A catchment, watershed, or drainage basin is the land area that drains to one outlet. Ridges and other topographic divides mark its boundary. Rain that falls on one side of a divide may enter a different river system from rain that falls only a few metres away on the other side.

A catchment is useful because it gives the water cycle a measurable boundary. Precipitation enters, evapotranspiration and streamflow leave, and water can be stored in snow, vegetation, soil, wetlands, lakes, and groundwater between those two moments.

The catchment water balance

For one catchment over one stated period,

$$P=ET+R+\Delta S,$$

where:

• $P$ is precipitation;
• $ET$ is actual evapotranspiration;
• $R$ is runoff exported at the outlet, expressed as an equivalent water depth over the whole basin;
• $\Delta S$ is the change in water storage.

Every term must use compatible units. Annual precipitation, evapotranspiration, runoff, and storage change are often written in $\mathrm{mm\,yr^{-1}}$. A positive $\Delta S$ means the basin ends the period with more stored water. A negative $\Delta S$ means storage fell and supplied part of the outgoing water.

Worked annual budget

A forested catchment receives $1100\ \mathrm{mm}$ of precipitation in one year. Actual evapotranspiration is $550\ \mathrm{mm}$, and soil plus groundwater storage ends the year $20\ \mathrm{mm}$ lower than it began. Therefore $\Delta S=-20\ \mathrm{mm}$ and

$$\begin{aligned} R&=P-ET-\Delta S\\ &=1100-550-(-20)\\ &=570\ \mathrm{mm}. \end{aligned}$$

The runoff coefficient for that year is

$$C_R=\frac{R}{P}=\frac{570}{1100}\approx0.52.$$

About 52% of the precipitation left as streamflow. The extra $20\ \mathrm{mm}$ did not appear from nowhere: it came out of storage.

This budget says how much water moved, but not how it reached the stream. That requires tracing the pathways through vegetation, soil, and groundwater.

What happens to precipitation

Precipitation can follow several paths after it enters a catchment:

1. vegetation intercepts some of it;
2. water reaches the ground as throughfall or stemflow;
3. it infiltrates into the soil, remains in surface depressions, or moves as overland flow;
4. soil water is taken up by roots, moves laterally as throughflow, or percolates downward;
5. groundwater stores and slowly transfers water toward springs and streams;
6. evaporation and transpiration return water to the atmosphere;
7. the remaining water leaves the basin as streamflow.

These paths operate on different timescales. Overland flow can reach a channel in minutes. Soil and shallow subsurface flow may take hours or days. Deep groundwater can take years, centuries, or longer.

Vegetation changes almost every term

Ecohydrology studies the connections between water and living systems. Vegetation is not a passive cover placed over the water balance. Leaves, stems, roots, litter, and plant physiology change how much water reaches the soil, how quickly it moves, and how much returns to the atmosphere.

Interception, throughfall, and stemflow

Interception is precipitation caught on leaves, branches, bark, litter, or other surfaces before it reaches the mineral soil. Some intercepted water evaporates directly back to the atmosphere.

Water that drips through the canopy is throughfall. Water that runs along branches and down trunks is stemflow. Together they deliver the non-evaporated part of canopy precipitation to the ground, but not uniformly. Throughfall can concentrate beneath canopy gaps and drip points, while stemflow directs water near plant bases.

Interception depends on plant form, leaf area, storm size and intensity, wind, and whether the canopy was already wet. Representative rainfall interception values from the course examples are about 30% for spruce–fir forest, 10% for birch, 5% for grass, and 50% for moss, but the ranges are broad. They are examples, not constants for every site.

Evapotranspiration: demand versus actual loss

Evapotranspiration combines evaporation from soil, water, and wet surfaces with transpiration through plants.

Potential evapotranspiration ($PET$) is the amount that could evaporate and transpire under the existing atmospheric demand if water were freely available to a healthy, fully covering vegetation surface. It depends on net radiation, air temperature, humidity, day length, and wind.

Actual evapotranspiration ($AET$) is what the landscape actually supplies. When soil water is plentiful, $AET$ can approach $PET$. When the soil dries, water supply limits plant uptake and evaporation, so

$$AET=PET-D,$$

where $D$ is the water deficit. At the wilting point, plants can no longer extract enough water to sustain transpiration.

Evapotranspiration can be measured or estimated in several ways:

• an evaporation pan represents open-water evaporation, with corrections needed before applying it to a landscape;
• a lysimeter tracks the mass or water balance of a contained soil and vegetation block;
• eddy covariance measures turbulent exchanges of water vapour, carbon dioxide, and energy above an ecosystem;
• climate-based formulas estimate $PET$ from radiation, temperature, humidity, and wind.

The surface energy balance and water balance meet in latent heat flux. A wet surface can direct much of its available energy into $Q_E$, while a dry surface directs more into sensible heat $Q_H$. Under similar available energy, the course examples give daily $ET$ of about $4.1\ \mathrm{mm}$ over a lake, $3.5\ \mathrm{mm}$ over forest, $2.2\ \mathrm{mm}$ over grassland, and $1.8\ \mathrm{mm}$ over an urban surface. The urban surface has the highest Bowen ratio in that comparison because sensible heat dominates over latent heat.

Infiltration controls rapid runoff

Infiltration is the entry of water through the soil surface. Percolation is its continued downward movement through soil and permeable rock. The two are related but not interchangeable: water can infiltrate, remain in the soil, move laterally, or return through evapotranspiration without recharging deep groundwater.

Infiltration capacity depends on:

• soil texture and pore size;
• soil structure and connected macropores;
• compaction, crusting, and raindrop impact;
• vegetation, roots, and litter;
• antecedent soil moisture;
• frozen ground;
• rainfall intensity and duration.

