Lewis J. Allison and Thomas J. Schmugge A Hydrologicai Analysis ol East Goddard Space Flight Center Greenbelt, Md. 20771 Australian Floods Using Nimbus-5 and Electrically Scanning Radiometer Gavin Byrne Commonwealth Scientific and Industrial Research Data Organization (CSIRO) Canberra, A.C.T., Australia Abstract The chronological development and diminution of six floods in eastern Australia during January, February, and March 1974 were mapped for the first time by the Nimbus Elec- trically Scanning Microwave Radiometer (ESMR). Day and nighttime ESMR (19.35 GHz) coverage was analyzed for the low gradient, flooded Darling River system in New South Wales. Apparent movement of surface water as indi- cated by low brightness temperatures (<250 K, day and <240 K, night) was easily followed around the curved runoff basin along the northern shoreline of the flooded Darling River during this 3-month period. This pattern was in good agreement with flood crest data at selected river height gage stations, even under cloudy conditions. 1. Introduction The Nimbus-5, a research and development satellite launched on 12 December 1972, carried the Electrically Scanning Microwave Radiometer (ESMR), which re- corded microwave brightness temperatures (TB) from which it has been possible to estimate meteorological and hydrologicai parameters on a global scale. The microwave brightness temperature (TB) of the earth's surface is essentially the product of the surface temperature and surface emissivity, with additional attenuation and emissions due to atmospheric water FIG. 1. The Nimbus-5 spacecraft and associated experiments. vapor and liquid water droplets (Allison, 1975 ; Zwally and Gloersen, 1977). Ice crystal clouds (cirrus) have little effect on microwave radiation and are essentially stant, to about 0.7 for wet soils, and to >0.9 for dry soils transparent at the 1.55 cm ESMR wavelength. Mo- and dense vegetation. Microwave emission from sur- lecular oxygen absorption can also be neglected at this faces is also affected by roughness, topographic slope, wavelength (Wilheit et al., 1977). Emissivity is the stratigraphy (layering of different materials), density ratio of the radiation from a surface at a measurable of rocks, and vegetation cover (Moore et al., 1975; temperature for a given wavelength interval to that Ulaby, 1977; Allison, 1977). For example, a smooth from a black-body source at the identical tempera- bare dry soil surface whose physical temperature was ture. When the emissivity is 1, the surface radiates 290 K and emissivity was 0.90-0.95 would indicate as a black body. When it is less than 1, the surface a TB of 261-275 K. This is to be contrasted to the behaves as a gray body. The emissivity of the surface low TB over the calm ocean, which is approximately depends on, among other things, the medium's di- 120 K, a result of low water emissivities of ~0.4 at electric properties, and for a smooth surface can range 19.35 GHz. The state of the sea surface (wave height, from 0.4 for water, which has a large dielectric con- roughness, and foam) would increase the observed brightness temperature for winds above 7 m/s at the rate of 1 K/(m s"1) (Nordberg et al., 1971). 0003-0007/79/121414-14$07.50 Nimbus-5 ESMR data have been used to detect © 1979 American Meteorological Society rainfall over the ocean by Allison et al. (1974), Rao 1414 Vol. 60, No. 12, December 1979 Unauthenticated | Downloaded 10/04/21 04:52 AM UTC Bulletin American Meteorological Society 1415 icejtfemraSQ4 0 30 FIG. 2. Annual mean rainfall (in) (Leeper, 1973). "0, Hobart 30 FIG. 3. Conventional photographs showing terrain and vegetation features in New South Wales and South Australia. (Laut et al., 1977). (a) Hill and plain with saltbush. (b) Gibber Plain with tufted grasses, (c) Dunes, claypans, and channels near Lake Eyre, (d) Dune with erosion remnants, and the lake in the background. Unauthenticated | Downloaded 10/04/21 04:52 AM UTC 1416 Vol. 60, No. 12, December 1979 FIG. 4. Major river systems with minor tributaries and lake locations, eastern half of Australia. Dot-dash box outlines the study area analyzed in this paper. (Department of National Develop- ment, 1956.) Unauthenticated | Downloaded 10/04/21 04:52 AM UTC Bulletin American Meteorological Society 1417 FIG. 5. A detailed chart of the Darling River system, New South Wales, Australia. et al. (1976), Kidder and Von der Haar (1977), Wilheit over the oceans (Rao and Theon, 1977; Rao et al., et al. (1977), Adler and Rodgers (1977), Austin and 1976). Geotis (1978), and Stout and Martin (1979). This is The extent of a disastrous regional flood in the possible because the oceans provide a relatively con- Mississippi Valley at varying recurrence intervals was stant, low TB 150 K) background for observing the mapped by use of Landsat (Rango and Anderson, atmosphere. Over land the