Water & Land Resources · Grade 12 Science
Chapter: Energy, Water & Material ResourcesWater & Land Resources
Learning goals
- I can explain why usable freshwater is a small and unevenly distributed fraction of Earth's water.
- I can describe how a watershed collects water and why protecting it protects the water supply.
- I can analyze how land use and agriculture affect water and soil resources.
Readings for this unit
Before this unit
Watersheds, withdrawal versus consumption, and percentages.
Use the detailed parent plan: two core 50–60-minute sessions per focused unit, with a third assigned for selected calculation/modeling topics. First study the core explanation and Parts 1–2 with their figures/checks; next study Parts 3–4, the whole-unit application, worked cases, varied practice and exit check. Attempt reasoning before opening guidance. Extend time when actual understanding requires it; investigations and chapter assessments have separate sessions.
Start with something to explain
Why can a region have abundant stored water and still face depletion?
Illustrative evidence case. Annual extraction exceeds net replenishment.
Notice and wonder
Record two observations using the stated quantities or relationships. Keep observations separate from possible causes. Write one question that a new measurement could help answer.
Why can a region have abundant stored water and still face depletion?
Your first model
Separate storage amount from recharge and withdrawal rates.
Build the language from the idea
- freshwater
- Water with very little dissolved salt, usable for drinking and agriculture; it makes up only a small fraction of all the water on Earth.
- watershed
- The entire area of land where all the water that falls on it drains to a common outlet, such as a river, lake, or bay.
- aquifer
- An underground layer of rock or sediment that holds and transmits groundwater, which can be pumped up through wells.
- irrigation
- Supplying water to crops artificially through channels, sprinklers, or drip systems; it is the single largest human use of freshwater.
A watershed is the land draining toward a common outlet. Water availability depends on precipitation, storage, surface flow, groundwater, quality, infrastructure, and timing. Freshwater percentage alone does not establish what is locally safe or accessible.
Much of Earth’s freshwater is in ice or groundwater rather than easily collected lakes and rivers. This course uses rounded distribution models for fraction practice; categories and accessibility definitions must be stated. Do not confuse a percentage of freshwater with the same percentage of all water.
Withdrawal is water removed from a source; consumption is the part not promptly returned to that same system. Agriculture, industry, and domestic shares vary by place, year, and definition. In a stated practice allocation 70% agricultural,20% industrial, the remainder is 10%; treat it as supplied data.
Irrigation can support crops but can deplete storage when withdrawals exceed recharge. Evaporation can concentrate salts and drainage can transport nutrients. Improved irrigation efficiency does not always lower total basin consumption if planted area or production expands; evaluate the larger boundary.
Land use changes infiltration, runoff, habitat, erosion, and water quality. Impervious surfaces can accelerate runoff, while vegetation and soil structure can retain water and filter some pollutants. A measure that works at one scale may have downstream consequences.
Use a water budget: storage change equals inputs minus outputs for a defined place and period. Compare conservation, reuse, treatment, and land-management options using measurements, seasonal reliability, ecological flow needs, and distribution of costs and benefits.
Model check: Water withdrawn can partly return; consumption is the portion not promptly returned to the same system.
Build your understanding
Use the detailed parent plan: two core 50–60-minute sessions per focused unit, with a third assigned for selected calculation/modeling topics. First study the core explanation and Parts 1–2 with their figures/checks; next study Parts 3–4, the whole-unit application, worked cases, varied practice and exit check. Attempt reasoning before opening guidance. Extend time when actual understanding requires it; investigations and chapter assessments have separate sessions.
Part 1 · Define a water boundary and competing uses
A watershed is an area draining toward a defined outlet, while a water-use planning boundary can differ from that physical basin. Storage changes with precipitation, imports, withdrawals, return flows, evaporation and outflow. Distinguish a withdrawal from consumptive use, which does not promptly return water to the selected available system. Competing needs include households, agriculture, industry and ecosystem flows; state whose needs and downstream effects enter the comparison.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Why can a withdrawal exceed consumptive use?
Visual reasoning — after your attempt
Some withdrawn water returns to the selected system, although timing and quality can change.
Water resource assessment depends on catchment, aquifer, interval, and the flows connecting them. Withdrawal is not necessarily consumption: some water returns, potentially at a different location, time, or quality. Environmental flows and downstream users belong in the decision account.
A 100-unit withdrawal with 60 returned differs from 100 consumed, but the return’s timing and quality can still affect availability.
Pause and explain
Why is withdrawal alone not the complete water-use impact?
Check your reasoning after answering
Consumption, return location and timing, quality, and downstream conditions also influence the system.
