Streams and aquifers exchange water continuously. Understanding the direction and magnitude of that exchange — and whether the systems are even hydraulically connected — is the starting point for every depletion analysis.
The water table sits above the stream stage on both sides, so groundwater flows toward the channel and discharges as baseflow. In Mediterranean and snowmelt climates, this baseflow is what keeps streams flowing through the dry season.
Pumping risk: reducing the gradient toward the stream cuts baseflow directly — the dominant SGMA depletion pathway.
The water table sits below the stream stage but still touches the streambed. Stream water leaks downward; the leakage rate depends on stream stage, streambed conductance, and the head gradient. Often seen on alluvial fans, recharge basins, and mid-elevation reaches.
Pumping risk: further drawdown increases the loss rate — still a depletion of surface water — until disconnection.
A vadose zone develops beneath the streambed. Seepage flows through the channel sediments at a rate fixed by streambed properties (unit‑gradient flow) and is independent of how deep the water table has fallen below. Common in heavily‑pumped semi‑arid alluvial basins.
Pumping risk: further pumping doesn't increase per-unit leakage but can dry the stream entirely by exceeding inflow.
A single reach can be gaining in one segment and losing 50 m away. Bedrock highs, paleochannels, beaver ponds, riffle-pool sequences, and clay lenses all create local reversals. Differential gaging picks up the net but misses the structure (Woessner 2020).
Practical implication: point measurements rarely scale up reliably. Multiple lines of evidence — gaging, temperature, head data, isotopes — are typically needed to characterize a reach.
Between the open channel and the regional aquifer lies the hyporheic zone — sediments saturated with a mix of stream water and groundwater. Exchange here drives oxygen, nutrient, and temperature dynamics that determine whether a stream supports aquatic life and riparian vegetation.
For SGMA, this is where "depletion" becomes ecological harm: a stream that loses its hyporheic exchange and shallow groundwater connection can lose its salmonids, frogs, and willows long before it goes visibly dry.
Hydraulic head is the energy elevation of groundwater, measured in a well. Gradient is the slope of head between two points. Discharge per unit area = hydraulic conductivity × gradient (Darcy's law). For a stream–aquifer interface, the exchange flux is Kbed · (hstream − haquifer) / bbed.
A stream is connected if a continuous saturated column exists from stream bottom to the regional water table; it is disconnected if a vadose zone has formed underneath. The simple picture places this threshold at the streambed, but Brunner et al. (2009, 2011) showed that the true threshold hcrit typically sits well below the streambed. The depth grows with: (a) the Kaquifer/Kclog contrast, (b) clogging-layer thickness, (c) capillary fringe (air-entry pressure), and (d) stream width. Morel‑Seytoux et al. (2018) propose a refined exchange formulation that gives a smoother, dimensionally consistent transition. Disconnection is also hysteretic — recovery requires the water table to rise back above the streambed, not just back above hcrit.
Baseflow = the portion of streamflow coming from groundwater discharge. Bank storage = water temporarily stored in alluvial banks during high flows and returned later. Both are first‑order budget terms; pumping affects baseflow directly and can convert temporary bank storage into permanent depletion.
SGMA term of art: "surface water that is hydrologically connected at any point by a continuous saturated zone to the underlying aquifer." A reach is ISW if and only if it is in the gaining or losing-connected regimes above. Once disconnected (per the regulatory test), it is no longer ISW — but it can become ISW again if heads recover.