C2 Drainage Basin Hydrology

INPUT: Precipitation

1. Affects the amount of water that circulates within the drainage basin and the proportion of water in the various hydrological processes

2. Determined by TAP and precipitation pattern 

3. Rainfall intensity of an individual rainfall event plays an important role in determining what happens to it when it reaches the Earth’s surface

   1. Higher the intensity of rainfall, shorter the lag time between precipitation reaching earth and entering rivers

INPUT: Snowmelt

1. Form of delayed precipitation, runoff from snowmelt

2. Climate change affects the timing and amount of snowmelt as input

STORAGE: Interception

• Interruption in arrival of precipitation at the ground surface

• Stored on leaves and tree trunks temporarily 

STORAGE: Soil moisture storage

1. Gravitational Water

When all soil pores are filled with water from rainfall, it becomes saturated. Under the action of gravity, water drains out of the larger pores and is replaced by air; when all has been drained, the soil is at field capacity.

2. Capillary Water

Water retained by small pores against the force of gravity, represents the majority of water available for plant uptake. Water remains in the soil due to cohesion and adhesion

3. Hygroscopic Water

Thin film of water held tightly around individual soil particles, unavailable for plants due to attraction between water and soil particles. When plants cannot withdraw the tightly held water, it is known as a permanent wilting point. 

STORAGE: Groundwater

• Groundwater stores are more stable and contribute to delayed stream baseflow

• Aquifers: body of rock that holds groundwater and provides a reservoir of water

STORAGE: Channel storage

water held by river channels

FLOW: Infiltration

• Process of water entry into the surface of a soil, plays a role in surface runoff, soil moisture storage, groundwater recharge, subsurface flows and evapotranspiration

Infiltration capacity: 

• Definition: maximum rate at which a particular soil under specific conditions can absorb water

• Decreases rapidly over time during a storm as pores in soil become progressively filled with water (soil moisture storage)

• Dependent on permeability

  • Porosity: measure of space between soil grains or rock pores

  • Perviousness: presences of cracks, joints and fissures

  • Soil texture depends on proportion of sand, clay and silt influences rate of infiltration 

FLOW: Percolation

• Rainwater that is not removed by throughflow or used by vegetation is pulled downwards by gravity

• Between ground surface and water table is zone of aeration (unsaturated zone)

• At a depth, when rock becomes filled with groundwater, forms a zone of saturation

FLOW: Baseflow

• Streamflow contribution from groundwater that sustains perennial rivers

• Important supplier of river discharge

FLOW: Throughflow

• As water infiltrates the soil, its ability to percolate decreases as it goes through compacted soil and layered rock

• Subsurface flow develops parallel to the ground surface and is known as throughflow

FLOW: Overland

When precipitation is unable to infiltrate into the ground

Infiltration excess overland flow

• When rainfall intensity is greater than infiltration capacity of the ground, excess water that cannot infiltrate will drain

• Can rapidly deliver water to stream channels in storms

Saturation overland flow

• When ALL pore spaces are full, soil is saturated and water table is at the surface

• Water cannot enter, and overland flow occurs

FLOW: Channel flow

Movement of water within the river channel (also known as river discharge)

Perennial rivers

Characteristics: water flows throughout the year, primary water source from groundwater and surface water

Where: humid tropical climates (Af, Am)

Intermittent

Characteristics: water flows for at least one month a year where runoff is produced

Where: Aw climates

Ephemeral

Characteristics: water flows after occasional storm, but usually dry

Where: Arid climates

OUTPUT: River discharge

loss of water from drainage basin

OUTPUT: Evapotranspiration

Rate is dependent on vegetation and climate

ACTET — actual amt of water removed, POTET — amount of water removed when there is no other interferences

Drainage basin equation

P = ET + Q ± ∆S

Humid climates: P > ET, hence river discharge is high and there is moisture surplus hence streams are full of water

Arid climates: ACTET < POTET as there is limited precipitation to meet evaporation demand. P < POTET, hence there is moisture deficit and streams are often dry