Audit a watershed balance without double-counting infiltration

Follow water across a boundary

Solar energy drives evaporation, and plants release water through transpiration. Condensation and precipitation move water through the atmosphere. Runoff moves over the surface; infiltration moves into soil; percolation moves through subsurface material. Groundwater can recharge, discharge to streams or cross a chosen watershed boundary. Gravity and differences in water potential help drive movement, while storage can delay a response.

A watershed budget is not a list of every process subtracted from rainfall. If infiltrated water remains inside the selected boundary, infiltration is an internal transfer. Subtracting it as a loss and later subtracting the same groundwater export would double-count the water. Draw the boundary before choosing the equation.

A one-year synthetic catchment

Our original catchment starts with 400 mm of accessible water storage expressed as an equivalent depth over its area. Precipitation is 900 mm/yr. Evapotranspiration is 500, surface runoff 250 and groundwater boundary outflow 50 mm/yr. There is no assigned external surface or groundwater inflow.

Storage change is 900−500−250−50=100 mm for the year. Final storage is 500 mm. The model’s storage is a deliberately aggregated accounting quantity, not the depth of a literal water layer above the ground or a direct groundwater-table elevation.

At precipitation 600 and evapotranspiration 600, the same other outputs cause a 300 mm drawdown. Final storage remains 100 mm. Total annual output can exceed precipitation during storage depletion without violating conservation. Repeating that budget indefinitely would become impossible; a bounded one-year example is not a sustainable long-term forecast.

Convert depth to a meaningful volume

One square kilometre is one million square metres. One millimetre is 0.001 m. Their product is 1,000 m³. Thus the fixed 250 mm runoff over 1 km² corresponds to 250,000 m³ per year. Keep time and area conversions visible before comparing communities or watersheds.

An observed decrease in streamflow could reflect lower precipitation, higher evapotranspiration, withdrawal, changed groundwater exchange or increased storage. A single stream gauge does not identify the cause. An impermeable land-cover change may increase rapid runoff and reduce infiltration locally, but downstream annual effects depend on other flows and storage.

Design and transfer

Propose a before-and-after comparison for a land-cover change with a matched untreated catchment. Compare precipitation as well as streamflow; otherwise a wetter year can masquerade as an intervention effect. Specify measurement intervals, area, uncertainty and which groundwater flows remain unmeasured. Do not claim the matched catchment makes all confounding impossible.

For precipitation 1,200 and evapotranspiration 300, calculate a storage gain of 600 mm and final 1,000 mm. Ask whether the real watershed can retain that much: flooding, drainage or altered evaporation may change the assigned flows. The calculation is a conservation screen, not proof of unlimited capacity. No outdoor sampling, entry into streams or well access is required for this virtual investigation.

Reservoir size is not accessible freshwater supply

Oceans hold the dominant share of Earth’s water. Ice and groundwater are major non-ocean stores; the stock in rivers is comparatively small even though its renewal can make river flow important to communities. Reservoir size does not establish accessibility, potability or sustainable withdrawal. In a fictional stock-and-flow packet, reservoir A contains 1,000 units but receives 2 per year, while B contains 100 and receives 20. Predict which has the larger stock/inflow timescale: A gives 500 years and B gives 5. Neither ratio alone is a withdrawal recommendation. If B’s inflow falls to 10, its ratio doubles to 10 years; its water has not become ocean water and its finite stock has not increased. Connect the distinction to solar-driven cycling and the actual boundary budget above.

Sources and investigation boundary

Original explanations and synthetic calculations aligned with AP Environmental Science topics 1.7. Framework checked 30 October 2026: College Board course at a glance and Fall 2026 clarifications. These are public teaching materials, not official exam questions. Computational captures do not establish field measurements, independent mastery or supervised hands-on laboratory time.

Additional process reference, checked 30 October 2026: USGS water-cycle stocks and fluxes.

Clause reconciliation checked 31 October 2026 against the official framework, with Fall 2026 corrections.