Long-term groundwater storage trends estimated from streamflow records: Climatic perspective

Base flow, also known as low flow or drought flow, in a natural river system originates mainly from drainage of the riparian aquifers in the upstream basin. Base flow data can therefore provide a quantitative estimate of the basin-scale groundwater storage changes that have taken place over the period of record. The concept was implemented and validated with data from two large river basins in Illinois. On average over the past 2/3 century, shallow groundwater storage in Illinois, derived from the drought flows, has been increasing at an average rate of around 0.05 to 0.10 mm a(-1). However, the trend has not been stationary throughout this period; for instance, the more recent trend from the mid nineteen nineties until now appears to have been negative. The groundwater storage changes inferred from the base flows are generally consistent with the average groundwater level changes measured in an observation well network over the same area.