Watershed Hydrology Final

510,000 km3

510,000 km3

global annual mean precipitation

air circulation, elevation changes, temp differences b/t land and water

factors affecting distribution of precipitation

vapor pressure deficit

the difference between saturated vapor pressure and the actual vapor pressure at a given temperature

specific humidity (g/kg)

ratio of the mass of water vapor to the mass of the total air (=Mw/Ma+Mw)

absolute humidity (g/m3)

mass of water vapor to volume of air (=Mw/V); independent of temperature and pressure

relative humidity (%)

amount of water vapor present in the air (actual vapor pressure ea) compared to the maximum amount of water vapor the air can hold (saturated vapor pressure es) at a given temperature (=ea/es *100)

convective precipitation

warm moist air rises and cools causing water vapor to condense into clouds that eventually form precipitation; common in areas with high levels of atmospheric instability, such as tropical regions or areas with large temperature variations

frontal precipitation

mixing/ collision of warm and cold air masses, precipitation forms along boundary; common in mid-latitude regions

orographic precipitation

moist air lifts above a mountain range, where it cools and condenses; windward side (facing the wind) receives majority of precipitation while leeward side has rain shadow

rain

droplet diameter >0.5 mm

drizzle

droplet diameter <0.5 mm

hail

regularly or irregularly shaped lump of ice

sleet

ice pellets

snow

frozen flakes

three characteristics used to describe drops of precipitation

shape, size, velocity

interception

loss of rain water to vegetation cover/ canopy

increases, canopy water holding capacity

canopy water balance concept: interception until __________ is reached

forest

litter interception is most important in ________ ecosystems

throughfall

precipitation that falls directly or indirectly onto the soil

gross rainfall - throughfall - stemflow

interception (canopy + litter) water budget balance equation

oceans

source of most water on earth

glaciers

largest source of freshwater on earth

higher

lower intensity rainfall leads to _____ rate of interception

higher

liquid precipitation causes ________ rate of interception than frozen precipitation

higher

high wind speed causes _______ rate of interception b/c it causes evaporation and decreases canopy water holding capacity

non-recording gage

measures total rainfall depth during a certain period of time

recording gage

continuously registers rate of rainfall and time of occurrence

arithmetic mean

method for calculating area total precipitation; sum point rainfall values and divide by number of points (ex recording stations)

Thiessen polygon method

method for calculating area total precipitation; divides basin into smaller polygons containing one recording station, value from the station is weighted based on area of the polygon (essentially a weighted average)

isohyetal method

method for calculating area total precipitation; draws contour lines of equal precipitation across watershed

exceedance probability

P=m/(n+1); probability that a rainfall event of a higher intensity than a given rank (m) will occur

return period

T=(n+1)/m; amount of time estimated for a rainfall event of a certain intensity to occur (ex “100 year flood”)

rooting, intermediate, capillary fringe

three zones of the unsaturated/ vadose zone of the soil profile in order from shallow to deep

saturated zone

area in the soil profile where pore spaces are filled with water

sand

2-0.05 mm, most coarse, drains quickly

silt

0.05-0.002 mm, in the middle in terms of coarseness

clay

<0.002 mm, most fine, holds the most water

soil porosity (f)

empty pore space compared to total volume of the sample

particle density

density of only the soil particles with all pore space compressed out

bulk density

density of the soil in its “bulk” or natural state (consisting of particles and pore space)

soil water content

volume or mass of water contained in a certain volume of soil

infiltrometer

single or double ring, used to directly measure infiltration

Horton’s equation

used to estimate infiltration given change in infiltration capacity, time, and recession constant

lower, runoff

higher intensity rainfall leads to ______ infiltration and higher __________ because soil becomes saturated more quickly

lower

vegetation cover can _______ intensity and therefore infiltration

unconfined aquifer

aquifer whose lower limit is impermeable rock but upper limit is the water table, allowing for interactions with the unsaturated zone and surface (can rise and fall, subject to infiltration, capillarity, and evaporation)

confined aquifer

aquifer that is under pressure between two impermeable layers of rock/soil, allows water to rise in wells, do not receive direct infiltration

perched zone

small area that is seasonally saturated very near the surface due to an impeding low permeability layer

specific yield (Sy)

ratio of volume of water that drains from saturated ground b/c of gravity to total volume of bulk soil; affects aquifer storage volume (porosity=Sy+Sr)

specific retention (Sr)

ratio of volume of water retained against gravity to total volume of bulk soil (porosity=Sy+Sr)

hydraulic conductivity (k)

the capability of a porous medium to transmit water (length of travel within certain time); remains constant under saturated conditions

