Hydrology Unit 5: Evaporation

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Questions and Answers

Why is it important to have a jug that holds more than 2 liters of water?

  • To prevent the jug from overflowing when adding water to the pan.
  • To ensure the jug can hold enough water to refill the pan after heavy rain.
  • To make it easier to measure small amounts of evaporation.
  • To allow for the evaporation of water from the pan during the measurement process. (correct)

How is the measuring jug scaled to measure less than 2 mm of evaporation?

  • The jug is filled with a known amount of water and the volume is marked.
  • The jug is weighed and the weight of the water is used to determine the volume and corresponding evaporation.
  • The distance from the top marker to the bottom of the jug is divided by 20 and marked with increments of 0.1 mm. (correct)
  • The jug is filled to the top mark and the water level is observed after a specific time period.

What is the purpose of the PVC pipe supporting the pointer during evaporation measurement?

  • To help maintain a constant water level in the pan.
  • To ensure that the pointer is always submerged in the water.
  • To reduce wave motion in the pan, allowing for a more accurate measurement. (correct)
  • To prevent the pointer from being moved by wind.

When rainfall exceeds the pan's capacity, what should be done to ensure accurate evaporation measurement?

<p>Empty the pan and refill it to the pointer level before adding water from the jug. (A)</p> Signup and view all the answers

In the text, how is evaporation calculated when rainfall occurs before evaporation measurement?

<p>The amount of rainfall is subtracted from the total water removed from the pan to determine evaporation. (B)</p> Signup and view all the answers

If 5 mm of rainfall occurs and 2 mm of water are added to the pan with the measuring jug, what is the evaporation measurement?

<p>7 mm (C)</p> Signup and view all the answers

What is the primary purpose of measuring both rainfall and evaporation?

<p>To assess the impact of weather conditions on water levels. (C)</p> Signup and view all the answers

What is the recommended time for performing evaporation measurements?

<p>9:00 am, when the day has just started. (B)</p> Signup and view all the answers

Which condition is essential for the appearance of subsurface flows?

<p>The hydraulic lateral conductivity is superior to the vertical conductivity. (C)</p> Signup and view all the answers

How does the drainage capacity of subsurface flows compare to superficial and groundwater flows?

<p>Faster than superficial flows but slower than groundwater flows. (C)</p> Signup and view all the answers

What type of flow occurs when infiltrated effective rainfall circulates in the superior soil layer?

<p>Subsurface flow (B)</p> Signup and view all the answers

In which regions is subsurface flow dominant?

<p>Humid regions with vegetal covering and well-drained soils (B)</p> Signup and view all the answers

What historical term was used to refer to subsurface flow?

<p>Hypodermic flow (D)</p> Signup and view all the answers

What does the term $H_n$ represent in the energy balance equation?

<p>Net heat energy received by the water surface (C)</p> Signup and view all the answers

Which of the following terms can be neglected when time periods are short?

<p>Net heat conducted out of the system by water flow (B), Heat stored in the water body (C)</p> Signup and view all the answers

How is the sensible heat term $H_a$ estimated?

<p>Through Bowen's ratio calculation (D)</p> Signup and view all the answers

In the context of evaporation estimation, which factor contributes to the latent heat of evaporation?

<p>Density of the water (B)</p> Signup and view all the answers

What is the purpose of the Mass-Transfer Method?

<p>To calculate mass water vapor transfer to the atmosphere (D)</p> Signup and view all the answers

Which variable is NOT included in the Bowen's ratio equation?

<p>Density of water (D)</p> Signup and view all the answers

What is the significance of the errors being around 5% in evaporation estimation?

<p>Reflects satisfactory results for short-term assessments (B)</p> Signup and view all the answers

What does the latent heat of evaporation formula depend on?

<p>Density of water times evaporation amount (A)</p> Signup and view all the answers

What is the recommended percentage open area for a well screen to prevent incrustation and corrosion?

<p>15 to 18% (D)</p> Signup and view all the answers

Which of the following is NOT one of the assumptions of Dupuit's equations?

<p>The aquifer has variable thickness. (D)</p> Signup and view all the answers

Which condition can lead to unsteady flow in a well?

<p>Prolonged pumping with no nearby recharge. (A)</p> Signup and view all the answers

What does the Theis equation assume about the water being pumped from a well?

<p>It is immediately released from the aquifer's storage. (D)</p> Signup and view all the answers

What does Dupuit's equation assume about well efficiency?

<p>It is assumed to be 100%. (A)</p> Signup and view all the answers

Which of the following characteristics of flow around a well is assumed in Dupuit's derivation?

