In Situ and Satellite Observation of CDOM and Chlorophyll-a Dynamics in Small Water Surface Reservoirs in the Brazilian Semiarid Region

Abstract

Abstract:We analyzed chlorophyll-a and Colored Dissolved Organic Matter (CDOM) dynamics from field measurements and assessed the potential of multispectral satellite data for retrieving water-quality parameters in three small surface reservoirs in the Brazilian semiarid region. More specifically, this work is comprised of: (i) analysis of Chl-a and trophic dynamics; (ii) characterization of CDOM; (iii) estimation of Chl-a and CDOM from OLI/Landsat-8 and RapidEye imagery. The monitoring lasted 20 months within a multi-year drought, which contributed to water-quality deterioration. Chl-a and trophic state analysis showed a highly eutrophic status for the perennial reservoir during the entire study period, while the non-perennial reservoirs ranged from oligotrophic to eutrophic, with changes associated with the first events of the rainy season. CDOM characterization suggests that the perennial reservoir is mostly influenced by autochthonous sources, while allochthonous sources dominate the non-perennial ones. Spectral-group classification assigned the perennial reservoir as a CDOM-moderate and highly eutrophic reservoir, whereas the non-perennial ones were assigned as CDOM-rich and oligotrophic-dystrophic reservoirs. The remote sensing initiative was partially successful: the Chl-a was best modelled using RapidEye for the perennial one; whereas CDOM performed best with Landsat-8 for non-perennial reservoirs. This investigation showed potential for retrieving water quality parameters in dry areas with small reservoirs.

Hydrological Guidelines for Reservoir Operation: Application to the Brazilian Semiarid Region

“…Should rationalization be linear with the licensed discharge? Another gap -still to be developed within the model framework -is the consideration of water quality [21] as a key parameter in the decision making. These problems are still technically unsolved, but a democratic and representative basin committee seems to be the best forum to decide such matters and provide proper water governance in reservoir-dependent regions [34].…”

“…The natural hydrological system constantly fails to provide enough water for that densely populated environment, which called for the construction of a reservoir-based water system [2][3]. Due to the considerable meteorological inter-and intra-annual variability, to the high number of reservoirs (one dam every 5 km² on average), and to the high residence time of the waters within the reservoirs (which causes low levels of water quality [21]), the Caatinga biome has become a challenging biome for water management [22]. Usually, River Basin Committees decide on water release shortly after the rainy season, the key information being the stored reservoir volume.…”

Simplified Method for the Assessment of Siltation in Semiarid Reservoirs Using Satellite Imagery

“…The proposed method combines satellite imagery with a simplified bathymetric survey. The small reservoirs in the Brazilian semiarid region deplete completely every year [32,33], whereas the large ones often reduce their storage to 20% or less at the end of after the annual dry season [6]. This feature hampers the use of traditional bathymetric surveys because it requires that the reservoir be full so as to achieve an accurate outcome, which does not happen frequently [12].…”

“…The classification is then accomplished on a single image, which is made up or not (when using only one spectral band) of the combination of three bands. Because of the scarcity of good-quality imagery over semiarid reservoirs [32][33][34][35], we recommend using the supervised classification and maximum likelihood method (MLM) to obtain informational classes from remote sensing images. The rationale for this approach is to get the most out of the available orbital data, which does not rule out the possibility of using other methods with similar end products [26][27][28]30,31].…”

“…The process occurs first, depending on the degree of eutrophication of the reservoir (recurrent in Brazilian semiarid region [4,45]), which promotes considerable changes in the spectral behavior of water [15,16,46,47]). Eutrophication contributes to the formation of a gradual stratified zone that connects the water body with surrounding vegetation; they become more prominent when there is aquatic vegetation (i.e., macrophytes- Figure 6) in the reservoir [33,34].…”