Tiger Workshop Cape Town River and Lake Water Level Monitoring in Africa Philippa Berry De Montfort University, UK & Jérôme Benveniste European Space Agency

Tiger Workshop Cape Town Overview Radar Altimetry Missions –ERS-1 –ERS-2 –ENVISAT Retracking –Waveform sequence over Africa Near Real Time Pilot Demonstration –Geographical River and Lake masks –Validation against river gauges –Validation against SGDR Capabilities and limitations –Examples over lakes –Examples over rivers The Envisat Radar Altimeter Ultra Stable Oscillator Correction Discussion

Tiger Workshop Cape Town ESA RA-1 & RA-2 ERS-2 long time series at 35 days sampling Envisat continues time series, also additional mode allows terrain to be sampled even over mountains. Near Real Time capability permits generating height data within within 3 days of measurement

Tiger Workshop Cape Town ERS-2 waveform sequencing across Africa Ocean type echoVery fat echo from high surface roughness Bright thin echo from still water

Tiger Workshop Cape Town South American NRT Mask To produce reliable NRT mask the Envisat SGDR data were run through the full mask to generate products, these were assessed and only the locations of clean time series were included in the final NRT mask. NRT products are now generated globally. Work continues on enhancing the system to retrieve more targets (grey parts of mask).

Tiger Workshop Cape Town NRT African Mask + Products

Tiger Workshop Cape Town Global NRT mask This is the first release of the global NRT mask. Work continues towards monitoring remaining targets by increasing the waveform analysis and retracking and enhancing the selections mask.

Tiger Workshop Cape Town Requirement of an Accurate mask To measure heights accurately using a fully automated system, a very detailed mask is needed, to select out only signals from water.

Tiger Workshop Cape Town Validation Extensive validation of results made against gauge stations. Example shown here from Amazon basin. Retracking enables accurate heights to be generated.

Tiger Workshop Cape Town VALIDATION Rio Negro

Tiger Workshop Cape Town Rio Amazonas VALIDATION

Tiger Workshop Cape Town NRT over Lake Debo This figure shows the continuing monitoring over lake Debo from ERS2 (blue), Envisat (red) with two NRT points plotted (cyan).

Tiger Workshop Cape Town Histogram of NRT to SGDR differences over Africa 2 cm

Niger & Benue

Niger River

Tiger Workshop Cape Town Time series over Congo ERS2 Envisat ERS2 Congo time series

Tiger Workshop Cape Town All Possible Measurements on the Congo These pictures show everywhere that we have useable waveform data over the Congo. Every circle is another measurement. We are working on the River and Lake system with the aim of retrieving measurements from every location shown.

Tiger Workshop Cape Town Example ERS2 Time Series over Africa Lake Kivu River Niger Lake Kariba River Nile

Tiger Workshop Cape Town Envisat RA-2 Time Series over Africa Lago do Cahoro Bassa Lake Kariba Lake Upemba Congo

Tiger Workshop Cape Town Time Series over African Rift Valley lakes from ERS2 Red dots show mean position of altimeter measurements - one time series for each dot. Note that the best time series are selected for distribution within the NRT system using the NRT mask.

Tiger Workshop Cape Town Time series from ERS2 Lake Malawi

Tiger Workshop Cape Town Time series from ERS2 Lake Tanganika

Tiger Workshop Cape Town Time series from ERS2 Lake Victoria

Tiger Workshop Cape Town Lake Volume Change - first results

Tiger Workshop Cape Town Radar Altimeter waveforms and geophysical corrections are fetched directly from the Envisat ground segment The near real time River&Lake processor is run as soon as the data is received The “River&Lake for Hydrology” (RLH) output product is stored in the web site The longest lag is the availability of the DORIS Interim orbit (3~4 days) This will improve to 3 hours when the DORIS Navigator orbit is fully processed in real time NEAR REAL TIME DEMONSTRATION PILOT

Tiger Workshop Cape Town Near-real Time Products: NRT system demo is on since October 2005 Stopped since March 2006, restarted November 2006 Data within 3 days Very good agreement indeed with SGDR data, meaning that the NRT heights are accurate within a few cm Outputs will be assessed and mask upgraded to include additional sites periodically

Tiger Workshop Cape Town River and Lake Products Website / riverandlake

Tiger Workshop Cape Town Each sample page has the same layout SAMPLE PAGE FOR HISTORICAL PRODUCTS Information Time series Products download A typical Time Series

Tiger Workshop Cape Town THE NEAR REAL TIME IS MADE AVAILABLE VIA THE RIVER AND LAKE WEB SITE SAMPLE PAGE FOR NEAR REAL TIME PRODUCTS

