Earth’s Critical Zone is the planet’s thin living skin — everything from the top of the vegetation canopy down to the base of the weathered bedrock. Rock, soil, water, air, and life all meet here, and this one lecture tours it at four spatial scales. Click a region of the landscape below (or a chapter card) to explore, experiment, and test yourself.
Companion to the Part 01 lecture slides · Evaristo Critical Zone Hydrology Lab, University of Georgia. Scores are self-assessment only — nothing is recorded, and progress resets if you reload.
Our tour starts with a single vertical core through the Critical Zone: the pedon. Everything a soil is — its layers, its feel, its plumbing — is written down this one column.
Soils are unconsolidated mineral and organic materials that serve as a medium for plant growth. No two soils are identical, yet every one of them is cooked from the same five ingredients — the five state factors of soil formation. Hover (or tap) each factor to see what it contributes.
A soil pedon is a vertical soil column extending from the ground surface down into the underlying regolith — a one-dimensional section used to represent the key characteristics of the soil profile. It is the smallest unit we can sample that still tells the whole vertical story.
Dig a pit and the soil sorts itself into layers — horizons — each made by decades to millennia of water moving material down the profile. Click each horizon in the column to read what it is; then prove you know the stack by building one from scratch.
Soil texture is the relative proportion of sand, silt, and clay particles — and it is destiny for water. Sandy soils drain rapidly; clayey soils resist water movement because their pores are so much smaller. Texture also sets how much water a soil can hold for plants. Loams are balanced mixtures of all three, and the adjectives (sandy, silty, clayey) say which fraction dominates.
Twelve textural classes are recognized, mapped on the textural triangle. Each corner is 100% of one fraction. Click (or drag) anywhere in the triangle — the composition and class update live. Try the lecture’s example: 20% sand, 25% silt, 55% clay.
In the field you won’t carry a laser particle-sizer — you’ll rub moist soil between your fingers. Gritty means sand, smooth/floury means silt, sticky means clay; the more clay, the more the ribbon holds together.
Texture is the ingredients; structure is the architecture — how those particles aggregate into peds. Structure matters because water moves along the faces between peds: plate-like peds that overlap impair permeability, while blocky and granular structures leave room for flow. Match each shape to its name, then check.
Run the five state factors for long enough, in enough combinations, and you get the 12 soil orders recognized by the US Department of Agriculture — the top level of soil taxonomy. Tap any order for its one-line personality.
Scored, with explanations after grading. Retake as many times as you like — questions reshuffle.
Now tip the pedon on its side. A catena is the chain of soils down a hillslope — ridgetop to channel — and it is where hydrology gets moving: water arrives, splits among flowpaths, and races (or crawls) to the stream.
From top to bottom, a catena runs: ridgeline (upland flat) → shoulder → backslope → footslope → toeslope → floodplain → channel. The water table sits deep under the ridge and shallow near the valley, so soil drainage grades from well drained upslope to very poorly drained in the floodplain. Click each position to see its drainage class and its job in the hillslope’s plumbing; toggle the season to watch the water table breathe.
Drainage classes are read from soil color: freely drained soils stay bright (reds, browns); the longer a horizon sits saturated, the grayer (“gleyed”, low-chroma) it becomes — gray at depth means moderately well or somewhat poorly drained, gray to the surface means poorly drained, and standing water much of the year means very poorly drained.
Rain hitting a hillslope has exactly four ways to reach the channel: fall directly into it; run over the surface as overland flow (surface runoff); slip sideways through shallow soil as interflow (subsurface stormflow/throughflow) above an impeding horizon; or soak deep and arrive years later as groundwater flow. Click each route to light it up — then switch to the tracer tab and answer the lecture’s trick question: is the water in the stream during a storm actually today’s rain?
Runoff is the portion of rainfall, snowmelt, or irrigation water that flows over the soil surface toward the stream rather than infiltrating. (Beware: some authors stretch “runoff” to include interflow too.) Whether runoff happens at all is a race between two rates: the rainfall rate and the soil’s infiltration capacity — the upper limit of the infiltration rate, both in mm hr−1.
Run the storm yourself. Two presets reproduce the lecture’s numbers: the intense Hortonian storm (25 vs. 15 mm hr−1) and the long, gentle saturating storm (10 mm hr−1 that fills the soil in 3 hours).
A hydrograph is the graph of discharge versus time at a point on a river — the watershed’s signature on a storm. Hover the curve to name its parts, then switch to Find it mode and click the features from memory.
