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A New Global Lakes Database for a Remote Sensing Program Studying Climatically Sensitive Large Lakes
J. Great Lakes Res. 21(3):307-318 Internal. Assoc. Great Lakes Res., 1995
A New Global Lakes Database for a Remote Sensing Program Studying Climatically Sensitive Large Lakes
Charon M. Birkett and Ian M. Mason Department of Space and Climate Physics University College London Mullard Space Science Laboratory Holmbury St. Mary Dorking, Surrey U.K. RH56NT
ABSTRACT. World wide climate related programs have calledfor hydrological studies to be put into a more global perspective, and yet the global study of large climatically sensitive lakes has been neglected. A remote sensing program at the Mullard Space Science Laboratory (MSSL) aims to monitor short and medium-term lake volume changes and interpret them in terms of aridity variations, as a measure of regional climate change. The program thus requires the global identification of all potentially closed and climatically sensitive lakes. We review the availability of current global lake (particularly closed lake) information and note its limitations with respect to the requirements of the large lakes study. As a result of this the MSSL Global Lakes Database (MGLD) has been constructed containing locational and lake type information for over 1,400 inland water bodies including, as far as possible, all lakes and reservoirs (but not lagoons), with surface areas ?100km 2 (approximately). Of these, the database identifies 857 open lakes (those with outflows), 226 reservoirs, and 320 potentially closed lakes. Approximate areas of the lakes are also noted. The identification of the closed lakes (59% on the Asian continent) is a cornerstone of the remote sensing program which aims to derive lake levels and lake areas using satellite radar altimeters and satellite imaging radiometers respectively. The MGLD therefore also notes the spatial coverage of all large lakes by past and current radar altimeters including that on ERS-1, which overflies almost half of the lakes. With the addition of referenced closed-lake information the MGLD is thus a unique data set which has already been applied to several remote sensing projects. INDEX WORDS:
Database, large lakes, remote sensing, closed lakes, lake levels, climate change.
INTRODUCTION Large lakes have been the subject of great interest not only because of their water resources role but also as indicators of local climate change. Closed lakes, which have no significant surface or subsurface outflow, are particularly climatically sensitive and have been used in the study of palaeoclimate (Street 1980, Guiot et al. 1993, Roberts et al. 1993). Current trends in precipitation have also been shown to be evident in more recent lake level records (Folland et al. 1990) while Williams (1993) brought to attention the limnological significance of falling lake levels for a number of closed lakes. Other studies of large lakes include the noting of both the anthropogenic and natural causes of the fluctuating levels of the Caspian Sea (Vali-Khodjeini 1991) and a study of the levels of the open
Lake Victoria (Kite 1981). From an economic and social point of view the potential impacts of future lake levels of the Great Lakes has recently been discussed (Changnon 1993, Cohen 1986). At the international level several climate/environmental programs have specifically outlined their interest in lake observations. The World Climate Program-Water (WCP-W) has interests in Lake Chad (Dozier 1992), the International Decade for the East African Lakes project (IDEAL) is concerned with the sensitivity of the rift valley lakes to climate change (Johnson 1993), and one of the aims of the Scientific Committee on Problems of the Environment (SCOPE) was to compile and publish information on all African lakes from geomorphological and hydrological viewpoints (Farmer and Wigley 1985). As these examples show, emphasis tends to be 307
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Birkett and Mason
placed on studies of one particular lake or at most several lakes in a localized region. Often, data and results are restricted due to the poor exchange or collation of hydrological data. Askew (1991) called for hydrological studies to be put into a much more global perspective with the use of remote sensing techniques, a point noted by both Dozier (1992) and Rodda et ai. (1993) who also called for the formation of more superior information archives with good distribution and exchange of data. At the Mullard Space Science Laboratory (MSSL) we have proposed a lakes remote sensing program which accords with these views (Mason et al. 1994). Its goal is to measure short and medium-term lake volume changes and interpret them in terms of aridity variations, as a measure of regional climate change. The project aims to monitor several hundred climatically sensitive open and closed lakes on a global scale. Satellite imaging radiometers will provide lake areas to ~1 % accuracy (Harris 1994) and satellite radar altimetry will provide relative lake level changes to an accuracy better than 0.1 m rms (Birkett 1994). Since there are practical limits to the size of lake measurable by altimetry, the program will initially be restricted to lakes of area ~100 km2, which we have deemed "large" lakes.
A REVIEW OF CURRENT GLOBAL LAKE INFORMATION To initialize the research program, the basic information we required was the distribution and size of closed and climatically sensitive open lakes whose areas were ~100 km 2. We also sought information such as lake type (open/closed or reservoir), the existence of any anthropogenic influences, the operation of any ground gauge-monitoring programs (accuracy/frequency and the means of data storage and accessibility), lake freeze/thaw dates, the possibility of ground seepage, and the effect of wind setup on lake levels. An extensive search via personal communications and literature sources was carried out to assess the global availability and accessibility of such information. Personal contacts included attempted communications with hydrological institutes in every country listed by the World Meteorological Organization (J.B. Miller 1992, personal communication) but the quantity and quality of the replies were poor. A number of computerized databases holding lake information were readily made known including MWDI (the U.S. Geological Survey National Water Data Exchange), HYDAT (Environment Canada), NORWIS (the Norwegian Water Information System), LISC/CLD (China National Natural Science Founda-
tion, Zhao et ai. 1992) and the Finnish lakes database (National Board of Waters and the Environment). Apart from the LISC/CLD, access to these lakes data is relatively simple. However, all of these databases tend to function as sources of water resources information and only exist on national scale. Although the United Nations Environment Program is developing a GEMS/Water global database, it currently holds only water quality information for some 82 watersheds (E.D. Ongley, Environment Canada 1994, personal communication) Table 1 lists published sources attempting to catalogue or categorize lake information. Note that the two earliest literary sources (Murray 1910, and Halbfass 1922) give only limited locational information and they are inevitably not comprehensive in their global coverage. No single reference gives all the information required for the remote-sensing program. For example, the International Lakes Environment Committee (ILEC 1988-1991) data volumes (an expanding and ongoing project) are an excellent source of information but are limited in the number of lakes (and in particular closed lakes) they contain. On the other hand Herdendorf (1982) gives an excellent global inventory, noting fresh/saline lake type, but the listings are limited to those lakes with areas >500 km2 . It was decided therefore to create a new global lakes database from scratch and use the located published information as validation only.
THE CONSTRUCTION OF THE MSSL GLOBAL LAKES DATABASE (MGLD) The MGLD is a computerized database containing textual information in the form of records. There is one record for each lake entered and each record consists of a number of information fields. By using a global set of 1: 1,000,000 Operational Navigation Charts (ONC) (USA/UK/Australian Air Chart Program), whose information dates from between the 1970s to the 1990s, those lakes having surface areas ~100 km2 (approximately) were identified. For certain geomorphological regions (e.g., Finland), it was difficult to define a single lake due to highly irregular shapes and the possibility of connecting pools. In these cases a lake record may well represent several sub-basins. In general, however, the MGLD records contain the lake name, estimated area, country and continent location, a latitude and longitude position within the lake (a central location if possible), lake type, and ONC map number. Alternative names, where given on the ONC, were also entered. Any lake which was not named on the ONC (the majority in the Asian conti-
TABLE 1.
Primary sources ofpublished lakes information for validation of the Global Lakes Database (MGLD).
