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Streamflow Drought Indicators
Streamflow levels are a useful indicator for drought as well as floods, but they must be used with considerable caution. For streams that are regulated, the streamflow may reflect management decisions at upstream reservoirs which will not reflect true drought conditions. Streams that are unregulated are better for drought monitoring. A hydrograph is used to show stream discharge (water level) over time. When a precipitation event occurs over a river basin, the discharge increases to a high level (peak flow), then gradually decreases to near the base flow after the precipitation event ends. The time it takes to reach peak flow and the magnitude of the peak flow are useful for flood forecasting. Base flow is the level the stream would have if no precipitation occurred over the basin and is generally fed by groundwater. Base flow is the best streamflow indicator for drought monitoring. In practice, however, the weather doesn't cooperate to provide widespread measurements of base flow at a network of river gauge stations on an operational basis — at any given time, streamflow measurements across the nation are a mix of peak flow, base flow, and everything in between. Consequently, streamflow measurements are averaged over several days to a month to indicate areas with low streamflow and, by implication, low groundwater levels. The values are compared to the local streamflow history, converted into percentiles, and grouped into above- and below-normal percentile classes to be used as an indicator for drought classification.