RADAR ALTIMETRY FOR STUDIES OF LARGE RIVER BASINS:HYDROLOGICAL REGIME OF THE EUPHRATES-TIGRIS RIVERS

Elena A. Zakharova (1), Alexei V. Kouraev (2,1), Jean-François Crétaux (2),Faiza Al-Yamani (3), Igor Polikarpov (3)

1) State Oceanography Institute (SOI), St. Petersburg branch, Vasilyevskiy ostrov,23 liniya, 2a, St Petersburg, Russia, zazatoulouse@mail.ru

2) Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS),14, avenue Edouard Belin, 31400 Toulouse, France, kouraev@legos.cnes.fr, cretaux@cnes.fr

3) Mariculture and Fishery Dept., Kuwait Institute for Scientific Research (KISR),P.O. Box 1638, 22107 Salmiya, Kuwait, fyamani@safat.kisr.edu.kw, ipolikarpov@mfd.kisr.edu.kw.

ABSTRACT

We present the results of analysis of hydrologicalregime of the Euphrates-Tigris river basin using asatellite radar altimetry. We use the data fromseveral radar altimetry missions: TOPEX/Poseidon(T/P) (1992-2002), Geosat Follow-On (GFO) (sinceJanuary 2000) and ENVISAT (since November2002) satellites. We analyze the variability of waterlevel for large reservoirs for the Tigris andEuphrates, as well as for the Tharthar Lake (Iraq).We also demonstrate how temporal evolution ofbackscatter coefficient could be used for waterdetection in the marshes and reservoirs and formapping of flooded areas.

1. INTRODUCTION

Water resources of the extensive Euphrates-Tigris(ET) river basin have vital importance for peopleliving on its watershed, and for its ecosystems. Thisriver basin also provides freshwater input into theArabian Gulf, affecting fishery, marine biology andbiogeochemistry. ET basin is shared by severalcountries (Turkey, Syria, Iraq, Iran and in small partby Saudi Arabia) and is extensively used forirrigation and other types for water consumption.Cascades of large reservoirs are constructed in eachof the four countries (Fig. 1).

The creation of large water reservoirs hastransformed the hydrology of whole river basin. Themajor reservoirs on the Euphrates River are Keban(constructed in 1974, 31 km3 of storage capacity),Karakaya (1987, 9.6 km3) and Ataturk (1992, 48.7km3). The largest Tigris reservoir in Mosul has beenfilled in 1985 and has a 13.5 km3 of total volume [1].For the Tigris River large-scale water regulationtakes place also on the influents. Since 1954, thefloodwaters of the Tigris have been transferred intothe tremendous Tharthar depression (72.8 km3 oftotal volume), that serve also as a water separatorbetween Euphrates and Tigris. One estimates that

actual total water storage in the reservoirs on theEuphrates is five times greater than the river’sannual flow, and twice that of the Tigris.

Water availability and water utilization in the ETbasin are always "under disputes" and, althoughsome agreements have been reached, the modernhydro-political problems are still far from beingsolved. Information on hydrological regime of theET basin (water level in the reservoirs, amount ofdiverted water, river level and discharge) hasparamount importance for studies of natural andanthropogenic influence on ET river system, andfreshwater input into the Arabian Gulf. For severallast decades such information, commonly obtainedusing in situ observations, has become very scarceand not available for scientific research.

In these conditions, satellite Earth Observationbecomes a useful tool to complement in situobservations, or to replace them (when no data isavailable). Recently, various remote sensingtechniques have been used for study and monitoringof water balance constituents of large river basins orwater bodies on time scales ranging from weeks todecades. One of the promising techniques is satelliteradar altimetry. Although the primary mission ofsatellite altimetry was the study of water level of theopen ocean, this technique have been successfullyapplied to monitor water level of inland seas such asCaspian and Aral seas in the Central Asia [2, 3, 4],large lakes [5, 6], as well as large rivers, wetlandsand floodplains [7, 8 ,9]. Recently, satellite altimetryhas been applied not only to derive river level, butalso to reconstruct river discharge from these datafor Ob' and Amazon rivers [10, 11]. In this work weshow how satellite altimetry help to study andanalyse a) variability of water level for large ETreservoirs and b) temporal variability of floodedareas in the ET marshes.

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Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

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Figure 1. Euphrates-Tigris (ET) river basin. Circles show water reservoirs for which altimetric data areavailable

2. DATA USED AND METHODS

We use altimetric data from several radar altimetrymissions. The earliest data are available from theTOPEX/Poseidon (T/P) satellite, operating since1992 up to 2002. We complement the T/P data byobservations from recent radar altimeters onboardGeosat Follow-On (GFO) (since January 2000) andENVISAT (since November 2002) satellites. Thealtimetry data were obtained from the Centre forTopographic studies of the Oceans and Hydrosphere(CTOH) at the LEGOS laboratory(http://www.legos.obs-mip.fr/en/observations/ctoh/).