Part 2 · Separate recharge from stored groundwater
An aquifer is water-bearing permeable material that can transmit groundwater. Recharge is water entering the groundwater system per time; storage is an amount already present. Pumping faster than net replenishment can deplete storage even when some recharge continues. Water-table changes also depend on aquifer properties and boundaries, so a simple amount budget alone does not give an exact well-level change. Account for connected streams and other groundwater flows where relevant.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
If recharge 40 and pumping 55 in matched annual volume units, what is Δstorage before other flows?
Visual reasoning — after your attempt
−15 volume units per year. Continuing recharge does not prevent depletion.
Groundwater storage changes through recharge, withdrawals, and natural exchanges. A large stored stock does not make continued extraction sustainable if the long-term balance is negative. Surface-water and groundwater interactions may complicate a simple independent-source picture.
An aquifer losing 10 million m³ each year can retain a large stock while experiencing a continuing decline.
Pause and explain
Does a large aquifer stock prove recharge matches pumping?
Check your reasoning after answering
No. Stock and annual replenishment are different quantities; a flow balance and observed storage trend are needed.
Part 3 · Account for land-cover mechanisms
Infiltration is water entering soil; runoff is flow over the land surface. Impermeable cover, compaction, vegetation and drainage design can alter partitioning among infiltration, storage, evapotranspiration and rapid runoff. The effect depends on rainfall intensity, soil and slope, so a single before/after photograph is not enough to quantify it. Compare matched events or an explicit model and distinguish peak flow from total event volume.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Does a lower runoff peak always mean less total runoff volume?
Visual reasoning — after your attempt
No. Flow can be delayed or spread out while event volume remains similar; measure both quantities.
Land cover can alter infiltration, runoff timing, soil retention, and habitat. Effects depend on rainfall, soil, slope, drainage, and scale; a simple always-versus-never rule is inadequate. Compare controlled or matched sites and distinguish a mechanism hypothesis from an established measured effect.
A paved area may increase rapid runoff under particular conditions, but engineered drainage and storage can change the local outcome.
Pause and explain
What should be controlled in a land-cover runoff comparison?
Check your reasoning after answering
Rainfall, contributing area, soil and slope where relevant, drainage, and measurement method should be documented or matched.
Part 4 · Compare allocation scenarios transparently
A scenario is an explicit set of assumptions, not a guaranteed forecast. These two illustrative water budgets have the same input but different use and selected outflow, giving different storage changes. A feasible annual total can still hide seasonal scarcity, poor water quality or unequal access. Identify affected groups and environmental requirements, compare uncertainty and monitor actual results. Explain which criteria are scientific estimates and which reflect chosen priorities.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Which scenario has the larger storage increase under these assumptions?
Visual reasoning — after your attempt
B:100−45−40=15 units versus 10 for A, but that alone does not decide every criterion.
Resource plans should name essential service, environmental constraints, users, and uncertainty. An allocation satisfying one annual sum can still fail during a dry season. Consider distribution across people and places without disguising selected priorities as measurements.
A conservation scenario should calculate changed consumption and returns, then compare seasonal supplies and ecological needs.
Pause and explain
Why is an annual balanced budget not sufficient for reliable supply?
Check your reasoning after answering
Timing and location matter; dry-season shortages can occur even when wet-season surpluses make the annual total balance.
Apply the whole unit · Evaluate a simplified aquifer budget
Illustrative annual recharge is 50 million m³, natural outflow 20 million m³, pumping 40 million m³, with negligible other exchanges. A plan reduces pumping by 25%.
Your task
Find current and proposed storage change, explain what the balanced proposal establishes, and identify seasonal, ecological, and quality evidence needed before calling it sustainable.
What your response should show
- Define the system, quantities, units, and relevant conditions before selecting a model.
- Show intermediate calculations or a labeled evidence account, including signs or denominators.
- Explain how the supplied evidence supports the conclusion rather than reporting only an answer.
- Identify a meaningful uncertainty, model limit, or measurement needed to test the conclusion.
Response guidance — after your attempt
Current change=50−20−40=−10 million m³/year.
Reduced pumping=40×0.75=30 million m³/year.
Proposed change=50−20−30=0 in the supplied annual account.
The proposal balances this simplified annual storage budget but does not establish reliable seasonal delivery, acceptable water quality, or unchanged ecological outflow. Recharge uncertainty and connected surface water also matter. Sustainability needs those additional constraints rather than a zero result from one equation.
Scientific references and source-model foundations
Parent / teacher teaching notes
Require a named boundary and units per year. Ask whether natural outflow is a dispensable loss or part of downstream ecological function. Ask the learner to explain each symbol, axis, unit and arrow before calculating. Calculated examples, supplied illustrative records and actual historical observations are labeled separately. Selected public OpenSciEd middle-school source figures are foundation examples, not complete high-school editions or an endorsement; all advanced explanations are supplied locally.
