water below the ground surface in pores spaces in soils and factures in rock.

any water stored in any form below the ground surface

the rate and amount of water flow through a porus medium (like rock or soil)

Q = KiA

or

water flow rate (in volume/time) = hydraulic conductivity x hydraulic head gradient x cross sectional area of the flow tube, - because it flows down hydraulic gradients

in this law we are concerned with hydralic head gradient, which is the change in pressure driving water flow over distance

the change in elevation over distance

a place with high topographic gradient - steep

a place with low topographic gradient = flat

the K in Q = KiA

a material property that describes how easily fluid moves through the material

the i in Q= KiA

the gradient in fluid potential that drives flow

the slope of the water table

there are conditions where the head gradient reflects pressures unrelated to the water table (in a confined aquifer - one sandwiched between two very impermeable aquiculdes, hydrolic head is not simply related to water table. you would know this if you drilled into an overpressured confined aquifer from the resulting gyser)

it will be high if the hydraulic conductivity is high or if the hydraulic gradient is high.

conversely, flow will be low if the hydraulic conductivity is low or the hydraulic gradient is low

1. pore size

2. grain shape and packing

3. fluid viscosity and density

coarser grains, bigger pores, higher k

*pore size itself depends on grain size

because we are only considering water, you can think of viscosity and density to be constant for the problems we will consider

grain size, which affects pore size

the K value for water moving through coarse sand is about 0.001m/s while for clay K valuse are more like 0.0000000001 m/s. these values mean that for a hydraulic gradient of 1, it will take a parcel of water 1000 seconds (17 min) to move 1 m through coarse sand, and it will take 10¹⁰ seconds (about 300 years) for the water to move the same distance through clay

a measure of the volume of water in an  aquifer that drains by gravity and can be pumped by a well

drainable water storage = volume of the aquifer x specific yeild

also called water in storage, althrough there is always more water in the aquifer than the drainable water in storage - it held too tightly by capillary forces to be easily extracted

the fraction of water in an aquifer that can freely drain

expressed as the volume of drainable water per volume of aquifer. it is similar to gravitational water in soils, the water that drains easily under gravity

174,000 square miles in parts of seven states

the source of 30% of all groundwater used for irrigation in the US, supplying water to 27% of the irrigated land in the us

shallow, keeps pumping costs low, provides good water quality

hosted primarily in the Ogallala formation (a tertiary age fluvial deposit of sand and gravel shed from the ancestral rockies) also contains quaternary age sand dunes of the nebraska sand hils. below the aquifer are less permiable rocks

average saturated thickness - 200ft (1980)

recharged by precipitation over the region

rainfall varies from east to west, ranging from about 16in/yr in colorado, new mexico, and wyoming to 28in/yr in nebraska and kansas

PET varies from north to south raging from 60in/yr in nebraska and wyoming to 105in/yr in texas

recharge rates range from 0.024in/yr (0.1% of precip) in texas to 6in/yr (20% of precip) in kansas and nebraska

average recharge over the whole aquifer is 0.6in/yr

80% of the recharge occurs in N part of aquifer

recharge highest where sand dunes present @ surface

flow from W to E so high recharge in Nebraska doesn't provide a source of water for Texas

important b/c it has high specifi yield of 15% with a range in different locations in the aquifer of 8-18%, lots of water that's easily pumped out