The comprehension of water level fluctuations and the
sustainability of the Inner Niger River Delta (IND) is a major concern for
the scientific community, but also for the local population. Located in the
centre of Mali, the heart of the Sahel, the delta is characterised by a
floodable area of more than 32 000 km2 during the rainy season, which
contributes very strongly to the vitality of local ecosystem, and is
consequently classified as a Ramsar site under the international Convention
for Wetlands. In addition, the Delta acts as an environmental and
socio-economic development barometer for the entire sub-region. Nowadays, we
can observe an increasing fragility of the delta due to climate change,
desertification and human activities, and justifies the need for permanent
monitoring. The present study is based on the recent successes of radar
altimetry, originally designed to monitor the dynamics topography of the
ocean, and now very frequently used to retrieve inland water levels, of
lakes, rivers, and wetlands. Previous studies evaluated the performances of
several radar altimetry missions including Low Resolution Mode (LRM)
(Topex-Poseidon, Jason-1/2/3, ERS-2, ENVISAT, and SARAL, and Synthetic
Aperture Radar (SAR) Sentinel-3A missions for water level retrievals over
1992–2017. More than 50 times series of water levels were build at the
crossing between water bodies and Sentinel-3A and 3B over 2016–2020.
Twenty-four comparisons between in-situ and altimetry-based time-series of
water levels were achieved over the IND. RMSE generally lower than 0.7 m and
r higher than 0.9 were obtained.
Introduction
Radar altimetry (RA) has been experiencing numerous inovations in terms of
acquisition mode (from Low Resolution Mode – LRM – to Synthetic Aperture
Radar – SAR –, and even Interferometry SAR – InSAR) and data processing
over all types of Earth surfaces
(Abdalla et al., 2021).
Owing to their availability over almost 30 years, since the launch of
Topex/Poseidon and ERS-1 missions in and 1991 and 1992, respectively, the
two first missions to provide high accuracy measurements (Stammer
and Cazenave, 2017), RA data are increasingly used, over land, to globally
monitor the water levels of rivers, lakes and reserveroirs, wetlands and
floodplains (Birkett et al.,
2011; Crétaux et al., 2017). To ensure the confidence in the reliability
of RA-based water levels, it is necessary to determine their quality through
comparisons against in-situ measurements. In these operations are routinely
performed over the ocean at different calibrration/validation sites (e.g.,
Bonnefond et al., 2011; Mertikas et
al., 2018), few of these facilities are available over land, except at Lake
Issykkul, in Kyrgyzstan (Central Asia)
(Crétaux et al.,
2009). Sentinel-3A and 3B are two of the most recently launched RA missions.
They operate in SAR and use the Open-Loop (OL) or Digital Elevation Model
(DEM) tracking mode designed to reduce the loss of tracking over hilly areas
(Biancamaria
et al., 2017; Taburet et al., 2020). Very few studies evaluated the accuracy
of both Sentinel-3A and 3B over inland water bodies, and mostly over lakes
(Frappart
et al., 2021; Kittel et al., 2021; Shu et al., 2021).
The Niger Inner Delta in Mali. Flooded areas appear in blue. They
are made available by http://floodobservatory.colorado.edu/ (last access: 1 June 2021).
Locations of in-situ gauges, and VS from Sentinel-3A and B are represented
using black dots, red and green triangles, respectively.
Owing the availability of a large number of in-situ measurements, the IND
has often been chosen as a study area for validating RA-based water levels
(Frappart
et al., 2015; Goita and Diepkile, 2012; Normandin et al., 2018). In this
study, RA-based water levels defined at the cross-sections of Sentinel-3A
and 3B ground-tracks and river and floodplains in the IND were compared to
in-situ water stages.
Study area and datasetsThe Inner Niger Delta
The study area the inner delta of the Niger River, located in the centre of
the Sahelian zone of West Africa, precisely in Mali (Fig. 1a). It extends
between latitudes 13 to 17∘ N, and longitudes
2 to 7∘ W (Fig. 1). Its floodable area is
estimated at 32 000 km2
(Goita and
Diepkile, 2012; Mahé et al., 2009; Seiler et al., 2009; Zwarts et al.,
2005). The Delta presents a rather particular ecological environment by the
fact that it separates two zones with very precarious climatic conditions:
on the one hand, the desert Sahara dominated by sand dunes, and on the other
hand, the Sahel which is characterized by recurrent droughts.
