Aquifer Hydraulics

• These are study notes on the topics of Runoff and Groundwater

Runoff

• Runoff includes the processes and pathways where excess water becomes streamflow.
• It is the part of total rainfall that flows off land, drains from soil, and isn't consumed through evapotranspiration.
• A streamflow hydrograph is a tabular or graphical representation of streamflow discharge against time.
• The hydrograph plots Discharge (Q) vs Time (T)

Types of Flow

• Surface runoff, or overland water flow, is a pathway where excess water flows over the soil surface.
• Shallow concentrated flow occurs when minor rivulets form just downstream from overland flow.
• Open channel flow is surface flow outside the confines of a stream channel.

Runoff Process

• Precipitation must meet the demands of evaporation, interception, infiltration, surface storage, surface detention, and channel detention before runoff.
• Runoff occurs when rainfall intensity exceeds infiltration rate.
• Water fills depressions on the soil surface before overland flow begins.
• Surface detention involves the buildup of water on the surface.
• Channel detention involves flow moving into defined channels.

Factors Affecting Runoff

• Rainfall impacts runoff through duration, intensity, and areal distribution.
• Duration influences total runoff.
• Intensity influences the rate and volume of runoff.
• Intense storms decrease infiltration rate because of destructive action on soil structure.
• Areal distribution of rainfall influences the rate and volume of runoff.
• Watershed characteristics impact runoff, including size, shape, orientation, topography, geology, and surface culture.
• Runoff decreases for storms moving upstream.
• Flat areas have low runoff.
• Geology, soil, materials, and vegetation affect the rate of infiltration, thus runoff.
• Structures, like dams, affect runoff rate.
• Topography depends on surface smoothness and slope.
• Steeper slopes cause quick flow with less evaporation and absorption, resulting in greater runoff.
• Catchments in mountainous areas on the windward side have more rainfall and runoff.
• Geological characteristics such as surface soil, subsoil, rock type, and permeability affect runoff.
• Rocky surfaces result in more runoff while rocks with fissures result in less runoff.
• Meteorological characteristics such as temperature, wind, and humidity impact runoff.
• Low temperature and saturated ground increases runoff.
• High temperature and greater wind velocity increases evaporation loss, reducing runoff.
• Storage characteristics such as artificial and natural storage tend to reduce peak flow and also increases evaporation losses.
• Artificial examples: dams, weirs, etc
• Natural storage examples: lakes, ponds

Measurement of Streamflow

• Stage refers to the height of water in a stream.
• A rating curve is a graph of the stage and streamflow discharge.
• Control section: Section of the stream for which a rating curve is developed
• A control section stream should be stable and have sufficient depth for velocity measurements.
• Stream should be straight for a distance upstream equal to 5x the width, and downstream equal to 2x the width
• Stream bed should be smooth and free from vegetative growth.
• A staff gauge is a graduated staff used for visual observation of water level.
• Continuous water level recorders can also be used.
• Float method determines the time required for a floating object to travel a specified distance on the surface of the water to measure velocity
• The formula for Velocity is distance/time
• Average velocity is considered 80-85% the surface velocity

Velocity Measurements

• The average velocity is then multiplied by the cross-sectional area of stream to estimate discharge
• Q = VA, Q = streamflow discharge, V = average flow velocity, A = cross sectional area of flow
• Current meter method utilizes an instrument with a wheel that rotates by the force of current.
• The speed of rotation depends on water velocity.
• The cross-sectional area of the stream is divided into a number of vertical sections where the streamflow velocity is estimated by the measured velocity w/ current meter.
• Area of each section its determined so that the average area and velocity can be obtained
• Q = Σ(AiVi), n = number of sections
• The use of pre-calibrated structures is for watersheds < 1,000 ha.
• Weirs are barriers placed across streams over which water flows that provide a simple, accurate means of measuring water
• Head is the height of water above the crest.
• Crest is the edge or surface over which the water flows
• The are rectangular, trapezoidal (Cipolletti), or triangular (V-notch) weirs
• A sharp crested weir has a blade w/ a sharp upstream edge, so the passing water touches only a thin edge
• Broad crested weir has a flat or broad surface over which the water flows
• Flumes are artificial open channels built to contain streamflows within designed cross-sectional areas and length.
• Parshall and Trapezoidal flumes are types of artificial open channels

Groundwater Basics

• Groundwater results predominantly from precipitation that infiltrates and percolates to the zone of saturation.
• In terms of water use priority from precipitation, it has the lowest priority and is recharged by surface waters.
• The zone of aeration consists of soil pores containing both water and air; the water in the upper layers is called soil moisture.
• The zone of saturation has soil pores or rock filled wit water; the water in this zone is called groundwater.
• The capillary fringe is a subsurface layer where groundwater seeps up from a water table by capillary action to fill pores.
• An aquifer a geologic formation containing water and transmits it at a rate sufficient to be economically developed for pumping.
• An aquiclude contains water, but transmits it at a rate insufficient to be economically developed for pumping.
• An aquitard, or confining unit, is a low-permeability unit storing groundwater and slowly transmits it from one aquifer to another.
• Water table: boundary between water-saturated and unsaturated ground.
• Influent stream loses water to the ground, contributing to the zone of saturation.
• Effluent stream gains water from groundwater.

Types of Aquifer

• Unconfined aquifer having a water table service as the upper surface of the zone of saturation
• Confined or artesian aquifer where the groundwater is confined by a relatively impermeable layer
• Semi-confined or leaky aquifer has an upper confining layer that is leaky or semi-permeable.
• Perched Aquifer’s lower limit is a relatively impermeable layer with an upper free water surface on water table

Aquifer Properties

• Porosity is the volume of void space that can hold water in the zone of saturation as proportion of the total volume
• n = Vv / Vt =0. m3 / 1.0m3 = 0.30, Vv= .Volume of voids(V), Vt = Total volume(V)
• Specific yield: volume of water that can be extracted of gravity flow, measure of the volume of water yield per unit soil volume
• Specific retention: water retained the soil formation against the force of gravity.
• Aquifer conductivity/Permeabiltiy: Ability of a porous material to allow fluids to pass through
• Transmissivity measures potential well discharge penetrating an aquifer, magnitude indicate economic value of aquifer as source of water supply.
• Storage cofficient: water yielding capacity of a confinement aquifer.

Threats to Groundwater

• Quantity of groundwater: An increased quantity of groundwater is being withdrawn to meet the demands of a growing population.
• Overdraft occurs when groundwater is removed faster than recharge can replace it, resulting in permanent capacity loss, unusable quality water, & salt water instruction.
• Subsidence can occur from over pumping so that the water pressure us reduced: Land Above Aquifer can sink causing Damage property
• Quality: Pathogens, Inorganic and organic compounds can harm wat quality: Scientists learn more to prevent contaminants.

Potential Sources of Contamination/Pollution

• Point source : contaminates that originates from a single pollution
• Non- point source: contaminations derived from multiple soures

Factors that effect the speed of the movment of groundwater

• Hydraulic granditent- is the slope of the water tabe
• Permeability- the ability of the pore material
• Darcys law: reported results of experiment used to enhance water of city and of France for water treatment
• Radial Groundwater Flow is the area used to confine and unconfined aquifer
• Steady state condition area with no changed that can occur w. time

Terms

• Radical symmetry - flow towerd well is symetric. The value depends on the direction
• Cones depression- pumped well surface were weilled level willed be lowered

Questions about radial flow, streamflow and aquifers. Topics include transmissibility, discharge estimation, and the impact of multiple pumping wells.