[pt] O presente trabalho teve como unidade de estudo a bacia hidrográfica do
rio Cachoeira, localizada na vertente sul do Maciço da Tijuca, município do Rio
de Janeiro. O objetivo geral do estudo foi analisar os efeitos das alterações na
cobertura vegetal dessa bacia sobre seu comportamento hidrológico. As
componentes hidrológicas selecionadas para análise foram a evapotranspiração
real e a vazão média na exutória da bacia. Através do método do balanço hídrico
de Thornthwaite e Mather, aplicado de forma sequencial, foi estimada uma série
mensal de evapotranspirações reais da bacia. Foram utilizados nesse método de
balanço hídrico dados de temperaturas médias do ar e totais precipitados na
região. A série de vazões médias na exutória da bacia foi obtida utilizando-se
como referência as estações fluviométricas Capela Mayrink e Itanhangá, ambas
situadas no interior da bacia. Por meio de tecnologias de geoprocessamento e
sensoriamento remoto foram mapeadas dez imagens do satélite Landsat-5/TM,
estimando-se assim as alterações ocorridas na cobertura vegetal da bacia.
Finalmente buscou-se estabelecer correlações entre as variações da cobertura
vegetal e das componentes hidrológicas selecionadas. O resultado obtido para a
evapotranspiração real foi satisfatório, indicando uma relação direta com a
dinâmica da cobertura da bacia. Entretanto não foi possível estabelecer para a
vazão média uma correlação de qualidade semelhante. Através desse estudo
adquiriu-se um melhor entendimento sobre a influência da variação da cobertura
vegetal no comportamento hidrológico da bacia hidrográfica do rio Cachoeira. / [en] The general objective of the study was to analyze the effects of land cover
changes on hydrological processes of the Cachoeira river watershed. The
hydrological components selected for analysis were the real evapotranspiration
and the mean flow at the exutory of the watershed. Through the Thornthwaite and
Mather water balance method, applied sequentially to the period between 1997
and 2010, a monthly series of real evapotranspiration was established. The choice
of this method was based on the compatibility of the data required by the method
with the hydrometeorological available data.
Monthly series of average air temperatures and total precipitations was
used to calculate the water balance of Thornthwaite and Mather. The average air
temperature data were obtained from the pluviometric station Alto da Boa Vista,
located around the watershed. In order to obtain a complete and consistent data
series of average air temperatures, a correlation was established of these data with
the data from the climatological station of the city of Rio de Janeiro. Through this
correlation was possible to confirm the consistency of the series of air
temperatures and fill any gaps from 1997 until the year 2010. In this period the
monthly series of average air temperatures had a mean of 22,1 Celsius degrees, ranging from a
maximum of 27,5 Celsius degrees and a minimum of 17,8 Celsius degrees. The pluviometric data used as reference for the average rainfall over the Cachoeira river watershed, was
recorded at the pluviometric station Capela Mayrink, located within the
watershed. The consistency of this series has been verified before the other
pluviometric stations around the watershed by the method of the double mass, which could confirm the consistency because no deviations were found in the
precipitation behavior over time. The average annual precipitation over the
watershed was 2.181 mm.
The highest estimated values of real evapotranspiration were found in the
rainy season, between December and March. The estimated average value of
annual real evapotranspiration during the studied period was 1.056 mm, ranging
between 1.007 mm and 1166 mm. This average value corresponds to
approximately 50 per cent of total annual precipitated, in other words, it is indicated that
half of the precipitation over the watershed would return to the atmosphere by the
processes of the hydrological cycle. Besides the real evapotranspiration, the water
balance also provided an estimation of the water extract of the watershed,
calculating components as water deficit, water surplus and soil water storage.
The mean monthly flow series at the exutory of the watershed was
obtained by referencing the data recorded at the fluviometric stations Capela
Mayrink and Itanhangá, both located within the watershed. The data from these
stations have gone through a consistency analysis, where their fluviometric levels
were verified together and their rating curves were elaborated to represent an
adequate adjustment to their liquid discharge measurements. After the consistency
analysis, the fluviometric levels data were transformed into flows, through the
rating curves. The methodology adopted to generate the flow series at the exutory
of the watershed consisted primarily in the extension of the flows series at
Itanhangá station through correlation with the flows at Capela Mayrink station.
