Correlations between reservoirs levels and the Palmer Drought Severity Index (PDSI) and Standardized Precipitation Index (SPI) are developed for the New York City Reservoir System. Regardless of the index chosen, correlation with reservoir level is relatively low owing to temporal changes in water consumption, population and total reservoir capacity. To account for these non-climatic influences on water storage, a method to adjust for trends in available reservoir storage and water use is developed and implemented.
Once adjusted, correlations between the climatic indices and reservoir levels improve substantially. During drought periods, a fairly strong (R2 = 70.9%) exponential relationship exists between adjusted reservoir levels and PDSI. Conversely, the relationship for the unadjusted levels is considerably weaker (R2 = 49.0%) indicating the benefit of the adjustment technique. Omitting the 1965 drought, these relationships become more linear with R2 values of 93.6 and 17.6 for the adjusted and unadjusted levels, respectively. The adjusted correlations are similar to those obtained for natural (i.e. undammed) stream flow data. Correlations between the SPI and adjusted reservoir levels and stream flows will also be evaluated to determine the feasibility of implementing this relatively new drought index as a drought monitoring tool in the Northeast