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Flow regimes
3.1
WATER RESOURCES
The hydrology of the Mekong shows no
evidence to date of significant impacts from
human induced land use changes that can
be detected against the natural background
variability, both long and short-term, of its
flow regime (Figure 3.1.1). When considering
the impacts of land-use change on the
hydrological environment, scale is an important
issue. As the area increases so does the
physical and biological complexity of the
catchment landscape.
Benchmark hydrological indices are
needed, against which any anticipated
changes can be measured, the consequent
ecological, environmental and socio-economic
impacts assessed and practical
management and mitigation measures developed.
A number of flow indices have been
developed which summarise the structure
and function of the flow regime in terms
of both its quantitative and temporal characteristics
(Adamson and King 2009). The
selection of these indices takes account of
issues relating to the geographic scale of
the basin, the nature of the hydrological
regime and the types of planned resource
development.
Four flow seasons were identified –
the flood and low-flow seasons and two
transition periods in between. Discharge
thresholds define the onset and end of each
one. Two flow indices were selected for
each season. The objectives were to employ
only a small number of parameters and to
identify those with a wide environmental
relevance, which define as completely as
possible the overall structure and pattern
of the flow regime, particularly the distribution
and volume of seasonal flows. The
onset and end dates of the four prescribed
seasons at Vientiane and Kratie over the
past 80–90 years (Figure 3.1.2) show that
not only are they unchanged but also that
they show a relatively narrow range from
year to year. In view of this, any small change
in these temporal variables would not only
be statistically significant but may potentially
have ecological and environmental
consequences that are disproportionate
to the size of the temporal shift. The refilling
of reservoir storages, for example, could
result in a delay of two weeks or more to the
onset of the flood
season which would
be easily detectable
given the consistent
historical natural
pattern.
From 1960 onwards, large-scale commercial
logging has been carried out in the LMB
and the area classified as undisturbed forest
has fallen by 20 per cent (MRC 2003).
The conventional view is that deforestation
results in a decrease in the natural water storage
capacity of a river basin which, in turn,
leads to an increase in water yield, the magnitude
of which varies with the local rainfall
climate, the topography and the proportion,
type and density of the removed forest cover
(Newson 1997). In principle therefore, two
potential hydrological impacts are possible –
total water yield is increased as annual water
loss through forests decreases and the seasonal
distribution of flows is modified as flood runoff
increases and dry season flows decrease.
In the Mekong region clear felling has not
been a significant factor in forest removal
as it has been elsewhere in Southeast Asia.
Benchmark hydrological indices are needed
in order to measure changes, assess
impacts and develop practical
management and mitigation measures
PRECEDING PAGE
The Mekong Basin,
with its range of
geographic and
climatic zones, is one
of the richest areas
of biodiversity in the
world.
FACING PAGE
Pollution is a major
issue that creates
severe health
problems.
STATE OF THE BASIN REPORT 2010 | 57