Hydrosphere II Flashcards
how do aquifers supply water simply
in terms of baseflow from groundwater
when does groundwater keep flowing
when p < et
how do the levels of p vs. et vary throughout the year
high river Q (discharge) november to may, P > PET
low river Q june to october when P < PET
what is the equation for river discharge
Q = P - ET +/- change in storage
what is the dry season due to in rivers
PET varies significantly more than P, rainfall isn’t especially seasonal in the Thames for example. Dry season is not due to lack of rainfall but more PET
define baseflow
baseflow is water released from basin storage such as groundwater, lakes/wetlands/reservoirs. Meltwater from alpine glaciers and seasonal snowpack is another groundwater storage.
what does the symbol Q mean
discharge and also flux
how is groundwater discharged
groundwater is commonly discharged to rivers as baseflow in humid environments
what does stormflow derive from
it derives from runoff via Horton and Hewlett hypothesis and overland flow
where is groundwater derived from
groundwater derives from precipitation that has infiltrated the subsurface and drain through the ‘unsaturated zone’ to the ‘water table’
how does groundwater return to the surface
via outflow to rivers (and springs)
what does the slope of the water table represent
it represents gravitational force during groundwater flow.
what is soil water and when does it become groundwater
if water has infiltrated the ground it is known as soil water, when it reaches the water table (saturated ground) it becomes groundwater.
what is the amount of baseflow determined by
discharge of groundwater to surface flow is baseflow, the amount is determined by gravity. water goes up because the water pressure below is higher than the pressure above. water flows under gravity and to the lowest water pressure.
define water pressure
Pressure is the force that pushes water through pipes. Water pressure determines the flow of water from the tap. The amount of pressure at your tap can depend on how high the service reservoir or water tower is above your home, or on how much water other customers are using.
how does groundwater flow
from places with hydraulic head to places with lower hydraulic head.
how do you measure water pressure
you need a common datum eg. metres above sea level
define hydraulic gradient
hydraulic gradient is change in hydraulic head divided by the distance between them
how do you calculate hydraulic gradient
hydraulic gradient (i) = h(1) - h(2) / L
h1 and h2 are higher and lower hydraulic head
change in hydraulic head divided by the distance between them
change in hydraulic head divided by the distance between the two points of measurement
What is hydraulic conductivity
measure of the ability of the rock/soil to transmit groundwater and varies from 10^-10 m/s for clay and to 10m/s for gravel.
symbol K
What is Darcys law
Darcy flux (velocity): v = -k x i
negative sign on k because hydraulic gradient will be negative so it cancels out
similar equation for that of groundwater or river discharge flow Q = v x A
define aquifer
geological formation that readily transmits groundwater (high k value)
define aquitard
geological formation that does not readily transmit groundwater (low k value)
what does readily mean in the context of readily transmitting groundwater
contextual, based on area
how can we divide the geology of the uk
into aquifers and aquitards
how can we deduce which areas have aquifers and those that dont
the higher proportions of the water supply using groundwater means the area has and is using aquifers.
e.g. 3% of water supply in scotland is groundwater so must be aquitards
are rivers more or less affected in areas of aquifers
rivers affected more by areas with less groundwater. areas underlain by aquifers use groundwater as their primary source of water supply.
what do areas underlain by aquitards rely on for water
rivers, lakes and, surface reservoirs
what is young groundwater
less than 50 years old (means its replenished)
how much water is stored in groundwater storage
groundwater storage is 3.5 times that stored in lakes, rivers, ponds and wetlands.
compare the amounts of surface water to groundwater
total groundwater storage is 200 times that of surface water
what two things is discharge made up of
stormflow + baseflow
what is the BFI
proportion of the total annual flow made up of baseflow in a river is base flow index (BFI)
what does a higher BFI mean
the river is much more resilient to climate variability
how much does BFI vary
BFI varies from 5% on aquitards to >80% for streams drained by aquifers such as chalk limestone.
how does baseflow change seasonally
baseflow from groundwater fluctuates seasonally.