<p>Laminar and radial flow. (D)</p> Signup and view all the answers

Under what circumstance can drawdowns in a well increase with prolonged pumping?

<p>When there is no natural recharge available. (A)</p> Signup and view all the answers

Which investigators contributed to the study of hydraulics of wells in unsteady flow conditions?

<p>Theis, Jacob, Chow, and others. (C)</p> Signup and view all the answers

Which geological formations serve as effective aquifers?

<p>Unconsolidated gravels and sandstones (A), Limestones with cavities and glacial deposits (C)</p> Signup and view all the answers

What is an aquiclude?

<p>A formation that cannot transmit significant amounts of water (A)</p> Signup and view all the answers

What characterizes an aquifuge?

<p>It can neither absorb nor transmit water. (C)</p> Signup and view all the answers

How is specific yield defined?

<p>The percentage of water that drains by gravity from a saturated aquifer. (D)</p> Signup and view all the answers

Which of the following best describes a confined aquifer?

<p>It is capped with an impermeable layer beneath a homogeneous porous layer. (A)</p> Signup and view all the answers

What is the formula relating porosity, specific yield, and specific retention?

<p>porosity = specific yield + specific retention (C)</p> Signup and view all the answers

What does specific retention refer to?

<p>Percentage volume of water that cannot drain by gravity. (B)</p> Signup and view all the answers

What is a key factor influencing specific yield in aquifers?

<p>Grain size, shape, and distribution of pores. (A)</p> Signup and view all the answers

Which of the following is NOT a factor that affects pan evaporation?

<p>Soil type (C)</p> Signup and view all the answers

What is the main advantage of large weighable lysimeters over smaller ones?

<p>They provide more precise measurements of evapotranspiration. (C)</p> Signup and view all the answers

Why is it important to use the same type of pan in measuring evaporation?

<p>Different pan types affect the evaporation rate differently. (B)</p> Signup and view all the answers

Which of these is NOT a technique used in hydraulic weighing systems for lysimeters?

<p>Magnetic resonance imaging (A)</p> Signup and view all the answers

What is the primary use of micro lysimeters?

<p>Measuring soil evaporation (C)</p> Signup and view all the answers

According to the passage, which of the following techniques would be most appropriate for accurate measurement of evapotranspiration in a research center?

<p>Large weighable lysimeters with load cells (A)</p> Signup and view all the answers

How can pan evaporation measurements be used in agriculture?

<p>Estimating the amount of water required for irrigation (A)</p> Signup and view all the answers

What is the main difference between weighable lysimeters used for short-term and long-term measurements?

<p>The size and design of the lysimeter (D)</p> Signup and view all the answers

Flashcards

Weighable Lysimeter

An instrument to measure evapotranspiration with high precision.

Micro Lysimeter

A small device for measuring soil evaporation.

Evapotranspiration

The process of water evaporation from soil and plant surfaces.

Pan Evaporation

A method to measure evaporation using an open pan.

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USWB Class A Pan

Standard evaporation pan used for uniform measurements.

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Hydraulic Weighing Systems

Measure weight using fluid pressure changes.

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Load Cells

Devices that measure weight using strain gauges.

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Evapotranspiration Modelling

Simulating evapotranspiration to estimate water needs.

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Jug capacity

The jug must hold more than 2 liters to prevent overflow.

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Evaporation scaling

For less than 2 mm evaporation, measure the jug's height and divide by 20.

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Evaporation measurement

Water is added to the pan until the pointer breaks the water's surface.

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Tracking jugs

Keep track of how many jugs refill the pan and the last jug's reading.

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Rainfall measurement

Measure rainfall alongside evaporation for complete data.

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Overflow management

Empty the pan after heavy rain to maintain pointer level.

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Evaporation calculation after rain

Subtract water removed from the pan from total rainfall for evaporation.

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Daily evaporation recording

Measure evaporation daily at 9:00 am and record results.

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Net Heat Energy (𝐻ₙ)

Total energy received by the water surface in a day.

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Energy Balance Equation

𝐻ₙ = 𝐻𝑑 + 𝐻𝑒 + 𝐻𝑔 + 𝐻𝑠 + 𝐻𝑖 shows heat distribution.

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Sensible Heat Transfer (𝐻ₐ)

Heat transfer from water surface to air.

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Heat Used in Evaporation (𝐻𝑒)

Heat energy consumed to convert water into vapor.

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Bowen's Ratio (𝛽)

Ratio used to estimate sensible heat transfer.

