Untitled DocumentRecharge/runoff partitioning from geochemical tracers
Macro Theme Area:
Basin Scale Water Balance [Project ID: B20]
PI:
Fred Phillips
CO-PI(s):
N/A
Basin focus:
Rio Grande
Specific area in
basin /
field sites:
Saguache Creek watershed, northern San Luis Basin
Summary/Goals: The objective of the Rio Grande transect study is to determine how components of the hydrological cycle, and particularly the influence of vegetation on the cycle, varies with elevation, and hence vegetation community, in the headwaters of the Rio Grande. This information is intended to serve as a basis for high resolution distributed-parameter modeling of the basin. One of the most important components of the system is the way that vegetation and soil/shallow bedrock interact to produce surface-water runoff and groundwater recharge. The objective of this componenent is to use a combination of monitoring of the vadose zne and runoff and measurements of environmental tracers to understand the mechanisms of this process, focusing on the distribution of subsurface flowpaths and associated residence times. Specifically, we intend to test the hypothesis that long flow paths with longer residence times in the subsurface play an important role in the generation of seasonal runoff.
Activities and outcomes during past year:
The work on applying geochemical tracers to estimating residence times and flowpath lengths has focused on the Rio Grande headwaters region in Colorado. This is a change from the initial plans and was forced by a survey of water quality in the Valles Caldera that demonstrated virtually pervasive geothemal influence on the water quality, precluding use of the intended tracers. Most of fall 2005 was spent surveying possible drainages in the San Luis Basin area. After measurement of longitudinal transects of water quality down several drainages, and comprehensive consideration of land use and hydrologic modifications, the Saguache Creek drainage was selected. A cooperative arrangement has been set up with the local BLM for the collection of water quality samples. During 2007 and 2008 Marty Frisbee conducted approximately monthly synoptic sampling campaigns on Saguache Creek, from close to its headwaters to close to the town of Saguache, as well as sampling numerous springs (winter sampling was less frequent due to heavy snowpack and the creek freezing over). We now have two full full annual cycles of chemical and isotopic data from the basin. Continuous monitoring instrumentation was installed beneath a bridge over the creek and was used to measure water chemistry from early spring through late fall 2008. In addition, there has been regularly scheduled sampling of wells and springs. In order to directly access baseflow generation, Marty installed piezometers in the stream bed at varous points along Saguache Creek and its tributaries. These have yielded 14C ages ranging from modern to several thousand years old. These data are a clear indication that long-residence time components are contributing significantly to runoff generation in Saguache Creek. In cooperation with Mark Person (Hydrology faculty, NMT), Marty has started modeling groundwater flow and discharge in the Saguache Creek basin.
Plans for the upcoming year:
The sampling phase of the project is nearly over. A few more carefully targeted samples may be collected and we will send some archived samples in for additional analyses, but there will be no regular sampling campaign as in previous years. Instead, our efforts are now focusing on modeling and interpreting the data. We will use a distributed basin-scale groundwater model, end-member mixing analysis, and mass-balance modeling to better understand the components of runoff throughout the seasonal cycle and to at least semiquantitatively assess the seasonal variation of residence times. Marty intends to defend his dissertation by December 2009..
Organization Involvement:
The BLM is actively cooperating with our research in the Saguache Creek watershed.
Shared Resources / Joint Activities:
BLM personnel will collect water samples for us on a regular basis and mail the samples to New Mexico Tech. They have shared with us the results of previous water quality studies. We will share our water quality data with them.
