Water 1 Groundwater Flashcards
How much of Dutch drinking water comes from groundwater and why is this?
65% because quality is better and more consistent than surface water.
drawdown
the groundwater table drop near an extraction tube.
desiccation
the process of becoming too dry. This is due to the extraction of too much groundwater.
water storage in root zone through the seasons
plants use water in root zone, which decreases soil moisture.
Water stored in root zone in the winter season is an important source of water durign crop growth in summer.
How does capillary rise vary?
depends on distance between groundwater table and root zone
soil type
evapotranspiration rate.
Negative impacts of a high/ shallow water table?
yield reductions - plants do not have enough oxygen
clayey, loamy adn peaty soils become soft and cattle/ cultivation vehicles compact or damage topsoil.
types of irrigation
sprinkling (spraying over a field)
drip-irrigation - water drips through a hose with holes. This limits loss, reduces ponding, surface runoff and evaporation
Impacts of long-term irrigation
- large amounts of dissolved salts
capillary rise transports salts to irrigated soils. High salinity reduces water extraction capacity for plants
which balance is needed for plants relating to groundwater?
you need oxygen and water so cannot be saturated and completely dry.
location factors
requirements for vegetation to grow. These are primarily determined by groundwater and human interference.
Sources of pollution for groundwater
agriculture, industry discharges, heavy metals and solvents.
What are the 4 dangerous questions about groundwater pollution
- do pollutants reach groundwater?
- when they reach groundwater, where do they go?
- how fast do they move in groundwater?
- effect on quality of water?
hanging water
water hld by the soil against gravity
how does groundwater affect spatial planning?
the drier or wetter areas just become nature reserves.
other uses of groundwater
heat exchange ti heat/ cool houses.
unconsolidated sediment
loose soil material such as sand, clay, gravel
consolidated sediment
bedrock
soil conductivity
a measure of how easily water can flow through soil material. It is used to compute groundwater flow.
saturated hydraulic conductivity
maximum conductivity when soil is saturated.
Does water flow more easily through saturated or unsaturated soil?
unsaturated.
anisotropic
soil conducivity is higher in one direction than the other bcause of the way the soil is layered.
isotropic
conductivity is the same in all directions.
porosity of soil
the fraction of soil volume that is not taken up by soil particles. In saturated soil, this gives the water fraction in the soil.
effective porosity of soil
the fraction of the soil volume that is filled with water than can move. This can be used to compute flow velocities and residence times.
larger or smaller grains have higher conductivity?
larger grains have higher conductivity
what does the product of conducitvity k and layer thickness D determine?
the capacity of a layer to transport water horizontally or to limit vertical waterflow
what is the unit of conductance/ transmissivity
m 2 d -1 where d is day
equation for conductance
kD= k x D
what is resistance
the capacity of water to flow vertically
equation for resistance c
c = D/ k
aquifer
a layer that consists of permeable material, can transport water easily and are characteristed by their high conductance.
aquitard
confining unit or poorly permeable layer
bad transporter of water and usually consists of clays or soil with lots of organic material. resistance is the measure for the ability to seal off an aquitard.
aquiclude
layer with so high resistance that no water flows through it.
phreatic aquifer
the upper aquifer, the groundwater table is located in this aquifer. part of the layer is unsaturated.
confined aquifer
the bottom aquifer, enclosed by a layer on top. It is always completely saturated.
semi confined aquifer
the aquitard on top does not cover the whole aquifer.
hydrological base
lower boundary beneath which water no longer takes part in the hydrological cycle.
effectivie conductivity
the conductivity of aquifers that lie on top of each other. This is a weighted average.
how do you calculate effective conductivity?
you take the weighted average of the conductivity of each of the aquifer layers.
other word for groundwater table
phreatic plane
groundwater level vs. groundwater depth
level - with respect to sea level - a.m.s.l.
depth - with respect to land surface
soil moisture, capillary fringe, groundwater table.
the soil moisture is found in the unsaturated zone. The capillary fringe nad groundwater are found in the saturated zone.
piezometer
instrument to measure groundwater
how does a piezometer work?
a perforated tube part - called the filter - allows an exchange of soil water until water pressure within the tube equals soil water pressure around the filter.
hydraulic head
the corresponding water height that shows the energy of a water droplet in a certain place. It consists of two parts, the elevation head, and the pressure head.
equation for hydraulic head
H = z + h
hydraulic head [m]
elevation head [m]
pressure head [m]
elevation head
the elevation of the water droplet compared to a reference level such as sea level.
pressure head
the local pressure like a pressure meter but converted into a certain height.
what is the reference pressure used for the pressure head?
atmospheric
what is one of the important hydraulic heads to remember?
at the groundwater table h = 0 , so H = z
monitoring tubes
deep piezometer, made of stronger material
flow direction due to hydraulic heads
differences in hydraulic heads cause groundwater flow. If the hydraulic head in two aquifers separated by an aquitard differs, you may get upward or downwards seepage. Always read the seepage directly off the diagram, do not compensate for height difference etc.
indications of groundwater level - peat
shows the groundwater level was very high during soil formation.
mineralisation/ oxidation of peat
transforms peat into gasses, occuring when water levels in ditches are artificially kept high in the summer.
groundwater level classses
numbered from wet to dry using roman numerals I - VII. They are based on the average highest and average lowest groundwater levels in 8 years.
AHG and ALG
AHG - average highest groundwater level. gw is measured every 14th and 28th day of month. the three highest values are averaged and this value is taken.
ALG - average lowest groundwater level.
hydromorphic characteristics
soil characteristics which are present as a result of wet soil conditions.
ASG
average spring groundwater level
ASG calculation
(0.83AHG) + (0.19ALG) +5.4
equipotential lines depict
spatial vartiations in hydraulic head. So points with the same hydraulic head are connected. They are used on maps and vertical cross sections.
isohypses
lines connecting points with equal groundwater levels for phreatic aquifers. They are a type of equipotential line.
flow lines
lines that show the direction of flow of groundwater, from high to low hydraulic head.
flow lines for isotropic soil
flow lines are perpendicular to equipotential lines as they follow the steepest slope of gw table.
flow lines for anisotropic soil
water follows route with lowest resistance, so the angle will not be perpendicular.
flow path/ flow tube
area between two flow lines.
flow net
network of equipotential lines and flow lines.
curvilinear flow net
shows equipotential and flow lines. It is a good idea to make these rouhghly square, then the maths is easier for isotropic soil ofc.
Dupuit’s assumption
for most circumstances, the error in the estimatio of groundwater flow will be relatively small when the vertical stream component is neglected.
Dupuit- Forchheimer assumption
flow in aquifers can be simplified to vertical flow, making maths easier.
recharge/ infiltration areas
water flows into the ground, low gwater level, slope of gwater level increases as does water flux
seepage areas
water exits the ground. gwater flux decreases. gwater slope decreases.
recognising infiltration/ seepage areas on a map with isohypses
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