Region Reference Global Global Global Global Global Global Global Global Global China China Global Global China Global
No. lakes
Murray, 1910 120 Halbfass, 1922 1755 Gresswel1&Huxley,1965 110 McCarraher,1972 434 UNESCO, 1978 82 Euguster&Hardie,1978 54 Showers, 1979 290 Herdendorf, 1982 253 Rapley et al., 1987 358 Chang, 1987 28 Wang, 1987 48 van der Leeden et al.,1990 155 ILEC,1988-1991** 183 Williams, 1991 48 MGLD, this paper (1995) 1403
Area No. in range MGLD >50km2 X >0.001 X >5 93 unk 46 >350 71 unk 31 >26 56 >500 223 -100 305 >450 27 >7 43 >100 142 > 0.5 87 >100 42 ~100 1403
Lake Type lakes/eph lakes/eph lakes/res/ephlcl lake/eph lakes/res/eph lakes/eph lakes/ephlcl lakes/ePhlcl lakes/eph lakes/eph lakes lakes/res/eph lakes/res/ephlcl lakes/eph lakeslres
Lake Alt. Lat Name Names Long Region Area y y y y y y y y y y y y y y y
y n
y y
ip ip
y
y y y y
y y y
y y y
y y y y y y y y y y y y y y y
Wind Alti- Freeze No. Vol. Setup tude ffhaw closed
y y y
y ip
y
y
y y y y y y y y
y Y y
ip
29 204 24 45 11
Y
y
y
y y y y
y y
y
y
y
y y y
53 32 40 28 11 19 26 6 42 320
Key: Lake Type: res = reservoirs, eph = ephemeral, cl = coastal lagoons, Note that excepting Rapley et at. 1987 and UNESCO 1978, all sources state whether lakes are freshwater or saline/closed. No. in MGLD = Note that only Herdendorf (1982) and the MGLD (this paper, 1995) claim to list all lakes within their size category. No. closed =Number oflakes in the source recognized as being closed or saline (excluding lagoons and ephemeral lakes) ip = in part unk = unknown ** ongoing project X = Unknown due to limited positional information give in the referenced source.
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Birkett and Mason
controlled by a dam). Reservoirs could easily be distinguished on the ONC maps, but the identification of a closed lake was a more difficult task. By inspection of surface drainage and contours the maps can only reveal information about surface, not subsurface, outflow. Therefore, at this stage, we deemed a closed lake to be one without surface outflow. If a lake could not be unambiguously defined as open or closed in this way, it was classed as "closedx" until further validation was possible. As we were mainly concerned with our own research program, coastal lagoons (those lakes cut off from the open sea via coral reefs or sand bars) were not entered into the MGLD. With the exception of Lakes Eyre, Torrens, and Frome, in which we have a special interest, those ephemeral lakes showing no indication of the presence of water on the ONC maps were also not included. However, lakes such as Bangkog Co (31 °42', 89°30') and Salines Grandes (-29°56', -64°46'), which were depicted as a definite body of water in an ephemeral area, have been included. The MGLD distribution of lakes is shown in Figure 1a)-c) and is summarized in Table 2. The defin-
nent) was entered as "unknown" unless a name could alternatively be identified from the Times Atlas of the World (1993) or various other reference sources. Except for Lake Walker, the Great Salt Lake, Lake Chad, and the Aral Sea, whose lake areas where taken from recent sources (Herczeg and Imboden 1988, Street-Perrott et ai. 1986, Grove 1993, Ellis 1990, respectively), approximate lake areas were deduced from their representation on the ONC maps. The accuracy of the lake areas estimated in this manner (typically to about 20%) is clearly likely to be worse for climatically sensitive lakes whose seasonal and year-to-year change in area may be considerably high. For example, over the time span 1985 to 1991, the lake area of Lake Abiyata (7°37', 38°36') changed by 40% (Harris et al. 1992). Therefore the entered values are first estimates and serve only a guidance to the approximate size category of the lake. In some cases accurate lake areas, derived from satellite images, can be substituted as the remote sensing program proceeds. Lake type refers to three possibilities-closed lake, open lake, and reservoir (Le., a body of water
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A New Global Large Lakes Database 1.50·W
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reservoirs, plottedfrom the MSSL Global Lakes Database (MGW). The open circles in a) depict "closedx" lakes while the solid circles depict "closed" lakes. See text for details.
(c)
312
Birkett and Mason TABLE 2.
MGW lake distribution ("2:100 km 2 approx.)
Region N. + C. America S. America Europe Africa Asia Australasia Total
All lakes
Open lakes
544 125 130 117 462 25 1,403
423 57 122 41 198 16 857
Reservoirs 78 34 3 31 75 5 226
Closed! Closedx*
Closed!closedx as % all lakes
43 34 5 45 189 4** 320
7.9 27.2 3.8 38.5 40.9 16.0 22.8
* see validation section for details ** excludes three ephemeral lakes, Eyre, Torrens, and Frome.
ition of "Region" is as defined by the Times Atlas of the World (1993). It can be seen that the majority of large lakes are to found in the North American and Asian continents. While North America contains 49% of the world's large open lakes, 59% of the potentially closed lakes are to be found in Asia.
EXPANSION OF THE MGLD FOR THE REMOTE SENSING PROGRAM For the majority of closed lakes there is a severe difficulty in obtaining in situ data on lake level or lake area variations. Remote sensing offers a way to monitor lake volume changes in climatically sensitive areas where such in situ data are non-existent or unobtainable. Imaging radiometers such as the Advanced Very High Resolution Radiometer (AVHRR) onboard the NOAA series of satellites and the Along Track Scanning Radiometer (ATSR) onboard the ERS-l satellite, with their relatively wide swath widths and :=::1 km image pixel size, will be able to provide area estimates for all the MGLD lakes, hindered only by cloud cover. Satellite radar altimeters however, though they have all-weather capability, have a very narrow beam width of a few kilometers. Thus level variations can only be obtained for a specific set of large lakes depending on the satellite orbit, in particular the orbit "repeat period" (the period after which the pattern of satellite ground tracks is repeated). The longer the repeat period, the greater the coverage but at a reduced sampling frequency. For example, ERS-l has spent much of its mission in an orbit with a repeat period of 35 days, which enabled almost half of the world's lakes to be covered by its ground tracks. To determine if a particular lake would be crossed by previously flown (Seasat and Geosat) or
current (ERS-l and TOPEX/Poseidon) radar altimeters, access to the U.S. World Data Bank II (WDBII) was obtained. This database is a digital representation of the world (including rivers, islands, and lakes) divided into five geographic areas. Unique line segments, defined by individually digitized points, can be plotted to gain the outline of each lake. The track positions of the various altimeter satellite orbits were overlaid on these lake outlines. Those lakes crossed by the satellites were noted and the information subsequently entered into the appropriate fields in the MGLD. The term "poss" (short for possible) was entered if a lake was not found in the WDBII (but was present on the ONC maps), or if the satellite ground track skimmed the lake edge. To test the ability of each altimeter to obtain level data for smaller lakes, 10 such closed lakes (with areas 60-80 km 2 ) have been additionally entered into the MGLD. Three other fields were also added to the MGLD in relation to the remote sensing program and their contents have for the present only been entered for the closed and for some other climatically sensitive open lakes. These fields are lake identification number (a unique but arbitrary lake designation), terrain type, and lake elevation. Terrain type is a rough guide to the topography surrounding the lake. In general the rougher the terrain the greater the difficulty in obtaining good radar echoes by a radar altimeter (Birkett 1994). A terrain type 1 indicates a level plain or plateau area, type 2 is a lake partially surrounded by mountains, and type 3 indicates a lake closely and completely surrounded by mountains. Lake elevation (in feet, with respect to mean sea level), has been taken from ONC spot heights or if these were unavailable, from the nearest ONC contour level.