background TB is higher, 1974; Rango and Salomonson, 1974; Otterman et al., typically >250 K, and more variable; as a result at- 1976), NOAA VHRR (Wiesnet et al., 1974), and tempts to measure rainfall over land with the Nimbus-5 Nimbus-5 ESMR data (Schmugge et al., 1974). ESMR have failed (Meneely, 1975). However, there However, a complete time sequence of a major flood has been some limited success in delineating rain over had not yet been demonstrated by satellite analysis. land using theoretical analysis and actual case studies This paper will present Nimbus-5 microwave data of the dual polarized Nimbus-6 ESMR (37 GHz) that were used to detect and map the movement of (Rodgers et al., 1978, Savage and Weinman, 1975). flooded areas in eastern Australia from January to The effect of rainfall over the ocean is to increase March 1974. the TB observed from space up to 250 or 260 K for high rain rates. The rate of increase in TB depends on the wavelength. At the 1.55 cm Nimbus-5 ESMR 2. Electrically-scanning microwave radiometer wavelength, this maximum is reached for a rain rate experiment of approximately 10 mm/h (Wilheit et al., 1977). This relationship has been used to determine rainfall rates The Nimbus-5 spacecraft (Fig. 1) was flown in a near- over the oceans, thus providing information for regions polar (81° retrograde) sunsynchronous circular orbit of the earth for which there were very few conven- of about 1100 km. Each orbit crossed the equator tional data available. These data have been combined northward at approximate local noon and southward to produce an atlas of the monthly averages of rainfall at midnight with a 27° longitude separation and a Unauthenticated | Downloaded 10/04/21 04:52 AM UTC 1418 Vol. 60, No. 12, December 1979 FIG. 6. Rainfall (in percent normal) for January and February 1974, Queensland and New South Wales. period of 107 min. The Nimbus-5 ESMR measures longitude at 30°N. The half-power beamwidth of the the emitted radiation from the earth's surface and antenna is 1.4° at nadir and the 2.2° at 50° edges intervening atmosphere in the 250 MHz band centered with resultant footprints of 22 km and 45 by 160 km, at 19.35 GHz (1.55 cm), with a noise equivalent tem- respectively. Since the ESMR senses horizontally po- perature difference (NEAT) of approximately 2 K. larized radiation, a scan-angle dependent correction It scans transverse to the flight path every 4 s from was applied to the data to make each observation 50° to the left through nadir to 50° to the right in more closely equivalent to a linearly polarized nadir 78 steps with some overlap. The scanning process is observation (Wilheit, 1973). The instrument and its controlled by an onboard computer and the radiome- basic physics are discussed in more detail by Wilheit ter scans a region whose width is approximately 30° of (1972), Wilheit et al. (1973), and Allison et al. (1974). Unauthenticated | Downloaded 10/04/21 04:52 AM UTC Bulletin American Meteorological Society 1419 FIG. 7. Nimbus-5 ESMR (19.35 GHz) photofacsimile pictures of the East Australian floods from 9 January 1974 to 21 March 1974. The arrows indicate the wet ground in the Darling River system during this period. Unauthenticated | Downloaded 10/04/21 04:52 AM UTC 1420 Vol. 60, No. 12, December 1979 Unauthenticated | Downloaded 10/04/21 04:52 AM UTC Bulletin American Meteorological Society 1421 FIG. 8. Nimbus-5 ESMR (19.35G Hz) grid-print maps of brightness temperatures (TB, K). The single arrow indicates the flooded Darling River system. The double arrow indicates the Lake Yamma Yamma and flooded Coopers Creek area, 10 January-14 March 1974. 3. The 1974 East Australian floods a. Background information Figure 2 shows the annual mean rainfall in inches for Australia. The wetter areas are indicated by grey tones (>20 in) and are located along the coastal strips to the north, northeast, east, southeast, and southwest. During the summer months (December- February) the rainy intertropical zone of convergence (ITC) is indicated by a low pressure monsoonal area that forms over the north equatorial coast. Extensive extremely dry desert regions (<10 in (250 mm)/yr) are shown covering large areas of central and western Australia. Evaporation is high and on the average is equivalent to 87% of the rainfall as compared to 60% for Europe and North America. Thus only a small proportion of rainfall becomes surface runoff except during heavy rain periods. During the winter months, June-August, the north coast is dry as the ITC moves FIG. 9. Landsat 1 (MSS-7, frame no. 1563-23530) recorded northward into the Northern Hemisphere while the on 6 February 1974. Lake Yamma Yamma (appendage lower southern coastal strips receive rain from the west- left) is shown near darker flooded Coopers Creek lowland to-east traveling lows in the "Roaring Forties" (Leeper, (center right).