In December 2025, the Groundwater Resources Association of California hosted a two-day webinar to address the complex relationship between Interconnected Surface Water (ISW) and groundwater. The event brought together experts to discuss the science, environmental implications, and policy challenges of ISW, focusing on how groundwater extraction impacts surface water flows and the ecosystems that depend on them.
The connection between surface water and groundwater is a critical component of sustainable groundwater management. When groundwater pumping occurs near a stream or river, it can lower the water table and cause surface water to seep downward, a process that can lead to stream depletion. This interaction affects various beneficial users of both surface water and groundwater resources.
Under the Sustainable Groundwater Management Act (SGMA), ISW is one of the most complex and least understood aspects for Groundwater Sustainability Agencies (GSAs) to manage. Regulations require GSPs to estimate “the quantity and timing of depletion of interconnected surface water systems due to groundwater pumping.” However, many GSAs have struggled to meet this requirement. SGMA defines “significant depletions of interconnected surface waters” as flow reductions that cause unreasonable adverse impacts on the beneficial uses of that surface water. If GSAs fail to manage these depletions by January 31, 2025, the State Water Resources Control Board may intervene.
Dr. Vivek Bedekar, a consultant with SS Papadopoulos and Associates, opened the webinar series by introducing the foundational concepts of ISW depletion. His presentation focused on how groundwater pumping alters streamflow and the need for effective models and management strategies to address these interactions.
The slide below presents four graphs that illustrate long-term trends for a representative California groundwater basin. The charts show a steady decline in streamflow. Both precipitation and evapotranspiration display downward trends, which further limit the amount of water entering the system and available for runoff. In contrast, groundwater pumping rates have increased throughout the same period, which accelerates the reduction in streamflow.
“Streams respond to multiple interacting factors: climate, vegetation, land use, surface water operations, and many others,” said Dr. Bedekar. “But for today’s discussion, our focus is specifically on isolating and understanding the effects of groundwater pumping.”
Under SGMA, streamflow depletion is one of the six sustainability indicators that every GSA must understand and manage. ISW depletion, surface water depletion, streamflow depletion,or simply depletion in this context means depletion caused by groundwater pumping.
From gaining to losing streams: The complex dynamics of pumping and stream depletion
Under natural conditions, groundwater flow feeds into the streams. This is a gaining stream. When a well begins pumping, the water initially comes from the aquifer storage. As pumping continues, the inflow to streams decreases because the water that would have entered the stream is captured by the well.
In some cases, if the well is close enough to the stream, it may reverse flow, the stream will start feeding groundwater, and the system will become a losing system. This transition from storage depletion to stream depletion is gradual and strongly tied to aquifer properties.
With a groundwater well located in close proximity to the stream, most of the pumping will lead to stream depletion within weeks to months. For the same aquifer system, pumping wells farther from the stream may take several years or even decades to affect streamflow.
Dr. Bedekar likened this to short-term and long-term loans. The wells in close proximity to the stream will affect the streams within weeks or months; that’s like taking out a one-year loan or using a credit card. The impact shows up quickly, and the system feels the effect right away. Other wells take years or even decades to influence stream flow. Those are like a 30-year mortgage; the obligation is still there, but it accumulates slowly and becomes increasingly significant over time.
Aquifer properties also play an important role. For instance, if the well is pulling water from a lava tube that feeds a spring, the effects on spring flow will be within hours or days, depending on the distance and the geologic properties.
In heterogeneous basins, depletion patterns can sometimes be non-intuitive. If there are faults and other features, they will need further detailed evaluation. The stream bed plays an important role; it is what controls how much flow occurs between the stream and the aquifer. Confining layers can buffer and redirect drawdown propagation.
Ppumping depth matters as well. Deeper pumping typically delays the depletion response and tends to spread those impacts across a larger area. These same concepts apply, whether it’s a single well, one well field, or pumping throughout the entire basin.
“For basin-wide assessment, hundreds or thousands of wells may be pumping from different locations, different depths, following different schedules, and influencing multiple stream reaches,” said Dr. Bedekar. “Deeper pumping can transmit drawdown farther throughout the aquifer. In interconnected basins, pumping in one basin can also contribute to stream depletion in neighboring basins. So this, as you can see, gets complicated, very, very quickly.”
Why models matter: estimating surface water depletion at the basin scale
While focused studies are useful for evaluating a specific well, stream reach, or time period, and field observations provide data on stream flows, stream stage, and stream-groundwater interaction, Dr. Bedekar noted that surface water depletion cannot be directly measured at the basin scale.
“This is why models are essential, because models do provide the counterfactual needed to estimate depletion,” he said.
Typically, two types of models are used:
- Numerical models such as MODFLOW and IWFM incorporate an aquifer’s heterogeneous properties, stream networks, pumping histories, and climate variability, and accommodate many complexities that are characteristic of real-world problems. These tools provide the most defensible basis for depletion and estimates used in SGMA planning and water management, assuming the model is well-calibrated and represents the system appropriately.
- Analytical models are based on simplifying assumptions. They are quick and appropriate for rapid assessments, though he noted that many advances have been made in recent years in analytical models that overcome some of these limitations.
Illustrating ISW depletion: A basin-scale perspective
Dr. Bedekar then gave an example to outline a general methodology to estimate depletion and to illustrate how the spatial scale of a basin matters.
