Groundwater · aquifers · the Floridan

Aquifer

Most of Florida's water is underground, hidden in rock. Learn how it's stored, how it moves, and how easily people break it.

An aquifer isn't an underground lake — it's rock full of holes, and water creeps through the holes. Two numbers decide everything: how much water the rock can hold (porosity) and how easily water can move through it (permeability). Get those, add a well, and you understand your water supply — and how a spring dies.

Nobody pumps groundwater for fun. Every region that depends on an aquifer does so because the water it needs is not available at the surface, when and where and in the condition it is needed. Three places, three completely different reasons, one shared answer.

Three controls on how much rain falls

Latitude — where air rises and where it sinks

Precipitation is high where air converges and rises — near the equator, and again at 40–50° where warm and cold air masses meet. It is low where air sinks: at 20–30° north and south, and at the poles. Descending air warms, its capacity to hold moisture rises, and it evaporates rather than rains. That subtropical sinking limb is why nearly every great desert sits in the same two bands — the Sahara, the Arabian, the Kalahari, the Atacama, the Australian interior. Cold air at high latitudes holds little moisture, which is the separate reason the poles are dry.

Distance from the ocean

Oceans are where atmospheric moisture comes from. Air moving inland loses water at every rainfall, so continental interiors dry out with distance from the coast — continentality. It is why the middle of a large continent is drier than its edges at the same latitude.

Mountains

Air forced up over a range cools, condenses and rains on the windward side. Having lost its moisture, it descends the far side warm and dry, creating a rain shadow. A range can put one flank in rainforest and the other in desert across a few tens of kilometers.

These three stack. A place that is subtropical and far inland and leeward of a range is about as dry as the planet gets — and the more of these that apply, the more certainly its people will be pumping.

Three regions

Three fictional regions, each built so one combination of the controls above dominates. Their numbers are plausible, not measured. Pick one to load its water balance below.

Timing beats total

An annual average hides the thing that actually matters. What a farmer needs is water in the months the crop is growing — so the comparison to make is monthly precipitation against potential evapotranspiration, the amount the atmosphere would remove given the chance. Where PET exceeds precipitation, the deficit has to come from somewhere.

Annual precipitation—mm

Annual PET—mm

Annual deficit—mm

Months in surplus—of 12

Drought is relative, and the record is longer than you think

A dry year in the delta would be a wet decade on the plains

The Monsoon Delta’s driest year in its record still brought about 1,450 mm — more than three times what the Leeward Plains get in an average year. That year is correctly called a drought, because drought is a shortfall relative to what the system expects: the crops planted, the irrigation built, the people supported. Drought is defined against demand, not against a universal threshold.

Instrumental records are too short

The Palmer Drought Severity Index combines precipitation with evapotranspiration estimates to express how anomalous a period is. Extend it back with tree rings — narrow rings in dry years, wide in wet — and the twentieth century stops looking exceptional. In western North America, ring-width reconstructions show megadroughts in roughly 900–1300 CE that lasted decades (Cook and others, 2004), and tree stumps still rooted in the beds of Sierra Nevada lakes grew while those lakes were low (Stine, 1994). Planning to survive the worst drought you have measured is planning for less than the worst drought you should expect.

And rainfall regimes change entirely

Today’s deserts were not always deserts. Around 18,000 years ago Lake Bonneville covered roughly 50,000 km² of what is now the dry Great Basin of Utah; the Great Salt Lake is what is left of it. Wetter past climates like that are when much of the groundwater under modern deserts was recharged — which is exactly why, once pumped, it does not come back.

Quantity, quality, or timing?

The question the whole subject turns on. All three regions end up on groundwater; none of them for the same reason.

Region	The problem	Why surface water won’t do	What can go wrong next
Leeward Plains rain-shadow interior	Quantity & timing	Semi-arid: PET far exceeds precipitation through the growing season, and few reliable rivers cross the plains	Depletion. Recharge is a small fraction of extraction, so the saturated thickness falls year on year until wells go dry
Basin Desert subtropical interior	Quantity — total	A few millimeters of rain a year and no permanent surface water at all	Mining fossil water. Recharged under a wetter past climate and receiving effectively none today, so every liter is mined rather than harvested
Monsoon Delta coastal lowland	Timing, and quality	Ample rain but nearly all of it in the monsoon; shallow surface water is easily polluted, so wells are the safer supply	Saltwater intrusion and subsidence. Dry-season pumping near the coast lowers heads, the salt wedge moves inland, and compacting delta muds let the land sink

The delta case is the one worth sitting with. Switching from contaminated surface water to wells can save enormous numbers of lives — but only if the aquifer’s chemistry and its connection to the sea are understood before pumping is scaled up. That is a hydrogeological problem, and it is why the boring parts of this subject matter.

