6. Drainage basins and runoff mechanisms Drainage basins Drainage basins The vegetation factor The vegetation factor Sources of runoff Sources of runoff Groundwater flow Groundwater flow

(a) Drainage basins (watersheds) Drainage basin (or watershed) is the area drained by a given network of stream channels (i.e. a given river). Drainage basin (or watershed) is the area drained by a given network of stream channels (i.e. a given river). Drainage divide is the “line” that separates one drainage basin from another. Drainage divide is the “line” that separates one drainage basin from another. Drainage network is the network of stream channels draining a particular area. Drainage network is the network of stream channels draining a particular area.

Fig 5:

Drainage network initiation

Stream order The size of the drainage basin or drainage network can be determined by its “order”. The size of the drainage basin or drainage network can be determined by its “order”. Headwater segments are called first order streams. When two 1 st order channels join, they form a 2 nd order stream, etc… Headwater segments are called first order streams. When two 1 st order channels join, they form a 2 nd order stream, etc…

Stream order

Drainage network composition As stream order increases, corresponding drainage area also increases (as well as stream length and channel size). As stream order increases, corresponding drainage area also increases (as well as stream length and channel size). N i-1 /N i = R b ≈ constant N i-1 /N i = R b ≈ constant e.g. (3 < R b < 4) e.g. (3 < R b < 4) where N i is the number of stream segments of a given order i and R b is called “bifurcation ratio”. where N i is the number of stream segments of a given order i and R b is called “bifurcation ratio”. L i /L i-1 = R L where L i is average length of streams of order I and average R L = 3.5 L i /L i-1 = R L where L i is average length of streams of order I and average R L = 3.5

Drainage density (D d ) D d = L/A D d = L/A where L is total length of all stream segments and A is drainage area where L is total length of all stream segments and A is drainage area D d can vary from D d can vary from < 1.0 km/km 2 to >100 Km/km 2 (in general, resistant surface materials or those with high infiltration capacities have widely spaced streams and, consequently, low drainage densities)

Fig 5:

Fig 10:

Streams in southern Ontario Most streams in southern Ontario have a drainage area between 100 and 1000 km 2. Most streams in southern Ontario have a drainage area between 100 and 1000 km 2. Near the shore of lake Ontario, usually between order 4 and 6. Near the shore of lake Ontario, usually between order 4 and 6. All originate from Oak Ridges Moraine (ORM). All originate from Oak Ridges Moraine (ORM). ORM is zone of ground water recharge (aquifer). ORM is zone of ground water recharge (aquifer).. Environmental issues associated with ORM, groundwater resources and stream water quality.

(b) Effects of vegetation - Interception loss… Interception loss: capacity of the plants to store water and lose it by evaporation Interception loss: capacity of the plants to store water and lose it by evaporation Rainfall through canopy divided into: Rainfall through canopy divided into: Throughfall (drip off the leaves of intercepting trees)Throughfall (drip off the leaves of intercepting trees) Stemflow (when water runs down the trunk)Stemflow (when water runs down the trunk) Factors to take into account: Factors to take into account: Type of vegetation and densityType of vegetation and density Intensity of rain as well as duration and frequencyIntensity of rain as well as duration and frequency windwind

Infiltration Process by which water enters the surface horizon of the soil Process by which water enters the surface horizon of the soil Controlled by: type of precipitation, soil properties, soil porosity, slope, vegetation, soil compaction, and pre- existing soil moisture Controlled by: type of precipitation, soil properties, soil porosity, slope, vegetation, soil compaction, and pre- existing soil moisture Excess water: surface storage and overland flow. Excess water: surface storage and overland flow.

Runoff generation Flow paths of water moving downhill: Flow paths of water moving downhill: (see diagram - p.6)

(c) Sources of runoff Horton Overland Flow (path # 1): movement of thin layer of water over the land surface (depth of overland flow generally less than 1 cm). When does it occur? Horton Overland Flow (path # 1): movement of thin layer of water over the land surface (depth of overland flow generally less than 1 cm). When does it occur? Uncommon in humid – temperate climates and well-vegetated areas. Uncommon in humid – temperate climates and well-vegetated areas. Also referred to as “infiltration- excess overland”. Also referred to as “infiltration- excess overland”.