The graphic shows two basins, side by side. A medium-sized basin with three streams and three subbasins with groundwater pumping throughout, and a much smaller basin located in a narrow valley.
As pumping has increased in these basins, a gaining stream over time has become a losing stream system for the most part. Similar effects are seen with the smaller basin, but with very different magnitudes. The pumping in the smaller basin in this example is about an order of magnitude lower than in a medium-sized basin, yet the effects are very similar.
When we have a model to analyze stream depletion, the model allows us to examine what the stream flow would be in the absence of pumping. The difference between the with- and without-pumping scenarios provides the surface water depletion, as the model allows us to isolate the effects of pumping. On the slide below, the charts on the bottom show depletion alone, which is the difference between the with- and without-pumping conditions. The chart shows that depletion is also proportional to pumping, and there’s a significant difference between the two basins.
The graphs on top are plotting pumping and depletion together; Dr. Bedekar noted that depletion in the larger basin for any given year is small; with pumping for the smaller basin, depletion increases in close succession as pumping increases.
“This is where the temporal and spatial scales are playing a role,” he said. “The plot below is just showing the cumulative pumping and cumulative stream depletion. Notice that, in general, depletion lags pumping by about 25 to 30 years in this example. In other words, in this system, the effect of pumping on streamflow depletion is spread over 25 to 30 years, whereas in the smaller basin, the response to pumping occurs within the same year.”
“One of the most misunderstood aspects is timing, and these examples illustrate that the depletion can lag by months, years, or even decades, depending on the basin scale, pumping locations, depth, aquifer and stream properties, and so on.”
Dr. Bedekar noted that the lagged effect of pumping is two-way; it also means that, at some point, even if all pumping is shut down, it will take about 25 to 30 years for the basin to return to natural conditions.
“This is just for illustration purposes,” he said. “I’m not suggesting that completely shutting off pumping is a practical management option, but this exercise helps managers develop a good understanding of the timing of the system to understand how the basin responds. Eventually, it is up to individual GSAs to decide how to manage their groundwater resources and how to set sustainable management criteria for their basins.”
So if a GSA, for example, decides to limit depletion to 10,000 acre-feet per year, or to a specific seasonal flow in CFS for a specific month or summer months, the model helps estimate the quantity of pumping curtailment and the timing of these impacts. Even at a smaller scale, the impacts could still be years away from when these policies or changes are implemented and when you would expect to see some results. These analyses can then be used to set minimum thresholds or measurable objectives.
Same as timing, the location of depletion is also important in larger basins with deeper pumping. He noted the impact can be unintuitive. For example, stream C is entirely within the East subbasin, but it is affected by pumping in both the East and West subbasins because of the basins’ interconnected nature.
As groundwater pumping intensifies, stream disconnection becomes increasingly common. Notably, these disconnections often begin in upstream reaches—even those situated upstream of the pumping locations. As pumping continues to rise, stream disconnection tends to occur more frequently, last longer, and affect larger portions of the stream network.
To summarize …
When we think about depletion, the first key point is that we cannot directly measure surface water depletion at the basin scale. What we can measure – stream flow, groundwater levels, stream groundwater interaction, base flow conditions- are all pieces of the system, but none of them isolate the pumping signal on their own. That is why we need models.
The timing of depletion depends heavily on the size of the basin and hydrogeological framework; larger basins respond more slowly, sometimes over years or decades, while smaller basins can respond more quickly. Similarly, the location of depletion depends on whether pumping occurs laterally or vertically relative to the hydrogeologic structure.
It’s also important to recognize that any pumping anywhere in the basin will show up at one or more boundaries, either as reduced stream flow, decreased evapotranspiration, or an adjustment to other inflows or outflows. Even wells far from the streams eventually contribute to the depletion of these boundary flows.
Management implications of depletion dynamics
A common misconception is that measuring streamflow is equivalent to measuring depletion; it isn’t, said Dr. Bedekar. Baseflow measurements tell us what is happening in the stream at that moment. But they don’t isolate the effect of basin-wide pumping from other influences, like climate variability, land use, or upstream controls.
Likewise, high groundwater levels in wet years does not erase the long-term debt accumulated during dry years. “This is where, again, the financial analogy comes in,” he said. “Near stream pumping is like your monthly credit card. It creates a quick, noticeable impact on stream flow. Basin-wide pumping, especially in large systems with long time lags, is like a 30-year mortgage. It creates a deeper, longer-lasting obligation that the aquifer must eventually pay back in the form of future depletion.”
In wet years, when groundwater levels rebound, it’s like having a great income year; you feel less pressure, but the mortgage hasn’t gone away, he said. And in dry years, when water levels fall, the pumping increases. It’s like needing to borrow more; you accumulate more long-term depletion, even if the immediate impact isn’t visible at the stream.
Near-stream solutions alone are generally not enough for long-term solutions. Mitigating short-term impacts may require managing pumps close to the stream, but long-term sustainability requires managing basin-wide pumping. Groundwater recharge may provide shallow storage augmentation and a near-term impact on alleviating surface water depletion. This analysis helps develop an understanding of the system’s dynamics, so that recharge or MAR facilities can be most appropriately designed for timing and related factors.
Ultimately, informed decision-making based on these analytical methods will be necessary to navigate and manage complex aquifer systems sustainably.