Radar altimetry data from Sentinel-3A and 3B
Sentinel-3 was developed by the European Space Agency (ESA) in the framework
of the COPERNICUS program. Two satellites are already operating: Sentinal-3A
and 3B launched on 16 February 2016 and 25 April 2018, respectively. They
are orbiting at 814.5 km altitude on a 98.65∘ inclination
sun-synchronous orbit with a 27 d repeat period and an equatorial
ground-track spacing of ∼ 105 km. Sentinel-3A and 3B were
placed on the same orbit with a phase difference of 180∘. The
satellites payloads is composed of SRAL (SAR Radar ALtimeter), a
dual-frequency SAR altimeter operating at Ku (13.575 GHz) and C (5.41 GHz),
bands, a Microwave Radiometer (MWR) sensor for wet path delay correction
over the ocean, and a triple system for Precise Orbit Determination (POD)
including a GPS receiver, a LRA and a DORIS instrument
(Donlon
et al., 2012). The data used in this study are the ones necessary to compute
water levels over land (see
(Frappart et al., 2021) and
Sect. 3). They are made available at: http://ctoh.legos.obs-mip.fr (last access: 15 May 2021).
In-situ water levels
Records from 10 gauge stations located in IND (see Fig. 1) were provided
by the Malian hydrological service, Direction Nationale de l'Hydraulique
(DNH). In this study, we used the time series of water levels from February
2016 to December 2020.
Methodology
Time-series of water level were produced at the cross-section between an
altimeter ground-track and a water body using the the Altimetry Time-Series
(AlTiS) software (Frappart et
al., 2021). From the Geophysical Data Records (GDR) which contain the
along-track altimetry data, AlTiS computed the altimeter height (h) as
described in (Crétaux et al., 2017) and Eq. (1):
h=H-R-ΔRion-ΔRdry-ΔRwet-ΔRsol-ΔRpol-N
where H is the altitude of the satellite on its orbit, R the range or
distance between the satellite and the surface, ΔRion, ΔRdry, ΔRwet, ΔRsol, ΔRpol, are
the corrections to apply to the range to account for the delay introduced by
the ionosphere, the troposphere (dry and wet components), and the effects of
the solid Earth and pole tides, and N is the geoid model. Following
Frappart et al., 2006), the ranges derived from Offset of
Center of Gravity (OCOG) (Wingham et al., 1986) were used
as they were found to provide more accurate water levels.
Then, the valid height values were manually selected through AlTiS Graphical
User Interface (GUI). Once the valid data are selected, the time-series of
water levels is generated computing the median of the heights for each
altimetry cycle (i.e., every 27 d for Sentinel-3A and B).
The altimetry-based time-series of water levels generated over the rivers in
the Inner Niger Delta were compared to in-situ water stages using the
Root-Mean-Square Error (RMSE), Relative (ratio of the RMSE to the mean
annual amplitude of the water level at the station multiplied by 100) RMSE
(RRMSE), and the Pearson correlation coefficient (r).
Time-series of water levels from the in-situ gauge
stations (blue) of Beneny Kegny (a) and Akka (b), Sentinel 3A (black
dots) and 3B (red squares).
Results
Several tenths of RA-based time-series of water levels were generated over
the IND (rivers and floodplains) using Sentinel-3A and 3B data: 28/24 on the
Sentinel-3A/3B RA ground-tracks (Fig. 1). They offer a dense network of RA
Virtual Stations (VS) were time-series of water levels are estimated. Two
examples of comparisons of in-situ and RA-based time-series of water level
are presented in Fig. 2. The first one to a cross-section of Sentinel-3A
and 3B located on the Bani River, a south-west tributary to the Niger River,
at (4.74∘ W, 13.51∘ N), the second one one to a
cross-section of Sentinel-3A and 3B located on the Niger River at
(4.27∘ W, 15.46∘ N), on the nort central part of the
IND. Comparisons were made with gauge records of Beneny Kegny
(4.917∘ W, 13.383∘ N) and Akka (4.233∘ W,
15.400∘ N) stations, respectively. The distance to the Sentinel-3
SV is 28 and 8 km, respectively.