A New Global Large Lakes Database VALIDATION OF THE MGLD For validation and other purposes, particularly with respect to the closed lakes, a "general infor~a tion" field was added to the MGLD. It contams brief notes, derived from external sources (published literature or private communication), on lake type (open/closed/reservoir/ and freshwater/saline), anthropogenic influences, the possibilit.y of ground seepage, freeze periods, wind setup/seIche effects, lake area and altitude, and typical lake level fluctuations. There is often a large range in the lake area values given by various external sources. Thus not all estimates are entered, but where estimates exist, at least one published value has been entered for each closed lake. Those closed lakes for which our own estimated area differs from the external reference value by more than 40% have been noted as a general indication of those lakes which may suffer large area changes. ONC lake altitudes were also checked against external estimates. If no spot height or nearest contour was given on the ONC, then the value from the external source was entered into the altitude field. For our remote sensing program it was important to validate lake type in respect of the true identification of a closed lake. By conventional definition (Euguster and Hardie 1978) a "closed" lake has no surface or subsurface outflow and is thus saline (salinity >3 gL- 1). Identification of "closed" lakes by ONC observation, however, is restricte~ to noting the absence of surface outflow only. ThIS can be a source of error in that freshwater (open) lakes do exist which have major ground seepage but no surface outflow. External information is thus required. However, most literature sources (excepting Murray 1910, Herdendorf 1982, and Van der Leeden et at. 1990) use fresh/saline terminology rather than open/closed and the classification may differ between sources. Furthermore, a saline lake can have restricted outflow in the form of ground seepage, so the definition of a saline lake is not a guarantee of a true closed lake. In view of these difficulties we used the following classification, pending further validation. A lake was defined in the MGLD as "closed" if either of the following criteria held: a) The lake had no surface outlets according to the ONC maps. b) The lake was defined as closed in the referenced literature.
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A lake was defined as "closedx" if any of the following criteria held: c) The lake was originally classed in the literature as closed but is now known to suffer from anthropogenic influence. d) As for a) but the lake was later discovered to have some form of minor ground seepage. e) The presence of surface outlets on the ONC was ambiguous and no external information could be obtained for validation. f) The lake had originally been defined as closed from ONC observation but was classed as "freshwater" in a literature source. g) The lake had been originally defined as open from ONC observation but was classed as "saline" in a literature source. h) The lake had been defined as both saline and freshwater in various literature sources. For the remote sensing program we do not wish to exclude any lake which may be climatically sensitive. Hence both "closed" and "closedx" lakes remain the main candidates for study. The references in Table 1 acted as the main sources of validation for the MGLD although many others with emphasis on, say, one particular lake were utilized. All the entered external information in the MGLD is linked to a separate reference database. The latter was created by the authors and contains information on ",,400 journal papers, books, conference proceedings, and reports. Note again that no single reference provides all the necessary information. Finally, two additional fields "level/area :efs" and "level/area pc" were created to regIster "yes/no" depending on. whether we had ~uc~essfully obtained, for each partIcular lake, quantitative level or area information, via published references or by personal communications, respectively. The complete contents of the MGLD is too large a data volume to be published in this paper but as an example, Figure 2 shows the form of a MGLD record for Lake Se-lin in China.
MGLD LAKE STATISTICS Though the validation of the MGLD is still ongoing, we can draw some prelimin~ry s~ati~tics. from the dataset. Figure 3 shows the SIze dIstnbutIOn of the lakes in the MGLD for all lakes and for closed/closedx lakes. The integral size distribution for all lakes (total number of lakes N> area A) approximates well to a power law over several
Birkett and Mason
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mapno.~
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!
Defined as saline, area 1628km2. a114530m (SAL.?). Defined as closed. area llOOkm2. a1146llm (CH.HYD.31).
SAL.? - Williams. 1991. CH.HYD.31- Halbfass, 1922.
FIG. 2.
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An example ofa database record.
decades, as noted by Rapley et ai. (1987). For the size range 10010 km 2 and z 85,000 lakes >1 km 2 • Interestingly, if there were slightly more lakes with smaller areas the power law index could become -1, and the distribution would then be scale invariant (i.e., if one could make a copy of a random tenth of the world's surface and then magnify it, so that its area was ten times larger, its distribution of lake sizes would be statistically identical to that for the whole world). This may imply that, as with many other natural phenomena, the distribution of lakes and their sizes is fractal in form. It is interesting to see from Figure 3 that the closed/closedx distribution also follows a power law with a similar index over the same range. Thus in any size interval the percentage of all lakes which are closed/closedx is approximately the same, at z 23%. The number of closed/closedx lakes in the MGLD is 320 although this is likely to be an overestimate considering the lack of external information. To date, we have not been able to locate any independent information for approximately half of these lakes. Also, just less than half of the closed lakes are termed "closedx." The regional breakdown of the numbers is given in Table 3. The majority of closed lakes are to be found in the Russian Federation, the New Independent States of the former Soviet Union, China and Mongolia-re-
10000 0 0
1000
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closed/x lakes
ItJ 0
0 0
TABLE 3. The global distribution of large closed lakes (~ km 2 approx.)
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000
10
Cb 0
1 10
0
100
1000
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o
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Area (km2)
FIG. 3. The size distributions of all lakes and closed/closedx lakes, based on lake area estimates in the Global Lakes Database.
Region N. America S. America Europe Africa Asia Australasia Total
Closed 14 26 1 24 105 0 170
Closedx
Total area (km2)
29 8 4 21 84 4 150
15,790 10,680 700 22,430 518,600* 800 569,000
*73% Caspian Sea, 8% Aral Sea
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A New Global Large Lakes Database TABLE 4. Closed lakes in the Tibetan Plateau Region-Latitude 30-35°N, Longitude 80-90 oE Name Ang-lajen ts'o Ang-tzu ts'o Bangdag Co Bangkog Ch'a-erh-ku-t'e Hu Cha-jih nan-mu-ts' 0 Cha-pu-yeh eh'a-k'a Dagze Hsu-ju ts'o Jen-eh'ing hsiu-pu-ts'o Ko-jen ts'o Kuo-mang ts'o La-ang ts'o Lixi' Oidain Co Lumajangdong Co Lung MuCo Ma-p'ang yung-ts'o Mo-k' o-yu Hu Mu-ts' o-ping-ni Na-Mu-Ts'o Namru Tso NauCo OrbaCo P'a-lung ts'o P'eng-Ts'o Pa-Mu-Ts'o Se-lin ts'o T'a-jo-ts'o Ta-tse-ts'o Tang-je-ts'o Ts'o-o unknown unknown unknown unknown unknown unknown unknown Xijir Ulan Hu Yaggain Caneo
Lat
Long
Area (km2 )
Altitude (feet)
Map no.
GLD no.
