Michael E. Campana

Seasonal variation in surface‐subsurface water exchange and lateral hyporheic area of two stream‐aquifer systems

Abstract. We used two-dimensional unconfined transient groundwater flow models to investigate the interface between stream and groundwater flow systems, or hyporheic zone, of two first-order streams that drain catchments with distinctly different alluvial sediments and bedrock lithology. Particle tracking showed that lateral hyporheic area (planimetric area of flow paths lateral to the stream that are recharged by and return to the stream with travel times of 10 days or less) differed between the two study streams and varied with discharge within each system. At the Rio Calaveras (welded tuff), lateral hyporheic area ranged from 1.7 to 4 m 2 over the annual cycle. In the Aspen Creek system (sandstone), lateral hyporheic area (1-1.5 m 2) was restricted to roughly half of that observed at Rio Calaveras. The size of the hyporheic zone lateral to the streams at both sites decreased by approximately 50% during high flows. Sensitivity analyses indicated that changes in the hydraulic conductivity of alluvial and streambed sediments and variation in recharge rates have greatest impact on the magnitude, direction, and spatial distribution of stream-groundwater exchange.

Alluvial Characteristics, Groundwater--Surface Water Exchange and Hydrological Retention in Headwater Streams

Conservative solute injections were conducted in three first-order montane streams of different geological composition to assess the influence of parent lithology and alluvial characteristics on the hydrological retention of nutrients. Three study sites were established: (1) Aspen Creek, in a sandstone–siltstone catchment with a fine-grained alluvium of low hydraulic conductivity (1·3×10−4 cm/s), (2) Rio Calaveras, which flows through volcanic tuff with alluvium of intermediate grain size and hydraulic conductivity (1·2×10−3 cm/s), and (3) Gallina Creek, located in a granite/gneiss catchment of coarse, poorly sorted alluvium with high hydraulic conductivity (4·1×10−3 cm/s). All sites were instrumented with networks of shallow groundwater wells to monitor interstitial solute transport. The rate and extent of groundwater–surface water exchange, determined by the solute response in wells, increased with increasing hydraulic conductivity. The direction of surface water–groundwater interaction within a stream was related to local variation in vertical and horizontal hydraulic gradients. Experimental tracer responses in the surface stream were simulated with a one-dimensional solute transport model with inflow and storage components (OTIS). Model-derived measures of hydrological retention showed a corresponding increase with increasing hydraulic conductivity. n n n nTo assess the temporal variability of hydrological retention, solute injection experiments were conducted in Gallina Creek under four seasonal flow regimes during which surface discharge ranged from baseflow (0·75 l/s in October) to high (75 l/s during spring snowmelt). Model-derived hydrological retention decreased with increasing discharge. n n n nThe results of our intersite comparison suggest that hydrological retention is strongly influenced by the geologic setting and alluvial characteristics of the stream catchment. Temporal variation in hydrological retention at Gallina Creek is related to seasonal changes in discharge, highlighting the need for temporal resolution in studies of the dynamics of surface water–groundwater interactions in stream ecosystems.

A general lumped parameter model for the interpretation of tracer data and transit time calculation in hydrologic systems

Abstract We present a general lumped parameter mathematical model for hydrologic tracer data interpretation and mean transit time calculation in hydrologic systems. The model takes the form of the three-parameter gamma distribution and accounts for different mixing types: perfect mixing; no mixing (piston flow); partial mixing (dispersive mixing, or the type between perfect mixing and no mixing); and various combinations of the above types. In these combinations, the different mixing types simulated by the model conceptually represent reservoirs in series. We introduce the mixing efficiency to characterize the extent or degree of natural mixing in hydrologic systems. This parameter equals zero for piston flow (no mixing), unity for perfect mixing, and a value in between these two extremes for partial mixing. The general model simulates the combination of perfect mixing, partial mixing, and piston flow. Six other models that simulate one or two of these mixing types can be obtained as special cases from the general model. Therefore, seven models are introduced in this effort. Of these, four (including the general model) are new, and three are currently existing in the field of tracer hydrology. The three existing models are the perfect mixing model, piston flow model, and the perfect-piston flow model which simulates the combination of perfect mixing and piston flow. The new models are the perfect-partial-piston flow model (the general model), perfect-partial mixing model, partial-piston flow model, and partial mixing model. Modeled mean transit times for three case studies agree with previous estimates: 21 and 2.4 years for two springs (sites 2 and 45, respectively) on Cheju Island, Republic of Korea; and 3.0 years for the Ottawa River basin, Canada.

