Changing demands for water from human activities – Agriculture
5.1 Global water resources & the human water cycle
Distribution of renewable freshwater (FAO AQUASTAT 2023):
≈ 70 % stored in rivers, lakes and reservoirs
≈ 30 % stored as groundwater
<1 % stored as ice in glaciers and seasonal snow
Human modifications to the natural water cycle (key examples):
Construction of dams and reservoirs – alters river discharge and timing of flows.
Inter‑basin transfers (e.g. China’s South‑to‑North Water Transfer) – redistributes water across regions.
Ground‑water abstraction for irrigation – lowers water tables, reduces base‑flow to streams.
5.2 Factors influencing water resources – supply and demand
Factor
Supply (physical)
Demand (human)
Climate
Precipitation amount, seasonality, temperature (controls evapotranspiration)
Higher temperatures raise crop water requirements; drought reduces natural availability.
Geology & relief
Aquifer capacity, permeability, basin shape
Flat plains often rely on shallow groundwater; mountainous terrain limits arable land.
Population & urbanisation
—
More people → greater domestic & industrial water use; urban spread reduces land for agriculture.
Economic development
—
Higher incomes increase demand for water‑intensive foods (meat, dairy) and for high‑yield irrigation.
Policy & institutions
Water‑allocation rules, pricing structures, water‑rights, IWRM frameworks
Regulations can curb or encourage abstraction; subsidies often promote water‑intensive crops.
Technology
—
Irrigation efficiency, drought‑tolerant varieties, remote‑sensing tools, soil‑moisture sensors.
Water‑security risk categories (Cambridge 9696)
Risk level
Definition
Typical examples
Very high
Severe, chronic shortage of water for basic needs and agriculture
North Africa (e.g., Egypt), Middle East (e.g., Jordan), parts of South‑Asia (e.g., Pakistan’s Indus basin)
High
Significant water stress; occasional deficits affect food production
Central Asia (e.g., Kazakhstan), western China, some sub‑Saharan river basins
Moderate / Low
Generally adequate water, but vulnerable to droughts or policy failures
Europe, North America, most of South‑America
5.3 Management of water resources
Key strategy groups (as required by the syllabus)
Increase supply
Dams, reservoirs and inter‑basin transfers
Desalination and large‑scale rain‑water harvesting
Managed aquifer recharge (MAR)
Manage demand
Water‑pricing, allocation reforms, tradable water‑rights
High‑efficiency irrigation (drip, precision sprinklers) and automated scheduling
Crop selection, breeding for drought tolerance, agronomic practices (mulching, conservation tillage)
Overall water‑security management
Integrated Water‑Resource Management (IWRM) at river‑basin scale
Stakeholder participation, public‑awareness campaigns
Monitoring & data‑sharing systems (remote sensing, groundwater observation networks)
Evaluation framework (successes, limitations, trade‑offs)
When assessing any strategy, students should consider:
Effectiveness – reduction in water abstraction or increase in reliable supply.Economic cost – capital and operating expenses, impact on farmer income.Environmental impact – effects on ecosystems, salinisation, greenhouse‑gas emissions.Social acceptability – equity, gender implications, community support.