Subsequently, the extended flow series at Itanhangá station was transferred to the
exutory location by proportionality between drainage areas. Due to the lack of
local data, it was not possible to obtain a mean monthly flow series without gaps.
The comparison of the flow data with the precipitation data, obtained for
the Cachoeira river watershed, showed a coherent behavior over the years. The
annual variation of rainfall in the watershed was accompanied by the flow.
It was possible to estimate the changes in land cover during the period
from 1988 to 2010 using geoprocessing and remote sensing technologies,
available at the extension Spatial Analyst Tool from the software ArcGIS 9.3. In
order to obtain this data, a geographic information system was developed for the
Cachoeira river watershed, composed by a digital terrain model, obtained from the
Shuttle Radar Topography Mission (SRTM), and by ten digital images obtained from the satellite Landsat-5/TM, spaced in average every two years during the
studied period.
The digital terrain model was used to generate the information grids of
Flow direction and Flow accumulation. Through these grids, the watershed and
the drainage areas of the fluviometric stations could be automatically delineated.
The delimitation of the fluviometric stations drainage areas was done in order to
verify the official areas mentioned in the inventory stations of the Brazilian
National Water Agency. Although the calculated values did show differences in
comparison with the official ones, they were used in the study, considering that
the relative errors are minimized when using the same geographic basis.
Initially the application of digital processing techniques on satellite images
consisted of a combination of bands 5, 4, 3, to form the color composite R, G, B.
All images were georeferenced at the same control points in the UTM projection
system, using the Datum WGS-84, Zone 23 South. Subsequently the images were
classified using the supervised classification maximum likelihood. To characterize
the dynamics of land cover over time, two thematic classes were chosen: Forest
Area, which has forest cover and others natural features not modified by human
activities and Non-Forest Area, which includes urbanized areas and most areas
that original feature has been changed as a result of human activities and. The
signature samples collected for each training were simple and spatially well
distributed, within the region of the studied watershed.
Since the supervised classification was an automatic process, the thematic
products generated showed errors, identified as isolated cells outside the context
of the classes, which left the areas fragmented. In order to work around these
errors and provide uniformity of the mapped classes a post-classification process
was done on the images by applying a majority filter, which replaces isolated cells
based on the majority of their contiguous neighboring cells. Even so, a small
portion of the thematic products still showed classification errors, so they were
manually edited to become more representative. Ten thematic maps of land cover
for the Cachoeira river watershed were generated as products of these processes.
The validation of each thematic map classification was verified through the
confusion matrix. Considering that only two thematic classes with distinct
characteristics were used, the performance of the confusion matrix was
tendentious and insufficient to ensure the accuracy of the classification. In order to evaluate the quality of the thematic maps obtained, the thematic map generated
for 2010 was compared with the official one, provided by the Municipal
Secretariat of Environmental of Rio de Janeiro (SMAC). This comparison could
validate the consistency of the thematic map of 2010, believing that the other
thematic maps also represent an estimate of the land cover reality from past
period.
The ten thematic maps could estimate the history of land cover changes on
the Cachoeira river watershed. It was observed at the maps that there were
changes in the shape of the occupation of the watershed, however, in accordance
with the estimated percentages, the evolution of land cover in the watershed had a
steady behavior over the years. The consecutive differences found did not exceed
the order of 3 per cent.
Finally, correlations were established between the variation of the areas
with forest coverage and the variation of the selected hydrological components.
The result obtained at the real evapotranspiration correlation was satisfactory,
which indicates a direct relationship between this hydrological component and the
watershed land cover dynamic. However it was not possible to establish a
correlation of similar quality with the mean flow.
This study could contribute as an exercise to aggregate knowledge about
the influence of land cover on hydrological processes over time.
| Identifer | oai:union.ndltd.org:puc-rio.br/oai:MAXWELL.puc-rio.br:21193 |
| Date | 20 February 2013 |
| Creators | RODRIGO JOSE COELHO PEREIRA |
| Contributors | ROGERIO RIBEIRO DE OLIVEIRA |
| Publisher | MAXWELL |
| Source Sets | PUC Rio |
| Language | Portuguese |
| Detected Language | English |
| Type | TEXTO |
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