Darcy’s law

describes the flow of groundwater under saturated conditions

percolation from unsaturated zone

aquifers are fed by _______

discharge region

area of a watershed where groundwater exits to the surface

groundwater

largest source of unfrozen freshwater on earth

direct runoff

consists of surface and subsurface runoff; input of precipitation that flows into bodies of water; measured in unit of length

baseflow

groundwater discharged into a surface water body

peak discharge

highest point of the hydrograph curve; highest discharge that occurs during a rainfall event

peak time

time from the start of the rainfall event to the peak discharge

lag time

time from the heaviest rainfall to the peak discharge (peak of rain curve to peak of hydrograph)

baseflow separation

part of the hydrograph that represents the streamflow that would exist without the rainfall event and subsequent runoff; draw straight line from beginning to end of direct runoff on hydrograph (horizontal line across bottom of curve, inexact)

time of concentration

length of time it takes for water to travel from the farthest extend of the watershed to the watershed outlet; depends on ground cover, slope, and distance

rainfall - infiltration - storage

runoff water budget balance equation

SCS curve number method

method for estimating runoff potential of different soil types with different land use/ cover

sheetflow

a thin layer of water flowing across the land surface under the influence of gravity, not confined to a channel

uniform flow

when the velocity pattern of flow does not change over the cross-section of the stream

laminar flow

smooth flow occurring at a Re <2000

Reynold’s number (Re)

ratio of inertial forces to viscous forces in order to classify flow as laminar (<2000) or turbulent (>3000)

Froude’s number (Fr)

ratio of inertial forces to gravitational forces in order to classify flow as slow (<1), critical (=1), or fast (>1)

perennial stream

stream that flows throughout the year, occurs where the groundwater table overlaps with the streambed

intermittent stream

stream that may not have flowing water at certain points during drier times

ephemeral streams

streams that have flowing water only after rainfall

QVA method

method for estimating discharge of a stream given velocity and cross-sectional area

propeller current meter

non-recording meter that uses spin-time to measure magnitude but not direction of flow velocity in a stream

Doppler current meter

measures change in source light/ sound frequency using frequency of reflections in order to determine flow velocity in a stream; recording meters allow for continuous measurement

slowest, fastest

velocity is _______ at the sides/ bottom of a stream and _______ at the middle/ surface

Manning-Chezy equation

used to estimate velocity of streamflow given hydraulic radius (from wetted area & perimeter), slope of channel bottom, and a roughness coefficient

stage-discharge rating curve method

a relationship can be calculated between height of water level above a certain established point (stage) and discharge of the stream; this relationship can then be used to determine discharge from a known stage level (similar to transmittance and absorbance in spectrophotometry)

latent heat (J/g)

energy a water molecule needs to break through the water surface and become vapor without a change in temperature

Penman equation

method of estimating evaporation including energy and mass transfer

precipitation - surface runoff - groundwater runoff + change in storage

evaporation water budget balance equation

evaporation pan

method to directly measure evaporation, adjusted with pan coefficient (Ce)

higher

faster wind speed leads to _____ evaporation

lower

higher relative humidity leads to ______ evaporation

gas exchange through stomata

mechanism by which plants lose water to transpiration

highest, lowest

evergreen forests tend to have the ______ transpiration rates while grasslands have the _______

potometer

method for measuring transpiration using cut plants

weighing lysimeter

method for measuring transpiration using weight changes in large, undisturbed soil profiles

sap flow

method of measuring transpiration by measuring changes in heat along the stems of woody plants

Bowen ratio

method of quantifying evapotranspiration from a moisture-containing surface; ratio of state changes measured in sensible heat to latent heat

sensible heat

change in temperature without an accompanying change of state (EX evaporation)

potential ET

evapotranspiration from an “ideal” situation: unlimited body of water (EX lake), water loss independent of vegetation and soil, available energy is only limiting factor

actual ET

evapotranspiration that occurs in situation where soil water supply is limited (instead of energy)

reference ET

evapotranspiration that occurs from a grass surface that is well-watered; estimated using meteorological data or evaporation pans

Thornthewaite equation

method of estimating PET using temperature, adjusted for unequal day lengths throughout the year

Penman equation

method of estimating evaporation using temperature, radiation, vapor pressure, wind speed, and latent heat (combines energy and mass balance)

Penman-Monteith equation

method of estimating actual evapotranspiration

erosion

movement of earth particles due to flowing agents (EX air, water)

mass wasting

movement of earth particles due to gravity

detachment, transport, deposition

three stages of erosion (in order)