31 °34' 31 °02' 34°57' 31 °42' 31 °49' 30°55' 31 °20' 31 °43' 30°17' 31°17' 31 °08' 31 °13' 30°40' 35°45' 34°02' 34°37' 30°40' 31 °02' 30°40' 30°45' 32°05' 32°50' 34°33' 30°54' 31 °30' 31 °15' 31 °45' 31 °08' 31 °54' 31 °00' 31 °35' 32°07' 35°34' 35°55' 30°13' 30°26' 31 °15' 30°56' 35°13' 33°03'
83°00' 87°10' 81 °34' 89°30' 88°14' 85°40' 84°00' 88°00' 86°25' 83°26' 88°21' 89°12' 81 °14' 90°10' 81°40' 80°29' 81 °28' 89°02' 86°12' 90°35' 90°52' 82°10' 81°03' 83°36' 90°58' 90°35' 89°00' 84°08' 87°32' 86°35' 88°45' 83°33' 82°45' 90°38' 84°48' 84°04' 84°57' 89°42' 90°20' 89°48'
450 300 100 100 100 900 250 250 200 150 300 100 250 250 250 100 300 100 130 1,500 175 100 100 150 100 150 1,600 400 200 700 150 100 100 100 150 100 120 100 300 100
16,000 16,000 17,000 15,000 15,000 16,000 15,000 15,000 16,000 16,000 16,000 16,000 15,510 17,000 17,000 17,120 15,510 16,000 16,000 15,060 15,000 15,000 17,000 17,000 14,830 14,830 15,000 15,000 15,000 15,000 14,360 15,000 17,000 17,000 18,000 18,000 16,000 15,879 17,000 17,000
H9 H9 G7 HlO H9 H9 H9 H9 H9 H9 H9 H9 H9 G8 G7 G7 H9 H9 H9 HlO HlO G7 G7 H9 HlO HlO H9 H9 H9 H9 H9 G7 G7 G89 H9 H9 H9 HlO G8 G8
142 156 176 168 186 152 147 185 153 143 157 161 141 192 179 174 140 160 154 163 167 184 177 144 165 187 158 146 151 155 159 149 182 193 138 139 145 162 191 169
gions from which ground data have not been particularly forthcoming. As an example of a region with a high density of closed lakes, Table 4 lists the identified closed (but not closedx) lakes for the Tibetan Plateau. The MGLD closed lakes range in size from Lake
Bosumtwi, Ghana, which has an area ",,60 km2 (entered as a test case for altimetry) to the Caspian Sea (area ""380,000 km 2 ). Approximately half of the closed/closedx lakes are to be found in areas with terrain types 2 or 3. This knowledge, together with the fact that a third of the closed/closedx lakes are
Birkett and Mason
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between "",100 km2 and "",200 km2 in area (see Fig. 3), will test the radar altimeter's ability to acquire good lake level information over mountainous terrain, a known potential difficulty with current altimeters, as noted earlier. The closed lakes range in elevation from 397 m below sea level (the Dead Sea) to 5,846 m above sea level (Lake Gyeza Caka 33°57', 80°54') and they predominately occupy the arid/semi-arid 3050° northern latitude regions. In terms of obtaining ground information either by personal communication or publications, lake area information (i.e., single area measurements) has been currently obtained for a third of all closed lakes, but level information (daily, weekly, or yearly means) for only "",10%. The number of lakes (closed or otherwise) crossed by each of the radar altimeters, Geosat, ERS-1 and TOPEXlPoseidon is shown in Figure 4. There is clearly a trade off between the satellite orbit repeat period and the number of lakes each altimeter passes over. A selection of the lakes identified by the MGLD for the Geosat mission has been studied (Birkett 1994) and are considered a test case for the application of radar altimeters in monitoring lake level changes. ERS-1 and TOPEX/Poseidon are in fact almost contemporary and although the TOPEXlPoseidon dataset has revealed interesting results (Birkett 1995) the main focus of
Lifetime I 992-cuITCnt
Lifetime 1986-1989
IOday repeat orhil
orbit
17day repeat
Lifetime 1991-1995 35day repeat orbit
FIG. 4. Venn diagram showing the number of large closed/closedx and open/reservoir lakes (the latter combination in brackets) crossed by the various satellite radar altimeters.
the program is now ERS-l. Not only does ERS-1 provide simultaneous altimeter and imaging observations but, during its 35-day repeat phase, it crossed over almost half of the world's closed lakes, potentially offering the strongest contribution to the global aspect of the lakes program.
CONCLUSIONS AND FUTURE WORK The MSSL lakes remote sensing program has led to the creation of the MSSL global lakes database (MGLD) containing information records for over 1,400 lakes and reservoirs. The database has proved a very useful tool in providing statistics on the world's large lakes (those with area ~100 km2 ) and can be considered a preliminary baseline for specific level and area information for any large lake in the world. The MGLD indicates that there are "",320 large, potentially closed lakes and that there is the capability, in principle, to measure the change in lake level of half of these lakes with the currently operating ERS-1 satellite radar altimeter. Imaging radiometers with their relatively wide swath widths will be able to provide area estimates for all the large closed lakes, hindered only by cloud cover. Future expansion of the database will incorporate knowledge pertaining to the potential crossings of all entered lakes by the radar altimeters ERS-2 (launched April 1995, 35 day repeat) and ENVISAT (launch 1998, 35 day repeat period). The validation of the database is ongoing and the authors would welcome comments and suggestions particularly regarding validation or information sources. The MGLD comprises a large data volume (currently 0.8 Mbytes using Filemaker Pro software for the Apple Macintosh) and it is hoped that it can be made available to the hydrological community via the Internet. Details of the availability of the database are being placed on the World Wide Web, Uniform Resource Locator (URL) http://msslsp.mssl.ucl.ac.uk/.
ACKNOWLEDGMENTS The authors wish to acknowledge Mr. Justin Mansley for the satellite orbit software and Mrs. Ellen Mason, Dr. Simon Brown, and Dr. Andrew Harris for assistance in the compilation of the global lakes database. This research was funded by the U.K. Natural Environment Research Council.
A New Global Large Lakes Database
REFERENCES Askew, A.J. 1991. Climate and water. Hydrological Sciences Journal 36:391-404. Birkett, e.M. 1994. Radar altimetry-A new concept in monitoring lake-level changes. EOS 75, No.24: June 14:273-275. _ _ _" 1995. The contribution of TOPEX/POSEIDON to the global monitoring of climatically sensitive lakes. J. Geophys. Res., in press. Chang, W.Y.B. 1987. Large lakes of China. J. Great Lakes Res. 13:235-249. Changnon, S.A. 1993. Changes in climate and levels of Lake Michigan: Shoreline impacts at Chicago. Climatic Change 23:213-230. Cohen, S.J. 1986. Impact of CO 2-induced climatic change on water resources in the Great Lakes Basin. Climatic Change 8:135-153. Dozier, J. 1992. Opportunities to improve hydrological data. Reviews of Geophysics 30:315-331. Ellis, W.S. 1990. A Soviet sea lies dying. National Geographic 177, No.2:70-92. Euguster, H.P., and Hardie, L.A. 1978. Saline lakes. In Lakes: Chemistry, Geology, Physics, ed. A. Lerman, pp. 237-293. New York: Springer Verlag. Farmer, G., and Wigley, T.M.L. 1985, Climatic trends for tropical Africa. Research Report for the Overseas Development Administration, Climatic Research Unit, University of East Anglia, U.K. Folland, e.K., Karl, T.R., and Vinnikov, K.Ya 1990. Observed climate variations and change. In Climate Change: The IPCC Scientific Assessment, ed. J.T.Houghton, G.J. Jenkins, and J.J. Ephraums, pp. 198-238. Cambridge Univeristy Press. Gresswell, R.K., and Huxley, A. 1965. Standard Encyclopedia of the World's Rivers and Lakes. London: Weidenfeld & Nicolson (Educational Ltd.). Grove, A.T. 1993. African river discharges and lake levels in the twentieth century. In Proceedings of the IDEAL Conference. Jinja, Uganda, In Press. Guiot, J., Harrison, S.P., and Pretice, I.e. 1993. Reconstruction of Holocene precipitation patterns in Europe using pollen and lake-level data. Quaternary Research 40:139-149. Halbfass,W. 1922. Die seen der Erde. Erganzungsheft, ed. Petermanns Mitteilungen, Gotha:Justus Perthes, No.185. Harris, A.R. 1994. Time series remote sensing of a climatically sensitive lake. Remote Sensing of the Environment 50:83-94. ____, Mason, I.M., Birkett, C.M., and Mansley, J.A.D. 1992. Lake remote sensing for global climate research. In Proceedings of the ISY Central Symposium, Munich, pp. 173-178. ESA-SP 341. Herczeg, A.L., and Imboden, D.M. 1988. Tritium hydrologic studies in four closed-basin lakes in the Great Basin U.S.A. Limnol. Oceanogr. 33:157-173.