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Evaporation Calculation

Evaporation can be estimated using heat terms and Bowen's ratio.

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Energy-Budget Method

Method for estimating lake evaporation with high accuracy.

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Mass-Transfer Method

Calculates water vapor transfer using turbulent flow theories.

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Subsurface Flow

Flow of water that circulates horizontally in the upper soil layer, often influenced by impermeable layers.

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Law of Darcy

A principle that describes the flow of fluid through porous media based on hydraulic gradient and conductivity.

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Confined Aquifer

An aquifer that is bounded by impermeable layers above and below, preventing water from easily flowing in or out.

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Unconfined Aquifer

An aquifer that has no upper impermeable layer, allowing water to flow freely and recharge from rainfall.

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Hydraulic Conductivity

A measure of a material's ability to transmit water through its pores or fractures.

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Aquifer

Geologic formation that can absorb and transmit significant water.

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Aquiclude

Layer that absorbs water but can't transmit it significantly.

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Aquifuge

Geologic formation with no interconnected pores, no water absorption or transmission.

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Aquitard

Formation that transmits water slowly, insufficient for pumping.

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Specific Yield (Sy)

Volume of water that drains by gravity from an aquifer.

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Specific Retention (Sr)

Volume of water held in an aquifer that won't drain by gravity.

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Porosity

Measure of the water-bearing capacity of a formation.

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Dupuit’s Equations

Mathematical equations used for groundwater flow analysis considering various conditions and assumptions.

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Assumptions of Dupuit's Equations

Conditions necessary for the validity of Dupuit’s equations, including stabilized drawdown and homogenous aquifers.

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Stabilized Drawdown

Condition when pumping has been ongoing long enough for the flow to reach a steady state.

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Homogeneous Aquifer

An aquifer with uniform properties throughout, such as constant permeability.

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Radial and Horizontal Flow

Flow pattern in aquifers where water moves outward in straight lines from a well.

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Laminar Flow

Smooth, orderly flow of groundwater that follows Darcy’s law.

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Transient Flow Conditions

Flow conditions that change over time, often seen when there is no nearby recharge source.

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Theis Equation

An equation describing unsteady radial flow into a well, assuming immediate release from aquifer storage.

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Study Notes

Preface

  • This hydrology module aims to detail the theory and application of hydrology.
  • The course covers the hydrologic cycle and related processes such as precipitation, evaporation, infiltration, overland flow, groundwater flow, and surface runoff generation.
  • The module development was influenced by COVID-19 pandemic comments and peer reviewer feedback, alongside faculty input intended to ensure quality module distribution.

Unit 5: Evaporation

  • Definitions: Evaporation is the process of a liquid changing to a gas below boiling point due to heat transfer.
  • Factors Affecting Evaporation:
    • Vapor Pressure: The rate is proportional to the difference between saturation vapor pressure (at the temperature) and actual vapor pressure in the air.
    • Temperature: A higher water temperature results in a faster evaporation rate.
    • Wind: Increased wind removes evaporated water vapor, increasing evaporation rate up to a critical wind speed.
    • Atmospheric Pressure: Lower pressure generally leads to faster evaporation.
    • Soluble Salts: The presence of salts in water lowers the evaporation rate.
    • Heat Storage: Deep water bodies store more heat, affecting seasonal evaporation patterns compared to shallow bodies.

Unit 5: Evaporation (Continued)

  • Measurement:
    • Lysimeters: Measure evapotranspiration by recording precipitation and water loss in soil-filled containers, used for long-term measurements of plants, often crops or trees.
    • Evaporation Pans: Standardised pans such as the Class A pan used for measuring evaporation from free water surfaces.

Unit 6: Basic Subsurface Flow

  • Introduction: Subsurface flow is the horizontal movement of infiltrated water. It's affected by the presence of an impermeable layer.
  • Darcy's Law: The velocity of groundwater flow is proportional to the hydraulic gradient. The equation is V = ki, where V is velocity, k is hydraulic conductivity and i is the hydraulic gradient.
  • Aquifers: Formations that transmit water efficiently.
    • Unconfined Aquifers: Water table aquifers, where the water level is at the top of the saturated zone.
    • Confined Aquifers: Artesian aquifers, where pressure forces water to rise above the water table.
  • Transmissibility: The flow capacity of an aquifer per unit width under a unit hydraulic gradient, and is equal to the product of permeability times the saturated thickness of the aquifer.
  • Water Conditions:
    • Steady radial flow into a well.
    • Steady state conditions; pumping has been continuous long enough to reach equilibrium.

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