Home > Uncategorized > Guest Post – Urbanization Changes Baseflow, But It Doesn’t Lower It – And Stream Gages Alone Can’t Explain Why

Guest Post – Urbanization Changes Baseflow, But It Doesn’t Lower It – And Stream Gages Alone Can’t Explain Why

Today, we have a guest post by Bill Simmons.

Bill is a retired health inspector blogging about public and environmental health at http://pehealthnj.blogspot.com/ and on Twitter as BillSimmonsNJ.

I asked Bill to Guest post after I heard him raise an interesting set of issues during the public comment period of last week’s monthly meeting of the NJ Water Supply Advisory Council about studies on stream base flow and land use. Base flow is an important aspect of fundamental hydrology and critical to water quality, quantity and the ecological health of our watersheds. Both NJ Acting State Geologist Hoffman and USGS representative on WSAC were aware of the research Bill summarized and interested in conducting watershed scale studies to get a better understanding, but lack funds to conduct that research due to diversion of research funds by the “Keep It Green” open space ballot deception (just one of the many water resource cuts we warned about that have yet to be restored in DEP’s budget).

There is a huge pool of retired environmental professionals out there with deep knowledge that could make an important contribution to public policy debates. We welcome guest posts from these folks – as well as active agency employees – and would be glad to provide anonymity to protect against retaliation. If interested, shoot me an email at bill_wolfe@comcast.net – Now to Bill’s post:

Urbanization Changes Baseflow, But It Doesn’t Lower It – And Stream Gages Alone Can’t Explain Why

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.”

gageYes 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.”


“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.


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