Case studies (contrasting contexts)
Case study
Context (income level)
Key strategies
Successes
Limitations
Indus Basin (Pakistan)
Low‑/middle‑income
Indus Water Treaty (1960) – surface‑water allocation
Canal‑line lining & bulk‑water pricing
Promotion of drip irrigation for high‑value crops
Groundwater monitoring & community abstraction licences
Canal conveyance losses ↓ from ~30 % to <15 %
Drip adoption ↑ wheat yields 20 % with 30 % less water
Weak enforcement of groundwater licences
Glacial melt threatens long‑term river reliability
California Central Valley (USA)
High‑income
Groundwater Sustainable Management Act (2014) – allocation limits & recharge mandates
Large‑scale adoption of precision drip and sprinkler systems
Water‑pricing tiered by volume; water‑bank trading
Investments in recycled‑water supplies and storm‑water capture
Groundwater extraction fell ~15 % after 2015
Crop‑yield per unit water ↑ 25 % in high‑value orchards
High capital cost limits uptake by small farms
Legal disputes over water‑rights persist
Link to the “systems” key concept
Water‑balance for an irrigated field (Figure 1) can be expressed using the syllabus language of inputs, stores, transfers, outputs :
$$\Delta S = P + I - ET - R - D$$
Inputs : Precipitation (P) and Irrigation (I)Store : Soil‑moisture storage (ΔS)Transfers : Water moving through the soil profileOutputs : Evapotranspiration (ET), Surface runoff (R), Deep percolation (D)
Global water use by sector
Sector
Share of total freshwater withdrawals
Typical water use per unit production
Agriculture
≈ 70 %
~ 1 500–2 000 m³ ha⁻¹ yr⁻¹ (irrigated cereals)
Industry
≈ 20 %
~ 300 m³ ton⁻¹ (manufacturing)
Domestic
≈ 10 %
~ 150 L person⁻¹ day⁻¹
Source: FAO AQUASTAT 2023.
Irrigation methods & water‑use efficiency (WUE)
WUE = crop yield ÷ water applied (kg m⁻³). Typical efficiencies:
Surface (gravity) irrigation – 30–50 % efficiency; high runoff & deep percolation losses.Sprinkler irrigation – 50–70 % efficiency; losses to wind drift and evaporation.Drip (trickle) irrigation – 70–90 % efficiency; water delivered directly to the root zone.
Moving to higher‑efficiency systems is a primary lever for reducing agricultural water demand.
Regional trends in agricultural water consumption (1990‑2020)
Region
Key crops
Trend in irrigated area (1990‑2020)
Major water‑management issues
South Asia
Rice, wheat
+ 45 %
Groundwater depletion, salinisation of shallow aquifers
North America
Corn, soybeans
+ 20 %
High energy demand for pumping; altered river flows
North Africa & Middle East
Wheat, dates
+ 60 %
Extreme water scarcity; reliance on desalination & food imports
Australia
Wheat, cotton
‑ 10 % (water‑pricing reforms)
Allocation efficiency, drought resilience, water‑rights trading
Strategies to reduce agricultural water demand (summary)
High‑efficiency irrigation (drip, precision sprinklers) with automated scheduling.
Crop selection and breeding for drought tolerance (e.g., drought‑resistant wheat, millet).
Agronomic practices: mulching, conservation tillage, crop rotation, cover crops.
Economic instruments: water pricing, allocation reforms, tradable water‑rights.
Technology‑driven decision support: remote sensing, soil‑moisture sensors, GIS‑based irrigation scheduling.
Cross‑topic links (AS‑Level integration)
Increased irrigation alters river discharge → impacts River hazards (Topic 1.3) .
Higher groundwater tables can trigger slope instability → links to Mass movements (Topic 3.3) .
Changes in sediment transport affect river‑channel morphology → relevant to River processes (Topic 1.2) .
Future outlook to 2050
Business‑as‑usual scenarios predict a 20–30 % rise in agricultural water withdrawals, even with widespread drip adoption. Meeting water‑security goals will require:
Integrated Water‑Resource Management (IWRM) at the river‑basin level.
Sustainable intensification – higher yields per unit water.
Policy coherence across agriculture, energy and climate mitigation.
Summary points (quick revision)
Agriculture accounts for roughly 70 % of global freshwater withdrawals.
Population growth, dietary shifts, climate variability and economic development drive rising water demand.
Water‑use efficiency: surface irrigation (30–50 %), sprinkler (50–70 %), drip (70–90 %).
Fastest irrigated‑land expansion occurs in water‑stressed basins (e.g., South Asia, North Africa).
Key management options: increase supply (dams, desalination, MAR), manage demand (pricing, efficient irrigation, drought‑tolerant crops), and basin‑scale water‑security governance (IWRM, stakeholder participation).
Understanding the water‑balance as a system (inputs, store, transfers, outputs) is essential for the Cambridge 9696 examination.
Figure 1 – Water‑balance system for an irrigated field (Inputs = P + I; Store = ΔS; Outputs = ET + R + D).
Water‑security definition (Cambridge 9696) :