The theoretical footprint of the altimeter data overthe open ocean is about 10-12 km (for Ku band,depending on surface roughness). However, forsmooth surfaces, that provide a quasi-specular returnsignal, the main part of the backscatter signal comesfrom a much smaller area. Among these surfaces are

a) ice cover, where largest part of signal comes fromthe area with a diameter of 1-2 km [12] and b) calmwater, which is often observed for small waterbodies, flooded areas etc. In order to minimisepotential contamination of the altimetric signal byland reflections, and at the same time to retain asufficiently large number of altimeter measurementson water, we performed a geographical selection ofthe data. We used GeoCover™ Landsat ThematicMapper orthorectified mosaics with 28.5 m pixelsize available from the MrSID Image Server [13] toselect the most appropriate intersections of waterbodies and satellite tracks (Fig. 2). We use data thatprovide the highest possible along track groundresolution, such as 10 Hz data for T/P and 18 Hz forENVISAT (distance between adjacent altimetricobservations is about 600 and 400 m,correspondingly).

Figure 2. ENVISAT ground tracks (thick white lines)No 242 (left) and 637 (right), and selected altimetric

measures for all cycles (white circles) over theupper part of the Ataturk water reservoir.

Background - Landsat TM image from ca 1990

As water flow within the ET river basin is almostcompletely regulated by network of channels and

reservoirs, assessment of water regime changes andvariability for this territory becomes problematicwhen few in situ data are available. In this respect,monitoring of water level of reservoirs from satellitealtimetry becomes a valuable source of information.

3. WATER LEVEL VARIABILITY INRESERVOIRS

We have processed more than ten reservoirs for theET basin, but here we discuss the three reservoirs forwhich we have the longest time series - Mosul,Karakaya and Tharthar (Fig. 3).

Seasonal variation of the water level on the Mosulreservoir on Tigris River has the highest amplitudethat reaches 25 meters. The lowest level wasobserved in the winter 2000, after the low flood thattook place in 1999. Since this unfavourable year, thereservoir has restored its volume and during the last4 years the minimal winter levels are supported onthe high grade, on average 5 m higher than duringthe 1992 – 1999.

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Figure 3. Time series of water level (m) for Mosul (a) and Karakaya (b) reservoirs, and for Tharthar lake (c).Note that the scale of vertical axes is the same for all four time series.

Karakaya reservoir on the Euphrates River has lowseasonal water level variability, not more than 5 m.But for this reservoir we observe two periods ofwater depletion: a short one in 1993–1994 and alonger one in 1997-2001. The first period occurredsoon after the start of Ataturk reservoir filling andwas characterised by insignificant (on 5 m) levelreducing. In the 1995 and 1996 the reservoir hasrestored its previous volume, but since 1997 waterlevel began to decrease gradually and finished with adramatic collapse (total decrease 15 m) in 1999. In2002 the refilling of reservoir have started and wascompleted by 2004. Now the Karakaya reservoiroperates in the normal regime, with seasonal waterlevel variation about 2 m.

The lake Thartar, situated between Tigris andEuphrates rivers was constructed to protect the cityof Bagdad from the floodwaters of Tigris. It is thelargest water body in the ET basin. Its water surfaceis of 2000 km2 and operational volume is 43.5 km3.

The seasonal water level variation is 2-7 m. Similarto the Karakaya reservoir, Tharthar experienced adramatic level decrease that has started in 1996. Thelowest level was reached in 2002 and was 14 mlower than the normal minimal winter stages for1992-1994. In 2002 the water level of Tharthar Lakestarted to rise and rose on 13 m in 2004. Now the

water level is relatively high, but still below thevalues of 1992-1994.

4. WETLANDS AND FLOODED ZONES

Satellite altimetry allows us not only to assess levelchanges of various water bodies, but also to estimatetemporal variability and extent of the flooded zones.This could be done using another parameter ofaltimetric measures - backscatter coefficient, whichis the ratio between the power reflected from thesurface and the incident power emitted by theonboard radar altimeter, expressed in decibels (dB).Land has typically a low backscatter coefficient,while flooded areas, where smooth water surfaceacts as a mirror providing quasi-specular return, arecharacterised by a high backscatter coefficient.