Interflow (subsurface flow; path # 3): when water beneath the surface flows laterally downslope. Interflow (subsurface flow; path # 3): when water beneath the surface flows laterally downslope. Subsurface stormflow occurs when a layer of low permeability is found at some depth into the soil. Subsurface stormflow occurs when a layer of low permeability is found at some depth into the soil. May also refer to macropores (“pipe flow”). May also refer to macropores (“pipe flow”).

Saturation overland flow (path # 4): combination of direct precipitation onto saturated areas (at the base of hillslopes) and return flow Saturation overland flow (path # 4): combination of direct precipitation onto saturated areas (at the base of hillslopes) and return flow Occurs at foot of hillslope, areas of thin soil (where moisture storage is reduced) and in hillslope hollows, where there is accumulation of soil water Occurs at foot of hillslope, areas of thin soil (where moisture storage is reduced) and in hillslope hollows, where there is accumulation of soil water Return flow: subsurface flow reaching the channel as overland flow (at a point of saturation) Return flow: subsurface flow reaching the channel as overland flow (at a point of saturation)

Partial area: refers to those saturated areas contributing to storm flows; saturated areas expand and contract during and after storm events (variable contributing areas). Partial area: refers to those saturated areas contributing to storm flows; saturated areas expand and contract during and after storm events (variable contributing areas). Stream response and runoff mechanisms: (overhead) Stream response and runoff mechanisms: (overhead)

Controls of storm runoff… Climate, soil type and characteristics and bedrock lithology control which type of runoff mechanism(s) will dominate in a particular watershed. Climate, soil type and characteristics and bedrock lithology control which type of runoff mechanism(s) will dominate in a particular watershed. Vegetation cover and topography important secondary controls (at the scale of hillslopes). Vegetation cover and topography important secondary controls (at the scale of hillslopes).

(d) Groundwater Flow… Groundwater flow direction and flow rate; Groundwater flow direction and flow rate; Groundwater movement is determined for the most part by what is called “Potential head” (H p ) Potential head: total potential energy of a fluid divided by its weight H p = z + y where z: elevation head (vertical distance above datum) where z: elevation head (vertical distance above datum) and y: pressure head (distance below water table) and y: pressure head (distance below water table)

Darcy’s Law – groundwater flow Darcy’s law of groundwater flow can be expressed as follows: Darcy’s law of groundwater flow can be expressed as follows: U = k (H 1 –H 2 )/L U: velocity K: constant that is a function of permeability; hydraulic conductivity H 1, 2 : Potential (head) at point 1 and 2 L: distance between two points

Groundwater flow… Flow in groundwater zones occurs in response to a gradient of potential energy Flow in groundwater zones occurs in response to a gradient of potential energy At any point, flow direction is parallel to the potential energy gradient (potential head) and flow rate is: At any point, flow direction is parallel to the potential energy gradient (potential head) and flow rate is: Directly proportional to magnitude of gradientDirectly proportional to magnitude of gradient Inversely proportional to magnitude of resisting forcesInversely proportional to magnitude of resisting forces

“Flow lines” and groundwater… Flow lines in groundwater zone are abstract representations of general flow directions. Flow lines in groundwater zone are abstract representations of general flow directions. Perpendicular to lines of equal potential energy (equipotential lines) Perpendicular to lines of equal potential energy (equipotential lines) Velocity and discharge of groundwater directly proportional to loss of potential that occurs as water flows from one point to another. Velocity and discharge of groundwater directly proportional to loss of potential that occurs as water flows from one point to another.

Darcy’s Law – groundwater flow Darcy’s law of groundwater flow can be expressed as follows: Darcy’s law of groundwater flow can be expressed as follows: U = k (H 1 –H 2 )/L U: velocity K: constant that is a function of permeability; hydraulic conductivity H 1, 2 : Potential (head) at point 1 and 2 L: distance between two points