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Herdendorf, e.E. 1982. Large lakes of the world. J. Great Lakes Res. 8:379-412. ILEC. 1988-1991. International Lake Environment Committee, Survey of the State of the World's Lakes: Data Books of the World Lake Environments, Volumes 1-4. Johnson, T.C. (ed.) 1993. IDEAL.·An International Decade for the East African Lakes, Science and Implementation Plan. PAGES Workshop Report, Series 93-2. Kite, G.W. 1981. Recent changes in the level of lake Victoria. Hydrological Sciences Bulletin 26:233-243. Mason, I.M., Guzkowska, M.A.J., Rapley, C.G., and Street-Perrott, F.A. 1994. The response of lake levels and areas to climatic change. Climatic Change 27:161-197. McCarraher, D.B. 1972. A preliminary bibliography and lake index of the inland mineral waters of the world. FAO Fisheries Circular, No.146. Murray, J. 1910. Characteristics and distribution of lakes. In Bathymetrical Survey of the Fresh Water Lochs of Scotland-Report on the Scientific Results, ed. Sir. J. Murray and L. Pullar, Volume I, pp. 514-648. Challenger Office, Edinburgh. Rapley, e.G., Guzkowska, M.A.J., Cudlip, W., and Mason, I.M. 1987. An exploratory study of inland water and land altimetry using Seasat data. ESA Report No. 6483/85/NUBI. Roberts, N., Taieb, M., Barker, P., Damnati, B., Icole, M., and Williamson, D. 1993. Timing of the younger Dryas event in east Africa from lake-level changes. Nature 366:146-148. Rodda, J.e., Pieyns, S.A., Naginda, S.S., and Mathews, G. 1993. Towards a world hydrological cycle observing system. Hydrological Science Journal 38:373-378. Showers, V. 1979. World Facts and Figures. WileyInterscience Publication. New York: John Wiley & Sons. Street, F.A. 1980. The relative importance of climate and local hydrogeological factors in influencing lakelevel fluctuations. Palaeoecology of Africa 12: 137158. Street-Perrott, F.A., Guzkowska, M.A.J., Mason, I.M., and Rapley, e.G. 1986. Response of lake levels to climatic change-Past, present and future. In Effects of changes in stratospheric ozone and global climate, Volume 3: Climate Change, pp. 211-216. United Nations Environment Program. Times Atlas of the World. 1993. Comprehensive Edition.Times Books/John Barthomolew & Son Ltd./Harper Collins Publishers, London. UNESCO 1978. Water balance of lakes and reservoirs. In World Water Balance and Water Resources of the Earth, pp. 533-544, Part 1, UNESCO. Vali-Khodjeini, A. 1991. Hydrology of the Caspian Sea and its problems. In Proceedings of the Hydrology of
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natural and manmade lakes, Vienna Symposium, pp. 45-54. IAHS Pub!. No. 206. Van der Leeden, F., Troise, F.L., and Todd, D.K. 1990. The Water Encyclopedia. Chelsea, Michigan: Lewis Publishers Inc. Wang, H. 1987. The water resources of lakes of China. Chin. J. Oceano!. Limnol. 5:263-280. Williams, W.D. 1991. Chinese and Mongolian lakes: a limnological overview. Hydrobiologia 210:39-66. _ _ _. 1993. The worldwide occurrence and limno-
logical significance of falling water-levels in large, permanent saline lakes. Verh. Internat. Verein. Limnol.25:980-983. Zhao, R., Zhao, H., and Yan, Z. 1992. The design and establishment of lake information system of China. Scientia Geographica Sinica 12:8-13.
Submitted: 22 March 1995 Accepted: 9 May 1995
A New Global Lakes Database for a Remote Sensing Program Studying Climatically Sensitive Large Lakes
Charon M. Birkett and Ian M. Mason Department of Space and Climate Physics University College London Mullard Space Science Laboratory Holmbury St. Mary Dorking, Surrey U.K. RH56NT
ABSTRACT. World wide climate related programs have calledfor hydrological studies to be put into a more global perspective, and yet the global study of large climatically sensitive lakes has been neglected. A remote sensing program at the Mullard Space Science Laboratory (MSSL) aims to monitor short and medium-term lake volume changes and interpret them in terms of aridity variations, as a measure of regional climate change. The program thus requires the global identification of all potentially closed and climatically sensitive lakes. We review the availability of current global lake (particularly closed lake) information and note its limitations with respect to the requirements of the large lakes study. As a result of this the MSSL Global Lakes Database (MGLD) has been constructed containing locational and lake type information for over 1,400 inland water bodies including, as far as possible, all lakes and reservoirs (but not lagoons), with surface areas ?100km 2 (approximately). Of these, the database identifies 857 open lakes (those with outflows), 226 reservoirs, and 320 potentially closed lakes. Approximate areas of the lakes are also noted. The identification of the closed lakes (59% on the Asian continent) is a cornerstone of the remote sensing program which aims to derive lake levels and lake areas using satellite radar altimeters and satellite imaging radiometers respectively. The MGLD therefore also notes the spatial coverage of all large lakes by past and current radar altimeters including that on ERS-1, which overflies almost half of the lakes. With the addition of referenced closed-lake information the MGLD is thus a unique data set which has already been applied to several remote sensing projects. INDEX WORDS:
Database, large lakes, remote sensing, closed lakes, lake levels, climate change.
INTRODUCTION Large lakes have been the subject of great interest not only because of their water resources role but also as indicators of local climate change. Closed lakes, which have no significant surface or subsurface outflow, are particularly climatically sensitive and have been used in the study of palaeoclimate (Street 1980, Guiot et al. 1993, Roberts et al. 1993). Current trends in precipitation have also been shown to be evident in more recent lake level records (Folland et al. 1990) while Williams (1993) brought to attention the limnological significance of falling lake levels for a number of closed lakes. Other studies of large lakes include the noting of both the anthropogenic and natural causes of the fluctuating levels of the Caspian Sea (Vali-Khodjeini 1991) and a study of the levels of the open
Lake Victoria (Kite 1981). From an economic and social point of view the potential impacts of future lake levels of the Great Lakes has recently been discussed (Changnon 1993, Cohen 1986). At the international level several climate/environmental programs have specifically outlined their interest in lake observations. The World Climate Program-Water (WCP-W) has interests in Lake Chad (Dozier 1992), the International Decade for the East African Lakes project (IDEAL) is concerned with the sensitivity of the rift valley lakes to climate change (Johnson 1993), and one of the aims of the Scientific Committee on Problems of the Environment (SCOPE) was to compile and publish information on all African lakes from geomorphological and hydrological viewpoints (Farmer and Wigley 1985). As these examples show, emphasis tends to be 307
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Birkett and Mason
placed on studies of one particular lake or at most several lakes in a localized region. Often, data and results are restricted due to the poor exchange or collation of hydrological data. Askew (1991) called for hydrological studies to be put into a much more global perspective with the use of remote sensing techniques, a point noted by both Dozier (1992) and Rodda et ai. (1993) who also called for the formation of more superior information archives with good distribution and exchange of data. At the Mullard Space Science Laboratory (MSSL) we have proposed a lakes remote sensing program which accords with these views (Mason et al. 1994). Its goal is to measure short and medium-term lake volume changes and interpret them in terms of aridity variations, as a measure of regional climate change. The project aims to monitor several hundred climatically sensitive open and closed lakes on a global scale. Satellite imaging radiometers will provide lake areas to ~1 % accuracy (Harris 1994) and satellite radar altimetry will provide relative lake level changes to an accuracy better than 0.1 m rms (Birkett 1994). Since there are practical limits to the size of lake measurable by altimetry, the program will initially be restricted to lakes of area ~100 km2, which we have deemed "large" lakes.
