# Shared Flashcard Set

## Details

5GEOG landscapes and water
Groundwater
21
Geography
04/05/2010

Term
 what is groundwater?
Definition
 water below the ground surface in pores spaces in soils and factures in rock.   any water stored in any form below the ground surface
Term
 what is the unsaturated zone (vados zone)?
Definition
 above the water table where pores or factures are not completely filled with water, but instead contain both air and water
Term
 what is the saturated zone?
Definition
 the area below the water table where all pores and fractures are completely filled with water
Term
 what is the water table?
Definition
 the top of the saturated zone
Term
 what is an aquifer?
Definition
 a layer of permeable rock or soil capable of providing groundwater to a well
Term
 what is an aquiclude
Definition
 a layer of rock or sediment that has very low permeablility such that it is a barrier to groundwater flow
Term
 What is Darcy's Law?
Definition
 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
Term
Definition
 a change in some property over a distance
Term
Definition
 the change in elevation over distance   a place with high topographic gradient - steep a place with low topographic gradient = flat
Term
 what is hydraulic conductivity?
Definition
 the K in Q = KiA   a material property that describes how easily fluid moves through the material
Term
Definition
 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)
Term
 what does Darcy's Law say about the amount of water moving through an aquifer?
Definition
 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
Term
 what are three characteristics that control the hydraulic conductivity (K) of a material?
Definition
 1. pore size   2. grain shape and packing   3. fluid viscosity and density
Term
 why does pore size matter for hydraulic conductivity?
Definition
 coarser grains, bigger pores, higher k   *pore size itself depends on grain size
Term
 why does grain shape and packing matter for hydraulic conductivity of a material?
Definition
 tortuosity (a property of curve being twisted; having many turns) of the flow paths around grains is set by packing and shape of grains, and this will have some effect on K
Term
 why does fluid viscosity and density matter for hydraulic conductivity of a material?
Definition
 because we are only considering water, you can think of viscosity and density to be constant for the problems we will consider
Term
 what is the most important control on hydraulic conductivity and why?
Definition
 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
Term
 why are clayey materials essentially impermeable?
Definition
 because the hydraulic conductivity is so low (assuming there are no fractures in the clay)
Term
 what is the drainable water in storage?
Definition
 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
Term
 what is the specific yeild?
Definition
 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
Term
 what is the high plains aquifer?
Definition
 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
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