Did
you know that increasing impervious surface in a watershed is clearly
linked with higher storm flow
- but not with lower baseflow?
And that this was first reported in NJ about ten years ago?
This
very counter-intuitive discovery by the USGS appears on page 132 of a
2008 technical report for NJ Highlands Master Plan, Water
Resources Volume II, Water Use and Availability:
“There
is a strong conceptual case that increased land development should
result in decreased stream base flow, but two USGS studies of
long-term base flow trends in New Jersey did not find many
statistically significant trends in low flows (Brandes and other
2005, Watson and others 2005). … An improved understanding of this
issue will allow for a more robust water availability modeling
approach in the future.”
Yes
it would.
Here’s
what the Brandes paper - Base
Flow Trends In Urbanizing Watersheds Of The Delaware River Basin
- states on pdf-page 15:
“The
results of this study suggest that stream base flow has not
systematically decreased in urbanizing watersheds of the lower
Delaware River basin over the past 60 years. The data do not support
the idea that low- to moderate-density land development typically has
a negative effect on base flow volumes and low flows at the scale of
a 25 to 200 km2 watershed. … one should not expect any single value
of percent impervious to emerge as a widely applicable threshold for
effects of urbanization on base flow. The implication of this study
is that the effect of low density to moderate density urbanization on
base flow is typically more subtle and less severe than its impact on
stormflow.”
Page
26 of Streamflow
Characteristics and Trends in New Jersey, Water Years 1897–2003,
by Watson and others: “The overall results of the trends analysis
show that high-flow trends for the regulated [developed] and
unregulated [undeveloped] gaging stations were upward. ... The
low-flow trend results for regulated gaging stations indicate that
most of the gaging stations had an upward trend … The relation of
development to low-flow trends does not appear to be as strong as
development to high-flow trends.”
Natural
and Artificial Sources of Baseflow
Brandes
and Watson were surprised by what the data told them. They took a
shot at explaining it. Since then, other states have found this
trending in their data as well, and the guesses are stacking up.
Here’s the Minnesota
Pollution Control Agency
in 2009, on page 59:
“… the
decrease in natural groundwater recharge in an urban watershed can be
unintentionally replaced by artificial recharge, i.e. infiltration of
imported water that has leaked from water supply and sewer pipes,
applied as excessive lawn irrigation, and infiltrated from septic
system drainage."
In
developed watersheds, baseflow isn’t just baseflow
anymore. That means baseflow doesn’t predict water levels in the
water-table aquifer the stream runs through. You could have adequate
baseflow but still have headwater wetlands and shallow wells drying
up during a drought.
The
most nuanced paper was published last year as part of the Baltimore
Ecosystem Study:
“Baseflow
signatures of Sustainable Water Resources. An Analysis of Maryland
Streamflow”.
Figure 9 on page 37 compares the traditional model of baseflow –
recharge in, baseflow out – with urbanized baseflow derived from
multiple processes. Some “artificial” sources of baseflow are
essentially interbasin transfers that obscure the volume of baseflow
that naturally flows from the water-table aquifer.
Wastewater
treatment plants can discharge effluent into a stream that is derived
from sources outside the watershed, “bypassing the groundwater
system”. Old urban drinking-water pipes can recharge the
water-table aquifer with pressurized water that came from sources
outside the watershed. To a lesser degree, even watering your lawn
can artificially recharge the water-table aquifer, if your well is
drilled deeply into a separate, confined
aquifer
that recharges far from the watershed and the water-table aquifer the
well is drilled through.
Storm
drains buried in areas with high water tables can accelerate the
discharge of groundwater into streams, like a french
drain
- and increase baseflow. Even impervious surface - roads, buildings,
and compacted soils – increases baseflow, because as it replaces
woods and fields, less water is lost to evapotranspiration.
The USGS estimates about one third of the precipitation that falls in
NJ returns to the atmosphere through evapotranspiration (Fig.
4)
instead of recharging the aquifer. Impervious surface leaves more net
groundwater in the water-table aquifer that can become baseflow
because it replaces vegetation(!)
Groundwater
can leak into sewer pipes in one watershed (I&I
- Infiltration and inflow)
but discharge from the treatment plant into another. When a watershed
is developed, how can a stream gage tell you if natural baseflow has
decreased - with all these artificial sources of baseflow?
“Regulatory
Paradoxes”: Now What
Page
73 of the Maryland paper:
“Where
baseflow signals reflect wastewater return flows that bypass the
subsurface hydrologic system, groundwater appropriations based on …
gauged streamflow may over-appropriate the resource and fail to
adequately protect the groundwater resource from depletion.”
Agreed.
“To
the extent leaking infrastructure truly recharges ground water, the
State faces the dilemma of whether or not to explicitly appropriate
this unintended interbasin transfer as an exploitable component of
regional groundwater system.”
Oh
yeah.
“The
limitations and potential risks from appropriating groundwater based
only on the characteristics of observed streamflow highlight the
value of a more process-based understanding of Maryland’s coupled
surface water- groundwater resource.”
That
answers “where do we go from here” for New Jersey as well. We
need to find out the unique combination of natural and artificial
baseflow in urbanized watersheds, so we can understand their specific
vulnerabilities to drought.
When
we can isolate natural baseflow from artificial baseflow, we can make
informed regulatory decisions about water
allocation.
Should the “unintended interbasin transfer” part of baseflow be
counted or excluded when deciding how much water can be safely
permitted to be withdrawn from a watershed?
Imagine a river - that is overly-dependent on treated
wastewater for maintaining its baseflow
- drying up someday because the municipalities discharging their
waste to the treatment plant implemented a successful water
conservation program, in another watershed.
Baseflow
data measured by a system of stream gages alone has become a black
box. We need a “more process-based understanding of [our] coupled
surface water- groundwater resource.” Baseflow data needs to be
augmented by data from a system of monitoring wells in the watershed
that record the levels of the water-table aquifer – especially in
the headwaters.
But
that’s expensive, and there are less
funding sources
in NJ for these research programs.
So
for now this ends up on the what-if list for climate change, or as
another reason for updating
the 1996 NJ Water Supply Management Plan,
or something, until it gets funded.
This blog was originally guest-posted on Wolfenotes.com