A REVIEW OF CURRENT GLOBAL LAKE INFORMATION To initialize the research program, the basic information we required was the distribution and size of closed and climatically sensitive open lakes whose areas were ~100 km 2. We also sought information such as lake type (open/closed or reservoir), the existence of any anthropogenic influences, the operation of any ground gauge-monitoring programs (accuracy/frequency and the means of data storage and accessibility), lake freeze/thaw dates, the possibility of ground seepage, and the effect of wind setup on lake levels. An extensive search via personal communications and literature sources was carried out to assess the global availability and accessibility of such information. Personal contacts included attempted communications with hydrological institutes in every country listed by the World Meteorological Organization (J.B. Miller 1992, personal communication) but the quantity and quality of the replies were poor. A number of computerized databases holding lake information were readily made known including MWDI (the U.S. Geological Survey National Water Data Exchange), HYDAT (Environment Canada), NORWIS (the Norwegian Water Information System), LISC/CLD (China National Natural Science Founda-
tion, Zhao et ai. 1992) and the Finnish lakes database (National Board of Waters and the Environment). Apart from the LISC/CLD, access to these lakes data is relatively simple. However, all of these databases tend to function as sources of water resources information and only exist on national scale. Although the United Nations Environment Program is developing a GEMS/Water global database, it currently holds only water quality information for some 82 watersheds (E.D. Ongley, Environment Canada 1994, personal communication) Table 1 lists published sources attempting to catalogue or categorize lake information. Note that the two earliest literary sources (Murray 1910, and Halbfass 1922) give only limited locational information and they are inevitably not comprehensive in their global coverage. No single reference gives all the information required for the remote-sensing program. For example, the International Lakes Environment Committee (ILEC 1988-1991) data volumes (an expanding and ongoing project) are an excellent source of information but are limited in the number of lakes (and in particular closed lakes) they contain. On the other hand Herdendorf (1982) gives an excellent global inventory, noting fresh/saline lake type, but the listings are limited to those lakes with areas >500 km2 . It was decided therefore to create a new global lakes database from scratch and use the located published information as validation only.
THE CONSTRUCTION OF THE MSSL GLOBAL LAKES DATABASE (MGLD) The MGLD is a computerized database containing textual information in the form of records. There is one record for each lake entered and each record consists of a number of information fields. By using a global set of 1: 1,000,000 Operational Navigation Charts (ONC) (USA/UK/Australian Air Chart Program), whose information dates from between the 1970s to the 1990s, those lakes having surface areas ~100 km2 (approximately) were identified. For certain geomorphological regions (e.g., Finland), it was difficult to define a single lake due to highly irregular shapes and the possibility of connecting pools. In these cases a lake record may well represent several sub-basins. In general, however, the MGLD records contain the lake name, estimated area, country and continent location, a latitude and longitude position within the lake (a central location if possible), lake type, and ONC map number. Alternative names, where given on the ONC, were also entered. Any lake which was not named on the ONC (the majority in the Asian conti-
TABLE 1.
Primary sources ofpublished lakes information for validation of the Global Lakes Database (MGLD).
Region Reference Global Global Global Global Global Global Global Global Global China China Global Global China Global
No. lakes
Murray, 1910 120 Halbfass, 1922 1755 Gresswel1&Huxley,1965 110 McCarraher,1972 434 UNESCO, 1978 82 Euguster&Hardie,1978 54 Showers, 1979 290 Herdendorf, 1982 253 Rapley et al., 1987 358 Chang, 1987 28 Wang, 1987 48 van der Leeden et al.,1990 155 ILEC,1988-1991** 183 Williams, 1991 48 MGLD, this paper (1995) 1403
Area No. in range MGLD >50km2 X >0.001 X >5 93 unk 46 >350 71 unk 31 >26 56 >500 223 -100 305 >450 27 >7 43 >100 142 > 0.5 87 >100 42 ~100 1403
Lake Type lakes/eph lakes/eph lakes/res/ephlcl lake/eph lakes/res/eph lakes/eph lakes/ephlcl lakes/ePhlcl lakes/eph lakes/eph lakes lakes/res/eph lakes/res/ephlcl lakes/eph lakeslres
Lake Alt. Lat Name Names Long Region Area y y y y y y y y y y y y y y y
y n
y y
ip ip
y
y y y y
y y y
y y y
y y y y y y y y y y y y y y y
Wind Alti- Freeze No. Vol. Setup tude ffhaw closed
y y y
y ip
y
y
y y y y y y y y
y Y y
ip
29 204 24 45 11
Y
y
y
y y y y
y y
y
y
y
y y y
53 32 40 28 11 19 26 6 42 320
Key: Lake Type: res = reservoirs, eph = ephemeral, cl = coastal lagoons, Note that excepting Rapley et at. 1987 and UNESCO 1978, all sources state whether lakes are freshwater or saline/closed. No. in MGLD = Note that only Herdendorf (1982) and the MGLD (this paper, 1995) claim to list all lakes within their size category. No. closed =Number oflakes in the source recognized as being closed or saline (excluding lagoons and ephemeral lakes) ip = in part unk = unknown ** ongoing project X = Unknown due to limited positional information give in the referenced source.
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Birkett and Mason
controlled by a dam). Reservoirs could easily be distinguished on the ONC maps, but the identification of a closed lake was a more difficult task. By inspection of surface drainage and contours the maps can only reveal information about surface, not subsurface, outflow. Therefore, at this stage, we deemed a closed lake to be one without surface outflow. If a lake could not be unambiguously defined as open or closed in this way, it was classed as "closedx" until further validation was possible. As we were mainly concerned with our own research program, coastal lagoons (those lakes cut off from the open sea via coral reefs or sand bars) were not entered into the MGLD. With the exception of Lakes Eyre, Torrens, and Frome, in which we have a special interest, those ephemeral lakes showing no indication of the presence of water on the ONC maps were also not included. However, lakes such as Bangkog Co (31 °42', 89°30') and Salines Grandes (-29°56', -64°46'), which were depicted as a definite body of water in an ephemeral area, have been included. The MGLD distribution of lakes is shown in Figure 1a)-c) and is summarized in Table 2. The defin-
nent) was entered as "unknown" unless a name could alternatively be identified from the Times Atlas of the World (1993) or various other reference sources. Except for Lake Walker, the Great Salt Lake, Lake Chad, and the Aral Sea, whose lake areas where taken from recent sources (Herczeg and Imboden 1988, Street-Perrott et ai. 1986, Grove 1993, Ellis 1990, respectively), approximate lake areas were deduced from their representation on the ONC maps. The accuracy of the lake areas estimated in this manner (typically to about 20%) is clearly likely to be worse for climatically sensitive lakes whose seasonal and year-to-year change in area may be considerably high. For example, over the time span 1985 to 1991, the lake area of Lake Abiyata (7°37', 38°36') changed by 40% (Harris et al. 1992). Therefore the entered values are first estimates and serve only a guidance to the approximate size category of the lake. In some cases accurate lake areas, derived from satellite images, can be substituted as the remote sensing program proceeds. Lake type refers to three possibilities-closed lake, open lake, and reservoir (Le., a body of water
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(c)
312
Birkett and Mason TABLE 2.
MGW lake distribution ("2:100 km 2 approx.)
