INTERACTIVE MODELFIELD NOTEHYDROGEOLOGY

The Borehole You Never Had to Drill

A live model accompanying “Eighty-One Cubic Kilometers of Water the Cascades Never Reported”

Study: Field note
Review: Peer-edited by the club review board
Article: 11 sectionsabout 23 min
Files: LaTeX source34 kB

The paper's whole method rests on one idea: water moving down through rock steals heat's job of warming that rock as you go deeper. Drag the slider below to send more or less water down a borehole and watch the temperature profile bend. Then try to reconstruct the paper's own 81 cubic kilometer estimate yourself, using its own reported numbers.

Model 1. Send water down a borehole

The curve is the standard steady-state solution for combined heat conduction and advection in a column of rock, θ(z*) = (e^βz* − 1)/(e^β − 1), the general physics behind reading vertical flow off a temperature log. It is background theory, not the paper's own computed curve: the 2025 study's actual diagnostic is simpler, just checking whether a measured gradient falls under 20 °C/km below 30 m depth. The dashed line marking that real threshold stays fixed as you drag.

θ(z*) = (e^β·z* − 1) / (e^β − 1) β = 0: pure conduction β large: strong downward flow

Downward flow strength β0.0Illustrative conductive gradient45°C/kmProbe depth300m

gradient at probe: 45.0 °C/kmflagged advective? noequivalent β: 0.0

At β = 0 there is no flow at all, and the profile is the straight conductive line every dry, old borehole shows. Drag β up.

β and the 45 °C/km default conductive gradient are illustrative parameters for exploring the mechanism, not values reported in the paper. The 20 °C/km threshold, the 30 m exclusion depth, and the ≈730 m depth marked for real drill hole SP 77-24 are the paper's own reported numbers.

Model 2. Reconstruct the 81 cubic kilometers yourself

The paper's headline number is just three quantities multiplied together: how much of the rock is actually open pore space (porosity), how deep the water-bearing layer runs, and how much area it covers. The paper reports the first two directly. It does not report the area it assumed, so the slider below lets you find the area that reproduces its own answer — a reconstruction, not a number stated in the paper.

Porosity6.0%Water-bearing depth1000mArea of the young Critical Zone1350km²

your volume: 81.0 km³in Lake Meads: 2.27in US gallons: 21.4 trillion

Defaults reproduce the paper's own 6% mean porosity and ≈1 km depth, with area set so the product lands on 81 km³.

Paper: heat-flow estimate: 81.0 km³
Paper: flux × transit-time estimate: 35.0 km³
Club: Lake Mead, full pool: 35.7 km³ (USBR)

Area is the club's back-calculation, not a figure the paper states. Porosity (6% mean, locally 15–20%) and depth (≈1 km at the crest) are its own reported numbers. Volume = porosity × area × depth.

What moving the sliders is supposed to show

The first model is the physical reason a thermometer can substitute for a well: once water is actually flowing down through rock fast enough, it dominates how that rock's temperature behaves, and the effect is large enough to be unmistakable in a real drill log. The second model is a caution in the other direction. An order-of-magnitude volume estimate like 81 km³ is only as solid as three multiplied assumptions, and small, entirely reasonable changes to any one of them — porosity from 4% to 10%, say — move the answer by more than the difference between the paper's own two independent estimates. That is exactly why the authors call 81 km³ a lower bound rather than a measurement.