Study guide notes Flashcards
Q=Qoe^-at
formula used to find discharge
Q= discharge at any time
Q0=peak in discharge
e=natural logarithm
t= amt of time since peak
a=recession constant
recession constant
k= (1/t) ln (Q/Q0)
D=A^.2
formula for finding the time to end of drainage flow
D=time in days
A=area of the drainage basin
Q=CiA
-the rational method used for predicting peak discharge/runoff given the amount of rain from a storm
Q=peak runoff
I=average rainfall intensity
C=runoff coefficient
A=drainage area
runoff coefficients for Q=CiA
Lawns: 0.3
Forest: .15
Cultivated land (farmland): .4
Meadow: .3
Urban environments: .7-.9/.95
-bigger impervious areas= bigger c coefficient
What is a hydrograph?
a graph that shows discharge over time at any specific point in time (discharge= y; time=x)
How to interpret an urban vs. rural hydrograph
urban hydrographs-discharge curve is higher and steeper due to faster and greater runoff because there is more overland flow due to paved roads, and other obstacles for water to be unable to penetrate the ground- results in less baseflow after the storm runoff is over
rural hydrographs have a more gentle falling limb because water will be returning to the river after many hours as it flows through soil and rocks as groundwater flow
overland flow
surface water; unconfined flow of water over the ground surface
baseflow
the portion of streamflow that is sustained between precipitation events, fed to streams via delayed pathways
interflow
the movement of water in the vadose zone that returns to the surface or enters a stream
watershed
an area or region that flows to a river or basin that is one contained area often separated by a ridge or mountains
evapotranspiration
the combined processes of evaporation and transpiration which move water from earth’s surface into the atmosphere
water table
the upper surface of the saturated zone
drainage divide
the divide that separates neighboring drainage basins/catchments/water sheds
infiltration
the movement of water into the soil driven by forces of gravity and capillarity
runoff
the quantity of water discharged in surface streams
What affects the shape of the hydrograph?
intensity of storm event, soil conditions, vegetation density and type, grade of slope/topography, urbanization, sediment type, channel width and depth, stream density
capillary action
the combination of cohesive forces of the liquid and the adhesive forces between the liquid and tube material
capillary fringe
the subsurface layer in which groundwater seeps up from a water table by capillary action to fill pores
porosity
Porosity or void fraction is a measure of the void (i.e. “empty”) spaces in a material
permeability
a measure of the ability of a porous material (often, a rock or an unconsolidated material) to allow fluids to pass through it
primary porosity
the spaces in between the grains in a sediment or sedimentary rock
-leftover space that never got filled
secondary porosity
additional porosity acquired after the original rock formation. It can be a result of weather, fracturing and can replace primary porosity or coexist with it
actual evapotranspiration
equals potential evapotranspiration when there is ample water present
-evapotranspiration can never be greater than potential evapotranspiration but can be lower if there is not enough water to be evaporated or plants are unable to transpire
potential evapotranspiration
the amount of water that would evaporate or transpire from a surface if water was available to that surface in unlimited supply
- is a measure of moisture demand for a region
ex-Sahara’s actual transpiration is 0 but its potential evapotranspiration is very high
What parts of the year does soil moisture/groundwater get depleted?
Summer
What parts of the year does soil moisture/groundwater recharge?
Winter and Fall
How is porosity and permeability related to sediment grain size?
increasing grain size=greater porosity and permeability
How does infiltration rate change over time at a constant rate of adding water to a soil surface?
-infiltration rate is a measure of how fast water enters the soil
-infiltration rate goes down as you constantly add water to soil and it becomes saturated. This leads to overland flow once the ground can no longer hold water
What is baseflow recession?
the portion of the streamflow that is maintained between precipitation events. Without any precipitation events this goes down naturally over time. Sooner or later more precipitation occurs which helps prevent the baseflow from depleting entirely
Basin scale equation
P-E-T-Ro= delta S (Ro- runoff leaving stream)
Groundwater scale equation
Rn (groundwater recharge) +Qi-T-Qo= delta S
what is delta S? How will the available amount of groundwater change if it does not equal o?
change in water storage (all subsurface water). When it is 0 there is no change in storage. If it is positive there will be an increase in water and negative a decrease in water storage
What is stream order?
A method of assigning a numeric order to streams based on their level of branching in a river system
How do you assign orders to streams?
1-any stream that has no tributaries and comes from its own headwaters, no branching upstream (nothing else comes into it) = stream order of 1
2-any time any two streams of the same order come together the resulting stream goes up one order = stream order of 2
3-if two streams of different orders come together the resulting streams retains the larger order
How do basin characteristics influence hydrograph shape?
-intensity of storm event (volume/depth over time; duration)
-soil conditions (saturation of the soil)
-vegetation density and type (percentage of rain will hit the vegetation and prevent water from hitting the ground)
-grade of slope/topography of the area (would affect velocity, gravity affecting movement of sediment) (steeper slope would mean more surface water entering the stream faster than a less steep one)
-urbanization (impervious surfaces, obstacles)
-sediment type (permeability, hydraulic conductivity; thick, impermeable sediment you would have a significant amount of water infiltrating it versus a thinner soil profile with bedrock beneath you won’t have much infiltration but mostly lateral movement)
-channel width and depth
-stream density (more streams in the watershed than a comparable watershed with fewer) (more streams=shorter overland paths before getting into the stream channel; channelized flow will be faster once flow hits the stream)
Darcey’s Law/formula for velocity for groundwater flow
A=-KA (H1-H2)/L
V= -K(H1-H2)/L
What is hydraulic conductivity?
the ability of the fluid to pass through the pores and fractured rocks
-depends on the type of soils and rocks
Isotropy
when the space around rocks is totally equal therefore ground water direction doesn’t make a difference in flow
anisotropy
when the shape of the rocks does make a difference in hydraulic conductivity and the void spaces are not the same in every direction
homogeneity
when grain sizes are all about the same size
heterogeneity
when grain sizes change throughout
aquifer
unit that can store water and can transmit water
aquitard
unit that can store water, can possibly transmit water but has low permeability, stores less than an aquifer
aquiclude
area with low permeability that can act as a barrier to flow
confined aquifer
an aquifer below the land surface that is saturated with water. Layers of impermeable material are both above and below the aquifer, causing it to be under pressure so that when the aquifer is penetrated by a well, the water will rise above the top of the aquifer
unconfined aquifer
a body of water formed from groundwater, rain water runoff and streams with its water table, or the upper surface, open to the atmosphere. They can be problematic as they fluctuate under atmospheric pressure. These aquifers also form at a faster rate than confined aquifers
artesian well
a well that brings groundwater to the surface without pumping because it is under pressure within a body of rock and/or sediment known as an aquifer
What is hydraulic head?
a value that measures the amount of mechanical energy available in water in a river, stream or even lake
-water moves from an area of high hydraulic head (or high potential energy) to an area of lower hydraulic head (or low potential energy)
-elevation of water table determines hydraulic head
What do we most commonly use as a proxy for hydraulic head?
we use the water level in a well that is in an unconfined aquifer to get the hydraulic head assuming there is no change in pressure
How does the elevation of the water table change with the elevation of the landscape? Do they mirror one another?
curves up under hills and drops under valleys. The groundwater found below the water table comes from precipitation that has seeped through surface soil.
Lane’s Balance
describes how changes in sediment load, sediment size, slope, and discharge determine whether a stream system will aggrade or incise
