Multiple timescales of streambed flux variability in two perennial mountain-front streams

Streambed fluxes are highly variable through time and space, having a range implications for stream-aquifer processes. This study investigated streambed fluxes over a 3-year study period to characterize short- and long-term variations and related implications for seepage recharge and hyporheic exchange. Time series of streambed fluxes were estimated through Darcy-based methods using water level and temperature inputs from shallow (<1.5 m) nested streambed piezometers installed in two mountain-front streams in Colorado, USA. Three predominant temporal scales of variability were characterized: sub-daily (<1 day), daily (>1 day; <1 year), and interannual (>1 year). Temporal variability was quantified using the median absolute deviation (MAD), a statistical measure that is resistant to extreme values associated with short-duration events. Sub-daily variability (MAD(S-D)) was related to ET, temperature-induced changes in hydraulic conductivity, and variable stream stage, and was quantified using the MAD of detrended fluxes (daily mean subtracted). Daily variability (MAD(D)), calculated as the MAD of daily median fluxes for a given year, exceeded sub-daily variability, and was influenced by strong seasonality at some sites. For individual sites and water years, the ratio of MAD(S-D) to MAD(D) ranged from 0.03 to 0.70, with an average of 0.25. Annual median fluxes at each site varied across years, but typically remained consistent in order of magnitude and direction. Results reveal a strong linear correlation between the daily variability and the median annual flux at individual sites. Within a year, summer months characterized by stronger losing conditions showed greater overall variability. 1D numerical heat and flow modelling was performed to calibrate hydraulic parameters and to investigate temperature-related controls for sub-daily variations. We discuss implications of documented temporal variability for analyses of hyporheic exchange and groundwater recharge. Results provide a basis for quantifying temporal variations in streambed fluxes and highlight the extent to which fluxes vary over multiple timescales.