On the timescales characterizing groundwater discharge at springs

In some regions, measurements made at springs can be used to study regional hydrogeologic processes, and determine hydraulic and transport properties of aquifers. Here, input–output models, spectral analysis, and time series analysis are used to identify three different timescales characterizing discharge at springs. First, the “hydraulic time scale ” depends on the trans-missivity of the aquifer and relates long term changes in discharge to long term changes in recharge. The hydraulic time scale will describe, for example, the secular decrease in discharge during a period of drought. A second hydraulic time scale, referred to as the “time lag”, measures the time lag between groundwater recharge (in the studied examples, recharge is snowmelt during the springtime) and the time of peak discharge at the spring. Finally, the “age ” or mean residence time of groundwater represents the transport time of tracers and the volume of water in the aquifers. As examples, large discharge springs (typically several m3/s) in the Oregon and California Cascades are studied. These springs are associated with volcanic-hosted fracture