W2 - Runoff Flashcards
How does water get into the river? Direct channel precipitation
Tends to be a relatively minor components of storm runoff as only a small proportion of a catchment is covered by stream
Ephemeral channels –> after rainfall, the channel may increase in size
How does water get into the river? Interception
Interception can vary between species
Affects speed at which the water will enter the channel
Time of year also impacts interception – summer = dense vegetation
Main hillslope pathways: Surface flow
As it rains, the gaps between the hummocks start to fill with water
Our pools will fill up more and water will travel towards the channel (gravity)
The volume of flow needs to be great enough to overcome barriers and connect to the river
- Infiltration excess overland flow
o e.g. Horton (1933) - Saturation excess overland flow
o e.g. Hewlett and Hibbert (1967)
Main hillslope pathways: Hortonian overland flow
- Difference between infiltration rate & infiltration capacity
o Rate high, capacity low –> infiltration excess overland flow
Where does Hortonian OLF occur?
Arid areas
o High rainfall intensities when it does rain
o Limited vegetation cover – a lot of rainfall is reaching the surface
o Soil surface crusts
Agricultural Fields
o Limited interception
o Raindrop compaction – forces out the air
o Swelling clays when they get wet
Cold environments
o When water enters soil it can freeze no more space
Urban environments
Problems with Horton hypothesis
The Partial Area Model (Betson 1964)
o Lots of research just at plot scale
o Spatial variability of surface produces patchy runoff…. known as the partial area model
——> Can be patchy in areas
——> Overland flow in agricultural areas tend to be more focussed around troughs and places where animals trample
o Emmett (1978)
In the temperate zone rainfall intensities rarely exceed infiltration capacity
Widespread sheetflow is rarely reported
There is little correlation between rainfall intensity and streamflow
o Yet…. stormflow responses are observed in streams
Main hillslope pathways: Saturation excess OLF
- Water can’t infiltrate because the soil is saturated
- Rainfall could enter the soil in an unsaturated area and throughflow to a saturated area in which it would displace previous water (piston displacement) – return flow
What is variable source area theory?
There are areas in a watershed especially prone to generating runoff and are therefore hydrologically sensitive with respect to their potential to transport contaminants to perennial surface water bodies
(Walter et al., 2000).
What is matrix flow?
Water moving through very small pore spaces
Tends to be slow
controlled by hydraulic head (the mechanical energy per unit weight of the fluid in the system) and hydraulic conductivity (a measure of how easily water can pass through soil or rock)
What is Macro-pore Flow?
o Holes in soil greater than 0.1mm
o Preferential flow pathways
o Water can move faster through larger spaces
What are soil pipes?
o Subsurface cavities greater in diameter than 1 mm
o Can extend over a large distance
o Can transport water, sediments and solutes through the soil and bypass the soil matrix
o Provide rapid connectivity of water
o Tend to be prolific in arid areas and peatlands
What is the lateral movement of water?
- Lateral throughflow and pipe networks
o Due to gravity or flowing from wet to dry - Percolation excess at hydraulic conductivity discontinuities promotes lateral throughflow
o Stops being able to travel vertically so moves across - Rapid throughflow in pipe networks (Holden 2017)
- Important role of sub-surface storm flow
What is groundwater flow?
- Percolation
- Rock needs to be porous and permeable (permeable = the pore spaces are connected)
- Slowest of all the flow pathways
- Aquifers – porous enough to store water and permeable enough for water to flow through in large quantities
- Maintains baseflow – long lag times
- Movement based on gravity and pressure
- Rate: Darcy’s Law
What controls hydrograph shape?
- Range of runoff generating processes
- Catchment size, shape, topography, soils, geology and land cover set the basic parameters
- But runoff production also varies between storms (due to antecedent conditions)
Difference in hydrograph shape from small catchment to large?
- Larger catchment travel time longer so…
- Larger lag time
- Lower peak discharge
- Total storm runoff –> greater area = more rainfall
- Rainfall runoff ratio –> longer travel time means more opportunities for water to be lost – larger absolute volume but smaller relative volume due to opportunities for water to be lost
As catchment size increases, lag time increases
Peak runoff rate decreases with size – more opportunities for loss
Key labels on a storm hydrograph
Lag time, Peak discharge, baseflow, rising limb, falling limb, storm flow, peak rainfall
What are flow duration curves used for?
comparison of flow characteristics
Key things about FDCs
- Typically based on daily mean flow data (> 5 years)
o Longer period than annual hydrograph
o Will produce one of these for each catchment and then compare them
o Ln flow = log scale
Stages in FDC creation:
- Order discharge from high to low
- Rank them
- Calculate exceedance probability (100*(rank/(no.values+1))
- Divide discharge values by the mean to normalise the data
- Plot on a logarithmic scale
How to read FDCs
Steeper curve = more variable flow
Why does the UK experience higher flows in winter?
in summer we have more vegetation = more interception and greater rates of evapotranspiration, evaporation also higher because it is warmer
Our autumns and winters are wet = antecedent conditions
climate cycles and river discharge
In positive phases of NAO, we get more rain correlate to high discharge
What is return flow?
piston displacement mechanism - soil water that has travelled through the soil reaches a saturated area and pushes it out of the soil
