Hydrology Midterms

Hydrology

The study of the occurrence, circulation, storage and distribution of surface and ground water on the earth.

Precipitation

Evaporation

Transpiration

Infiltration

Ground water

Runoff

The Hydrologic Cycle

Precipitation

Rainfall, snow,etc

Evaporation

conversion of water to water vapor from a water surface

Transpiration

loss of water vapor through plant tissue and leaves

Infiltration

water entering the soil system, function of soil moisture, soiltype

Groundwater

flows in porous media in the subsurface

Runoff

Overland flow, portion of precipitation that does not infiltrate

Evaporation

When high temperatures turn liquid drops to gaseous molecules.

Condensation

When low temperatures turns gaseous molecules to liquid drops.

Precipitation

When liquid drops fall to the surface that were formed in a saturated atmosphere

Infiltration

Absorption and downward movement of water from the surface of the soil

Percolation

Vertical movement of water down the bedrock

Atmoshpere
Ocean
Cryoshphere
Biosphere
Lithosphere

The climate system

Highly complex global system consisting of 5 major interlinked components:

Atmoshpere

The most unstable and rapidly changing-where weather occurs

Ocean

High thermal inertia– important in regulating atmospheric variations

Cryoshphere

Ice sheets and sea ice

Lithosphere

Geosphere–solid earth

Ideal Gas Law

Describes behavior of gas under different conditions

Gas law

Pressure and Temperature are directly related at constant density

Temperature and Air Density (n/V) are inversely related

Decrease in temperature increases density

Affects movement of air masses

High pressure moves toward low pressure

Humidity

Measure of amount of water vapor in atmosphere

Specific Humidity

the mass of water vapor in a unit mass of moist air at a given temperature

Relative Humidity

ratio of (air’s actual water vapor content) to (amount of water vapor at saturation for that temperature)

less

Cool air “holds” ___ water

Vapor Pressure

partial pressure exerted by water vapor

Dew Point Temperature

temperature that an air mass with constant pressure and moisture content becomes saturated

Precipitation

Condensed water vapor that falls to earth

Occurs when airparcel reaches saturation

      i.e. the Dew Point Temperature is reached

Heat must be removed from moist air to allow for condensation

      Latent Heat

      Major energy source for storm systems

Moisture source

Lifting and resultant cooling

Phase change occurs with condensation onto small nuclei in the air

Range from 0.1u–10u

Come from ocean salt,dust, etc

Droplets grow large enough to overcome drag and evaporation

Requirements for Precipitation Formation

Convective

Cyclonic

Orographic

Lifting Mechanisms

Precipitation often classified by vertical lifting

Convective

Intense heating of the ground expansion and vertical rise of air

Cyclonic

Movement of large air-mass systems (warm/cold fronts)

Orographic

Mechanical lifting of moist air masses over the windward side of mountain range

Thunderstorms

Heavy rainfall, thunder, lightning, hail

Result from strong vertical movements or warm, moist air

Low-pressure systems

Surface heating

Forced ascent over mountains

Thunderstorms Generally occur due to instability caused by:

Cumulus Stage

Mature Stage

Dissipating Stage

Thunderstorm Stages

Cumulus Stage

Moist air rises, cools, and condenses into cumulus clouds and continues to rise and condense

Updraft

Mature Stage

Rain begins to fall

Surrounding dry air is drawn into storm, evaporates some drops and cools the air Denser, cold air descends (downdraft) and creates cool gusts of wind at ground level