Comparisons between in-situ and altimetry- based water
levels for Snetinel-3A (upward-pointing triangles) and 3B (downward-pointing
triangles) over the IND: (a) distance between in in-situ stations and VS, (b) RMSE (m), (c) R.
RA-based water levels exhibit similar temporal variations as the in-situ
gauge stations, with a well-marked seasonal cycle that can reach 6 m (Fig. 2). RMSE of 0.35 m/0.30 m, and r of 0.99/0.99 were found at Beneny Kegny
Akka for Sentinel-3A and 3 B, respectively, and RMSE of 0.45 m and r of 0.98
at Akka for Sentinel-3A. As too few in-situ measurements were collected or
made available for the Akka gauge station during the period of operation of
Sentinel-3B, we were unable to estimate RMSE, RRSME and r for Sentinel-3B.
Nevertheless, as RA-based water levels from Sentinel-3A and Sentinel-3B
exxhibit very similar temporal variations and taking into account the good
agreement between in-situ and Sentinel-3A-based water levels, similar
perfomances can be expected from Sentinel-3B based water stages. This latter
example brings into focus the strong interest of radar altimetry to provide
a continuous monitoring of water levels in case in-situ gauge stations
stopped operating for a reason or another.
The same evaluation parameters (distance between the VS and the in-situ
station, RMSE, RRMSE and R) were estimated for 12 Sentinel-3A VS and 12
Sentinel-3B VS. They are presented on Fig. 3, except the RRMSE not shown
here. Comparisons were made on distances between the in-situ station and the
VS ranging from 7 to 126 km (Fig. 3a). The RMSE range from 0.15 to 1.39 m,
with 13 values over 0.5 m (Fig. 3b). If these values can be considered
quite high, the corresponding RRMSE are all lower or equal 20 %, and 19
RRMSE values out of 24 are below 15 %. Correlations are all higher or
equal to 0.79 (Fig. 3c). Twenty one out of 24 r values are above 0.9, and
even, 16 of them are higher than 0.95, confirming the good accuracy of
Sentinel-3 based water stages reported in previous studies over rivers
(e.g.,
Bogning
et al., 2018; Kittel et al., 2021; Normandin et al., 2018). The high RMSE
values can be accounted for the distances between the in-situ gauges and the
SV. If the temporal variations are likely to exhibit a similar pattern over
distances of several tenths of kilometers, the seasonal amplitude can be
affected by changes in depth and width of the river along of the river
network. As a consequence, the correlation can still present high values
when the RMSE increases. This situation is what is observed here: high
correlation values were estimated, slightly decreasing as the distance
increases, but RMSE generally increases with the distance (e.g., RMSE of
0.93 and 1.39 m were obtained fro distances of 126 and 93 km, respectively,
for corresponding r of 0.79 and 0.91).
Conclusions
An extensive assessment of the quality of the data acquired by Sentinel-3A
and 3B was achieved over the IND over the period 2016–2020. Fifty two VS
were defined over the rivers and the floodplains using the newly developed
AlTiS software, and 24 comparisons against in-situ water levels were made.
An overall very good agreement with r values ranging from 0.79 to 1, but
generally over 0.90 or 0.95, and RRMSE lower than 20 %, and, most of the
times, lower tha 15 %. The high RMSE obtained for some VS (RMSE
>0.50 m) are, most of the time, due to the long distance, above
50 km, between the in-situ gauge stations and the VS. Owing to the density
of the in-situ gauge stations over the IND and the rapid availability of the
water stages, The Inner Niger Delta is a good site for the validation of the
recently launched (e.g., Sentinel-6/Jason-CS in November 2020) or to be
launched missions (e.g., Surface Water and Ocean Topography – SWOT in fall
2022).
Code availability
AlTiS is an Open Source project under CeCill license (IDDN certification: IDDN.FR.010.0121234.000.R.X.2020.041.30000 – https://www.iddn.org/cgi-iddn/certificat.cgi?IDDN.FR.010.0121234.000.R.X.2020.041.30000, last access: 3 June 2021, CTOH, 2020).
AlTiS software is provided by CTOH along with the radar altimetry data using the following request form: http://ctoh.legos.obs-mip.fr/applications/land_surfaces/altimetric_data/altis (last access: 4 July 2021, Normandin et al., 2018).