Tiger Workshop Cape Town THE NEAR REAL TIME IS MADE AVAILABLE VIA THE RIVER AND LAKE WEB SITE SAMPLE PAGE FOR NEAR REAL TIME PRODUCTS

Tiger Workshop Cape Town Lake Debo Validation NRT NRT SAMPLE PAGE FOR NEAR REAL TIME PRODUCTS

Tiger Workshop Cape Town AFRICA - Lake Volta NEAR REAL TIME PRODUCTS

Tiger Workshop Cape Town AFRICA - Lake Rudolph NEAR REAL TIME PRODUCTS

Tiger Workshop Cape Town amount of data for one day NEAR REAL TIME PRODUCTS

Tiger Workshop Cape Town Location of data since 25 October All Data generated in Near Real Time over one 35 day cycle NEAR REAL TIME PRODUCTS

Tiger Workshop Cape Town ENVISAT RA2 USO Anomaly RA2 has shown (since Feb 2006) an anomalous behaviour of the Ultra Stable Oscillator (USO) clock period. The anomaly consists of an offset of about 5.6 m and orbital oscillations of ~20 cm in the EnviSat RA-2 range values Consequence  geophysical applications of RA-2 sea level measurements have been hindered since USO anomaly started –The River and lake NRT system has been halted

Tiger Workshop Cape Town 20 cm 1 orbit

Tiger Workshop Cape Town EnviSat RA2 USO anomaly Solutions: Two different models and corrective algorithms have been implemented since then: –The first model, developed by CLS (France) currently being used to correct the geophysical range values (at 1s rate). Only the corrections (and not the corrected output L2 products) have been provided (through the ESA web portal so that users can correct their own products. –The second model, developed by R. Scharroo (Altimetrics LLC, USA) has been implemented and used at ESRIN to correct the L1b RA2 products, that are given as input to the River & Lake processor for further computation of the elevations.

Tiger Workshop Cape Town EnviSat RA2 USO anomaly A comparison between the two corrective models has been performed in order to assess their validity. The following plot illustrates the differences between the two models, applied to ~6 h of data on 14 th October Differences of up to ~100 picoseconds in the Ku-band window delay are shown, that are equivalent to a difference of up to 13 mm in range or elevation

Tiger Workshop Cape Town EnviSat RA2 USO anomaly 13 mm

Tiger Workshop Cape Town River and Lake processor results The uso corrected Level 1b products, as well as non-USO-corrected L2 IGDR products (needed for the geophysical corrections) are now given as input to the River & Lake processor for a correct determination of river and lakes water level.

Tiger Workshop Cape Town River and Lake processor results The following African rivers are processed in near real time by the River&Lake system : –Senegal, Niger/Benue, Volta, Nile, Congo, Zambezi, Limpopo and Orange The following African lakes are processed in near real time by the River&Lake system: –Albert, Aswuan, Bangweulu, Cahora Bassa, Chad, Chilwa, Ndombe, Edward, George, Kainji, Kariva, Kivu, Kyoga, Lac de Buyo, lac de Kossou, lac de lagdo, lac de Manantali, lac de Selingue, Malawi, Mbakaou, Mweru, Mweru Wantipa, Nasser, Niangay, Nyasa, Shiroro, Tana H’ay’k, Tanganyika, Turkana, Victoria

Tiger Workshop Cape Town

River and Lake processor results RA2 data from 2005 have been given as input to the R&L processor to illustrate the outputs of the processor over Africa The plotted values are elevation differences with respect to climatological mean values.

Tiger Workshop Cape Town River and Lake processor results [m]

Tiger Workshop Cape Town THE MASK CAN BE ENHANCED TO ATTEMPT RETRIEVAL OVER YOUR PROJECT’S AREA NEAR REAL TIME PRODUCTS

Tiger Workshop Cape Town SIGN UP IN THE DISTRIBUTION LIST OF THE NRT PILOT DEMO VIA THE RIVER AND LAKE WEB SITE or write at NEAR REAL TIME PRODUCTS

Tiger Workshop Cape Town BURST ECHOES - RESEARCH CONTINUES

Tiger Workshop Cape Town BURST ECHOES - RESEARCH CONTINUES

Tiger Workshop Cape Town Discussion The ERS2 measurements have allowed generation of long time series of inland water heights over Africa. ENVISAT continues this valuable time series: additionally, its Near-Real Time capability allows generation of heights within 2-3 days of measurement. The spatial sampling permits data over a huge number of inland water targets to be gathered; the drawback is the low temporal sampling which permits annual and seasonal variations to be observed but does not allow high frequency changes to be identified. These data are provided by ESA to Users: work continues to enhance this pilot system and recover heights from all the available water crossings worldwide.