Region N. + C. America S. America Europe Africa Asia Australasia Total
All lakes
Open lakes
544 125 130 117 462 25 1,403
423 57 122 41 198 16 857
Reservoirs 78 34 3 31 75 5 226
Closed! Closedx*
Closed!closedx as % all lakes
43 34 5 45 189 4** 320
7.9 27.2 3.8 38.5 40.9 16.0 22.8
* see validation section for details ** excludes three ephemeral lakes, Eyre, Torrens, and Frome.
ition of "Region" is as defined by the Times Atlas of the World (1993). It can be seen that the majority of large lakes are to found in the North American and Asian continents. While North America contains 49% of the world's large open lakes, 59% of the potentially closed lakes are to be found in Asia.
EXPANSION OF THE MGLD FOR THE REMOTE SENSING PROGRAM For the majority of closed lakes there is a severe difficulty in obtaining in situ data on lake level or lake area variations. Remote sensing offers a way to monitor lake volume changes in climatically sensitive areas where such in situ data are non-existent or unobtainable. Imaging radiometers such as the Advanced Very High Resolution Radiometer (AVHRR) onboard the NOAA series of satellites and the Along Track Scanning Radiometer (ATSR) onboard the ERS-l satellite, with their relatively wide swath widths and :=::1 km image pixel size, will be able to provide area estimates for all the MGLD lakes, hindered only by cloud cover. Satellite radar altimeters however, though they have all-weather capability, have a very narrow beam width of a few kilometers. Thus level variations can only be obtained for a specific set of large lakes depending on the satellite orbit, in particular the orbit "repeat period" (the period after which the pattern of satellite ground tracks is repeated). The longer the repeat period, the greater the coverage but at a reduced sampling frequency. For example, ERS-l has spent much of its mission in an orbit with a repeat period of 35 days, which enabled almost half of the world's lakes to be covered by its ground tracks. To determine if a particular lake would be crossed by previously flown (Seasat and Geosat) or
current (ERS-l and TOPEX/Poseidon) radar altimeters, access to the U.S. World Data Bank II (WDBII) was obtained. This database is a digital representation of the world (including rivers, islands, and lakes) divided into five geographic areas. Unique line segments, defined by individually digitized points, can be plotted to gain the outline of each lake. The track positions of the various altimeter satellite orbits were overlaid on these lake outlines. Those lakes crossed by the satellites were noted and the information subsequently entered into the appropriate fields in the MGLD. The term "poss" (short for possible) was entered if a lake was not found in the WDBII (but was present on the ONC maps), or if the satellite ground track skimmed the lake edge. To test the ability of each altimeter to obtain level data for smaller lakes, 10 such closed lakes (with areas 60-80 km 2 ) have been additionally entered into the MGLD. Three other fields were also added to the MGLD in relation to the remote sensing program and their contents have for the present only been entered for the closed and for some other climatically sensitive open lakes. These fields are lake identification number (a unique but arbitrary lake designation), terrain type, and lake elevation. Terrain type is a rough guide to the topography surrounding the lake. In general the rougher the terrain the greater the difficulty in obtaining good radar echoes by a radar altimeter (Birkett 1994). A terrain type 1 indicates a level plain or plateau area, type 2 is a lake partially surrounded by mountains, and type 3 indicates a lake closely and completely surrounded by mountains. Lake elevation (in feet, with respect to mean sea level), has been taken from ONC spot heights or if these were unavailable, from the nearest ONC contour level.
A New Global Large Lakes Database VALIDATION OF THE MGLD For validation and other purposes, particularly with respect to the closed lakes, a "general infor~a tion" field was added to the MGLD. It contams brief notes, derived from external sources (published literature or private communication), on lake type (open/closed/reservoir/ and freshwater/saline), anthropogenic influences, the possibilit.y of ground seepage, freeze periods, wind setup/seIche effects, lake area and altitude, and typical lake level fluctuations. There is often a large range in the lake area values given by various external sources. Thus not all estimates are entered, but where estimates exist, at least one published value has been entered for each closed lake. Those closed lakes for which our own estimated area differs from the external reference value by more than 40% have been noted as a general indication of those lakes which may suffer large area changes. ONC lake altitudes were also checked against external estimates. If no spot height or nearest contour was given on the ONC, then the value from the external source was entered into the altitude field. For our remote sensing program it was important to validate lake type in respect of the true identification of a closed lake. By conventional definition (Euguster and Hardie 1978) a "closed" lake has no surface or subsurface outflow and is thus saline (salinity >3 gL- 1). Identification of "closed" lakes by ONC observation, however, is restricte~ to noting the absence of surface outflow only. ThIS can be a source of error in that freshwater (open) lakes do exist which have major ground seepage but no surface outflow. External information is thus required. However, most literature sources (excepting Murray 1910, Herdendorf 1982, and Van der Leeden et at. 1990) use fresh/saline terminology rather than open/closed and the classification may differ between sources. Furthermore, a saline lake can have restricted outflow in the form of ground seepage, so the definition of a saline lake is not a guarantee of a true closed lake. In view of these difficulties we used the following classification, pending further validation. A lake was defined in the MGLD as "closed" if either of the following criteria held: a) The lake had no surface outlets according to the ONC maps. b) The lake was defined as closed in the referenced literature.
313
A lake was defined as "closedx" if any of the following criteria held: c) The lake was originally classed in the literature as closed but is now known to suffer from anthropogenic influence. d) As for a) but the lake was later discovered to have some form of minor ground seepage. e) The presence of surface outlets on the ONC was ambiguous and no external information could be obtained for validation. f) The lake had originally been defined as closed from ONC observation but was classed as "freshwater" in a literature source. g) The lake had been originally defined as open from ONC observation but was classed as "saline" in a literature source. h) The lake had been defined as both saline and freshwater in various literature sources. For the remote sensing program we do not wish to exclude any lake which may be climatically sensitive. Hence both "closed" and "closedx" lakes remain the main candidates for study. The references in Table 1 acted as the main sources of validation for the MGLD although many others with emphasis on, say, one particular lake were utilized. All the entered external information in the MGLD is linked to a separate reference database. The latter was created by the authors and contains information on ",,400 journal papers, books, conference proceedings, and reports. Note again that no single reference provides all the necessary information. Finally, two additional fields "level/area :efs" and "level/area pc" were created to regIster "yes/no" depending on. whether we had ~uc~essfully obtained, for each partIcular lake, quantitative level or area information, via published references or by personal communications, respectively. The complete contents of the MGLD is too large a data volume to be published in this paper but as an example, Figure 2 shows the form of a MGLD record for Lake Se-lin in China.
MGLD LAKE STATISTICS Though the validation of the MGLD is still ongoing, we can draw some prelimin~ry s~ati~tics. from the dataset. Figure 3 shows the SIze dIstnbutIOn of the lakes in the MGLD for all lakes and for closed/closedx lakes. The integral size distribution for all lakes (total number of lakes N> area A) approximates well to a power law over several
Birkett and Mason
314
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Defined as saline, area 1628km2. a114530m (SAL.?). Defined as closed. area llOOkm2. a1146llm (CH.HYD.31).
SAL.? - Williams. 1991. CH.HYD.31- Halbfass, 1922.
FIG. 2.
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An example ofa database record.
decades, as noted by Rapley et ai. (1987). For the size range 100
10000 0 0
1000
"-<.....
all lakes
closed/x lakes
ItJ 0
0 0
TABLE 3. The global distribution of large closed lakes (~ km 2 approx.)
0'
100
o,oo~
~
z
0
000
10
Cb 0
1 10
0
100
1000
0
10000
o
0
-
100000 - 1000000
Area (km2)
FIG. 3. The size distributions of all lakes and closed/closedx lakes, based on lake area estimates in the Global Lakes Database.
