Water in the Soil–Plant–Atmosphere Continuum

Landscape soils and surface environments - Week 4 Workshop 2a

Raphael Viscarra Rossel, Lewis Walden

2026-03-11

Recap: Week 4 Workshop 1

  • The soil, plant, atmosphere continuum

Part A: Water movement through the continuum

Part B: Carbon movement through the same system


Today – Part A focus: - Water cycle in the soil, plant, atmosphere continuum, using patterns as evidence for processes - Exploring the water balance in contrasting Australian environments

Learning goals for this workshop

By the end of this workshop you should be able to:

  • Use the water balance equation ( P = ET + Q + S ) to explain how water is partitioned in different ecosystems
  • Explain how PAW, rooting depth, and climate control seasonal water availability and surplus
  • Predict how changes in vegetation alter water fluxes in the soil–plant–atmosphere continuum (SPAC)

The water balance

Water is conserved: Water in = water out + water stored

Important

\[ P = ET + Q + \Delta S \]

  • \(P\): Precipitation – the only input
  • \(ET\): Evapotranspiration – soil evaporation + plant transpiration
  • \(Q\): Runoff + deep drainage below the root zone
  • \(\Delta S\): Change in soil and groundwater storage

ET-dominated system: Mountain Ash forest

Water balance:

  • \(P \approx\) 1200 mm yr\(^{-1}\)
  • Deep roots (>10 m), dense canopy, long growing season
  • \(ET \approx\) 1000 mm (~85% of P)
  • \(Q \approx\) 150 mm (~12%)
  • \(\Delta S \approx\) 50 mm (~3%, essentially steady state over the long term)

Dense canopy + deep roots = almost all precipitation returned to atmosphere

Recharge-dominated system: Annual cropping, WA wheatbelt

Water balance:

  • \(P \approx\) 350 mm yr\(^{-1}\)
  • Shallow roots (<1 m), sparse canopy cover, fallow periods
  • \(ET \approx\) 230 mm (~65% of P)
  • \(Q \approx\) 20 mm (~6% – still low on permeable soils)
  • \(\Delta S \approx\) +100 mm yr\(^{-1}\) (~29%, significant net recharge → rising water table)

Shallow roots + bare soil = water passes below root zone and accumulates in groundwater

Applying the water balance in Australian ecosystems

For many native WA ecosystems, two simplifications often hold:

  • Q is small – permeable sandy soils, low surface runoff
  • Long-run \(\Delta S \approx\) 0 – mature systems in approximate steady state

So the balance often simplifies to:

\[ P \approx ET \quad \text{(over many years)} \]

The key question becomes: what controls ET?

But: Some years or sites may show significant surplus or deficit.

Activity (40 min): The water balance explorer

The explorer presents a simple monthly bucket model of soil water, to help you understand how water coming in as rain is split into evapotranspiration, changes in soil storage, and surplus (recharge/runoff).

It captures the key dynamics of soil moisture and plant-water availability over time without modeling complex landscapes or streams.

Click this link: The soil water balance explorer

or copy this link into your browser:

https://ravr19.github.io/lsse_teaching/water_balance_app.html

Follow the instructions in the app and answer the questions provided.

Key takeaways

  1. Water balance structure ( P = ET + Q + S ) helps us predict where water goes in any ecosystem
  2. PAW and rooting depth control how long plants can sustain transpiration after rain stops
  3. Contrasts in ET vs recharge dominance reflect differences in vegetation, soil, and climate—not just rainfall amount

Next: Carbon cycles, disturbance scenarios, and assessment introduction.

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