Test it

Precipitation & demand

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Converting, because sources never agree

Precipitation is reported in mm almost everywhere and in inches in the United States, and you cannot compare three regions until they are in the same unit.

inches = mm ÷ 25.4

So the Leeward Plains at 449 mm/yr get about 17.7 in; the Basin Desert at 8 mm/yr gets about 0.3 in; and the Monsoon Delta at 2,099 mm gets about 82.6 in. Stated in one unit the spread is obvious — the delta gets nearly 5× the plains and more than 250× the desert.

Rain that falls is not water you can pump. It has to survive three competing claims first — the atmosphere takes some back, the land sheds some to streams, and only what is left crosses the water table. Recharge is the remainder, and it is usually the smallest term.

How little of it there is

Before anything else, the scale of the resource. Of all the water on Earth:

saltwater — 97.5% ice & glaciers — 1.7% groundwater — 0.7% surface water — 0.03%

Only 2.5% of Earth’s water is fresh, and most of that is locked in ice. Groundwater is about 0.7% of all water — and yet it is more than twenty times the volume held in every lake and river combined. That ratio is the entire reason this subject exists: the accessible surface reservoir is a rounding error, and the usable freshwater store is almost all underground.

The recharge equation

Recharge = Precipitation − Evapotranspiration − Runoff

Four terms, one bookkeeping identity — everything that falls must go somewhere. Drag the sliders, or load a setting, and watch how the same rainfall splits very differently depending on climate, slope, vegetation and what the ground is made of.

Precipitation1350

Evapotranspiration55%

Runoff13%

evapotranspiration runoff recharge

The four fates of a raindrop

1 · Stored on the surface — in depressions, intercepted on leaves, or as snow and ice. Mostly short-term, a season or less, except in glaciers.

2 · Evaporated or transpired — returned straight to the atmosphere. Usually the largest term.

3 · Run off — over the surface to streams, lakes and the ocean.

4 · Infiltrated — into the unsaturated zone. Even then it is not groundwater yet; roots can still take it back. Only what gets past the root zone and crosses the water table counts as recharge.

In Florida, despite abundant rain, under a third of precipitation becomes recharge. The rest evaporates, transpires, or runs off.

Infiltration, and why pavement matters

Infiltration capacity is a rate, and it falls as you use it

The maximum rate at which soil can take water in. Sand and gravel accept water readily; clay and asphalt barely at all. Crucially it is not constant: as rain continues, pores fill and capacity declines. Measured in the field with an infiltrometer — a ring driven into the soil, filled with water, and the drop rate monitored.

Exceed it and you make a stream

Once rainfall intensity outruns infiltration capacity, water ponds in surface depressions. When those fill, it moves downslope as sheet flow — a thin, roughly even layer. Flow then concentrates itself, the way paint running down a can gathers into drips; concentrated flow erodes, and erosion deepens the channel that concentrates it further. In hydrology any channelised flow is a stream, whatever it is called locally.

What sets the runoff-to-rainfall ratio

Four things. Infiltration capacity of the surface, the intensity of the rainfall — the same total delivered slowly may all soak in, delivered in an hour it will not — the steepness of the basin, since steep slopes give water less time to infiltrate, and vegetation, which slows overland flow, intercepts rain on leaves, and transpires water back to the atmosphere.

Measuring the terms

Every term in the equation is estimated by a physical instrument with its own error, which is worth remembering before treating a water budget as exact.

