Drainage Basin SystemDrainage Basin System

1. Introduction 1. Introduction • The circulation of water between atmosph

ere, land and ocean is referred as the hydrological cycle

• The drainage basin (river basin or catchment) is the land-based part of the hydrological cycle and forms the fundamental unit of study

What is a drainage basin?What is a drainage basin?

• A drainage basin is an area of earth’s surface bounded by a watershed or stream divide

• Individual streams and their catchment areas joined together to form larger basins as the individual streams combines to form tributaries of the larger streams and rivers.

• The surface of the land is therefore covered by a nested hierarchy of drainage basins

The drainage basin as a systemThe drainage basin as a system

• It receives inputs and responds to these with a serious of outputs and therefore can be viewed as open system

• The watershed forms the boundary of the basin

• The inner working of the basin such as the slopes, the drainage network, and stream channels can be regarded as a separate sub-system

Input and output Input and output

• The main inputs are solar energy and precipitation

• The main outputs are evaporation, transpiration, runoff and erosional products

2. The hydrological cycle in a drainage basin2. The hydrological cycle in a drainage basin

• Precipitation falls as interception, throughfall, stemflow, drips on trees

• The moisture that passes through the vegetation subsystem next reaches the surface subsystem. It may be stored in puddles, ponds or lakes as surface storage and return to atmosphere as evaporation and transpiration

• It may also move by overland flow to stream channel or it may sink into the ground (infiltration) and enter the soil subsystem

• In the soil, water may flow laterally through the soil (throughflow), interflow and baseflow

3.Hydrological processes in a drainage basin3.Hydrological processes in a drainage basin

A. Precipitation :

1. Amount and seasonality of precipitation

2. Precipitation in different environments:

Manaus (瑪腦斯 )- tropical rainforest

Atbara (阿特巴垃 ,蘇丹 )- tropical desert

Yakutsk (雅庫次克 ,蘇聯 )- tundra

B. Interception B. Interception

• Rain under most conditions does not fall directly upon the ground but is retained by the branches and foliage of plants. This is termed interception

• It may also fall as secondary interception, stemflow and return to the atmosphere by evaporation from the plant leaves

• Amount of moisture retained on the vegetation during a storm varies according to

1. The character of the storm

2. The character of the vegetation

C. Evapotranspiration :

1. Evaporation

2. Transpiration

3. Evapotranspiration

4. Actual and potential evapotranspiration

5. The soil- water budget

Soil water budget in different environmentSoil water budget in different environment

1.The tropical rainforest region

~ Water need is uniformly high throughout the year, because air temperature are uniformly very warm with abundant sunlight

~ Soil water storage remains close to field capacity

The desert regionThe desert region

• The true desert is area have an annual precipitation of less than 250mm while large areas have less than 100mm

• The values of water need are always the larger • A soil-water deficit prevails throughout all

months of the year • Soil water storage is close to zero at all times

The tundra regionThe tundra region

• Evapotranspiration is effectively zero for 8-9 month period when soil moisture is frozen.

• Evapotranspiration rises sharply in summer,exceeding precipitation and giving rise to a moisture deficiency in July and August

• Water-logging is widespread in summer due to the presence of permafrost (permanently frozen subsoil)

D. InfiltrationD. Infiltration

• Litter and soil will take up moisture as retention storage

• If the rainfall intensity exceeds the soil’s infiltration capacity ,a thin water layer forms on the surface and run downslope,surface runoff/overland flow is initiated

The infiltration capacity of soil is controlled by:The infiltration capacity of soil is controlled by:

A. Time or level of saturation

B. Soil Texture

~ Porosity

~ Permeability

C. Presence or absence of a near-surface

impermeable subsoil or rock layer

D. Rainfall intensity

• E. Ground surface (rough or level)

• F. Presence of vegetation

• G. Human activities

E. Throughflow (interflow)E. Throughflow (interflow)

• 1. Soil texture• 2. Gradient • 3. Evaporation rate and density of root growth• 4. Vegetation, animal burrows and drying • 5. Obstructed by hardpans and so deflected laterally

F. Baseflow (groundwater flow)F. Baseflow (groundwater flow)

• Beneath the soil, some water may penetrate into the underlying permeable rock,percolating vertically under the force of gravity.This downward movement of water is know as percolation

• The degree of percolation depends on the primary permeability and secondary permeability or hydraulic conductivity

G. Overland flowG. Overland flow

The rate of runoff depends on 5 factors :

1. Duration of rainfall

2. Vegetation cover

3. Raindrops size/ intensity of rainfall

4. Soil porosity

5. Slope angle

H. Channel FlowH. Channel Flow

• Water falling within a drainage basin arrives at the basin floor and becomes concentrated into channel flow. The stream or river which eventually drains the water out of the basin is supplied with water from 4 sources:

1. Direct precipitation over the basin 2. Runoff from the slope on the two sides of the c

hannel 3. Throughflow 4. Groundwater flow

In a drainage basin ,there may be 3 types of

streams :

• Perennial streams

• Intermittent streams

• Ephemeral streams

Water Balance Water Balance

• P=Q+E ± ΔSP- Precipitation

E- Evapotranspiration

Q- Runoff

S- Storage

General importance of hydrological cycle to General importance of hydrological cycle to

landform developmentlandform development

1. Nutrients cycles,since all nutrients are usually in the dissolved state or else carried by moving water before they can be cycled.eg. Carbon cycle,nitrogen cycle

2. Development of plant communities and animal communities

3. Development of soils,which cannot take place without the presence of water

4. Weathering of rocks

5. Erosion and evolution of hill slopes

6. Processes of erosion,transportation and deposition of undissolved sediments and hence the generation of landforms on the earth surface

7. Contribution of the salinity of sea and ocean,which is due to the dissolved minerals carried seawards by river water

8. Development of general circulation of the atmosphere,and contribution to the various kinds of climates on the earth surface