Undergraduate program focuses on international issues in water resources

For the past two summers, faculty from the University of Notre Dame, the University of Nevada, Reno, and the University of New Mexico have directed a U.S.National Science Foundation (NSF) Research Experience for Undergraduates (REU) site focusing on issues in international water resources. (See REU Site on Water Resources in Developing Countries, www.nd.edu/~reuwater/). The overarching objective of this project is to engage and educate U.S. students in the issues and problems facing the worlds nations in water resource development and potable water supply. n nThe stated goals of the REU program are to expand student participation in all areas of research, and specifically, to attract a diverse group of students into the fields of science and engineering, including graduate-level studies. In addition, international REU sites often seek to develop students who can be “globally competent;” that is, understand science and engineering in frameworks other than a North American perspective. (More information on international REU sites and site development can be found at www.nsftokyo.org/ and www.nsf.gov/sbe/int/.)

Reallocation of water and the hydrological effects of climate change: the upper Rio Grande Basin, Southwestern USA.

Abstract The Southwestern United States is a semi-arid region that has experienced rapid population growth in the past few decades. This growth shows little sign of abating, so much so that there is concern among water planners and managers as to where additional water supplies will be found. The few large existing perennial streams—such as the Colorado River and the Rio Grande—are overallocated, meaning that additional surface water supplies cannot be developed. Ground-water supplies are also heavily used, leading to excessive water-level declines and incipient land subsidence; some areas also have ground-water quality problems (e.g., arsenic). To exacerbate matters, the federal governments Endangered Species Act (ESA) may require that water be left in some streams to preserve aquatic and raparian flora and fauna, thus reducing the amount available for agriculture and municipal and industrial uses. Preliminary research indicates that parts of this region are entering a drought, which may not reach its driest period unitl 2020–2030. The upshot is that reallocation of water is inevitable. One approach to reallocation involves water marketing, which is already in effect in some areas. Water marketing will certainly not increase the total amount of water, but presumably will provide a market-based mechanism to sell or lease water rights. As an example, irrigated agriculture, the largest user of water, is perceived by some to be an inefficient and low-value use of water. Under the water market approach, a farmer might decide to retire some of her acreage, thereby freeing some water which could be sold or leased to a municipality or perhaps dedicated to instream ecological use. Clearly, increased population, ESA considerations and impending drought bode difficult times ahead for water managers and planners who must reallocate precious water supplies among competing uses. Reallocation decisions will be politically sensitive and must be done with as much certainty regarding water supply and availability as possible (note that “availability” pertains to both hydrological and legal availability). Informed decisions with strong scientific bases can mitigate some of the political issues. Sophisticated models that can simulate the effects of global change, drought, and reallocation are clearly needed. In response to that need, we propose a GIS-based (Geographic Information System) integrated physical hydrological—behavioral model of the upper Rio Grande Basin of New Mexico and Colorado. The model is unique in that it is process-based with the physical processes embedded within the GIS. Land-atmosphere interactions, surface water and ground water are represented, as is the legal availability of water. The physical model will determine the hydrological availability of water in the face of reallocation and climate change (drought); the behavioral model can assess stakeholders responses to reallocation and reduced/increased hydrological flows. A preliminary model of a portion of the basin, the Conejos River Basin in the heavily agricultural San Luis Valley in southern Colorado, will be available by January 2003 for economic simulations. Future plans call for the integration of water quality and instream ecological flow components into the basic physical hydrological-behavioral model. Our approach will find applicability elsewhere, whether they be arid or humid regions; there is nothing inherent in the model that restricts its use to semi-arid climatic zones. The model will provide a stronger scientific foundation for the difficult water reallocation decisions facing humankind in the 21 st century.