Remote Sensing Utility for Water Quality and Riparian Vegetation Monitoring in Mediterranean Reservoirs: Impact of Ecological Flow Regimes Management

Abstract

Significant progress has been achieved since the 2013 implementation of ecological flow rates due to the Water Framework Directive in Spain (WFD). Nevertheless, certain shortcomings exist, such as adequately monitoring compliance and analyzing the ecological response post-implementation. This is especially evident in areas characterized by complex meteorology, with extended periods of drought, as observed in regions affected by the Mediterranean climate. Moreover, it is crucial to combine minimum flows with pollution issues, whether anthropogenic or natural, to attain the good ecological status of water bodies. Our study aims to address three distinct questions: 1) How does implementing various environmental flow regimes impact the levels of hydrological alteration in terms of water quality and riparian vegetation downstream of the reservoirs? 2) How can we use remotely sensed information to complement existing water monitoring networks to assess changes in water quality and riparian vegetation? 3) What is the required spatiotemporal resolution needed to monitor these alterations?   The pilot reservoir to conduct this study is within the Guadalquivir River Basin (southern Spain). This basin has relevant problems of reservoir silting and water pollution arising from high erosion and human intervention rates. To assess the evolution of water quality, quantity, and vegetation state, we evaluate different indexes derived from high, medium, and low spatial resolution VIS/NIR satellite images, with temporal resolution ranging from daily to biweekly. The analysis spans the period from 2018 to 2023, and we correlate remotely sensed information with ground data series of reservoir inlet and outlet flows, volume, and water level, provided by the regional government's Automatic Hydrological Information System (SAIH), but also with water quality data provided by the regional government’s DMA network (WFD approach). This also allowed for evaluating the relationship between flow regimes and the estimated water and vegetation parameters. Higher spatiotemporal scales proved crucial in studying changes in riparian vegetation, capturing the natural characteristics of Mediterranean riversides, which are not very wide and exhibit marked seasonal patterns. Due to the homogeneous land use of the basin, coarse-resolution indexes accurately reflect nearby vegetation patterns, serving as a proxy for the basin's ecological status. For water quality indexes, a spatial resolution of meters becomes necessary because, in this reservoir, invasive species proliferation and clogging levels are low. The lower resolution water index aligns with water level fluctuations, allowing us to use this information for longer-term analysis. Our ultimate goal is to provide effective metrics based on observations and simulations, accessible in quasi-real time, to support operational decision-making processes. We will apply the methodology to different reservoirs of the Guadalquivir River's upper, middle, and lower areas. This work has been funded by the project TED2021-130937A-I00, ENFLOW-MED “Incorporating climate variability and water quality aspects in the implementation of environmental flows in Mediterranean catchments” with the economic collaboration of MCIN/AEI/10.13039/501100011033 and European Union “NextGenerationEU”/Plan de Recuperación