Region N. America S. America Europe Africa Asia Australasia Total
Closed 14 26 1 24 105 0 170
Closedx
Total area (km2)
29 8 4 21 84 4 150
15,790 10,680 700 22,430 518,600* 800 569,000
*73% Caspian Sea, 8% Aral Sea
315
A New Global Large Lakes Database TABLE 4. Closed lakes in the Tibetan Plateau Region-Latitude 30-35°N, Longitude 80-90 oE Name Ang-lajen ts'o Ang-tzu ts'o Bangdag Co Bangkog Ch'a-erh-ku-t'e Hu Cha-jih nan-mu-ts' 0 Cha-pu-yeh eh'a-k'a Dagze Hsu-ju ts'o Jen-eh'ing hsiu-pu-ts'o Ko-jen ts'o Kuo-mang ts'o La-ang ts'o Lixi' Oidain Co Lumajangdong Co Lung MuCo Ma-p'ang yung-ts'o Mo-k' o-yu Hu Mu-ts' o-ping-ni Na-Mu-Ts'o Namru Tso NauCo OrbaCo P'a-lung ts'o P'eng-Ts'o Pa-Mu-Ts'o Se-lin ts'o T'a-jo-ts'o Ta-tse-ts'o Tang-je-ts'o Ts'o-o unknown unknown unknown unknown unknown unknown unknown Xijir Ulan Hu Yaggain Caneo
Lat
Long
Area (km2 )
Altitude (feet)
Map no.
GLD no.
31 °34' 31 °02' 34°57' 31 °42' 31 °49' 30°55' 31 °20' 31 °43' 30°17' 31°17' 31 °08' 31 °13' 30°40' 35°45' 34°02' 34°37' 30°40' 31 °02' 30°40' 30°45' 32°05' 32°50' 34°33' 30°54' 31 °30' 31 °15' 31 °45' 31 °08' 31 °54' 31 °00' 31 °35' 32°07' 35°34' 35°55' 30°13' 30°26' 31 °15' 30°56' 35°13' 33°03'
83°00' 87°10' 81 °34' 89°30' 88°14' 85°40' 84°00' 88°00' 86°25' 83°26' 88°21' 89°12' 81 °14' 90°10' 81°40' 80°29' 81 °28' 89°02' 86°12' 90°35' 90°52' 82°10' 81°03' 83°36' 90°58' 90°35' 89°00' 84°08' 87°32' 86°35' 88°45' 83°33' 82°45' 90°38' 84°48' 84°04' 84°57' 89°42' 90°20' 89°48'
450 300 100 100 100 900 250 250 200 150 300 100 250 250 250 100 300 100 130 1,500 175 100 100 150 100 150 1,600 400 200 700 150 100 100 100 150 100 120 100 300 100
16,000 16,000 17,000 15,000 15,000 16,000 15,000 15,000 16,000 16,000 16,000 16,000 15,510 17,000 17,000 17,120 15,510 16,000 16,000 15,060 15,000 15,000 17,000 17,000 14,830 14,830 15,000 15,000 15,000 15,000 14,360 15,000 17,000 17,000 18,000 18,000 16,000 15,879 17,000 17,000
H9 H9 G7 HlO H9 H9 H9 H9 H9 H9 H9 H9 H9 G8 G7 G7 H9 H9 H9 HlO HlO G7 G7 H9 HlO HlO H9 H9 H9 H9 H9 G7 G7 G89 H9 H9 H9 HlO G8 G8
142 156 176 168 186 152 147 185 153 143 157 161 141 192 179 174 140 160 154 163 167 184 177 144 165 187 158 146 151 155 159 149 182 193 138 139 145 162 191 169
gions from which ground data have not been particularly forthcoming. As an example of a region with a high density of closed lakes, Table 4 lists the identified closed (but not closedx) lakes for the Tibetan Plateau. The MGLD closed lakes range in size from Lake
Bosumtwi, Ghana, which has an area ",,60 km2 (entered as a test case for altimetry) to the Caspian Sea (area ""380,000 km 2 ). Approximately half of the closed/closedx lakes are to be found in areas with terrain types 2 or 3. This knowledge, together with the fact that a third of the closed/closedx lakes are
Birkett and Mason
316
between "",100 km2 and "",200 km2 in area (see Fig. 3), will test the radar altimeter's ability to acquire good lake level information over mountainous terrain, a known potential difficulty with current altimeters, as noted earlier. The closed lakes range in elevation from 397 m below sea level (the Dead Sea) to 5,846 m above sea level (Lake Gyeza Caka 33°57', 80°54') and they predominately occupy the arid/semi-arid 3050° northern latitude regions. In terms of obtaining ground information either by personal communication or publications, lake area information (i.e., single area measurements) has been currently obtained for a third of all closed lakes, but level information (daily, weekly, or yearly means) for only "",10%. The number of lakes (closed or otherwise) crossed by each of the radar altimeters, Geosat, ERS-1 and TOPEXlPoseidon is shown in Figure 4. There is clearly a trade off between the satellite orbit repeat period and the number of lakes each altimeter passes over. A selection of the lakes identified by the MGLD for the Geosat mission has been studied (Birkett 1994) and are considered a test case for the application of radar altimeters in monitoring lake level changes. ERS-1 and TOPEX/Poseidon are in fact almost contemporary and although the TOPEXlPoseidon dataset has revealed interesting results (Birkett 1995) the main focus of
Lifetime I 992-cuITCnt
Lifetime 1986-1989
IOday repeat orhil
orbit
17day repeat
Lifetime 1991-1995 35day repeat orbit
FIG. 4. Venn diagram showing the number of large closed/closedx and open/reservoir lakes (the latter combination in brackets) crossed by the various satellite radar altimeters.
the program is now ERS-l. Not only does ERS-1 provide simultaneous altimeter and imaging observations but, during its 35-day repeat phase, it crossed over almost half of the world's closed lakes, potentially offering the strongest contribution to the global aspect of the lakes program.
CONCLUSIONS AND FUTURE WORK The MSSL lakes remote sensing program has led to the creation of the MSSL global lakes database (MGLD) containing information records for over 1,400 lakes and reservoirs. The database has proved a very useful tool in providing statistics on the world's large lakes (those with area ~100 km2 ) and can be considered a preliminary baseline for specific level and area information for any large lake in the world. The MGLD indicates that there are "",320 large, potentially closed lakes and that there is the capability, in principle, to measure the change in lake level of half of these lakes with the currently operating ERS-1 satellite radar altimeter. Imaging radiometers with their relatively wide swath widths will be able to provide area estimates for all the large closed lakes, hindered only by cloud cover. Future expansion of the database will incorporate knowledge pertaining to the potential crossings of all entered lakes by the radar altimeters ERS-2 (launched April 1995, 35 day repeat) and ENVISAT (launch 1998, 35 day repeat period). The validation of the database is ongoing and the authors would welcome comments and suggestions particularly regarding validation or information sources. The MGLD comprises a large data volume (currently 0.8 Mbytes using Filemaker Pro software for the Apple Macintosh) and it is hoped that it can be made available to the hydrological community via the Internet. Details of the availability of the database are being placed on the World Wide Web, Uniform Resource Locator (URL) http://msslsp.mssl.ucl.ac.uk/.
ACKNOWLEDGMENTS The authors wish to acknowledge Mr. Justin Mansley for the satellite orbit software and Mrs. Ellen Mason, Dr. Simon Brown, and Dr. Andrew Harris for assistance in the compilation of the global lakes database. This research was funded by the U.K. Natural Environment Research Council.
A New Global Large Lakes Database
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Submitted: 22 March 1995 Accepted: 9 May 1995