The Basin Hydrologic Cycle

Inputs - precipitation (rainfall, snowfall, hail, rime, dew)

Outputs - Evapotranspiration, channel runoff (strreamflow at basin outlet) and leakage (loss to another basin - water transferred under ground from one basin to another)

Storage - Vegetation (roots, stems, leaves etc), surface water (lakes, glaciers, marshes etc), soil moisture (in pores in upper layers of soil), groundwater (in saturated zone at greater depth), channel water (water in the stream channel network)

Transfer

• Interception (trapped by vegetation)
• Stemflow (water down stems)
• Throughfall (water drip from leaves)
• Infiltration (downward movement into soil)
• Percolation (downward movement through soil)
• Channel Precipitation (precipitation directly onto stream)
• Overland flow (water flow over ground surface)
• Throughflow (movement of water just below the surface)
• Groundwater flow (slow movement of deep groundwater)
• Capillary Rise (upward movement of water in the soil)
• Recharge (replenishment of groundwater)

Quantifies input, output and storage components of a basin. 

Basin water balance = P+(E+R)+ change in S

P = Precipitation

E = Evapotranspiration

R = Runoff

S = Storage

1. Leakage is assumed to be negligible.

2. Change in storage may be positive or negative.

3. Over time, the change in storage is mainly change in groundwater storage.

(E+R) > P = negative change in storage

P > (E+R) = positive change in storage

Over several years, the change in storage is often very small.

Climate Change may affect the water balance

• Increased evapotranspiration through warming so change in S is negative.
• Increased precipitation so change is S is positive.

Human Interference may also affect the water balance. 

• Urbanisation
• Afforestation/Deforestation
• Water Storage (Resevoirs)
• Soil Drains
• Vegetation Burning
• Tillage

Primary Process: Condensation of water vapour as a result of air cooling.

Temperature at which condensation begins is the saturation point or dew point. It can no longer take any more water vapour.

Relative Humidity (RH) at a given temperature:

Actual Vapour pressure x100 / Maximum Potential Vapour Pressure

Condensation is produced by adiabatic cooling.

• Adiabatic Cooling: Cooling by the rising and expansion of a body of air.
• Adiabatic Lapse Rate: Rate of cooling with altitude.

If RH < 100%: dry adiabatic lapse rate (DALR) = 9.8 degrees/1000m

If RH = 100%: latent heat is released during condensation, giving a saturated adiabatic lapse rate (SALR) where: SALR < DALR

• Below condensation level = RH <100%: DALR applies
• Above condensation level = RH = 100%: SALR applies

Dust particles act as condensation nuclei, forming cloud droplets (0.001-0.1mm) in diameter.

Form in 2 ways:

1. The Bergeron-Findeisen Process:

• < 0 degrees, ice crystals grow at the expense of water droplets.
• They fall through the clouds to form snowflakes
• Snowflakes melt to form raindrops.

2. Capture Process

• Collision and coalescence (merging of droplets during contact) of rising cloud droplets.
• They are held together by surface tension within the water.
• Small droplets become large until they reach a mass that cannot be supported and they fall as raindrops.
• Fast updraft allows larger drops to form before falling.

Causes of rising air

• 1. Direct heating of lower atmosphere - convectional precipitation
• 2. Forcing up of air at weather fronts - cyclonic precipitation
• 3. Forcing up of air at mountains - orographic precipitation

Convectional Precipitation

Environmental lapse rate (ELR): drop in air temperature with increasing altitude; averages 6.4 degrees / 1000m - static air not rising.

ELR varies greatly and may even be negative (temperature inversion).

• For example, there may be colder air in the valley than the mountain. 
• Cold air forms in the mountain and is denser than warm air 
• Slides off mountain into valley
• Colder air at bottom than top

As SALR < DALR, there are 3 possibilities:

• 1. SALR > ELR = absolute stability.
• 2. SALR < ELR < DALR = conditional instability (most conventional precipitation).
• 3. DALR < ELR = absolute instability. 

Consequences of instability:

• Strong updrafts.
• Strong downdrafts.
• Torrential downpour.

Factors favouring convectional precipitation

• Intense heating of moist air
• Steep ELR
• Gentle winds

Cyclonic (frontal) Precipitation

Results from horizontal convergence of warm and cold airmasses: precipitation results from air being forced to rise at fronts.

• 1. Warm Front 
• Warm air pushed up over cold air.
• Warm air is less dense.
• Prolonged light to moderate precipitation.
• 2. Cold Front 
• Oncoming wedge of cold air rises over warm air.
• Much steeper than warm front.
• Rapid rise of air. 
• 3. Occluded Front 
• Warm air moving upwards at advancing cold front.
• Cold front collides with warm front.
• Brief intense showers. 

Orographic (relief) Precipitation

When layer of moist air is forced to rise (and cool) over a mountain barrier.

As the air rises and cools, orographic clouds form and serve as the source of the precipitation, most of which falls upwind of the mountain ridge. 

On the lee side of the mountain, rainfall is usually low, and the area is said to be in a rain shadow.

Very heavy precipitation upwind of a prominent mountain range that is orientated across a prevailing wind from a warm ocean. 

Orographic effects often act in combination with cyclonic effects: the three can occur together.

Conclusions

• Primary process: condensation.
• Rapid cooling when RH < 100%
• Slow cooling when RH = 100%
• Cloud droplets form ice crystals or raindrops by capture and collision.
• Adiabatically.