Land-use change does not add a new set of physical laws. It changes the terms in the water and energy balances already developed in §§10 and 13. Remove a forest, drain a peatland, or pave a neighbourhood and the same precipitation and net radiation are partitioned differently.

The useful question is therefore not just “did the land get warmer or wetter?” Ask which flux changed, why it changed, and over what timescale.

Forest removal changes water yield

A forest intercepts precipitation, moves soil water to the atmosphere through transpiration, protects soil structure, and slows surface flow. Cutting trees immediately reduces canopy interception and transpiration. More water reaches the ground, soil moisture rises, and a larger part of precipitation can leave as streamflow.

The response is not determined by tree removal alone. It also depends on:

• the proportion of the catchment cut;
• whether roots, litter, and harvest debris remain;
• soil compaction and road construction;
• precipitation and evaporative demand;
• slope and soil permeability;
• how quickly vegetation regrows.

Harvest debris can shade the soil and act as mulch, reducing evaporation even though the open surface receives more sunlight. Heavy machinery can compact soil and increase rapid runoff, while a low-disturbance winter cut can leave infiltration pathways mostly intact.

What Hubbard Brook showed

The Hubbard Brook Experimental Forest used paired watersheds: a treated basin was compared with a nearby control whose pre-treatment relationship to the treated basin had been measured for years. This helps separate the treatment effect from wet and dry years that affected both basins.

Three treatments produced different annual water-yield responses:

Most of the extra yield appeared as greater growing-season low flow, although some peak flows also increased. Snowmelt volume changed little. The annual increases disappeared within about 7–9 years as vegetation recovered.

The long record also shows why “deforestation increases runoff” is not a permanent rule. Regenerating stands later contained more species with low stomatal resistance and used enough water that annual yield fell below the control for years 13–30 after the clear-fell treatment and years 8–25 after strip cutting. Recovery changes species composition, leaf area, rooting, and transpiration, so the hydrologic effect changes through time.

Forest removal also changes the surface energy balance

The radiation balance is

$$Q^*=SW^\downarrow-SW^\uparrow+LW^\downarrow-LW^\uparrow,$$

and a simplified surface energy balance is

$$Q^*=Q_H+Q_E+Q_G.$$

After forest removal:

• the brighter exposed surface often has higher albedo, so $SW^\uparrow$ rises and absorbed shortwave falls;
• canopy roughness falls, changing turbulent exchange;
• transpiration drops, so latent heat flux $Q_E$ usually falls;
• more available energy is partitioned into sensible heat $Q_H$ and ground heat $Q_G$;
• the surface and near-surface air often become hotter and drier;
• less evapotranspiration returns water vapour to the atmosphere, weakening local moisture recycling.

The higher albedo does not guarantee cooling. The loss of evaporative cooling can be larger than the reduction in absorbed shortwave, leaving the cleared surface warmer. In tropical regions, widespread forest removal can also reduce humidity and recycled precipitation, but the size of that effect depends on circulation and the scale of clearing.

Wetland water levels control carbon and heat

Wetlands and peatlands store carbon because waterlogged soils contain little oxygen. Slow anaerobic decomposition allows partly decomposed plant material to accumulate as peat.

Drainage lowers the water table and lets oxygen enter. Aerobic microbes then decompose peat quickly:

• carbon-dioxide emissions rise;
• methane production falls because methanogenic microbes require anaerobic conditions;
• the peat surface subsides as stored organic matter is lost;
• evapotranspiration and latent heat can fall;
• sensible heating increases and the surface warms.

Rewetting reverses many of those changes. Waterlogging slows aerobic decay and can restore net carbon-dioxide uptake, but anaerobic methane production rises. A restored wetland can therefore be a carbon-dioxide sink and a methane source at the same time.

A measured land-use comparison

One course case compared drained crops and pasture with rice and restored wetlands. Positive net $\mathrm{CO_2}$ means emission to the atmosphere; negative values mean uptake.

Methane is more powerful per unit mass than carbon dioxide over a century, so the gases must be combined with a stated global-warming-potential convention before comparing total climate effects. Using the convention in the course example, including harvested carbon, the net greenhouse-gas budgets were approximately $2086$, $1575$, $702$, $645$, and $-392\ \mathrm{g\ CO_2eq\,m^{-2}\,yr^{-1}}$ for corn, pasture, rice, younger wetland, and older wetland respectively.

The younger wetland was already a strong carbon-dioxide sink but remained a net greenhouse-gas source because methane was high. The older wetland was a net sink even after methane was included. Restoration outcome therefore depends on wetland age, vegetation, water regime, and the accounting period—not simply whether the site is flooded.

Wetlands also cool the surface. At Prairie Pothole sites, open water and wetlands had median daytime surface temperatures about $1.4$–$3.0\,^{\circ}\mathrm{C}$ below nearby cropland, with cooling up to $5.4\,^{\circ}\mathrm{C}$ on the hottest days. Water availability lets more $Q^*$ become latent heat $Q_E$ and less become sensible heat $Q_H$.

Climate change can alter these fluxes without direct drainage. Warming, drought, and fire can change water tables, plant productivity, decomposition, and methane production. In one boreal–Arctic estimate, wetland methane emissions rose 8.9% from 2002 to 2021, with warming and increased plant productivity both contributing. This is a feedback: warming can raise methane emissions, which adds further warming.

