ENSO-Induced Streamflow Connected to Anoxia in Water Reservoirs

Water quality is of great concern for arid areas that rely on reservoirs, so it is equally important to understand the factors that might affect the water quality. ENSO (El Niño Southern Oscillation) affects the amount of rainfall certain regions receive which could alter the streamflow into reservoirs (Marcé et al. 2010). Analyses showed that when streamflow decreased during El Niño the oxygen content—a measure of water quality—of the reservoir also decreased. — Luisana Hernandez 

Marcé, R., Rodríguez-Arias, M., Garcia, J., Armengol, J. 2010. El Niño Southern Oscillation and climate trends impact reservoir water quality. Global Change Biology, 1–9.

 Marcé et al. investigated the possible connection between ENSO and reservoir water quality in Sau Reservoir in Spain from 1964–2007. The study consisted of two parts, first the authors tested to see if streamflow—and therefore water quality—were sensitive to ENSO. Water samples were taken almost on a monthly basis since the initial fill of the reservoir in 1964 to measure the oxygen content. The oxygen readings were then expressed as Anoxic Factors (AF)—a standardized appraisal of the absence of oxygen in water (anoxia). After a spectral analysis, the final analysis of coupling time-series was done using continuous wavelet transform (CWT), cross-wavelet transform (XWT), and wavelet coherence (WTC). Secondly, the researchers tried to quantify the effect of the decreasing streamflow they observed that would enter the reservoir. Rainfall-runoff models were used to measure climate variability while another model was built to observe river chloride concentration and mean annual water residence time in the reservoir.

The spectral analysis showed that streamflow, AF, and ENSO occurred between 5.2 and 2.4 years during 1964–1991. Streamflow also oscillated inter-annually during the spring and autumn, though AF only matched these peaks during 1964–1991 and this trend was most prominent during dry years. This data suggests that anoxia is sensitive to streamflow but loses its affectability when there is high human impact—like with remediation. The wavelet analyses also supported the ideas that ENSO affects reservoir water quality through streamflow. Another aspect of this study that needs to be researched is the influence of riverine labile organic matter.