Hydrological Drought–Water Quality Interactions in Lakes and Streams of Türkiye: Implications for Eutrophication and Agricultural Water Quality

Hydrological drought can alter water quality by affecting nutrient concentrations and ionic composition. This study aimed to investigate the relationships between hydrological drought and water quality characteristics in the lakes (Mogan and Eymir) and the streams (Botbaşı and Porsuk) in Türkiye, which are important in terms of their trophic status and salinity conditions. Monthly chlorophyll-a, Secchi depth, and total phosphorus data for the period 1993–2002 were used for Eymir and Mogan Lakes, while monthly Ca, Mg, Na, K, pH, and EC data for the 1979–1988 period in the Porsuk and Botbaşı Streams were analyzed. Monthly streamflow data for the 1970–2000 period were used to calculate the Streamflow Drought Index (SDI). Significant decreasing trends in SDI and Sodium Adsorption Ratio (SAR) values were identified in the Porsuk Stream. No significant correlations were found between hydrological drought and the water quality indices in the investigated lakes or streams, except for a significant negative correlation between SDI and Na% in the Botbaşı Stream (p = 0.018, ρ = −0.354) and a significant positive correlation between SDI and Trophic State Index (TSI) in Mogan Lake (p = 0.006, ρ = 0.323). Therefore, drought has limited influence on water quality.