(a) Drainage basin water balance
– input: precipitation
– pathways: overland flow, infiltration, percolation, throughflow and baseflow
– water stores: interception and biological water storage, soil water storage, groundwater storage and channel storage
– output: evapotranspiration and river discharge
(b) Fluvial and runoff processes
– fluvial processes: erosion, transportation and deposition
– storm runoff generation: infiltration excess and saturation overland flow
(c) Variations in river channel morphology
– factors affecting channel morphology: discharge and sediment regime
– drainage density and channel patterns – river equilibrium and longitudinal profile
Drainage Basin Water Balance
The drainage basin water balance refers to the relationship between input, outputs, and storages in a river basin, governed by the principle of conservation of water, as the input matches the output.
Drainage basin water balance varies from climatic region to another and across time.
INPUT: Precipitation
Precipitation into the catchment system takes the form of water, snow, and ice. The quantity and intensity of precipitation is determined by climatic characteristics of each region and varies within the region. The quantity and intensity affects the presence and conditions of subsequent differing flows and storages in the cycle.
PATHWAY: Overland Flow
Two types of overland flow, namely infiltration excess flow and saturation excess flow.
Infiltration excess flow is generated when rainfall intensity exceeds rainfall capacity
-Infiltration rate = Infiltration capacity
-Excess water accumulates on soil surface, occupying small irregularities, forming depression storage.
-Water overflows to form a continuous sheet of water moving downslope, known as surface detention.
-Depth and velocity of water increases downslope.
Saturation excess flow is generated when downward movement of water is impeded. This results in saturation and the lateral movement of water takes place.
-When rain falls directly on saturated ground, it cannot be absorbed, surface water accumulates and overland flow begins.
-Down the slope, water increases due to migration of water from upslope by throughflow, forming returnflow.
PATHWAY: Infiltration
Infiltration is the seeping or the entry of water into the soil through interconnected pores in the soil. This is driven by gravity and capillary action. This is possible because soil consists of minerals and organic particles that are separated from each other by small spaces known as pores.
Infiltration rate is the rate at which water seeps into the soil.
Infiltration capacity is the maximum rate at which the soil can absorb water.
Due to its nature, infiltration is affected by several factors, namely Soil Texture, Vegetation, and Preciptation Characteristics.
Soil Texture influences infiltration capacity, and is determined by the proportions of particles within the soil. If soil contains mainly coarser materials (gravel and sand), the pore spaces are larger and soil is more porous, increasing infiltration capacity. If soil contains mainly finer materials (i.e. silt or clay), pore spaces are smaller and soil is less permeable, resulting in lower infiltration capacity.
Vegetation influences both infiltration rate and capacity, in mainly 3 ways.
Firstly, plant and soil fauna sustaining on organic matter provided by decaying vegetation churn through mineral particles and organic material. The resultant plant roots, animal burrows, and worm holes provide ready-made passages, thus increasing infiltration.
Secondly, organic material from vegetation in soil binds particles together in a crumb like structure, creating larger pores, increasing infiltration. In comparison, change in land use from forested to agriculture leads to compaction of soil by tractors or trampling by humans and cattle, giving rise to platy structure which impedes infiltration.
Thirdly, soil surface is protected from packing by raindrops. Direct rainfall leads to rainsplash action, whereby force of falling drops displaces particles, which get lodged into pores and seal soil, producing a surface crust that impedes infiltration. If vegetation is present, interception absorbs the wforce, decreasing the velocity and impact of droplets hitting ground. This prevents breaking up of soil aggregates, enhancing infiltration.
Preciptation Characteristic influences
PATHWAY: Percolation
As water leaves soil moisture storage, it exits by throughflow or percolation. Percolation refers to the vertical movement of water downward through soil followed by rock towards the water table. The water table is a boundary between saturated and unsaturated rock.
Percolation rate is mainly determined by permeability of rock it is passing through. It can be divided into primary and secondary permeability.
Primary permeability relates to the passage of water through pore spaces in the rock.
Secondary permeability relates to the passage of water through fractures like joints and bedding planes in the rock.
PATHWAY: Throughflow
Throughflow refers to the lateral, downslope flow of water through the soil, generated when there is a decrease in permeability due to increasing soil depth, caused by difference in soil compaction (or soil of less density), difference in material composition, and bedrock. In these cases, rate at which water enters soil surface is greater than rate at which water moves through layers.
Throughflow may join channel under surface, or seep back onto surface as return flow, depending on height of water table.
PATHWAY: Channel Flow
The 4 components contributing to channel flow include channel precipitation, overland flow, through flow and baseflow
There are 3 types of channel flow.
