Droughts, which are difficult to predict, are a natural feature of the hydrology in most regions.Climate change, however, has the potential to increase the frequency and magnitude of future droughts. While the lack of water availability during droughts is widely publicized, there are equally severe water quality impacts that occur during and after droughts as well. Recent droughts have led to water quality implications for drinking water supplies including turbidity, taste and odor, pathogen concerns, and challenges in managing disinfection byproducts (DBPs). This paper presents the results from a series of case studies prepared for a Water Research Foundation study on the effects of extreme weather on drinking water quality in order to help utilities prepare for vulnerabilities under future climate change. A key finding from the case studies is that droughts can fundamentally alter nutrient cycling and biota within both watersheds and reservoirs that influence water quality for months or years after the event. A few of the critical management actions for responding to degraded water quality related to droughts include awareness of potential impacts, increased monitoring during and after the event, and capacity to quickly adjust treatment processes.

Natural organic matter (NOM) is present in all raw water supplies and is the major precursor to chlorinated disinfection by‐products (DBPs). Reducing the amount of NOM entering treatment plants and better understanding the character of NOM in raw water supplies are essential for optimizing treatment and meeting more‐stringent regulations. A study of the unfiltered New York City (NYC) water supply system evaluated the extent to which alternative raw water selection strategies could reduce finished water DBP concentrations. A two‐year monitoring program was conducted to establish spatial and temporal patterns of DBP precursors in NYC's upstate reservoirs. System operations modeling was driven with a statistically generated, long‐term time series of dissolved organic carbon concentrations for each reservoir as a proof‐of‐concept evaluation of alternative operating rules to minimize precursor transport to terminal reservoirs. Results found that DBP precursors varied widely among reservoirs and over time but that modified reservoir operations could substantially reduce finished water DBP concentrations.