Measuring the flow that makes this curve is its own craft. Runoff spread across a hillslope is nearly impossible to catch, so we let the stream collect it for us and measure there:
Quantitative hydrograph analysis comes in Part 5 (Surface Runoff); groundwater flow gets the same treatment in Part 6.
Scored, with explanations after grading. Retake as many times as you like — questions reshuffle.
Zoom out once more. A watershed (basin, drainage basin, catchment) is the area of land where precipitation collects and drains to a common outlet. It begins at the drainage divide and ends at the basin outlet — and every drop inside it must be accounted for.
Hydrology’s deepest law is bookkeeping: over any time interval, change in storage = inflow − outflow. For a watershed the inflow is precipitation P and (sometimes) groundwater seeping in, Gin; the outflows are streamflow Q, evapotranspiration ET, and groundwater leaking out, Gout. Move the sliders and watch the storage respond.
Storage wobbles with the seasons — but average Eq. 1 over decades and the wobbles cancel: ΔS → 0. In most watersheds the net groundwater exchange is also negligible (Gin ≈ Gout), leaving the most-used equation in hydrology:
That is remarkable: evapotranspiration — the hardest flux to measure directly — falls out of a rain gauge network and a stream gage. The widget above has a North Oconee preset and an “average over decades” button; use them to reproduce the lecture’s answer, μET = 1,232 − 220 = 1,012 mm yr−1.
Every measured term carries uncertainty: rain gauge networks are typically good to about ±10% (95% confidence), stream gages to about ±5%. Since ET is computed from P and Q, their errors propagate into it — independent errors add in quadrature (as the square root of summed squares). Drag the error sliders to see which term controls the answer.
Residence time is the time required to completely renew a water body’s volume — storage divided by throughflow, τ = V/Q. It is the bathtub question: with the faucet and drain matched, how long until the tub holds entirely new water? Inject dye and watch the old water flush out.
Residence times and transit times get their own problem-based assignments later in the course (PBAs 6 & 7).
Flowing waters are lotic; standing waters are lentic. A stream that flows year-round is perennial; one that periodically dries is ephemeral. Reservoirs are lentic waters engineered to store, regulate, and control flow. Sort the examples, then classify the river network below.
Networks get a hierarchy too: stream order (Strahler). Order-1 headwaters start the network; where two streams of equal order meet at a confluence, the order steps up by one; where unequal orders meet, the larger order simply continues. The river ends at its terminus — a lake, the ocean, or a sink. Click each segment to assign its order, then check your network.
Scored, with explanations after grading. Retake as many times as you like — questions reshuffle.
The Critical Zone doesn’t stop at the soil. Beneath nearly every landscape sits groundwater — invisible, slow, and vastly larger than every river and lake combined.
Before the definitions, calibrate your intuition. Guess each number first — drag the slider, lock it in, then reveal. Most people guess low.
Beyond people and crops: many rivers, lakes, and wetlands — and the ecosystems that depend on them — are kept alive between rains by groundwater discharge.
First, four definitions worth memorizing. The water table is the surface of saturated groundwater where pore-water pressure is atmospheric — above it lies the unsaturated (vadose) zone, below it the saturated (phreatic) zone. Hydraulic head is the total energy of water at a point, expressed as the height of a static water column. Permeability is a material’s ability to transmit fluid (units of L² — it belongs to the rock alone), while hydraulic conductivity folds in the fluid’s properties too (units of L T−1).
An aquifer is a geologic unit saturated enough — and permeable enough — to yield significant water to wells or springs. Drag the drill rig across the valley and choose which aquifer to complete the well in. Watch where the water level rises to — and find the spot where the well flows without a pump.
Porosity is the fraction of a material’s volume that is pore space — where groundwater actually lives. But storage is not the same as delivery: water only moves if the pores are large and connected. Pick a material and compare its two bars — then find the paradox.
Groundwater occupies both unconsolidated formations (loose, uncemented sediments) and consolidated formations (particles cemented or firmly packed into rock). Sort them:
Geology sets not just aquifer type but aquifer size. The Ogallala (High Plains) Aquifer spans eight US states and stores roughly 2.9 billion acre-feet (≈3,580 km³) — about as much water as Lakes Huron and Ontario combined — and irrigation draws it down by about 15 million acre-feet each year.
And groundwater doesn’t always surface in a stream. Submarine groundwater discharge (SGD) is direct groundwater outflow across the ocean–land interface into the sea:
Scored, with explanations after grading. Retake as many times as you like — questions reshuffle.