Dissipating Stage

When the updraft is cutoffcut off

Rate of precipitation decreases

Downdrafts die-off

Clouds dissolve

Hurricanes

Intense cyclonic storms

Form over tropical oceans

Energy comes from the condensation of very warm, humid, tropical air

Categorized by the Saffir -Simpson Hurricane Windscale

Hurricane

Cyclone
Typhoon

Baguio

Hurricanes localized names

N america

India

East asia

china sea

Minor

Saffir -Simpson Wind Scale

Damage Description

Some flooding

Minimal

Saffir -Simpson Wind Scale

Damage Description

Limited damage, unanchored mobile homes, trees

Moderate

Saffir -Simpson Wind Scale

Damage Description

Some roof, door and window damage

Extensive

Saffir -Simpson Wind Scale

Damage Description

Some structural damage to residences and utility buildings

Extreme

Saffir -Simpson Wind Scale

Damage Description

Extensive curtainwall failures, complete rooffailures, all signs blown down

Catastrophic

Saffir -Simpson Wind Scale

Damage Description

Complete rooffailure and some complete building failures

Seasonal variation or monthly distribution

Precipitation Trends of rainfall is important in water resources planning particularly in analyzing reservoir structures.

Hourly or even more detailed variations of rainfall

Precipitation Trends are important in planning water resources projects especially urban drainage systems.

Rain gage

radars

satellite data

Three ways of measuring rainfall data

Weighing type

Float and siphon

tipping-bucket

Type of Recording Rain Gauges

Radars

Unlike rain gages, they provide spatial and temporal variations of rainfall
However data acquired must be adjusted with rain gage measurements

Sensing rainfall

Recording the data

Transmitting to central location

Translating data

Editing or checking for errors

Storing in database

Retrieving for further use

Measurement Process

Barometer

Measurement Devices for Atmospheric pressure

Psychrometer

Measurement Devices for Relative Humidity

Gages

Measurement Devices for Precipitation

Radar

Measurement Devices for Rainfall Rates

Hyetograph

Plot of rainfall intensity (in./hr) vs. time

Often used as input to hydrologic comouter models for predicting watershed response to input rainfall

Double mass Analysis

It is a plot of successive cumulative annual precipitation of a suspect gauge versus the cumulative annual precipitation of other gauges in the same region for the same duration.

Double mass Analysis

A change in proportionality between the measurements at the suspect station and those of the region is reflected in a change in the slope of the trend of the plotted points

Areal Precipitation

The average depth of precipitation over a specific area (watershed)

Use point measurements to determine avg

Arithmetic Mean

Thiessen Polygon Method

Isohyetal Method

three methods of areal precipitation

Arithmetic mean

Takes arithmetic mean of rainfalls from available gages

Not accurate for large areas with variable distribution

Only works if gages are uniformly distributed

Thiessen Polygon Method

Areal weighting of rainfall for each gage

Series of polygons created by lines connecting each gauge and perpendicular bisectors

Uses ratio of polygon area to total area of interest

Most widely used method

Isohyetal Method

Draw contours of equal precipitation based on gauge data

Uses area between each contour

Needs an extensive gauge network

Most accurate method

ABSTRACTIONS

Water that does not appear as surface runoff

EVAPORATION

Phase change from liquid to vapor

Critical for large water storage reservoirs

Surface pan

OBTAINING EVAPORATION

U.S. Weather Bureau Class A Pan

Most common type. 4 ft diameter, 10 in deep. Galvanized steel, set 6 in above ground on a slatted platform.

Minimal heat storage; allows easy correction for advected energy.

Floating Pan

OBTAINING EVAPORATION

Mounted on a raft to sit on the water surface.

Conceptually ideal (same environment as water body). Difficult to maintain, inaccessible, and subject to splashing

Sunken Pan

OBTAINING EVAPORATION

Placed in the ground with the water surface near ground level.

Stable environment. Difficult to maintain (trash), complex heat transfer calculation, and risk of inaccurate estimates if the pan leaks.

TRANSPIRATION

Water vapor escape from living plant leaves. Measured using devices like the phytometer

EVAPOTRANSPIRATION

Combined loss from soil evaporation and plant transpiration Potential ET : Max possible loss

INFILITRATION

Water penetrating from the ground surface into the soil

Primary component of total loss

Capacity is the maximum rate the soil can absorb.