when P > ET (uk winter and spring), water tables rise and hydraulic gradients steepen, groundwater flow increases, amplifying baseflow.
when P < ET (Uk summer) water tables decline, reducing the hydraulic gradient and thus baseflow (groundwater)
water tables rise faster than rivers during precipitation, rise is much more slow
where can river flow be seasonal or episodic
in drylands (semi-arid, arid and hyper-arid environments)
how can groundwater be replenished
by rivers as water tables are often deeper and below the riverbed.
what are episodic heavy rain events crucial for
episodic heavy rain events causing river flow are crucial for sustaining groundwater resources in drylands.
the effects occurs every few years so the flood is essential to replenish groundwater
why is groundwater more resilient to climate change than river flow
because groundwater derives from recharge occurring over years to decades (and longer)… it is more resilient to climate than river flow.
define artisan bore hole
An artesian aquifer is a confined aquifer containing groundwater under positive pressure. An artesian aquifer has trapped water, surrounded by layers of impermeable rock or clay which apply positive pressure to the water contained within the aquifer. water can flow up and out of the ground like a tap
what favours groundwater recharge in many environments
intensification of rainfall in a warming world favours groundwater recharge
what is bad about more intense and extreme rainfall events for groundwater
fewer but more intense rainfall increases infiltration at the expense of soil moisture.
what is groundwater
groundwater is a natural freshwater store that sustains river flow during summer and seasons when P is less than ET
What does darcys law desribe
describes a groundwater flow that is a function of the hydraulic gradient (i) and the hydraulic conductivity (k)
where is the contribution of baseflow important
contribution of baseflow (from groundwater) to river flow is substantial in landscapes underlain by aquifers (e.g chalk, sandstone) but minimal where the geology comprises aquitards (eg. mudstone, crystalline rocks)
in drylands how can groundwater be replenished
by episodic river flow
groundwater is resilient to…
climate variability and change
what is the worlds largest store of freshwater
glaciers but the worlds largest distributed source is groundwater
how much domestic water is groundwater
36% of domestic water is groundwater.
what is fossil groundwater
groundwater in a lot of places has not been recharged. recharge occurred under cooler climates before - fossil groundwater
what type of groundwater is dominating water supply
current recharge rates are not responsible for a lot of total groundwater storage - fossil aquifers storage dominated rather than recharge - flux dominated
are fossil aquifers sustainable
fossil aquifers provide reliable groundwater source which is resilient to climate variability, but it is unsustainable like oil.
land use change can indirectly affect groundwater
over time what will groundwater recharge be influenced by
climate variability
what tends to reduce seasonal duration and magnitude of recharge
changes in snowmelt regimes due to climate change
how does irrigation affect groundwater
it affects groundwater storage and accounts for 70% of global freshwater withdrawals!
what could increased recharge result in
it may degrade groundwater quality through the mobilisation of salinity in soil profiles but also flush natural contaminants such as arsenic from groundwater systems.
what will climate change do instead of directly affecting grounwater
it will influence the demands of irrigation water which will influence groundwater stores.
define groundwater irrigation
groundwater is naturally recharged by rain water and snowmelt or from water that leaks through the bottom of some lakes and rivers. groundwater also can be recharged when water supply systems leak and when crops are irrigated with more water than required.
what can we regard the hydrosphere as
a global system consisting of four reservoirs linked by the hydrological cycle, these are the world ocean, polar ice, terrestrial waters and atmospheric waters.
how is the general wind circulation important to precipitation
the general wind circulation is very important in explaining the observed global precipitation patterns. Particularly high precipitation regions are found where moist winds from oceanic areas either flow onto windward continental coasts or converge onto low pressure troughs such as that found in the equatorial zone.
what are the 3 components of evaporative losses
evaporative losses from land surfaces have 3 components; transpiration, interceptions losses and soil surface evaporation.
define transpiration
water is extracted from the soil by roots of actively growing vegetation and transpired through small holes in vegetation leaves called stomata. transpiration is therefore strongly controlled by nature and the condition of vegetation.