Term	Method	How it works	The catch
Precipitation	Rain gage	Accumulates water, read like a graduated cylinder; or weighed automatically and logged	A point measurement — one gage speaks for a whole area
	Tipping bucket	Rain fills a small bucket to a set volume, which tips and empties; each tip is counted	Can under-record very intense rain; snow needs a heater
	Doppler radar	Emitted energy scatters off droplets; the return gives intensity over a wide area	Hail and terrain distort the return, so it still needs ground-truthing against gages
Evapo- transpiration	Land pan	An open pan of water; the loss is measured directly	Metal sides warm in the sun and overstate evaporation, so readings are multiplied by a pan coefficient of roughly 0.58–0.78
	Lysimeter	An instrumented plot where every inflow and outflow except ET is measured; ET is the residual	Expensive, fixed, and small relative to a catchment
	Energy balance	Track the energy going in and out; evaporation removes a known amount of heat	Indirect, and demanding instrumentation
Infiltration	Infiltrometer	A ring pushed into the soil, water added, the rate of fall recorded	Also a point measurement, and disturbed by installing it

Potential versus actual

PET is a demand, not a loss

Potential evapotranspiration is what would be lost given an unlimited supply of water — computed from solar radiation, temperature, humidity and wind. It is a measure of atmospheric thirst, and it is what tells you how much irrigation a crop will need.

Actual ET is usually less

In real soil the supply is limited. As the root zone dries, surface tension holds the remaining water against the roots’ pull; plants respond by narrowing their stomata and transpiring less, then by wilting — which reduces leaf area and cuts transpiration further — and finally, if drought persists, by dying. So actual ET falls below PET exactly when water is short, which is the moment the difference matters most.

Most plants cannot reach the water table

Ordinary vegetation draws only from the upper unsaturated zone. Phreatophytes are the exception — they extend roots into the saturated zone and pump groundwater directly, which is why riparian vegetation can measurably draw down a shallow water table.

Where the water goes afterwards

Consumptive or not

A withdrawal is non-consumptive if the water returns to the source, and consumptive if it is evaporated, transpired or discharged somewhere it cannot be reused. The distinction matters more than the raw withdrawal figure: in the United States energy production withdraws the most water, almost all of it for cooling and almost all of it returned — whereas globally, agriculture is the largest consumptive use, because irrigation water largely leaves as vapour and does not come back.

Surface water still does most of the work

In the U.S. in 2015, surface water supplied about 77% of freshwater withdrawals and groundwater about 23%. Surface water is easier to reach — but it is less reliable across seasons and droughts, and far more exposed to contamination. That trade-off is exactly what pushed each of the three regions in the previous tab underground.

Residence time spans everything

Groundwater flows from higher head to lower, and discharges to streams, lakes, the ocean, wells, or evapotranspiration where the table is shallow. The time between recharge and discharge ranges from days to millions of years — and that single number decides whether an aquifer is a renewable flow or a finite store being mined.

A word that means two things

Discharge is used for water leaving an aquifer, and also for a volume per unit time flowing within an aquifer or a stream. Both are standard, and context is the only thing separating them. Watch for it in exam questions.

A cross-section of a Florida aquifer system. Rain soaks in, sits in a shallow sandy layer, and — where it can — leaks down through a clay confining unit into the great limestone Floridan aquifer below. Tap a feature to learn it.

Build a cross-section from boreholes

Hydrogeologists can’t see between boreholes; they interpolate. Tap each layer to call it an aquifer (high hydraulic conductivity) or a confining unit (low), check yourself, then show the interpretation. A fictional valley; real hydrogeologic cross-sections are read the same way.

Tap a layer: first tap = aquifer, second = confining unit, third = clear.

Read the section

Here's the trap that catches everyone: porosity is not permeability. Clay is full of water yet barely lets any through. Pick a material and see both — then drive Darcy's law and watch the flow.

Material

Porosity — water it holds—

Permeability — water it passes—

Hydraulic gradient (slope of the water table)

0.030steeper = faster push

Flow Pick a material to compute Darcy flow.

Darcy's law: Q = K · A · (dh/dl) — flow rises with permeability (K) and with a steeper gradient.

Darcy problem · 0 solved

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Not all of it comes out: specific yield

Drain an unconfined aquifer and some water stays behind, clinging to the grains. The share that drains is the specific yield; the share that stays is the specific retention. Together they make the porosity — so specific yield is always less than porosity, and fine sediments hold on to most of theirs.

Material

grainsstays (retention)drains (specific yield)

Porosity: middle of the range in the course’s Table 1. Specific yield: averages compiled by Johnson (1967), USGS Water-Supply Paper 1662-D.

Water table drops by

2.0 mover the whole area

Area of the aquifer

10 km²

Primary porosity

Space left from when the rock or sediment formed: the gaps between sand grains, the gas bubbles in a lava. Burial and cement shrink it.

Secondary porosity

Space made afterwards: fractures from tectonic stress or unloading, caves and conduits dissolved out of limestone. It is why granite and limestone can be aquifers at all.

