Drainage Basins and Rivers Flashcards
Drainage basin
an area of land drained by a river and its tributaries
Watershed
The boundary marked by a ridge of highland beyond which any precipitation will drain into adjacent basins
Evaporation
Physical process by which moisture is lost directly into the atmosphere from water surfaces, due to the effects of air movement and the suns heat
Transpiration
The biological process by which water is last froma plant throught the minute pores in the leaves
Potential evapotranspiration
In theory what could happen based off the different factors
Interception storage
First raindrops of a rainfall event fall on vegetation which shelters the underlying ground
Throughfall
Water reaches the ground by dropping off the leaves
Stemflow
Water reaches the ground by flowing down the trunk
Secondary interception
Water reaches the ground by hitting leaves/other and then hitting the undergrowth
Surface storage
Due to dryness, when the ground can’t absorb the water and so the water that lies on the surface
Surface runoff
When the ground is so saturated that it can’t absorb anymore water, so excess water flows over the surface
Infiltration capacity
The maximum rate which water can pass through the soil. The more pores the higher the infiltration capacity
Porosity
The space between soil particles (air pockets)
Capillary action
System by which water is drawn up to the surface through small air pockets in the soil
Groundwater storage
Water collects above an impermeable rock layer under the earths surface (created by percolation)
Zone of saturation
The area above the permeable rock which is filled with water
Positive water balance
Precipitation input exceeds evapotranspiration loss
Negative water balance
When evapotranspiration exceeds precipitation
Soil moisture budget
Balance of water in the soil, as a result of precipitation and evapotranspiration
Soil moisture surplus
Precipitation exceeds transpiration resulting in considerable surface runoff and rise in water levels
Field capacity
The maximum amount of saturation the soil can hold
Soil moisture deficit
Plants can only survive at this point if they are either drought resistant or have proper irrigation due to the lack of water in the soil
Recharge
When precipitation does exceed potential transpiration after the summer, rain is needed to replace and recharge that taken from the soil
Channel precipitation
Rain falls directly into the channel in the river
Channel storage
When water reaches the river it becomes channel storage
Groundwater flow/baseflow
Water underground that flows laterally against the earths impermeable surface
Watertable
Upper boundary of the saturated material that can then be transffered laterally as groundwater
Water balance formula
P=Q+E
P - precipitation measured using rain gauges
Q - runoff measured by discharge fumes in the river channel
E - evapotranspiration (far more difficult to measure)
Aquifers
Underground layer of water bearing permeable rock, rock fractures of unconsolidated materials from which ground water can be extracted using a water well
Hydrograph
Means of showing the discharge of a river at a given point of a short period of time
Hydrology
The study of water (precipitation, runoff, evaporation etc
Discharge
The amount of water, originating as precipitation which reaches the channel by surface runoff, throughflow and baseflow
Discharge formula
Q=AxV
Q - discharge (measured in m)
A - Cross sectional area of the river (measured in m2)
V - Velocity (m3/sec or cumecs)
Approach segment
Shows the discharge of the river before the storm (antecedent flow rate)
Rising limb
On the hydrograph, the rapid increase in discharge os indicated by the beginning line
Lag time
The period between maximum precipitation and peak discharge
Several factors that regulate ways a river responds to precipitation
- Types of precipitation
- Temperature
- Tides and storm surge
- Basin shape/size/relief
- Landuse
- Rock type
- Soil type
- drainage density
Falling or recession limb
Is the segment of the graph where discharge is decreasing and river levels are falling. This segment is usually less steep than the rising limb because throughflow is being released relatively slowly into the channel
Storm flow
The discharge, both surface and subsurface attributed to a single storm
Baseflow
Very slow to respond to a storm, but by releasing groundwater, it maintains the rivers flow throughout periods of low precipitation
Bankfull discharge
Occurs when a rivers water level reaches the top of its channel and any further increase in discharge will result in the flooding of the surrounding land
Types of precipitation (River responds)
Prolonged - When flooding most frequently occurs after. Ground has maximum saturation and surface runoff
Intense storms - Rain intensity may be greater than the infiltration capacity of the snow. Rapid rise in rain levels due to lots of surface runoff (flash floods)
Snowfall - Water is held in storage and rain levels drop. After it warms up ground may stay frozen for a while, surface runoff increases
Basin size, shape and relief (River responds)
Size - small basin means that rainfall will reach the main channel more quickly. Lag time is greater in larger basins
Shape - Circular basins are more likely to have a shorter lag time and higher peak flow than an elongated basin
Relief - Steep-sided upland valleys, water is likely to reach the river more quickly than in a gently sloping lowland areas
Temperature (River responds)
Extremes of temperature can restrict infiltration and therefore increase surface area runoff. If evapotranspiration rates are high then there will be less water available to flow into the main river
Landuse (River responds)
Vegetation can help prevent flooding by intercepting rainfall. Also can reduce through flow by taking up water from the soil.
Urbanization can increase flooding because water can’t infiltrate through the tarmac
Rock type (River responds)
Permeable rock (porous or pervious) permit rapid infiltration which means little surface runoff and limited number of surface streams. Impermeable rocks do not allow water to pass, therefore more surface streams and runoff
Soil Type (River responds)
Can control the rate and volume of infiltration, the amount of soil measure storage and rate of throughflow. Sandy soils allow more infiltration and do not encourage flooding. Clays have smaller pore spaces which reduce infiltration rate and encourages runoff
Drainage density
Refers to the # of surface streams in a given area. The density is higher on impermeable rocks and clay. The higher the density the greater the probability of flash floods
Tides and storm surges
High spring tides prevent river flood water from escaping into the sea, therefore causing floodwater to build up in the valley.
High tides + gale force winds = storm surge
