Geoscience Reference
In-Depth Information
16
Groundwater controls and processes
David J. Nash
16.1
Introduction
halite and gypsum crusts (see Chapter 8), in the geomor-
phology of dryland slopes (Chapter 11), drainage net-
works (Chapter 12), badland gullies (Chapter 11) and
in pans and playas (Chapter 15). In addition to these
various direct roles as an agent of weathering and ero-
sion, groundwater may also act as an important control on
the operation of specific processes (e.g. where the max-
imum extent of aeolian deflation is limited by the depth
to the regional water table). This chapter will identify
three areas of groundwater influence in drylands, com-
plimenting discussion in other chapters where reference
is made to specific landform suites. The water resource
implications arising from the influence of geomorphol-
ogy upon groundwater availability in drylands are not
discussed as they have been considered elsewhere (e.g.
Berger, 1992; Carter, 1994; Carter
et al.
, 1994). Karst
processes and landscapes in arid environments are also
not considered as, while dissolution processes operate
on limestone surfaces even under conditions of limited
available moisture (Smith, 1988, 1994), many dryland
karst landscapes are largely relict at a macroscale (Smith,
1987; Palmer, 1990). Within the phreatic zone (the zone
beneath the water table in which all voids are completely
filled with water), groundwater processes and morpholo-
gies in karst terrain are virtually independent of climate
(Palmer, 1990). There may be reduced dissolution in dry-
land regions owing to the higher salinity, low levels of
recharge and subsequent antiquity of many desert ground-
water sources (Lowry and Jennings, 1974). For a wider
discussion of the role of groundwater in geomorphol-
ogy in general, readers are referred to La Fleur (1999)
and the excellent summaries provided in the volumes
edited by La Fleur (1984), Higgins and Coates (1990) and
Brown (1995).
Beyond the arid zone, the role of groundwater as a ge-
omorphological agent has received considerable atten-
tion in the literature. Subsurface water has long been
recognised as an important factor in processes such as
weathering (particularly carbonate dissolution), soil de-
velopment and hillslope stability, and as a component of
river discharge. However, the influence of groundwater in
sculpting arid landscapes has been either underestimated
or ignored (Higgins, 1984). Dryland landscapes are often
viewed as an end-product of the long-term interaction of
wind and surface water operating under different struc-
tural and tectonic settings, with little reference to sub-
surface activity. That is not to say that the importance of
groundwater as a geomorphological agent in arid areas
has been completely overlooked - many of the pioneering
studies of piping and tunnel scour (e.g. Bryan and Yair,
1982; Parker and Higgins, 1990, and see Chapter 11), salt
weathering (e.g. Cooke
et al.
, 1982, and see Chapter 6)
and groundwater seepage erosion (e.g. Peel, 1941) were
based upon observations made in desert environments.
However, unless the geomorphological impacts of sub-
surface water are manifest at relatively short timescales
and have an impact upon engineering structures, they are
largely overlooked. This can, in part, be explained by the
difficulties in identifying the long-term role of groundwa-
ter in drylands, particularly where groundwater processes
contributed to early landscape development and have been
overwritten by more easily observed surface-water and
aeolian processes (Nash, Thomas and Shaw, 1994).
Groundwater plays an important role in dryland land-
scapes, which is reflected in other chapters: in the for-
mation of nonpedogenic calcrete, silcrete, sodium nitrate,
