beginning of the wet season in December. The peak period is from late January to early March. In late February, visitors have the highest chance of seeing the mirror-like effect, as approximately 50% of the bursts are radar smooth. From late April to November, the surface becomes radar smooth only on very rare occasions.
A metric based on sidelobes in radar altimeter waveforms is also introduced to better refine the decision whether the water is smooth or alternatively not smooth or there is no water. Supplementary Fig. 1 shows the Sentinel-3 radar response during 1 March 2024. The lowest panel highlights that the average waveform is close to the theoretical one by applying Hamming window. In order to see the slight broadening of the main peak, no post-processing correction for the beat-frequency drift is applied. The central panel displays the along track RCS, with an average value of around 120 dBsm. Supplementary Fig. 2 is from a Sentinel-3 passage 27 days later (28 March 2024). The average RCS has dropped to around 108 dBsm and there is a raised level on the sidelobes, presumably due to the roughness of the dry lake surface. We analyse 385 passes, which are then divided into two categories (specular and quasi-specular), based on the sidelobe levels. The water surface is assumed specular if the sidelobes raise less than 1 dB from the theoretical value. If sidelobes raise more than 3 dB the water surface is assumed quasi-specular. Supplementary Fig. 3 shows the probability density function (PDF) for the two categories. The majority of the data fall in the quasi-specular category. The specular cases sharply peak with a median of around 119.6 dBsm. Only 12 cases are identified in the wet category. There are two in January, six in February and four in March, confirming that February is the most likely month for smooth water.
The RCS values corresponding to radar-smooth surfaces reflect the rainfall cycle (Fig. 5) in the closest city of Uyuni (Fig. 1), where a meteorological station operates. These events are consequent to the rainfall events over the study area during the wet season. Pearson correlation coefficients were computed considering different temporal lags for rainfall accumulation and RCS representative values (see Supplementary Note 1 for methodological details). The best agreement (r = 0.74 ± 0.05) occurred when 35 days of accumulated rainfall and mean RCS values greater than 110 dBsm were considered.
The only important exception during the dry season was August 2018. The area reached 9.5 mm and 4.5 mm of water depth on 4th and 5th August 2018, respectively, which was enough rain to cover a surface of 1.4 cm, assuming uniform accumulation.
During the dry season, the westerly wind typically prevents easterly moisture from precipitating in the Altiplano, where the Salar de Uyuni is located. We suggest that the exception is associated with a disruption of atmospheric conditions, possibly due to large-scale climate anomalies. This unexpected rainfall contributed to the temporary flooding of the Salar de Uyuni, and Sentinel-3 flew over the water layer on the 5 and 6 August 2018. There was a long range of peaks at approximately 120 dBsm (Fig. 6), a clear indication of the existing radar-smooth surface. A GPS-tagged photograph (retrieved from an on line source) taken on 13 August 2018 at a latitude of -20.414313° and longitude of -67.090105° confirms the presence of water inside the Salar de Uyuni southwest of Plaza de las Banderas, which is along the road leading west from Colchani (the gateway to the salt flat).
Figure 7 shows the spatial distribution of bursts on a year-to-year basis during the wet season from late 2021 to 2024 when the Sentinel-3 satellites were fully operating. The RCS values decreased following lower smoothness as the colour changed from blue to yellow. In this figure, the focus is on the rainiest and driest years. From December 2021 to March 2022, the Salar de Uyuni received a large amount of rain (222 mm), which occurred mostly in December (102 mm) and that gradually decreased in January (84 mm), with a slow decline in February (32 mm) until a minimum was reached in March (4 mm). The rise in water during December led to a rapid RCS increase in some areas of the Salar de Uyuni. An increase was especially observed on the western edge. Smoothing remained stable in the middle (track 0167 of S3A) from January to March.
The Salar de Uyuni had the driest period from December 2022 to March 2023, with a total rainfall of 87 mm. Rainfall started in December, with a value of only 25 mm. The lowest value was observed in January (8 mm), so visitors had fewer chances to see the mirror-like effect. Almost half of the total rain (42 mm) that fell in March (12 mm) was recorded in February. High RCS values were concentrated only in the middle of the Salar de Uyuni during December. The limited input of free water during January slightly decreased the smoothness of the Salar de Uyuni because of evaporation. The RCS values rapidly increased in response to the enhanced rainfall during February, with the Salar de Uyuni becoming fully filled and smooth in the central part. The RCS then quickly decreased in March following the reduced amount of rain.
December 2023 to March 2024 was the wettest part of the analysed period, with 265 mm of accumulated rain. The wet season started with approximately 25 mm in December, increasing to approximately 37 mm in January and 64 mm in February, and finally, an exceptional peak of 139 mm occurred in March; this pattern meant that visitors would have had a longer time to see the reflection. During the same period, the RCS values follow the rainfall variations, with the specularity starting in December, with peaks at approximately 120 dBsm when the rainfall is concentrated mostly in the northwestern part of the Salar de Uyuni. In January, the RCS increases and spreads to the east, with most of the Salar de Uyuni becoming completely specular in February and March 2024.