what is interception loss
during rainfall, water is intercepted in vegetation canopies and evaporated back to the atmosphere during and just after the rainfall event. this is interception loss and is dependant on atmospheric condition.
what is soil surface evaporation
water evaporated directly from moisture held on or just below the surface is known as soil surface evaporation,
what is evapotranspiration the sum of
the sum of transpiration, interception loss and soil surface evaporation
define a drought
a drought is considered to be a period of abnormally dry weather that cause serious hydrological imbalance in a specific region.
serious and abnormally dry depend on the nature of the local climate and the impact of the drought on the local society. difficult to produce a definition that applies to a variety of climates
what 4 categories does drought fit into
meteorological or climatological, agricultural, hydrological and socioeconomic
what is a meteorological drought
atmospheric conditions result in absence reduction of precipitation over several months or years cause a meteorological drought
what is an agricultural drought
a few weeks dryness in the surface layers (vegetation root zone) which occurs at a critical time during the growing season, can result in an agricultural drought that reduces crop yield.
what is a hydrological drought
precipitation deficits over a prolonged period that affects the surface or the subsurface water supply, thus reducing stream flow, groundwater, reservoir and lake levels will result in a hydrological drought
what is the widely accept meteorological drought index
the palmer drought severity index (PDSI) is a simplistic model of the cumulative anomaly of moisture supply and demand at the land surface.
what is the porosity of a soil or rock
the porosity of a soil or rock is that fraction of a given volume of material that is occupied by void spaces or interstices. porosity is indicated by the symbol, n and is usually expressed as the ratio of the volume of void, V(subscript v), to the total unit volume V(subscript t) of a soil or rock such that n = V(v) / V(t)
how can porosity be determined in the lab
porosity can be determined in the lab from knowledge of the bulk mass density and the particle mass density of the porous material.
what is primary porosity
primary porosity is the inherent character of a soil or rock matric that developed during its formation.
what is secondary porosity
secondary porosity may develop as a result of secondary physical and chemical weathering along the bedding planes and joints of indurated sediments such as limestones and sandstones, or as a result of structurally controlled regional fracturing and near-surface weathering in hard rocks such as igneous and metamorphic rocks.
what is a dual-porosity system
where both primary and secondary porosities are present, a dual-porosity system is recognised for example, as a result of fracturing and fissuring in porous sandstone or limestone.
define specific yield
in an aquifer with a water table, it is the volume of water released from groundwater storage per unit surface area of aquifer per unit decline in the water table.
what is the specific retention
the fraction of water that is retained in the soil or rock against the force of gravity is termed the specific retention. the sum of the specific yield and the specific retention is equal to the total porosity
what is the total porosity
the total porosity relates to the storage capability of the material
what is effective porosity
relates to the transmissive capability of the material
what is specific surface area
can be define as the ratio of total surface area of the interstitial voids to the total volume of the porous material
what is hydraulic conductivity
it is a measure of the ease of movement of a water through a porous material
how do we recognise aquifers, aquitards and aquicludes
natural variations in the permeability and ease of transmission of groundwater in different geological materials lead to the recognition of them
define aquifer
an aquifer is a layer or layered sequence of rock or sediment comprising one or more geological formations that contains water and is able to transmit significant quantities of water under an ordinary hydraulic gradient.
how can aquifers be exploited
aquifers have sufficient permeability to transmit groundwater that can be exploited economically from wells or springs. good aquifers are usually developed in sands gravels, solutionally weathered limestones and fractured sandstones
define aquitard
used to describe a formation of lower permeability that is incapable of transmitting significant quantities of water under hydraulic gradient and can act as a barrier to regional groundwater flow
what are hydraulic properties
water contained within the interconnected voids of soils and rocks is capable of moving, and the ability of a rock to store and transmit water constitutes its hydraulic properties.
what did Darcy find
Darcy found that the total flow, Q, is proportional to both the difference in water level, measure in manometer tubes at either end of the column and CSA of flow and inversely proportional to column length,
