Geology topics
R. Webb
Publications and source records attributed to R. Webb.
Ground water occurrence and contributions to streamflow in an alpine catchment, Colorado Front Range
Ground water occurrence, movement, and its contribution to streamflow were investigated in Loch Vale, an alpine catchment in the Front Range of the Colorado Rocky Mountains. Hydrogeomorphologic mapping, seismic refraction measurements, and porosity and permeability estimates indicate that talus slopes are the primary ground water reservoir, with a maximum storage capacity that is equal to, or greater than, total annual discharge from the basin (5.4 ± 0.8 × 10 6 m 3 ). Although snowmelt and glacial melt provide the majority of annual water flux to the basin, tracer tests and gauging along a stream transect indicate that ground water flowing from talus can account for ≥75% of streamflow during storms and the winter base flow period. The discharge response of talus springs to storms and snowmelt reflects rapid transmittal of water through coarse debris at the talus surface and slower release of water from finer-grained sediments at depth. Ice stored in permafrost (including rock glaciers) is the second largest ground water reservoir in Loch Vale; it represents a significant, but seldom recognized, ground water reservoir in alpine terrain. Mean annual air temperatures are sufficiently cold to support permafrost above 3460 m; however, air temperatures have increased 1.1° to 1.4°C since the early 1990s, consistent with long-term (1976–2000) increases in air temperature measured at other high-elevation sites in the Front Range, European Alps, and Peruvian Andes. If other climatic factors remain constant, the increase in air temperatures at Loch Vale is sufficient to increase the lower elevational limit of permafrost by 150 to 190 m. Although this could cause a short-term increase in streamflow, it may ultimately result in decreased flow in the future.
Application of XTOP_PRMS model in Green Lakes Valley, Colorado front range: Runoff simulation and flowpath identification
Stream runoff was simulated from 1996 to 2000 using XTOP_PRMS (coupling of TOPMODEL and Precipitation Runoff Modeling System) model under Modular Modeling System at Martinelli and Green Lake 4 catchments in Green Lakes Valley, Colorado Front Range. Two flowpaths determined by XTOP_PRMS model, surface flow (infiltration-excess overland flow) and subsurface flow, were compared against the flowpaths determined by mixing model using isotopic and chemical tracers. Three tracers (DOC, K/Si, and ??18O) were used in mixing model to identify four flowpaths, i.e., overland, upper soil horizon, lower soil horizon, and base flow. The results showed that the runoff simulation using XTOP_PRMS model is reasonably successful for Martinelli catchment (8 ha in drainage area). The Nash-Sutcliffe efficiency is 0.76. The t-test of two means for paired sample showed that the difference between the observed and modeled runoff was not significantly different at ??=0.05 at Martinelli catchment (n = 1611, p = 0.6). The flowpaths identified by XTOP_PRMS model matched the flowpaths determined by the tracer-mixing model reasonably well in magnitude, but poorly in pattern. The surface flow primarily occurred in the beginning of snowmelt at Martinelli as illustrated by the tracer-mixing model. Both runoff simulation and flowpath identification using XTOP_PRMS model were relatively poor at Green Lake 4 catchment, which has a drainage area of 220 ha. The runoff peaks observed in May and June were not captured in runoff simulation. The problem may be caused by poor understanding of behaviors of flowpath parameters and insensitivity of snowmelt to daily mean air temperature.