The evaporation rate is important to understand the short time between a specular and following quasi-specular pass. Borsa suggested that evaporation over Salar de Uyuni is spatially and temporally uniform at approximately 2 mm/day. The satellite passage on 10 March 2017 occurred after a 27-day period in which 7.5 cm of rainfall was recorded. Based on the previous evaporation rate, around 2 cm of surface water remained on the Salar de Uyuni at the time of the satellite. During the following 27 days, rainfall was negligible. At the time of the next satellite pass on 6 April 2017, quasi-specular reflections were observed, indicating a drying tendency. A similar pattern was observed in early 2019. Prior to 14 January, no rainfall was recorded. This was followed by several days of rainfall totalling about 8.5 cm. By 10 February, the satellite detected specular reflections over the Salar de Uyuni, with an estimated 3.1 cm of surface water. An additional 3.3 cm of rainfall occurred before the next satellite passage on 9 March, when about 1 cm of water remained. After that, rainfall ceased, and on 5 April 2019, the surface again exhibited quasi-specular reflections. These observations highlight the important role of evaporation during the relatively short transition from specular to quasi-specular conditions.
The rainfall intensity in the Salar de Uyuni has shown quite unpredictable schedules in recent years, which suggests a connection to climate change due to El Niño‒Southern Oscillation (ENSO) conditions that fluctuated from 2021-2024. The La Niña event began in September 2020, with pronounced effects from 2021 through 2022, resulting in increased precipitation in the Altiplano. Then, La Nina shifted to a weaker status from 2022-2023, explaining the below-average rainfall over the region. WMO reported that La Niña ended and El Niño conditions began to develop in early 2023; however, during the latter part of 2023, the El Niño conditions weakened, which might explain the possible enhanced moisture transport to the Altiplano, causing above-average precipitation over the Salar de Uyuni.
To more effectively evaluate the relationship between rainfall and ENSO variability, cumulative rainfall during the wet season (December-February, DJF) was calculated using two complementary approaches. First, we extended the observational time series from the Uyuni station to cover the period 1976-2025, allowing for an analysis of long-term inter-annual variability in DJF rainfall (Fig. 8).
Second, we extracted recent DJF precipitation estimates (2021-2024) from the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) satellite-based data, spatially averaged over the Salar de Uyuni Basin (Fig. 9). In both cases, the values were compared against the Oceanic Niño Index (ONI), using thresholds of ONI ≥ +0.5 to indicate El Niño conditions and ONI ≤ -0.5 to represent La Niña events.
The extended La Niña from late 2020 to 2022 coincided with a notably wet DJF in 2021-2022, which recorded 222 mm at the Uyuni station and was associated with early and persistent radar-smooth surface conditions. However, in 2022-2023, despite the continuation of La Niña (ONI = -0.7), DJF rainfall decreased sharply to 87 mm, and specular reflections were minimal. This breakdown in the expected ENSO-rainfall relationship underscores the importance of examining longer records and suggests a potential weakening of the ENSO teleconnection or the influence of additional drivers. The long-term comparison between ONI phases and DJF precipitation from 1976 to 2024 reveals that La Niña phases often align with wetter summers and El Niño events with suppressed rainfall, but these patterns are not consistent every year. Such discrepancies highlight the complexity of ENSO's influence on the Altiplano's hydrology and justify the need to incorporate multi-decadal data in climate-rainfall analyses.
Rainfall records from Potosí Los Pinos, a representative station in the southern Altiplano, indicate that approximately 77% of the annual rainfall occurs between December and March. Future climate projections suggest that global warming could intensify ENSO variability, potentially leading to more extreme El Niño and La Niña events. Such changes may disrupt current precipitation patterns in the Altiplano, altering the seasonal dynamics of the Salar de Uyuni. Also, additional modes of variability, such as the Atlantic Meridional Mode (AMM) and the North Atlantic Oscillation (NAO) may also modulate regional moisture fluxes and contribute to observed anomalies. One example is the off-season rainfall event in August 2018, during a moderate La Niña, which led to the temporary flooding of the Salar de Uyuni.
Our results indicate that during the wet season, water typically sits on the surface, but its smoothness evolves spatially and temporally. The Salar de Uyuni is not a vast uniform mirror for the radar altimeter. Therefore, it is also likely that it is not a large mirror for optical wavelengths, as suggested in the literature. Satellites indicate how the Salar de Uyuni transforms into a smooth wet surface early on, thereby helping visitors to better decide when to travel to see the mirror effect. As radar echoes from a smoothed water surface tend to be very strong, the required radar power can be considerably reduced with a paradigm change away from large spacecraft towards a swarm of small satellites that would provide better coverage with more frequent revisits.
We anticipate a parallel study to explain why no waves were generated initially, despite considerable winds. According to Munk, there is a threshold wind speed below which waves do not form and the flow of water is laminar. Although several theoretical and experimental studies have been conducted, some discrepancies are found in the reported thresholds. A wave model will be used to establish a threshold for wave generation, given the depth and viscosity of the water samples collected at the survey site.