If rainfall intensity exceeds capacity, runoff occurs

Capacity is time-variable, greatest at start of storm

Land use

soil type (texture, structure)

vegetative cover

initial soil moisture content

depth to ground water table

Intensity and duration of rainfall

INFILITRATION FACTORS

PHI – INDEX METHOD

INFILITRATION MODELS

A constant abstraction rate used to partition rainfall into net rain and loss, such that the volume of net rain equals measured direct runoff

W-Index

INFILITRATION MODELS

An average rate of infiltration over the period of rainfall excess.

Infiltration Capacity Curves (Horton’s)

INFILITRATION MODELS

Empirical approach where capacity decays exponentially from initial to final capacity

Capacity Curves (Green-Ampt Model)

INFILITRATION MODELS

An operational model based on unsaturated flow theory. Widely used in runoff simulation. Requires parameters like hydraulic conductivity and wetting front suction head

Infiltrometers

INFILITRATION MEASUREMENT

Double-Ring Infiltrometers measure infiltration capacity by isolating a sample area. Sprinkler Infiltrometers simulate natural rainfall intensity

Hydrograph Analysis

INFILITRATION MEASUREMENT

The infiltration capacity can be determined indirectly by accurately measuring varying rainfall intensities and the resulting runoff hydrograph

Watershed

Contiguous area that drains to an outlet, specifically in regards to precipitation

Basic hydrologic unit within which measurements, calculations & predictions are made

Catchment boundary

line that separates the watershed from its adjacent watershed

Drainage Basin / Catchment Area

The entire area where surface runoff collects to drain into a single river.

Drainage Divide

The boundary line along a topographic ridge separating one drainage basin from adjacent ones

Concentration Point (Measuring Point)

The location where all surface drainage concentrates and flows out of the basin.

Time of Concentration (tc)

The time required for rain falling at the most distant point on the catchment fringe to reach the concentration point

StormCharacteristics

Four major factors affecting flow

The nature and distribution of the rainfall event itself.

Meteorological Characteristics

Four major factors affecting flow

Broader atmospheric conditions during the event.

•BasinCharacteristics

Four major factors affecting flow

Fixed physical properties of the watershed.

StorageCharacteristics

Four major factors affecting flow

Natural features that temporarily hold water

Intensity & Duration

Storm &Meteorological Impacts:

High intensity storms over small areas increase runoff because losses (like infiltration) are less pronounced. Low intensity, long-spell storms contribute more to groundwater storage.

Antecedent Precipitation

Storm &Meteorological Impacts:

If a storm follows a previous one closely (succession of storms), the initial wetness of the soil leads to greatly increased runoff.

Temperature/Frozen Ground

Storm &Meteorological Impacts:

Rain during winter or over frozen ground greatly increases runoff, as infiltration is restricted

Size and Slope:

Basin characteristics and the Flood intensity:

Peak runoff decreases as the catchment size increases, due to higher time of concentration. Steep, rocky catchments produce more runoff compared to flat, vegetated areas.

Land Use/Vegetation

Basin characteristics and the Flood intensity:

Thick vegetation promotes greater water absorption and interception, resulting in less runoff. Poor land management (like converting forests to urban areas) increases runoff sharply.

Shape Fan-Shaped Catchment

Basin characteristics and the Flood intensity:

The geometry of the basin determines flow synchronization.

Produces greater flood intensity because flow contributions from all parts arrive quickly and simultaneously.

Shape Fern-Shaped (Elongated) Catchment

Basin characteristics and the Flood intensity:

The geometry of the basin determines flow synchronization.

Distributes discharge over a longer period, resulting in lower flood intensity.

Storm Direction

Basin characteristics and the Flood intensity:

If the storm moves down the stream, it produces a greater flood discharge than if it moves upthe stream

Surface Storage

Storage characteristics and Runoff moderation:

Includes depressions (puddles), pools, and lakes.

Channel Storage (Valley Storage)

Storage characteristics and Runoff moderation:

Water held within the main stream channels themselves, including temporary bank storage in permeable banks during floods.

Upstream Structures

Storage characteristics and Runoff moderation:

Reservoirs, tanks, and check dams intentionally moderate flood magnitudes due to their storage effects.