1 The spatiotemporal patterns of surface water temperature in a Brazilian hydroelectric reservoir Enner Herenio Alcântara José Luiz Stech João Antônio Lorenzzetti Evlyn Márcia Leão de Moraes Novo Brazilian Institute for Space Research, Remote Sensing Division E-mails: {enner, stech, loren, evlyn}@dsr.inpe.br Abstract The water temperature plays an important role in the ecological functioning and controlling the biogeochemical processes of a water body. Conventional water quality monitoring is expensive and time consuming. Particularly problematic if the water bodies to be examined are large. Conventional techniques also bring about a high probability of undersampling. Conversely, remote sensing is a powerful tool to assess aquatic systems. Based on this, the objective of this study was to map the surface water temperature and improve understanding of spatiotemporal variations in a hydroelectric reservoir. In this work the MODIS land-surface temperature (LST) level 2, 1-Km nominal resolution data (MOD11L2, version 5) was used. All available clear-sky MODIS/Terra imagery between 2003 and 2008 were used, resulting in a total of 786 daytime and 473 nighttime images. Descriptive statistics (mean, maximum and minimum) was computed for the historical images, so as to build a time series of daytime and nighttime monthly mean temperature. The thermal amplitude and the anomaly were also computed. In-situ meteorological variables were used from 2003 to 2008 to help us understand the spatiotemporal variability of the surface water temperature. The surface energy budget and the depth the wind can distribute a given surface heat input were also measured. A correlation between daytime and nighttime surface water temperature and the meteorological parameters and a linear regression computed. These relationship and the causes of the spatiotemporal variability was discussed. Keywords: Water surface temperature; heat flux; mixed depth layer; thermal amplitude. Introduction Reservoirs, or man-made lakes, are usually built to store water for later use, water supply, flood control or power generation (Casamitjana et al., 2003). In Brazil, there are approximately 31 hydroelectric reservoir buildings by electric sector with volume more than 1 billion of m 3 . The hydroelectric sector is responsible for 97% of energy generation and considered the largest hydroelectric park of the world (Kelman et al. 2002). The building of these dams, however, causes greatest environmental, social and economics impacts (Tundisi, 1994). Over the time, the functioning of reservoirs affects its retention time. Ford (1990) INPE ePrint: sid.inpe.br/mtc-m18@80/2009/09.29.19.24 v1 2009-09-30
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The Spatiotemporal Patterns of Surface Water Temperature In a Brazilian Hydroelectric Reservoir
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The spatiotemporal patterns of surface water temperature in a Brazilian hydroelectric reservoir
Enner Herenio Alcântara José Luiz Stech
João Antônio Lorenzzetti Evlyn Márcia Leão de Moraes Novo
Brazilian Institute for Space Research, Remote Sensing Division
E-mails: {enner, stech, loren, evlyn}@dsr.inpe.br Abstract The water temperature plays an important role in the ecological functioning and controlling the biogeochemical processes of a water body. Conventional water quality monitoring is expensive and time consuming. Particularly problematic if the water bodies to be examined are large. Conventional techniques also bring about a high probability of undersampling. Conversely, remote sensing is a powerful tool to assess aquatic systems. Based on this, the objective of this study was to map the surface water temperature and improve understanding of spatiotemporal variations in a hydroelectric reservoir. In this work the MODIS land-surface temperature (LST) level 2, 1-Km nominal resolution data (MOD11L2, version 5) was used. All available clear-sky MODIS/Terra imagery between 2003 and 2008 were used, resulting in a total of 786 daytime and 473 nighttime images. Descriptive statistics (mean, maximum and minimum) was computed for the historical images, so as to build a time series of daytime and nighttime monthly mean temperature. The thermal amplitude and the anomaly were also computed. In-situ meteorological variables were used from 2003 to 2008 to help us understand the spatiotemporal variability of the surface water temperature. The surface energy budget and the depth the wind can distribute a given surface heat input were also measured. A correlation between daytime and nighttime surface water temperature and the meteorological parameters and a linear regression computed. These relationship and the causes of the spatiotemporal variability was discussed. Keywords: Water surface temperature; heat flux; mixed depth layer; thermal amplitude. Introduction Reservoirs, or man-made lakes, are usually built to store water for later use, water supply,
flood control or power generation (Casamitjana et al., 2003). In Brazil, there are
approximately 31 hydroelectric reservoir buildings by electric sector with volume more than 1
billion of m3. The hydroelectric sector is responsible for 97% of energy generation and
considered the largest hydroelectric park of the world (Kelman et al. 2002). The building of
these dams, however, causes greatest environmental, social and economics impacts (Tundisi,
1994). Over the time, the functioning of reservoirs affects its retention time. Ford (1990)
The Itumbiara hydroelectric reservoir (18°25’S, 49°06’W) is located in a region stretched
between Minas Gerais and Goiás States (Central Brazil) originally covered by tropical
grassland savanna. Damming the Parnaiba River flooded backward its main tributaries:
Araguari and Corumbá River. The basin geomorphology resulted in a lake with a dentritic
pattern covering an area of approximately 814 Km² and volume of 17.03m³ (Figure 1).
(a) (b)
(c)
Figure 1: Localization of Itumbiara hydroelectric reservoir on Brazil’s central (a), on state context (b) and on regional scale (c) with the bathymetric map. On regional scale is showing the flooded area over a SRTM (Shuttle Radar Topography Mission) image.
The reservoir was built in 1979 and started its operation in 1980. Figure 2 shows the reservoir
area before the flooding (Figure 2-a) and after (Figure 2-b) flooding.
Figure 2: (a) MSS-Landsat-3 imagery from 11-08-1978 show the area before inundation; and (b) TM-Landsat-5 imagery from 26-05-2007 actual period. The figure also shows the position of dam on reservoir.
The climate in the region is characterized by precipitation ranges from 2.0 mm in the dry
season (May - September) to 315 mm in rainy season (October - April). In the rainy season
the wind intensity ranges from 1.6 to 2.0 ms-1 and reaches up to 3.0 ms-1 in the dry season
(Figure 3-a). The air temperature in the rainy season ranges from 25 to 26.5 ºC and breaks
down to 21ºC in June as the dry season starts. The relative humidity has a pattern similar to
that of the air temperature, but with a little shift of the minimum value towards September
(47%). Moreover during the rainy season the humidity can reach 80% (see Figure 3-b).
(a) (b)
Figure 3: Climate patterns on Itumbiara reservoir: mean monthly of (a) precipitation (mm month-1) and wind intensity (m s-1), (b) air temperature (ºC) and humidity (%).
The following section will describe the methodology needed to explain the spatiotemporal
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