03 — Capture vs. Depletion: Time and Space

← All summaries · 02 Concept

Figure 1. The Theis (1940) water-budget framework. Every unit of water pumped must be balanced by a change in one of four terms. Together, the last three are called capture. At steady state, storage release goes to zero and capture equals pumping. After Lohman (1972), Bredehoeft (1982 — "Hydraulic budget myth"), and Barlow & Leake (2012).

Time matters: depletion lags pumping and persists after it stops

A single year of pumping can keep depleting a stream for many years afterward. Capture spreads through the aquifer slowly and unwinds slowly. This means the well that turned on yesterday isn't necessarily the well taking water from the stream today — today's depletion is a legacy of decades of pumping superposed across the basin.

Pumping rate Q 1000 gpm

Pumping season 6 mo

SDF (lag scale) 365 days

Pumping years 5

Total water pumped over study	— AF
Total depletion during pumping	— AF (—%)
Legacy depletion after last pump-off	— AF (—%)
Years until depletion < 10% of peak	— yr

Figure 2. Top trace: an annual pumping schedule. Bottom trace: the resulting depletion at the stream, computed by superposing Glover responses. Depletion lags the onset of pumping, smooths the seasonal cycle, and persists after pumping stops — a phenomenon often called legacy depletion. The longer the SDF, the more pronounced these effects.

Space matters: not all wells capture the same water

Figure 3a. A capture-fraction map tells you, for any potential well location, what fraction of pumping would come from each downstream feature at some future time. Adapted from Leake, Reeves & Dickinson (2010).

Figure 3b. A capture zone (wellhead protection) asks "what region supplies water to this well?". It is not the same as a capture-fraction map, which asks "what feature loses water to this well?". Mixing the two leads to wrong conclusions (Barlow, Leake & Reeves 2018).

Three ideas every depletion analysis hinges on

1. Capture is a flow, not a volume

"Capture" is the rate at which pumping causes inflows to increase or outflows to decrease across the aquifer's natural boundaries. Per Bredehoeft, the "perennial yield" myth (treating recharge as a fixed water supply) ignores this — at steady state, recharge is unchanged; pumping is balanced by changes in discharge, leakage, and ET.

2. Direction and "groundwater flow" don't matter

Capture is a superposed response: it's the change relative to no-pumping. The natural groundwater flow direction is irrelevant — only the head field induced by pumping matters (Leake, 2011). A well upgradient of a stream can still cause stream depletion if the drawdown cone reaches the stream.

3. Steady state may be unreachable

Bredehoeft (2002) and Konikow & Leake (2014) point out that, for many basins, the time to reach steady-state capture is centuries. Management decisions based on steady-state pictures can dramatically understate the transient deficit being created.

Implications for SGMA

An "interim period" of unsustainable pumping does not end when pumping stops — it ends when depletion recovers, which can take decades to centuries.
"Average annual capture = average annual pumping" only holds in long-term steady state — many SGMA basins are not at steady state and may not reach it within the planning horizon.
For monitoring, both the magnitude and the timing of capture matter: short, intense pumping causes different ecological harm than steady, distributed pumping.
Capture maps (Leake 2010) are an excellent communication tool: they translate from "well location" to "which stream reach is affected" — a question every GSA must answer.

Key references in the project library

Theis, C.V. (1940). The source of water derived from wells. Civil Engineering 10(5): 277–280.
Bredehoeft, J. (2002). The water budget myth revisited: why hydrogeologists model. Groundwater 40(4): 340–345.
Leake, S., Reeves, H.W. & Dickinson, J.E. (2010). A new capture fraction method to map how pumpage affects surface water flow. Groundwater 48(5): 690–700.
Leake, S. (2011). Capture — rates and directions of groundwater flow don't matter! Groundwater 49(4): 456–458.
Barlow, P.M. & Leake, S.A. (2012). Streamflow depletion by wells. USGS Circular 1376.
Konikow, L.F. & Leake, S.A. (2014). Depletion and capture: revisiting "the source of water derived from wells." Groundwater 52(S1): 100–111.
Barlow, P., Leake, S. & Reeves, H. (2018). Capture versus capture zones: clarifying terminology for sustainable groundwater management. Groundwater 56(5): 694–696.