We have used this approach for the ET Marshes(Fig. 4). During Iran-Iraq war, this area has beenpartly dried out, and causeways to ease militarytransport have been constructed here [14]. Later,both the Al Hammar and Central marshes have beenpartitioned into polders using dikes and large parthas been evaporated or emptied. It is estimated thatmore than 90 % of marshes surface has beentransformed into bare land and salt crust [14].

A

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Figure 4. Satellite imagery of region covering eastern and central marshes. Left panel - Landsat TM cicrca 1990,right panel - Landsat ETM+ circa 2000. Lines show altimetric griound tracks for T/P (red), GFO (green) and

ENVISAT (orange). Numbers on T/P track are the reference points (the same as in figure 5). Letters denoteregions of interest: A - Al Hammar (southern) marshes, B - confluence of Tigris and Euphrates, C - part of Al

Hawizeh marshes, drained out between 1990 and 2000, D – Al Hawizeh marshes

Temporal evolution of backscatter coefficient forT/P (Fig. 5) shows that this parameter could be usedfor water detection and for mapping of flooded areasalong the track. Land has low backscatter values(blue and light blue dots, less than 15 dB), and forthis type of surface altimetric values are often rare oreven absent (see region between reference points460 and 500) due to quality control. For southernmarshes (region A) and confluence of Tigris andEuphrates (region B) we see a well-marked seasonalsignal, with backscatter values rising up to 20-42 dBin winter and spring. Note also the growth of theflooded area, seen as larger zones of yellow and reddots along the track.

A B C D

Backscatter in Ku band, 100*dB

-1000 to 0 0 to 1000 1000 to 1500 1500 to 2000 2000 to 3000 3000 to 4200 4200 to 6000

Figure 5.Temporal evolution of radar backscatter atKu band (13.6 GHz) for T/P track 31. X axis -

reference points numbers along the track (see figure6), Y axis - time (years). The drainage of the part ofeastern marshes (region C) occured between 3 and

23 June 1996

Northern part of eastern marshes (region D) ischaracterised by high values throughout the year(more than 30 dB), with maximal values amountingup to more than 42 dB in spring. Period of 1995-1999 is marked by overall high backscatter values,while after 1999 seasonal variability becomes morepronounced. Southern part of eastern marshes(region C), as it is observed for region D, has highbackscatter values in the beginning of 1990s. But weobserve dramatic changes in 1996 - this area hasbeen drained completely and we are able to estimatethe date of the drainage as between 3 and 23 June

1996 (T/P repeat period is 10 days, but data for onecycle is missing). Thus satellite altimetry dataprovide us with the tool monitor flooded zones andto assess their temporal variability and extent.

5. CONCLUSION

Using satellite altimetry we can reliably monitorwater levels of large water bodies, as well ashydrological regime of the flooded zones. For ETriver basin, poorly constrained by in situobservations, this information provides valuablecontribution for studies of water budget and itsvariability. It will also help to reduce uncertainty inpredicting response of ET river system to naturalchanges and anthropogenic water management, and,ultimately, to estimate the amount of water comingto the Arabian Gulf, and its relation to the fishery,marine biology and biogeochemistry.

In future we plan to assess water surface and vilumevariation associated with water level change fordifferent water bodies in the ET basin using multi-temporal satellite imagery in the visible range.

6. REFERENCES

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4. Cretaux, J-F, Kouraev, A.V., Papa, F., Berge-Nguyen, M., Cazenave, A., Aladin, N.,Plotnikov. I. S., 2005, "Water balance of theBig Aral Sea from satellite remote sensing andin situ observations," J. of Great Lakes Res.,31(4), pp. 520-534.

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10. Kouraev A.V., Zakharova E.A., Samain O.,Mognard-Campbell N., Cazenave A. "Ob’river discharge from TOPEX/Poseidon satellitealtimetry data", Remote Sensing ofEnvironment, 93, 2004, pp. 238-245

11.Zakharova, E. A., Kouraev, A. V., Cazenave, A.,Seyler, F., 2006, "Amazon river dischargeestimated from Topex/Poseidon satellite waterlevel measurements," Comptes Rendus -Geoscience, 338(3), pp. 188-196.

12. Legresy, B., and Remy, F., 1997, "Altimetricobservations of surface characteristics of theAntarctic ice sheet," J. of Glaciology, 43(144),pp. 265-275.

13. MrSID Image Server, 2006, "NASA EarthScience Application Directorate,"https://zulu.ssc.nasa.gov/mrsid/

14. UNEP, 2001, Partow, H., "The MesopotamianMarshlands: Demise of an Ecosystem,"Division of Early Warning and Assessment,United Nations Environment Programme,Nairobi, Kenya.