Hydrology Midterm #2

What is the importance of groundwater?

It stabilizes water temperature, sustains base flow of rivers, contains nutrients, and allows for interaction with riparian vegetation

What percentage of unfrozen freshwater in the world does groundwater make up?

97%

What is capillary action?

Electrostatic forces somewhat offset forces due to gravity, which attracts the water to the soil and allows it to move upwards

What percentage of freshwater on Earth is groundwater?

30%

Darcy’s Law

Q = -KA(Δh/Δl)

Hydraulic conductivity (K)

Measure of how easily a fluid (usually water) can move through pore spaces or fractures in soils, rocks, and other porous media

Specific discharge (q)

The volume of fluid flowing per unit time through a unit cross-sectional area normal to the direction of flow. q = Q/A

Hydraulic head (dh)

Measurable quantity used to describe flow in porous media (delta h = change in hydraulic head between 2 points)

Hydraulic gradient (dl)

The rate at which fluid energy (hydraulic head) drops over a specific distance as it flows through a pipe, channel, or groundwater aquifer (delta l = The actual path length or distance the water travels between those points)

Aquifer definition

Saturated geological formations that contain & transmit “significant” quantities of water under normal field conditions

Aquiclude definition

Formations that may contain water but do not transmit significant quantities (impermeable layer)

Aquifuge definition

Formations that neither contain nor transmit significant quanitites of water

Aquitard definition

General term for formations that are of relatively low permeability (may include both aquicludes and aquifuges)

Unconfined aquifers are…

… bound at the top by the water table, but open to atmospheric pressure

Confined aquifers are…

… bound at the top by an overlying aquitard

What is the piezometric surface?

An imaginary surface representing the level to which water will rise in a well drilled into a confined aquifer

What is the water table?

The upper boundary of an unconfined aquifer where pore water pressure equals atmospheric pressure

Groundwater characteristics

Stable, reliable source of water (less prone to short term flux than shallow water). It has a long residence time/slow movement (abt 30 cm/day)

Water in the unsaturated zone is…

… soil moisture

Water in the saturated zone is…

… groundwater

Hydraulic head equation

Hydraulic head = elevation head + pressure head (h = z + ψ)

Elevation head (z)

The potential energy a water particle has just because of its physical height above a chosen baseline or reference point

Pressure head (ψ)

The height of a water column that the internal pressure of the fluid can support. Measures how hard the water is being pushed or squeezed. Occurs in saturated zones (p>atm pressure)

Specific Yield

Ratio of the water that drains by gravity to the total volume of the sample

Storativity

Describes the ability of an aquifer to store or release water. Measures the volume of water an aquifer releases from or takes into storage per unit surface area per unit change in hydraulic head

Unconsolidated aquifers

Made of loose/unconsolidated earth materials like sand, gravel, silt, and clay. Typically have high porosity and high hydraulic conductivity, allowing water to move through easily

Consolidated aquifers

Made of solid/compacted bedrock such as sandstone, limestone, basalt, granite, or marble. Permeability varies but is worse than unconsolidated aquifers

What is a perched aquifer?

An aquifer that sits above the regional water table

Recharge area (for groundwater)

Area where the groundwater increases and raises the water table (uplands)

Discharge area (for groundwater)

Area that loses groundwater (valleys, depressions)

Hydraulic potential

Contours of constant hydraulic head

Stream line/flow line

Perpendicular to hydraulic potentials (water doesn’t cross streamlines)

Streamtube

Region of flow bounded by streamlines (discharge through a streamtube is Q = -Kbdh)

Flownet

Diagram with streamlines and hydraulic potentials

Field capacity

Amount of water left behind after gravitational drainage

Groundwater inputs

Deep percolation of precipitation, seepage from streams, & artifical recharge

Groundwater outputs

Evapotranspiration from plants with roots in the saturated zone, seepage to lake, stream, wetland, etc, & pumping

Effect of groundwater pumping

Recharge increases, storage and discharge decreases. Can impact water levels in streams/lakes, lead to saltwater intrusion, and subsidence

Cone of depression

Drawdown of water near a pump, which changes the hydraulic gradient

Calculating the volume of the cone of depression

Volume of the cone = volume of water pumped / specific yield/storativity

Soil water zone (of the water table)

Roots can reach

Hydroscopic water (wilting point)

Thin film of water held tightly along mineral grains (plants cannot access)

Bank storage

As streamflow pushes up, water is stored in stream banks (occurs during seasonal/event-based flooding)

Baseflow definition

Unconfined aquifer flow

Stormflow definition

Additional water that causes peaks in hydrograph but returns to baseflow

Hydrograph separation

Differentiates between baseflow and stormflow

Groundwater-surfacewater interactions

Flow can be upward or downward in the hyporheic zone

Basic properties of soils: bulk density

The oven-dry mass of soil divided by its total volume

Basic properties of soils: porosity

The fraction of total soil volume occupied by pore space rather than solid particles

Basic properties of soils: water content

The amount of water stored in the soil, measured either by mass or by volume

Influence of soil texture on soil moisture

Soil texture dictates pore size distribution, total porosity, and surface area

Hydraulic conductivity relationship to soil water content

As soil water content decreases, hydraulic conductivity decreases

Hydraulic conductivity relationship to soil type

Coarser soils have higher hydraulic conductivity, and finer soils have lower hydraulic conductivity

Hydraulic/intrinsic permeability (k)

Describes the ability of the geologic formation or rock alone to transmit a fluid. Does not depend on the fluid type

Root zone/soil water zone

Uppermost layer of the vadose zone, starting directly at the ground surface. Contains plant roots, organic matter, and soil moisture. Water here moves via infiltration and is lost to the atmosphere through evapotranspiration

Intermediate zone

Middle layer of the vadose zone (between root zone & capillary fringe). Acts as a transmission path where percolating precipitation moves downward toward the water table due to gravity

Capillary fringe

Transition layer immediately above the water table. Water here experiences capillary action

Groundwater zone

Subsurface region starting at the water table and extending downward, pore spaces are fully saturated

Equipotential lines

Contours connecting points of equal hydraulic head in a groundwater system. Groundwater naturally flows from areas of higher equipotential to areas of lower equipotential

Groundwater residence time

Length of time a water molecule spends inside an underground aquifer before returning to the surface (2 weeks to 10,000+ years)

Groundwater recession

Gradual decline in underground water levels or baseflow to streams when there is little to no active recharge

Groundwater recession equation

Q = Q0e^-ct

Surface recharge

Water is flooded into large basins or released into natural river channels and washes, letting it slowly percolate down through the soil into the aquifer

Subsurface recharge

Water is injected directly into deep wells to store in the aquifer, which usually requires much cleaner, higher-compatibility source water because it skips natural soil filtration

Tension head

Same as pressure head, but occurs in unsaturated zones (pressure head is negative)

Regional groundwater flow and effects of geology (Mendenhall Valley)

Unconfined upper aquifer composed of permeable glacial and outwash deposits extending down to roughly 215 ft