the movement of water into the soil

* rate = high at beginnning of a rainstorm on a dry surface, declines over time to a constant value

(more water can enter the soil more rapidly at the beginning of a storm)

soil texture (grain size)

rainfall rate

how much moisture is in the soil

it sets the maximum rate of infiltration as long as there is no ponding of water on the surface

(you can't infiltrate more water than is available)

it matters because dry soil can basorb more water than wet soil (due to both capilary effects and having storage space for more water) - hence has a higher infiltration rate

changes in soil moisture are a reason for the reduction in infiltration rate over time in a storm

named after Robert Horton

flow of water over the ground surface

occurs when or where rainfall rate is greater than the infiltration rate

excess water (rainfall rate-infiltration rate) ponds on ground surface and at great enough depths, will begin to run down the soil surface

(parking lots)

occurs on bare rock surfaces, clayey soils, frozen soils (all surfaces with low infiltration rate) or in exceptionally intense rainstorms

fast flow mechanism

some delay in the onset of HOF in a storm while ponding starts and little ponds connect

once flow in the thin sheets begins, water rapidly reaches the stream because it is relatively unimpeded in its progress over the ground surface, rather than through the filter of the soil. flow reaches the stream quickly, and flow stops rapidly after the rainstorm stops

uncommon in natural settings (found over frozen soils, compacted soils, or bare rock) --> urbanization creates impervious surfaces like parking lots, building roofs, roadways (even dirt ones have low infiltration rates) from which HOF is produced (impacts streamflow and groundwater recharge)

shallow flow through either unsaturated soil or through perched saturated layers found above the water table

flow rates through unsaturated soils are much much slower than through saturated soil (the hydraulic conductivity decreases with water content)

this tends to be a slow route to the stream channel with a long delay from teh onset of rainfall, and is responsible for slowly rising flow during storms, and long duration flows following the end of rain

associated with permeable soils and steep slopes

flowpath across the ground surface, but differs from HOF

overland flow occurs due to the water table rising to the ground surface

sheet of SOF that develops contains water emerging from the soil itself (sometimes called exfiltration, the oppsoite of infiltation) and rainwater that cannot infiltrate because the soil is already full (saturated). this latter bit is called direct precipitation on saturated areas, DPSA

occurs on low sloping regions, particularly at the base of slopes along the margins of stream channels

usually considerable lag in the onset of this flow route - water table must rise first - once it begins to operate, water moves quickly to the stream

SOF has a short sharp peak that is the direct precipitation on saturated areas contribution to SOF, followed by a longer sustained peak associated with return flow (the water emerging fromt eh subsurface to flow over the surface. the peaks are delayed and smaller than for a HOF hydrograph. finally, SSSF produces the most delayed and subdued peak, but also the most sustained flow.

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the flow of water

the amount and type of sediment in motion

the character of the materials that make up the bed and banks

it exerts a shear stress

it is stable when the shear stress is approximately balanced by the resistance of the bed and bank

the form is the same even as it migrates from side to side

there is a balance between erosion and deposition to maintain this form

the volume passing through a cross-section per unit time

it has units of volume/time, as in cubic feet per second or meters cubed per second (m³/s sometimes abbreviated as cumec)

measure the hight of the water surface with an electronic measuring device, the elevation of the water surface above some datum is called the stage.

measurement of stage is automated in slect locations, suing a float in a stilling well (a well connected to the river channel so that water level in the stilling well is the same as in the river)

and a recording device.

sometimes stage is measured with a pressure sensor. possible to use an acoustic sensor that measures the distance down to the water surface (through the air) to measure stage. this approach has the advantage of not needing to stick anything in the flow, a good thing in the turbulent sometimes ice-charged rivers that need gauging

we measure discharge by measuring the channel cross sectional area (width x depth) and measuring the average velocity of the water flowing through the channel

Q=v(wd)

Discharge = velocity x (width x depth)

velocity = L/t

all measurements must be reported in the same units (if velocity is in m/s then measure width and depth in m so cross section area is in m²)

the idea is that discharge increases with depth of water in the channel, and that stage is a measure of water depth in the channel

the relationship between stage and discharge must be known, this si done empirically - by measurement, rather than theoretically

the measurements of discharge at different stages are used to produce a rating curve for the channel - a graph and an equation that show the relationship between discharge and stage

velocimeter - has cups that spin in the flow, can be attached to a staff that a hydrologist holds in the flow, or can be hung on a cable and suspended in the flow from a bridge or cableway. a weight is attached if suspended from bridge to hold velocimeter down in the flow

acoustic Doppler current profilers (ADCP) - send sound waves out and measure return times. can be attached to a boat (for a big river) or a small tethered raft and used to get velocity profiles accross a channel

orange peel on the surface - toss a biodegradeable float (snowball = awesome) and measure its speed with a stop watch over some pre-measured length of stream channel. this gives the surface velocity, so must be adjusted to approximate the mean velocity

they divide the stream channel into sections and measure the cross sectional area and velocity in each of these sections

at least ten sections are used, often more

measure of width and depth will be made, and a measurement of average velocity of water. a discharge will be computed for each vertical section, and then these are added to find the discharge of the river

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