Firstly, perennial channels are occupied by flowing water throughout the year. Such channels are most common in humid tropics, where precipitation is abundant and little seasonal variation. This leads to high water table and baseflow is constant throughout year.
Secondly, ephemeral channels are mostrly dry, only becoming occupied after a storm. These normally occur in arid regions where preciptation is low. The lack of preciptation means that water table is far below the surface and disconnected from channel flow. This is known as influent condition, leading to ephemeral stream. Channel flow is temporary and only occurs immediately after rain. Water is lost rapidly to evapotranspiration and infiltration, leading to a decrease in discharge downstream.
Thirdly, intermittent channels are seasonally occupied by water. These channels are found in areas with strong seasonal contrasts such as the wet and dry seasons in the monsoon regions. During dry summer season, water table lies below stream beds, rises to intersect only during wet winter season. Streams are only fed by baseflow/groundwater during wet winter season during which they experience effluent condition. During summer season, seepage and influent condition occurs and streams are thus called intermittent streams.
STORAGE: Interception
Interception storage refers to the water collected by the vegetation canopy and litter layers. Water not intercepted reaches ground through throughfall.
Interception reduces the amount of water reaching the ground, and the component of water in the interception storage is known as interception loss when it evaporates and is returned to the atmosphere.
Interception storage is highly variable and dependent on factors like precipitation and vegetation type.
Low intensity rainfall over longer duration allows for saturation of interception storage. In comparison, high intensity rainfall over short duration leads to less interception storage as water stored on vegetation surface initially is displaced by force of subsequent raindrops.
Vegetation cover influences amount of water intercepted. Grasses and crops have been measured to remove 10-20% precipitation, while forest canopies measured to remove up to 50% precipitation. In areas where vegetation cover (density, type) changes over the year, interception storage also varies. In urban areas where vegetation cover is minimal, interpcetion is carried out by buildings and urban infrastructure. Therefore, changes in land use lead to changes in amount of interception storage and interception loss.
STORAGE: Surface
STORAGE: Soil Moisture
Three scenarios: Wilting point, Field Capacity, and Saturation
The condition of soil is dependent on patterns of precipitation in each climate, which differs with space and time.
At saturation capacity, which is usually immediately after a storm, saturation moisture content is equal to porosity of soil. However, surface tension is unable to keep holding all the water present in the soil, thus gravity drains water downwards.
The soil is now at field capacity, where capillary water is held in thin films around each particle.
Over a period of time, more water is lost to atmosphere through evapotranspiration as well as to plant roots. If preciptation does not occur, wilting point is reached, where no more water can be removed. Only hygroscopic water remains.
STORAGE: Groundwater
Groundwater refers to water below the water table. The soil is saturated only with water.
The amount of groundwater and the height of water table is affected by two main factors.
Firstly, amount of groundwater varies with changes in surface topography. If surface is flat, water table is relatively flat. If there are changes in relief, water table changes with the relief. The water table lies at a greater depth beneath surface of hills compared to beneath surface of valleys. This is due to effect of gravity pulling water down from hill to valley, known as process of throughflow.
At points where water table intersects the surface, channel flow occurs. This is known as baseflow.
Secondly, seasonal fluctuations in precipitation affect amount of groundwater. In humid tropics, water table height shallow due to high amounts of precipitation and remains roughly same throughout year due to little seasonal variation. In seasonally humid tropics, water table may drop below level of channel during dry season, creating intermittent channels.
There is fossil groundwater within arid tropics that is not replenished at sustainable rate. For example, major aquifers can be found at considerable depths in central Saudi Arabia. These were formed from past climates. The continuous extraction for human use in such areas leads to lowering of water table/ For example, irrigation in Australia
STORAGE: Channel
OUTPUT: Evapotranspiration
Evaporation is the physical process of molecular transfer whereby a liquid is changed into a gas. This process takes place from preciptation that has accumulated on ground surface, soil and from interception storage. Transpiration is a biological process whereby water is lost from a plant through minute pores in its leaves. This occurs when soil water absorbed by roots and translocated up the plant system exits the plant.
Potential evapotranspiration refers to evapotranspiration losses when soil moisture is maintained at field capacity. It represents maximum rate at which evapotranspiration can take place when water is continually available in the soil.
Actual evapotranspiration refers to measured rates of evapotranspiration.
Evapotranspiration is dependent on mainly 3 factors - temperature, relative humidity, and type of vegetation cover.
