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 dis charge at springs. First, the "hydraulic time scale" depends on the transmi ssivity of the aquifer and relates long term changes in discharge to long t erm changes in recharge. The hydraulic time scale will describe, for exampl e, the secular decrease in discharge during a period of drought. A second h ydraulic time scale, referred to as the "time lag", measures the time lag b etween groundwater recharge tin the studied examples, recharge is snowmelt during the springtime) and the time of peak discharge at the spring. Finall y, the "age" or mean residence time of groundwater represents the transport time of tracers and the volume of water in the aquifers. As examples, larg e discharge springs (typically several m(3)/s) in the Oregon and California Cascades are studied. These springs are associated with volcanic-hosted fr acture-flow systems. (C) 1999 Elsevier Science B.V. All rights reserved.