Large connected pores in sandy or well-structured soil can transmit water quickly. Roots and soil animals create macropores, while litter protects the surface from sealing. Bare clay-rich or compacted ground generally accepts water more slowly. In a well-forested soil, infiltration rates around $1$–$2\ \mathrm{cm\,h^{-1}}$ are possible, so only intense rainfall may exceed capacity. That value is a representative course example, not a universal forest rate.

Two mechanisms create overland flow:

• Hortonian, infiltration-excess, or precipitation-excess overland flow occurs when rainfall intensity exceeds infiltration capacity;
• saturation-excess overland flow occurs when the soil is already saturated to the surface, so additional water cannot enter storage even if the soil is otherwise permeable.

These mechanisms can occur in different parts of the same basin during one storm. A compacted path may produce infiltration-excess runoff while a low riparian area produces saturation-excess runoff.

Soil water: drainage, storage, and plant access

Soil pores hold water with different strengths.

• Gravitational water occupies large pores after saturation and drains under gravity.
• Field capacity is the water content left after rapid gravitational drainage has mostly stopped.
• Capillary water is held in smaller pores and is generally the main source available to roots.
• Hygroscopic water is held tightly on particle surfaces and is not available to plants.
• The permanent wilting point is the water content below which plants cannot recover their turgor without new water.

A useful approximation is

$$\text{plant-available water} =\text{field capacity}-\text{wilting point}.$$

Texture affects both terms. Sand has large pores and drains quickly, so both field capacity and wilting point are low. Clay retains much more water, but also holds a large fraction too tightly for roots. Loam and silt loam often provide the largest difference between field capacity and wilting point.

The seasonal soil-water cycle

A midlatitude soil commonly passes through four stages:

1. surplus: precipitation exceeds atmospheric demand and storage is full, so extra water becomes runoff or recharge;
2. storage withdrawal: $PET>P$, but roots maintain $AET$ by using stored soil water;
3. deficit: soil water becomes too limited for $AET$ to meet $PET$;
4. recharge: precipitation again exceeds $AET$, but it first refills soil storage before creating a new surplus.

Climate changes the timing and size of these stages. Vancouver’s wet winters refill storage and create surplus, followed by a dry summer withdrawal and possible deficit. Phoenix has low precipitation and very high summer $PET$, so a large deficit dominates much of the year.

Groundwater connects hillslopes to streams

The water table separates the unsaturated zone above from the saturated zone below. It is rarely a flat underground surface. It usually follows a subdued version of topography, rising beneath uplands and approaching the ground in valleys.

A local low-permeability layer can hold water above the regional water table, creating a perched water table. Groundwater can emerge naturally where the water table intersects the land surface, producing a spring or wetland.

A stream can exchange water with its aquifer in either direction:

• in a gaining stream, the water table slopes toward and intersects the channel, so groundwater supplies baseflow;
• in a losing stream, the channel sits above the water table and stream water leaks into the subsurface.

The same stream can gain in one reach, lose in another, and switch seasonally as groundwater levels change.

From discharge to runoff depth

Stream discharge is a volume flow rate. At a cross-section,

$$Q=vA,$$

where $v$ is mean water velocity in $\mathrm{m\,s^{-1}}$ and $A$ is cross-sectional area in $\mathrm{m^2}$. Discharge is therefore measured in $\mathrm{m^3\,s^{-1}}$.

To compare streamflow with precipitation, convert the total discharge volume over a period to a depth over the basin:

$$R_{\mathrm{mm}}=\frac{V_Q}{A_b}\times1000,$$

where $V_Q$ is total discharge volume in $\mathrm{m^3}$ and $A_b$ is basin area in $\mathrm{m^2}$. If discharge is constant for a time interval $\Delta t$,

$$R_{\mathrm{mm}}=\frac{Q\Delta t}{A_b}\times1000.$$

The time integration matters. Dividing an instantaneous $Q$ by basin area gives a runoff rate, not a runoff depth.

Reading a hydrograph

A hydrograph plots discharge against time. A storm hydrograph usually contains:

• a rainfall input shown above the discharge curve;
• a rising limb as quick-flow pathways reach the channel;
• peak discharge;
• lag time between rainfall input and the flow peak;
• a recession limb as temporary stores drain;
• baseflow, the slower groundwater-supported part beneath the storm response.

The same $40\ \mathrm{mm}$ thunderstorm would produce different hydrographs in two equal-size catchments:

The same storm in two catchments

Swipe horizontally to compare both hydrographs.

High-intensity rain, steep slopes, short routing distances, bare or compacted ground, and impervious surfaces tend to make a hydrograph flashy. Vegetation, permeable soil, gentle slopes, wetlands, and groundwater storage tend to lower and delay the peak.

Check the mechanism

1. A basin receives $900\ \mathrm{mm}$ of precipitation, loses $500\ \mathrm{mm}$ through $ET$, and gains $50\ \mathrm{mm}$ of storage. What are runoff depth and the runoff coefficient?
2. Why can a clay soil hold more total water than a sandy soil but provide no more plant-available water?
3. Explain how one storm can produce both Hortonian flow on a road and saturation-excess flow beside a stream.
4. Sketch two hydrographs for the same storm before and after soil compaction. Label peak discharge, lag time, recession, and baseflow.
5. If a stream is gaining during spring but losing during late summer, what must have changed in the water-table relationship?

The goal is not to memorize a list of pathways. Start with the boundary and budget, then ask what controls the partition between fast runoff, temporary storage, evapotranspiration, and delayed groundwater flow.