OUTPUT: River Discharge
Additional Material:
Detailed Document for revision (39 pages):
https://www.google.com.sg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwiS-MLwk-_SAhVIK48KHfGDDNYQFggkMAI&url=http%3A%2F%2Frocky.ess.washington.edu%2Fgrg%2Fcourses10_11%2Fess326%2Ftextbook%2FChapter_1_Revised.doc&usg=AFQjCNG1oVfi8FqQYuUPvJb2FodlCeiQQA&sig2=xpY0dWGC_K_pdWhTTemPNg&bvm=bv.150475504,d.c2I
Precipitation into the catchment system takes the form of water, snow, and ice. The quantity and intensity of precipitation is determined by climatic characteristics of each region and varies within the region. The quantity and intensity affects the presence and conditions of subsequent differing flows and storages in the cycle.
PATHWAY: Overland Flow
Two types of overland flow, namely infiltration excess flow and saturation excess flow.
Infiltration excess flow is generated when rainfall intensity exceeds rainfall capacity
-Infiltration rate = Infiltration capacity
-Excess water accumulates on soil surface, occupying small irregularities, forming depression storage.
-Water overflows to form a continuous sheet of water moving downslope, known as surface detention.
-Depth and velocity of water increases downslope.
Saturation excess flow is generated when downward movement of water is impeded. This results in saturation and the lateral movement of water takes place.
-When rain falls directly on saturated ground, it cannot be absorbed, surface water accumulates and overland flow begins.
-Down the slope, water increases due to migration of water from upslope by throughflow, forming returnflow.
PATHWAY: Infiltration
Infiltration is the seeping or the entry of water into the soil through interconnected pores in the soil. This is driven by gravity and capillary action. This is possible because soil consists of minerals and organic particles that are separated from each other by small spaces known as pores.
Infiltration rate is the rate at which water seeps into the soil.
Infiltration capacity is the maximum rate at which the soil can absorb water.
Due to its nature, infiltration is affected by several factors, namely Soil Texture, Vegetation, and Preciptation Characteristics.
Soil Texture influences infiltration capacity, and is determined by the proportions of particles within the soil. If soil contains mainly coarser materials (gravel and sand), the pore spaces are larger and soil is more porous, increasing infiltration capacity. If soil contains mainly finer materials (i.e. silt or clay), pore spaces are smaller and soil is less permeable, resulting in lower infiltration capacity.
Vegetation influences both infiltration rate and capacity, in mainly 3 ways.
Firstly, plant and soil fauna sustaining on organic matter provided by decaying vegetation churn through mineral particles and organic material. The resultant plant roots, animal burrows, and worm holes provide ready-made passages, thus increasing infiltration.
Secondly, organic material from vegetation in soil binds particles together in a crumb like structure, creating larger pores, increasing infiltration. In comparison, change in land use from forested to agriculture leads to compaction of soil by tractors or trampling by humans and cattle, giving rise to platy structure which impedes infiltration.
Thirdly, soil surface is protected from packing by raindrops. Direct rainfall leads to rainsplash action, whereby force of falling drops displaces particles, which get lodged into pores and seal soil, producing a surface crust that impedes infiltration. If vegetation is present, interception absorbs the wforce, decreasing the velocity and impact of droplets hitting ground. This prevents breaking up of soil aggregates, enhancing infiltration.
Preciptation Characteristic influences
PATHWAY: Percolation
As water leaves soil moisture storage, it exits by throughflow or percolation. Percolation refers to the vertical movement of water downward through soil followed by rock towards the water table. The water table is a boundary between saturated and unsaturated rock.
Percolation rate is mainly determined by permeability of rock it is passing through. It can be divided into primary and secondary permeability.
Primary permeability relates to the passage of water through pore spaces in the rock.
Secondary permeability relates to the passage of water through fractures like joints and bedding planes in the rock.
PATHWAY: Throughflow
Throughflow refers to the lateral, downslope flow of water through the soil, generated when there is a decrease in permeability due to increasing soil depth, caused by difference in soil compaction (or soil of less density), difference in material composition, and bedrock. In these cases, rate at which water enters soil surface is greater than rate at which water moves through layers.
Throughflow may join channel under surface, or seep back onto surface as return flow, depending on height of water table.
PATHWAY: Channel Flow
The 4 components contributing to channel flow include channel precipitation, overland flow, through flow and baseflow
There are 3 types of channel flow.
Firstly, perennial channels are occupied by flowing water throughout the year. Such channels are most common in humid tropics, where precipitation is abundant and little seasonal variation. This leads to high water table and baseflow is constant throughout year.