Data availability
Radar altimetry data are made freely available by CTOH. They can be obtained using the following request form: http://ctoh.legos.obs-mip.fr/applications/land_surfaces/altimetric_data/altis (last access: 4 July 2021, Normandin et al., 2018).
Author contributions
ATD has made the In-situ data collection for the IND. He has also performed the statistical comparisons, and contributed to the writing of the manuscript.
FE has performed the data processing of radar altimetry data from Sentinel-3.
FB has extracted radar altimetry data.
EM has contributed to the manuscript writing.
FF has made the statistical comparisons. He has produced the figures, and carried out the writing of the manuscript.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “Hydrology of Large River Basins of Africa”. It is a result of the 4th International Conference on the “Hydrology of the Great Rivers of Africa”, Cotonou, Benin, 13–20 November 2021.
ReferencesAbdalla, S., Kolachina, A., Adusumilli, S., Bhowmick, A., Alou-Font, E., Amarouche, L., Andersen, O. B., Naeije, M. C., Simons, W. J. F., et al.: Altimetry for the future: Building on 25 years of progress, Adv. Space Res., 68, 319–363, 10.1016/j.asr.2021.01.022, 2021.Biancamaria, S., Frappart, F., Leleu, A.-S. S., Marieu, V., Blumstein, D.,
Desjonquères, J.-D., Boy, F., Sottolichio, A., and
Valle-Levinson, A.: Satellite radar altimetry water elevations performance
over a 200 m wide river: Evaluation over the Garonne River, Adv. Sp. Res.,
59, 128–146, 10.1016/j.asr.2016.10.008, 2017.
Birkett, C., Reynolds, C., Beckley, B., and Doorn, B.: From research to
operations: The USDA global reservoir and lake monitor, in Coastal
Altimetry, Springer Berlin Heidelberg, Berlin, Heidelberg, 19–50,
2011.Bogning, S., Frappart, F., Blarel, F., Niño, F., Mahé, G., Bricquet,
J. P., Seyler, F., Onguéné, R., Etamé, J., Paiz, M. C., and
Braun, J. J.: Monitoring water levels and discharges using radar altimetry
in an ungauged river basin: The case of the Ogooué, Remote Sens., 10,
350, 10.3390/rs10020350, 2018.
Bonnefond, P., Haines, B. J., and Watson, C.: In situ Absolute Calibration
and Validation: A Link from Coastal to Open-Ocean Altimetry, in Coastal
Altimetry, Springer Berlin Heidelberg, Berlin, Heidelberg, 259–296,
2011.CTOH: Centre de Topographie des Océans et l’Hydrosphère, AlTiS – Altimetric Time Series Software, [code] available at: https://www.iddn.org/cgi-iddn/certificat.cgi?IDDN.FR.010.0121234.000.R.X.2020.041.30000 (last access: 4 July 2021) 2020.
Crétaux, J.-F., Nielsen, K., Frappart, F., Papa, F., Calmant, S., and
Benveniste, J.: Hydrological applications of satellite altimetry: rivers,
lakes, man-made reservoirs, inundated areas, in: Satellite Altimetry Over
Oceans and Land Surfaces, edited by: Stammer, D. and Cazenave, A.,
CRC Press., 459–504, 2017.Crétaux, J. F., Calmant, S., Romanovski, V., Shabunin, A., Lyard, F.,
Bergé-Nguyen, M., Cazenave, A., Hernandez, F. and Perosanz, F.: An
absolute calibration site for radar altimeters in the continental domain:
Lake Issykkul in Central Asia, J. Geod., 83, 723–735,
10.1007/s00190-008-0289-7, 2009.Donlon, C., Berruti, B., Buongiorno, A., Ferreira, M. H., Féménias,
P., Frerick, J., Goryl, P., Klein, U., Laur, H., Mavrocordatos, C., Nieke,
J., Rebhan, H., Seitz, B., Stroede, J., and Sciarra, R.: The Global
Monitoring for Environment and Security (GMES) Sentinel-3 mission, Remote
Sens. Environ., 120, 37–57, 10.1016/j.rse.2011.07.024, 2012.Frappart, F., Calmant, S., Cauhopé, M., Seyler, F., and Cazenave, A.:
Preliminary results of ENVISAT RA-2-derived water levels validation over the
Amazon basin, Remote Sens. Environ., 100, 252–264,
10.1016/j.rse.2005.10.027, 2006.Frappart, F., Fatras, C., Mougin, E., Marieu, V., Diepkilé, A. T.,
Blarel, F., and Borderies, P.: Radar altimetry backscattering signatures at
Ka, Ku, C, and S bands over West Africa, Phys. Chem. Earth, 83, 96–110,
10.1016/j.pce.2015.05.001, 2015.Frappart, F., Blarel, F., Fayad, I., Bergé-Nguyen, M., Crétaux,
J.-F., Shu, S., Schregenberger, J., and Baghdadi, N.: Evaluation of the
Performances of Radar and Lidar Altimetry Missions for Water Level
Retrievals in Mountainous Environment: The Case of the Swiss Lakes, Remote
Sens., 13, 2196, 10.3390/rs13112196, 2021.