The key idea
Water availability needs a common-boundary stock/flow account. Withdrawal, consumption, recharge and storage differ; timing, quality, ecosystems and access shape allocation.
Worked examples
Example 1
- The accessible surface freshwater is 1% OF the 3% that is freshwater, so multiply the two fractions.
- Write each percent as a decimal: 3% = 0.03 and 1% = 0.01.
- Multiply: 0.03 x 0.01 = 0.0003.
- Convert back to a percent: 0.0003 x 100 = 0.03%.
Example 2 · Worked application
- Initial deficit=120−100=20.
- Reduced demand=105 after saving 15.
- A 5-unit deficit remains unless another flow changes.
Efficiency improvements need to meet the full service balance.
Example 3
- Identify the relevant quantities or mechanism.
- 10% for the remaining category in that stated model. Do not treat these shares as universal current observations.
- State the model and conditions.
Example 4 · Find and fix the mistake
- Test the claim against the stated conditions: Efficiency improvements need to meet the full service balance.
- Savings may be insufficient; availability, demand, quality and timing all matter.
Efficiency improvements need to meet the full service balance.
Example 5 · Reason from the visual model
- Infiltration is water entering soil; runoff is flow over the land surface. Impermeable cover, compaction, vegetation and drainage design can alter partitioning among infiltration, storage, evapotranspiration and rapid runoff. The effect depends on rainfall intensity, soil and slope, so a single before/after photograph is not enough to quantify it. Compare matched events or an explicit model and distinguish peak flow from total event volume.
- No. Flow can be delayed or spread out while event volume remains similar; measure both quantities.
Original land-cover mechanism comparison, not a universal numerical effect size.
Avoid these mistakes
- Water withdrawn can partly return; consumption is the portion not promptly returned to the same system.
Check your understanding
A model allocates 70% farming and 20% industry. What remains?
Discuss the answer
10% for the remaining category in that stated model. Do not treat these shares as universal current observations.
Return to the phenomenon
Return to the initial case. Why can a region have abundant stored water and still face depletion? Revise your drawing or explanation using two named ideas and one stated limitation.
Compare explanations after your attempt
A stock can support use temporarily while declining; seasonal availability, quality and downstream flows add constraints.
What should we investigate next?
Which comparison would test a remaining uncertainty? State what you would change, keep comparable and measure.
Investigate, calculate and revise
Keep a first model and an evidence-supported revision. Attempt each task before opening guidance; all required explanations and supplied-data routes are available locally.
Activity 1 · Return to the opening case
Use the opening phenomenon, unit definitions and explained Parts 1–4.
Your task
Return to the initial case. Why can a region have abundant stored water and still face depletion? Revise your drawing or explanation using two named ideas and one stated limitation.
Success criteria
- Use the stated observations and appropriate units.
- Connect a labeled model with the physical/environmental mechanism.
- State assumptions and an evidence limitation.
- Explain a substantive revision or next test.
Hint
Keep object/material identity, material transfer and energy account distinct.
Response guidance — discuss after your attempt
A stock can support use temporarily while declining; seasonal availability, quality and downstream flows add constraints.
Activity 2 · Transfer to a changed case
Cover the worked answer and use the explained models, relationships and notebook.
Your task
An aquifer receives 8 million m³/year recharge, loses 2 million to streams and is pumped at 9 million. Find annual storage change and a limitation of predicting well depth from it.
Success criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the physical or environmental mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Hint
Name known quantities and conditions, show intermediate steps and check conservation.
Response guidance — discuss after your attempt
Δstorage=8−2−9=−3 million m³/year. Exact water-level change also needs aquifer geometry, storage properties and spatial pumping/flow patterns.
Words to know
- freshwater
- Water with very little dissolved salt, usable for drinking and agriculture; it makes up only a small fraction of all the water on Earth.
- watershed
- The entire area of land where all the water that falls on it drains to a common outlet, such as a river, lake, or bay.
- aquifer
- An underground layer of rock or sediment that holds and transmits groundwater, which can be pumped up through wells.
- irrigation
- Supplying water to crops artificially through channels, sprinklers, or drip systems; it is the single largest human use of freshwater.
In the real world
Farmers in California's Central Valley irrigating crops during drought must weigh bigger harvests against draining the rivers and aquifers they share — a real, high-stakes water-resource trade-off.
Exit check
A model allocates 70% farming and 20% industry. What remains?
Check your explanation after answering
10% for the remaining category in that stated model. Do not treat these shares as universal current observations.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.