Urbanization makes catchments flashier

Roads, roofs, parking lots, compacted ground, storm drains, and buried streams replace infiltration and storage with rapid routing.

A useful measure is the runoff coefficient,

$$C_R=\frac{R}{P}.$$

Representative values for temperate land uses show the contrast:

The ranges overlap because storm intensity, soil, slope, antecedent moisture, drainage design, and connected impervious area still matter. The table is not a lookup rule for every basin.

After urbanization, the storm hydrograph usually has:

• a steeper rising limb;
• a shorter lag time;
• a higher, earlier peak;
• more direct storm runoff;
• less infiltration and groundwater recharge;
• lower baseflow between storms.

How urbanization changes a storm hydrograph

Swipe horizontally to compare both hydrographs.

Storm sewers amplify the timing change by connecting impervious surfaces directly to channels. A city can therefore experience both larger flood peaks during storms and reduced low flow during dry periods.

Green infrastructure tries to restore some of the missing pathways. Street trees intercept rain and transpire water. Rain gardens and bioswales slow flow and encourage infiltration. Permeable pavement stores water beneath the surface. Wetlands and detention basins temporarily store stormwater. Their effectiveness depends on soil, maintenance, available storage, groundwater conditions, winter freezing, and whether they are connected to the runoff source.

The urban heat island is an energy-balance change

An urban heat island is the relative warmth of an urban area compared with a representative non-urban surroundings over a stated period. It can refer to surface, canopy-layer air, boundary-layer air, or subsurface temperature. Those are different measurements and should not be mixed.

Surface thermal-infrared maps show apparent surface temperature, not air temperature. Day and night images can also use different colour scales, so matching colours across panels do not necessarily mean matching temperatures.

A simplified urban energy balance is

$$Q^*+Q_F=Q_H+Q_E+\Delta Q_S,$$

where $Q_F$ is anthropogenic heat from buildings, traffic, industry, and other activity, and $\Delta Q_S$ is heat stored in buildings, pavement, ground, air, and vegetation.

Several mechanisms make cities warm:

• dark roofs and pavement absorb shortwave radiation;
• street-canyon geometry creates multiple reflections and reduces the view of the cold sky;
• concrete, brick, and asphalt store daytime energy and release it after sunset;
• impervious surfaces and sparse vegetation reduce $Q_E$, leaving more energy for $Q_H$;
• buildings, vehicles, and air conditioning add $Q_F$;
• aerosols and the urban atmosphere can increase downward longwave radiation.

The canopy-layer heat island is often strongest at night. Rural surfaces can lose heat quickly after sunset, while urban materials release stored heat and street canyons reduce longwave loss to the sky. Human heat and low evaporation continue to support the difference.

Heat hazard is not the same as heat risk

A temperature map locates the hazard. Risk also depends on exposure and vulnerability. In Montreal, hotter industrial and commercial surfaces do not always coincide with the greatest residential exposure, while nearby neighbourhoods may still contain many vulnerable people. Age, illness, isolation, income, housing condition, access to cooling, and air quality can turn the same thermal hazard into very different health outcomes.

A useful planning sequence is:

1. locate heat exposure and vulnerable populations;
2. identify the local mechanisms—low canopy, dark surfaces, heat storage, waste heat, or poor ventilation;
3. choose measures that match that site;
4. evaluate benefits, costs, maintenance, and possible harms;
5. measure the result rather than assuming the intervention worked.

Cooling strategies change different terms

No single strategy fixes every urban heat problem.

• Trees and other vegetation add shade, intercept rainfall, and raise $Q_E$ when water is available.
• Green roofs and living walls add evapotranspiration and insulation, but their effect depends on substrate depth, vegetation, irrigation, and scale.
• High-albedo roofs and pavement increase reflected shortwave radiation and can lower surface temperature.
• Permeable pavement supports infiltration and evaporation while reducing storm runoff.
• Efficient buildings, transport, and anti-idling measures reduce $Q_F$.
• Shaded or cooled public spaces reduce exposure even when they do not change the whole city’s climate.

Bright surfaces need context. Reflected sunlight can increase glare or be absorbed by nearby walls and people inside a street canyon. A surface can become cooler without lowering a pedestrian’s mean radiant temperature. Urban cooling works best as a site-specific combination of shade, vegetation, water-sensitive design, suitable materials, and reduced waste heat.

Check the mechanism

1. Forest removal raises albedo but often raises surface temperature. Explain why using $Q^*$, $Q_E$, and $Q_H$.
2. Why did Hubbard Brook water yield return toward the control and later fall below it?
3. A restored wetland is a carbon-dioxide sink but emits methane. What information is needed before calling it a net climate sink?
4. Sketch the hydrograph before and after urbanization. Label the change in peak, lag time, recession, and baseflow, then connect each change to a pathway.
5. On a calm clear night, downtown is warmer than nearby fields. Name three urban-energy terms or mechanisms that maintain the difference after sunset.
6. Why might a high-albedo pavement cool itself without improving pedestrian thermal comfort?

The connecting idea is partition. Land use changes where water goes, where energy goes, how carbon is stored or released, and how quickly the system responds.