Secondly, ephemeral channels are mostrly dry, only becoming occupied after a storm. These normally occur in arid regions where preciptation is low. The lack of preciptation means that water table is far below the surface and disconnected from channel flow. This is known as influent condition, leading to ephemeral stream. Channel flow is temporary and only occurs immediately after rain. Water is lost rapidly to evapotranspiration and infiltration, leading to a decrease in discharge downstream.
Thirdly, intermittent channels are seasonally occupied by water. These channels are found in areas with strong seasonal contrasts such as the wet and dry seasons in the monsoon regions. During dry summer season, water table lies below stream beds, rises to intersect only during wet winter season. Streams are only fed by baseflow/groundwater during wet winter season during which they experience effluent condition. During summer season, seepage and influent condition occurs and streams are thus called intermittent streams.
STORAGE: Interception
Interception storage refers to the water collected by the vegetation canopy and litter layers. Water not intercepted reaches ground through throughfall.
Interception reduces the amount of water reaching the ground, and the component of water in the interception storage is known as interception loss when it evaporates and is returned to the atmosphere.
Interception storage is highly variable and dependent on factors like precipitation and vegetation type.
Low intensity rainfall over longer duration allows for saturation of interception storage. In comparison, high intensity rainfall over short duration leads to less interception storage as water stored on vegetation surface initially is displaced by force of subsequent raindrops.
Vegetation cover influences amount of water intercepted. Grasses and crops have been measured to remove 10-20% precipitation, while forest canopies measured to remove up to 50% precipitation. In areas where vegetation cover (density, type) changes over the year, interception storage also varies. In urban areas where vegetation cover is minimal, interpcetion is carried out by buildings and urban infrastructure. Therefore, changes in land use lead to changes in amount of interception storage and interception loss.
STORAGE: Surface
STORAGE: Soil Moisture
Three scenarios: Wilting point, Field Capacity, and Saturation
The condition of soil is dependent on patterns of precipitation in each climate, which differs with space and time.
At saturation capacity, which is usually immediately after a storm, saturation moisture content is equal to porosity of soil. However, surface tension is unable to keep holding all the water present in the soil, thus gravity drains water downwards.
The soil is now at field capacity, where capillary water is held in thin films around each particle.
Over a period of time, more water is lost to atmosphere through evapotranspiration as well as to plant roots. If preciptation does not occur, wilting point is reached, where no more water can be removed. Only hygroscopic water remains.
STORAGE: Groundwater
Groundwater refers to water below the water table. The soil is saturated only with water.
The amount of groundwater and the height of water table is affected by two main factors.
Firstly, amount of groundwater varies with changes in surface topography. If surface is flat, water table is relatively flat. If there are changes in relief, water table changes with the relief. The water table lies at a greater depth beneath surface of hills compared to beneath surface of valleys. This is due to effect of gravity pulling water down from hill to valley, known as process of throughflow.
At points where water table intersects the surface, channel flow occurs. This is known as baseflow.
Secondly, seasonal fluctuations in precipitation affect amount of groundwater. In humid tropics, water table height shallow due to high amounts of precipitation and remains roughly same throughout year due to little seasonal variation. In seasonally humid tropics, water table may drop below level of channel during dry season, creating intermittent channels.
There is fossil groundwater within arid tropics that is not replenished at sustainable rate. For example, major aquifers can be found at considerable depths in central Saudi Arabia. These were formed from past climates. The continuous extraction for human use in such areas leads to lowering of water table/ For example, irrigation in Australia
STORAGE: Channel
OUTPUT: Evapotranspiration
Evaporation is the physical process of molecular transfer whereby a liquid is changed into a gas. This process takes place from preciptation that has accumulated on ground surface, soil and from interception storage. Transpiration is a biological process whereby water is lost from a plant through minute pores in its leaves. This occurs when soil water absorbed by roots and translocated up the plant system exits the plant.
Potential evapotranspiration refers to evapotranspiration losses when soil moisture is maintained at field capacity. It represents maximum rate at which evapotranspiration can take place when water is continually available in the soil.
Actual evapotranspiration refers to measured rates of evapotranspiration.
Evapotranspiration is dependent on mainly 3 factors - temperature, relative humidity, and type of vegetation cover.
OUTPUT: River Discharge
Additional Material:
Detailed Document for revision (39 pages):
https://www.google.com.sg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwiS-MLwk-_SAhVIK48KHfGDDNYQFggkMAI&url=http%3A%2F%2Frocky.ess.washington.edu%2Fgrg%2Fcourses10_11%2Fess326%2Ftextbook%2FChapter_1_Revised.doc&usg=AFQjCNG1oVfi8FqQYuUPvJb2FodlCeiQQA&sig2=xpY0dWGC_K_pdWhTTemPNg&bvm=bv.150475504,d.c2I
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