INFILTRATION VARIABILITY AND SHALLOW GROUNDWATER RESPONSE TO SNOWMELT IN HEADWATER CATCHMENTS, NIWOT RIDGE, COLORADO

Interactions between surface and groundwater are a critical component of the water budget for snowmelt-dominated headwater catchments of the western United States. Flow into alpine and subalpine subsurfaces in the Colorado Front Range is changing as climate warms, precipitation patterns evolve at the same time as downstream demands for water increase. This study reports and analyzes hydraulic conductivity (K) values derived from 130 soil infiltration measurements on 13 transects and 14 falling head slug tests conducted on 10 closely spaced piezometers at the Niwot Ridge LTER site. K-values at the atmosphere-soil interface are similar to those measured in the shallow aquifer. Infiltration values vary spatially (transect medians range from 2.09x10^-7 to 7.74x10^-5 ms^-1) and increase with elevation. Modified Hvorslev K values range from 4.86x10^-7 to 1.77x10^-4 ms^-1 in the subalpine unconfined aquifer, which is developed in late-Pleistocene glacial till. Measured rates overlap with infiltration K-values from lower elevations. The apparent increase in infiltration capacity with elevation suggests that the unglaciated alpine area acts as a “leaky roof “ that allows the shallow subsurface to absorb and transport the maximum rate of snowmelt. Analysis of groundwater flow during the 2016 snowmelt season in the glaciated subalpine suggests that channel infiltration from Como Creek, a snowmelt-fed, seasonally connected stream, contributes more to groundwater recharge than direct infiltration through the unsaturated zone. However, direct infiltration likely wets the vadose zone and initiates groundwater recharge before the pulse of channel infiltration takes over. These findings, combined with infiltration results, suggest an elevational gradient from (alpine) high infiltration capacity where infiltration feeds surface water into periglacial deposits and snowmelt streams to lower elevations where surface and channel infiltration feed water into forested glacial deposits and evapotranspiration is significant. This study illustrates how interactions between surface and groundwater differ in alpine and subalpine environments based on elevation because of rocks materials, snowpack distribution and vegetation.