Effective porosity

Only the connected pores. Isolated gas bubbles in a basalt count as porosity, but water can’t get in or out of them.

Pore size beats pore volume

Clay has more porosity than sand and far less permeability. Compaction lowers permeability mainly because it shrinks the pores, not because it removes pore space.

Same everywhere? Same in every direction?

Homogeneous or heterogeneous: does K change from place to place? Isotropic or anisotropic: does K change with direction? In layered sediment it is far easier to flow along the layers than across them — so water no longer flows straight down the head gradient. It veers toward the layering.

Horizontal ÷ vertical conductivity, K_h/K_v

101 = isotropic

Direction the head drops (below horizontal)

45°perpendicular to the head contours

Converting hydraulic conductivity

K turns up in m/s, m/day, ft/day and cm/s, and you can’t compare aquifers until they’re in the same units. The two conversions that matter: 1 m/s = 86,400 m/day (seconds in a day) and 1 m = 3.28 ft.

Hydraulic conductivity

Unit practice · 0 solved

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Ranges from the course’s Table 1 (water at 20 °C). The line is the value in the converter. “Aquifer” and “confining unit” are relative terms: a silt is a confining unit above a gravel and an aquifer above a granite.

Check yourself · water underground

Question

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Pump a well and the water table sags into a cone of depression. Pump too hard and the cone reaches the spring and dries it — or pulls the coastal saltwater inland. Supply the town without wrecking either.

Pumping rate

Q = 20supplies a small town

SteadyRaise the pump rate and watch the cone.

Town demand met 0%

The town · 0 / 6 solved

Ichetucknee —

How a well is built

A well has to stay open, keep sediment out, and keep surface water and other aquifers from getting in. Pick a setting and tap each part.

Tap a part

Measuring hydraulic head

Survey the elevation of the top of the casing, lower a tape until it beeps at the water, and subtract: head = casing elevation − depth to water. Two wells at one spot, screened at different depths, tell you which way water moves vertically: always from higher head to lower.

Top of casing (both wells)

52.0 mabove sea level

Depth to water · shallow well (unconfined)

6.4 mbelow top of casing

Depth to water · deep well (confined)

3.1 mbelow top of casing

Three wells are enough to map the water table. Contour the heads, and groundwater flows downhill and at right angles to the contours — which also tells you whether the river nearby is being fed by the aquifer or leaking into it.

Three-point problem

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Correct 0

Divides, capture and the cone of depression

An unconfined aquifer between a stream and a lake, recharged by rain. Groundwater flows from high head to low, at right angles to the contours (the aquifer is isotropic). A groundwater divide splits the rain that reaches the stream from the rain that reaches the lake. Turn on the pump and the cone of depression captures water, shifts the divide, and starves the lake. Tap the map to release a drop and follow it.

Pumping rate

0 m³/daywell near the lake

ends in the streamends in the lakeends in the well

Dupuit model: K = 10 m/day, recharge 0.001 m/day (about 365 mm/yr), stream at 20.0 m and lake at 20.5 m above the aquifer base. Contours every 0.05 m.

Drawing contours by hand

Contours of equal head (equipotential lines) are placed by linear interpolation between wells: the 140 ft contour lies halfway between a well at 145 ft and one at 135 ft. Use only wells in the same aquifer, measured over a short time. In an unconfined aquifer, include the stream and lake levels — they are part of the water table. Contours never cross or merge; a closed low around a pumping well is a cone of depression, and it won’t carry hachure marks, so read the numbers.

Contour practice · 0 solved

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Check yourself · wells and flow

Question

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The numbers and the vocabulary — porosity and permeability by material, plus the terms that show up on every hydrogeology exam.

Porosity & permeability by material

Notice how porosity and permeability come apart — clay holds the most water and passes the least.

Material	Porosity	Permeability (K)	Aquifer?

Three controls on how much rain falls

Three regions

Timing beats total

Drought is relative, and the record is longer than you think

Quantity, quality, or timing?

Test it

How little of it there is

The recharge equation

Infiltration, and why pavement matters

Measuring the terms

Potential versus actual

Where the water goes afterwards

Build a cross-section from boreholes

Not all of it comes out: specific yield

Same everywhere? Same in every direction?

Converting hydraulic conductivity

Check yourself · water underground

How a well is built

Measuring hydraulic head

Divides, capture and the cone of depression

Drawing contours by hand

Check yourself · wells and flow

Porosity & permeability by material

Key terms