Stream Flow Generation Flashcards
What is stream flow generation
How rainfall moves quickly through a catchment to produce riverflow in a storm
What is direct in channel precipitation
Rainfall directly into river. Discounted bc most area is covered by vegetation so less than 1%
What is infiltration excess (Hortian) overland flow
Dominant mechanism as thought by engineers is water that hits the ground surface and runs over it quickly to stream. Hortian overland flow
What did Horton find
Rainfall intensity is bigger than infiltration capacity.
Soil surface acts as a seperatint surface and bite in ground act as a flow between storms.
Small quantity of water enters soil and keeps rivers flowing between storms.
Why is hortian overland flow not the dominant mechanism for routing rainfall to rivers
Rarely seen or even measured in significantly quantities . Less than 5% of river flow at Danum.
Soil infiltration capacity usually bigger than rainfall intensities so it all goes in ground
Conservative tracers very variable in rainfall but damped in rivers.
What is infiltration capacity also known as
Surface, saturated hydraulic conductivity
What is surface saturated hydraulic conductivity
Permeability or the surface specifically at the surface
How do we measure conservative tracers in rainfall
Natural tracers in rainfall itself - water enters soil and mixes with subsurface water in storage.
If catchment was impermeable rainfall would hit the ground and run into river, same chemistry. If not water mixes with that In catchment and chemistry changes.
Where is hortian overland flow often localised to
Tree stem bases
Compacted surface roads
Desert surfaces (hydrophobic)
How does water in the soil get to rivers in storms
Subsurface pathways
Evidence that water goes through soil in subsurface pathways
Water table during flood event comes to surface as it is fed by water further up the slope. May even rise to surface before it gets to channel so you get streams.
What happens as you look down the catchment as you go through a storm or season
The channel system that is contributing water expands or contracts. Downslope flow is more lost into channel. Saturation to soil.
What is a computer model based on VSA concept
TOPMODEL (developed at Lancaster) sits under met office and formulates weather
What is VSA
Variable source area concept by Hewlett
What happens when the water channel fills up underground
It connects with other channels and extends. Expansion of stream networks in ephermal and intermittent streams.
What are the water paths in river side areas
Channel bed inflow
Direct precipitation on saturated areas
Return flow
What is channel bed inflow
Wet bits near the river. First mechanism is the loss of water from soil through the bed. But if you pour enough water down the slope it can’t get out into river fast enough and backs up the hill slope all the way to the surface and it’ll start to come out of the ground (exfiltration) called return flow which runs over slope before returning to river
What is direct precipitation on saturated areas
Could have permeable soul but water is coming out or surfer bc the pores are full of water. If all things are wet the rain falling into surface can’t go in and goes into catchment
What is return flow
Saturation excess overland flow. Water emerges at valley bottom before going into stream. Backs up.
What happens when water is less than saturated
It delivers water quite slowly so most of the catchment isn’t wet. Important bc these pathways take contaminants with them
What are recharge pathways for saturated soil
Water coming into the ground at different points of the catchment
What are rapid paths in unsaturated soil
Might have high permeability on surface but meet something less permeable down into the profile. Can’t get in any further so gets thrown down into river. Very shallow. Unconsolidated geologic material throws water laterally. Laterally.
Example of shallow water paths
Impeding layers in podzolic soils
Head deposits from siltstones
Natural soil pipes
Wave propagation
How are head deposits from granite made
In Uk had freeze thaw which froze souls and smashed it up. Souls slid down hill side and froze again, smashed again. In profile you get layers pointing down stream. Solidfluction process
What is a natural soil pipe
Formed by seepage forces of the water. Few cm in diameter and it will go a few m up hill slope. Soil around is dry even though there’s a stream going through it
What do roofs a soil pipes collapse
They are unstable and create channels that are 5-10cm in diameter
Where do you get soil pipes
Peaty souls Soil in deep peat are big at 1m Through podzol close to surface Acidic mountain soils In deserts Caves in limestone
Examples of soil pipes
Desert in south east Spain
Israel - so big buildings fall through
Baru catchment in Borneo
What is wave propogation
Environment with discrete cave passage, see the pulse of water going into cave system will be delayed. When we try to model this we think about darcys law but channel movement is manning equation. Weir responds instantly to input of rainfall, how we measure most things is wrong bc of this
What is difficult about stream flow measuring
Deciding how much of each of the paths matter
