|
SAHRA
– Recharge Workshop
New Mexico Tech, Socorro, New Mexico
NOTE:
The linked presentations are in Adobe
Acrobat format
On
March 22-23, 2001, ~30 participants gathered at New
Mexico Tech in Socorro New Mexico for a SAHRA-sponsored
workshop on recharge in semiarid regions. The purpose
of the workshop was to bring together SAHRA researchers
investigating recharge in order to 1) present recent
advances in the understanding of groundwater recharge
and vadose-zone processes in semiarid regions and 2)
foster cross-thrust area integration to develop a unified
approach to understanding this key parameter. The workshop
was broken into five sessions as well as a period for
discussion at the end of the workshop. Listed below
is a summary of the presentations that were made during
these five sessions, a summary of the concluding discussion
and a list of participants.
Introductory
comments presentation - Fred Phillips, James Hogan
New approaches
to investigating vadose-zone processes.
Session
Leader: Brenda Ekwurzel
This
session focused on the use of isotopic methods to
study vadose zone processes. Brenda Ekwurzel provided
an introduction to the isotopic tracers currently
in use (3H, dD,
d18O, 36Cl,
etc). She noted that if these tracers are to be applied
to the southwestern US what is critically needed is
a better understanding of their atmospheric deposition
rates. Chris Eastoe provided an introduction to the
use of stable chlorine isotopes (d37Cl)
in the hydrology of arid regions. Michelle Walvoord
illustrated, through some recent modeling work, that
fractionation of stable chlorine isotope caused by
diffusion may provide evidence for non-recharge in
thick desert vadose zones. Devendra Lal provided
an introduction to the rational for why the cosmogenic
nuclides 10Be and 32Si can be
used as a tracer of groundwater recharge.
Mountain-block
recharge: can it be quantified?
Session
Leader: Chris Duffy
This
session examined mountain block recharge using a variety
of isotopic and numerical methods. Chris Eastoe presented
the results of a multi-isotope study of the Tucson
basin (dD, d18O,
3H, 14C, d32S)
which has help to delineate zones of recharge (both
along stream channels and from the mountain front)
within the Tucson basin and the flowpaths of groundwater.
Brent Newman showed that SSA (Singular Spectrum Analysis)
can be used to examine the underlying structure of
a very noisy data set for discharge from a spring
near LANL. This led to a conceptual model of a two
component flow system with one component having a
low frequency (seasonal) and a longer residence time
whereas the other is higher frequency (storm event)
and has a shorter residence time. Oxygen isotopes
were used to test this model. Chris Duffy illustarted
a variety of methods to investigate recharge at the
mountain from and how it translates to discharge in
the main drainages. These included SSA, Concentration-Discharge
patterns and the “hydrologic phase plane”
(T-Q-P space).
Recharge
along ephemeral stream: scaling up from the borehole
to the basin.
Session
Leader: Dave Goodrich
Dave
Goodrich presented some recent work tying to quantify
groundwater recharge along ephemeral channels at Walnut
Gulch during monsoon events using a combination of
well water-level data, stream flow information and
geochemical / isotopic tracers. Stan Leake provided
some thoughts about how recharge may be different
in different hydrogeologic regions of the southwest
(i.e. basin and range versus Colorado plateau) as
well as an introduction to the USGS recharge study
sites and the methods that they are employing. Ty
Ferre briefly discussed some of the new “meta-scale”
geophysical techniques he is using to quantify ephemeral
channel recharge (e.g. ERT – electrical resistance
tomography and GPR – ground penetrating radar).
Recharge
data: how does it fit into the model?
Session
Leader: Larry Winter
Larry
Winter presented an introduction to the modeling environment
that LANL is developing. He noted that what is needed
is a list of important recharge mechanisms that need
to be supported in the model toolbox (i.e. what type
of model elements are needed to support recharge research
within SAHRA?). Possibilities would include a stream
element that allowed for groundwater recharge. Peng-Hsiang
Tseng and Regan Murry showed some work on some of
the components of the modeling environment. Regan
discussed modeling the stream-aquifer interface and
the importance of selecting proper boundary conditions
to ensure mass and momentum transfer across this boundary.
The basin
floor: “Swiss cheese” or “vegetation
sucks”?
Session
Leader: Fred Phillips
Fred
Phillips presented an overview of possible controls
on recharge in desert-floor environments. One hypothesis
is that the extremely negative matric potentials imparted
to desert soils in the 1 – 2 m depth range by
plants are the dominant control. These may produce
pervasive upward gradients in the top 20 to 50 m and
thus virtually preclude recharge. On the other hand,
heterogeneities in soil structure or topography may
focus infiltration in small areas and thus overcome
the vegetative sink. Bruce Harrison showed pedological
evidence that focusing of shallow soil water at the
heads of ephemeral drainage basins may permit deep
infiltration, and Graciela Rodriguez illustrated “pipes”
through otherwise indurated pedocalcic horizons.
Her modeling results indicated that under unusually
wet conditions, later flow of soil water along the
top of the calcic horizon might focus enough flow
through the pipes to penetrate to the water table.
Eric Small presented data showing that at the small
scale, infiltration was significantly greater under
desert shrubs than in the spaces between the shrubs.
Michelle Walvoord compared her linked water and vapor
flow simulations using the code FEHM with data on
matric potential and chloride concentrations that
she collected in West Texas. These showed a strong
correlation between the vegetation type and the vadose-zone
characteristics. These presentations gave support
for both hypotheses. The next step toward resolving
these questions would appear to be to identify sites
characterized by specific surficial heterogeneities
and drill at these sites to compare vadose-zone profiles
within and outside of the sites, and then to attempt
to match the data using numerical simulations.
The
workshop ended with a discussion of 1) what steps
should be taken to follow up on the recharge workshop
and 2) some recommendations for the planning of future
SAHRA workshops.
The participants
decided on the following plan of action:
1.
SAHRA document on recharge meeting / post on web.
2.
AGU Special Session on recharge at Fall 2001 meeting.
3.
Summary of special session published in EOS.
4.
Present summary at annual meeting.
5.
AGU monograph on recharge.
6.
Second Recharge Workshop - ~1-2 years time.
The participants
made the following recommendations for future SAHRA
workshops:
1.
Make an effort to incorporate economic / policy aspects
of SAHRA
2.
Identify stakeholders to attend / participate in SAHRA
workshops
3.
Identify the decisions that stakeholders need to make
and how they will drive research.
The participants
suggested the following topics for future SAHRA workshops
1.
Basin integrating meetings (San Pedro, Rio Grande)
2.
Biogeochemical cycling (nutrient and solute balances)
3.
Ecohydrology
4.
Impacts of agriculture of hydrologic systems
5.
“Tools” – Isotopes etc.
|