Goita, K. and Diepkile, A. T.: Radar altimetry of water level variability in
the Inner Delta of Niger River, in 2012 IEEE International Geoscience and
Remote Sensing Symposium, IEEE., 5262–5265, 2012.Kittel, C. M. M., Jiang, L., Tøttrup, C., and Bauer-Gottwein, P.: Sentinel-3 radar altimetry for river monitoring – a catchment-scale evaluation of satellite water surface elevation from Sentinel-3A and Sentinel-3B, Hydrol. Earth Syst. Sci., 25, 333–357, 10.5194/hess-25-333-2021, 2021.Mahé, G., Bamba, F., Soumaguel, A., Orange, D., and Olivry, J. C.: Water
losses in the inner delta of the River Niger: water balance and flooded
area, Hydrol. Process., 23, 3157–3160, 10.1002/hyp.7389, 2009.Mertikas, S. P., Donlon, C., Féménias, P., Mavrocordatos, C.,
Galanakis, D., Tripolitsiotis, A., Frantzis, X., Tziavos, I. N., Vergos, G.
and Guinle, T.: Fifteen years of Cal/Val service to reference altimetry
missions: Calibration of satellite altimetry at the permanent facilities in
Gavdos and Crete, Greece, Remote Sens., 10, 1557,
10.3390/rs10101557, 2018.Normandin, C., Frappart, F., Diepkilé, A. T., Marieu, V., Mougin, E.,
Blarel, F., Lubac, B., Braquet, N., and Ba, A.: Evolution of the performances
of radar altimetry missions from ERS-2 to Sentinel-3A over the Inner Niger
Delta, [code, data set], Remote Sens., 10, 833, 10.3390/rs10060833, 2018.Seiler, R., Schmidt, J., Diallo, O., and Csaplovics, E.: Flood monitoring in
a semi-arid environment using spatially high resolution radar and optical
data, J. Environ. Manage., 90, 2121–2129,
10.1016/j.jenvman.2007.07.035, 2009.Shu, S., Liu, H., Beck, R. A., Frappart, F., Korhonen, J., Lan, M., Xu, M., Yang, B., and Huang, Y.: Evaluation of historic and operational satellite radar altimetry missions for constructing consistent long-term lake water level records, Hydrol. Earth Syst. Sci., 25, 1643–1670, 10.5194/hess-25-1643-2021, 2021.
Stammer, D. and Cazenave, A.: Satellite altimetry over oceans and land
surfaces, Taylor & Francis, Boca Raton, FL., 2017.Taburet, N., Zawadzki, L., Vayre, M., Blumstein, D., Le Gac, S., Boy, F.,
Raynal, M., Labroue, S., Crétaux, J.-F., and Femenias, P.: S3MPC:
Improvement on Inland Water Tracking and Water Level Monitoring from the
OLTC Onboard Sentinel-3 Altimeters, Remote Sens., 12, 3055,
10.3390/rs12183055, 2020.
Wingham, D. J., Rapley, C. G., and Griffiths, H.: New Techniques in Satellite
Altimeter Tracking Systems, Proc. IGARSS Symp. Zurich, (SEPTEMBER 1986),
1339–1344, 1986.
Zwarts, L., Beuering, V. B., Kone, B., and Wymenga, E.: The Niger , a
lifeline: effective water management in the Upper Niger Basin., 2005.