Drought impacts on the water quality of freshwater systems; review and integration

Droughts are increasing in frequency and severity in many regions of the world due to climate change. The meteorological drivers of drought often cause subsequent hydrological effects such as reduced catchment runoff, river flows and lake levels. Hydrological droughts may also result in significant changes in water quality. This review provides a synthesis of past observational research on the effects of drought on the water quality of freshwater systems (rivers, streams, lakes, reservoirs). Over the last 10–20 years there has been an increasing amount of studies on the water quality effects of drought, mostly in North America, Europe, and Australia. In general droughts, and the immediate recovery period, were found to have profound water quality effects. These effects were varied, depending on the characteristics of the water body and its catchment. Key drivers of water quality change were identified and integrated across different systems using quantitative analysis where possible. Water flow and volume decreases during drought typically led to increased salinity due to reduced dilution and concentration of mass. Temperature increases and enhanced stratification occurred during drought in some systems due to air temperature increases and longer hydraulic residence times. This also enhanced algal production, promoted toxic cyanobacterial blooms, and lowered dissolved oxygen concentrations. Nutrient, turbidity and algal levels also often increased in lake systems due to reduced flushing and enhanced productivity, and resuspension in some shallow lakes. In contrast, nutrients and turbidity often decreased during droughts in rivers and streams with no significant loading from point and agricultural non-point sources. This was due to disruption of catchment inputs and increased influence of internal processes (e.g. biological uptake of nutrients, denitrification, settling). Where point sources of pollution were present, water quality generally showed deterioration due to less dilution, particularly for nutrients. Storage and buildup of material and changed geochemistry (e.g. sulfide oxidation) in catchments during drought resulted in mobilisation of large post-drought flood loadings of constituents such as major ions, nutrients and carbon. In some cases this caused severe downstream water quality effects such as deoxygenation. Key areas for further research are process-level understanding of the key drivers of water quality change in catchments and receiving water bodies during drought, development of predictive models, and studying the resilience of systems to the predicted increase in frequency of drought and floods. The maintenance of long term water quality monitoring programmes is also critical.