The knowledge of the hydrological processes of a hydrographic basin is essential for directing water resources management actions related to water use.
The water balance can be understood as the accounting of water inflows and outflows from a given space. The balance can be calculated for a layer of soil, a stretch of river or a watershed.
The hydrographic basin is an adequate space for evaluating the water behavior, as it has the geographic locations of the entrances and exits well defined.
Understanding the water balance depends on several factors such as knowledge of the hydrological cycle (precipitation, runoff, evapotranspiration, infiltration), climatic variables, soil conditions and its use, hydrogeology of the basin, existing water uses, among others.
The most recent water balance recorded for the basins of the Guandu, Guarda and Guandu-Mirim rivers was carried out in the diagnostic stage of the Strategic Plan for Water Resources for the Hydrographic Basins of the Guandu, Guarda and Guandu-Mirim rivers (PERH-Guandu).
The preparation began in 2016 and its conclusion and approval took place in 2018, through Guandu Resolution No. 139, of December 6, 2018. The diagnosis of the PERH included the quantitative and qualitative balances, as can be seen in SIGA-GUANDU.
Below, it is possible to check a summary of the conclusions of the qualitative-quantitative water balance of PERH-Guandu.
Quantitative Water Balance
The Large-Scale Hydrology Research Group (HGE) of the Hydraulic Research Institute of the Federal University of Rio Grande do Sul (IPH-UFRGS) has been developing some products aimed at the integration of decision support systems aimed at the management of water resources with hydrological models and GIS, based on the Large Basin Hydrological Model (MGB), also developed in the same research center.
The model has already been applied in the basins of the Ibicuí and Ijuí rivers, in Rio Grande do Sul, in two tributary basins of the São Francisco river, in Minas Gerais, and also in the Macaé river basin, in the State of Rio de Janeiro. the great ease of implementation in any hydrographic basin. The current version of the SiGBAH-IPH is available for download on the research group’s page, as well as the instruction manual and published articles.
The water balance scheme adopted in this study is constituted by a balance of water inflows and outflows in each of the microbasins of the hydrographic network. The details of the methodology used can be found in Volume II of the Diagnosis Stage of PERH-Guandu.
The most expressive commitment occurs in the São Pedro river (Planning Hydrological Unit – UHP Santana and São Pedro rivers), in which in its outflow, the demand exceeds the level of 50% of water availability, being caused almost entirely by abstractions for supply purposes.
The second most expressive impairment occurs in the Queimados River, where the impairment in its outlet is 34%, mostly due to water supply. Observing the water balance maps, there are also other points of high criticality, however, less expressive results are considered, as they represent drainage areas in general very small.
Regarding the course influenced by the transposition, there is a commitment greater than 50% in the São Francisco Channel from the TKCSA catchment point. An alternative scenario is also presented, in which the release adopted in Pereira Passos is 70 m³/s, corresponding to the exceptional minimum verified in the series of deflows in some periods in 2015. In this scenario, the commitment exceeds 50 % starts right after the Guandu ETA catchment point, with a remaining flow of 31 m³/s at this point. In the channel’s outlet, the commitment would exceed 90%, with a remaining flow of only 3.9 m³/s.
Qualitative Water Balance
The water quality is presented in the diagnosis in terms of the average Water Quality Index (WQI), indicating great heterogeneity in the Guandu Hydrographic Region, with stretches classified as good, average, bad and very bad. It is noticed that the UHPs with the most critical classification are those with the greatest economic potential (in terms of GDP): Queimados and Ipiranga rivers, Guarda river, Guandu-Mirim river and coastal basins (ME).
Only UHP Ribeirão das Lajes, upstream from the reservoir, has a good IQA, where it is clear that the regional economic potential is low, with no urban headquarters. The effect of the dilution flow is clearly visible in the Rio Guandu and Canal de São Francisco UHPs, which present medium IQA, even with a large polluting load coming from the affluent basins, which have a worse IQA, but suffer dilution by the remaining flow from the transpositions.
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