GEOG 322 Midterm Review PDF

Midterm Review Lecture 1 Hydrology - Study dealing with the occurrence, circulation, distribution, and properties of the water of the Earth and its atmosphere (the study of water) - Focuses of freshwater at various scales - Water-related disasters - Phase changes of wate...

Midterm Review Lecture 1 Hydrology - Study dealing with the occurrence, circulation, distribution, and properties of the water of the Earth and its atmosphere (the study of water) - Focuses of freshwater at various scales - Water-related disasters - Phase changes of water Lecture 2 Distribution of Earth’s water - 3% of earth’s water is freshwater - Of which 68.7% is icecaps/glaciers, 30.1% is groundwater, 3% is surface water Water cycle - Conceptual model describing the storage and movement of water between the biosphere, atmosphere, lithosphere, and hydrosphere - Model that simply describes water circulation - Can be examined at many spatial scales Watersheds - Spatial unit most used by hydrologists - An area of land from which all runoff drains into a single point, like a lake or ocean - Also known as: catchment, drainage/water/river basin - Outlined by a divide and the outlet  o Divide: ridge or strop of high elevation that separates drainage basins o Outlet: outflow or ‘exit’ point of the watershed - Surface water is the visible watershed, and groundwater is watershed we don't see - Watersheds can be nested (larger watersheds contain smaller watersheds) - Waterbody + riparian zone make up = floodplain (terraces indicate flood plain) Streams - Stream order is the relative size of a stream o A number indicating the level of branching associated with a stream in a drainage network (order 1, 2, 3, etc.) Lecture 3 Watercycle - Precipitation, evaporation, infiltration, storage, runoff, river discharge, interception, transpiration, evapotranspiration, percolation, recharge, groundwater flow - Reservoirs include surface water, soil moisture, groundwater storage, glaciers and snow, biomass - Fluxes INTO hydrologic systems: rainfall, snowmelt, groundwater inflow, irrigation water (if applicable) - Fluxes OUT OF hydrologic systems: evapotranspiration, catchment runoff, groundwater outflow - Residence time: Tr = Vol/Q where Vol = volume of water in reservoir and Q = rate at which water exists the reservoir - Oversimplifications in diagrams: humid bias, blue-water bias, neglect of human role Water balance equation - Water budget for any system: input – output = change in storage o Equation is fixed but what goes into input/output needs to be evaluated in each situation o Steady-state water balance: change in storage = 0 o Transient-state water balance: change in storage ≠ 0 - P + Gin – ET – Q – Gout = ΔS o Input: precipitation (P) + groundwater inflow (Gin) o Output: evapotranspiration (ET) + discharge (Q) + groundwater outflow (G out) o Change of storage: change in storage (ΔS) o Steady state: P + Gin - ET - Q - Gout = 0 o Without groundwater: P - ET - Q = 0 o Transformed: Q = P - ET and ET = P – R - Rewritten: Runoff = total precipitation – hydraulic abstractions (or losses) o Hydraulic abstractions: interception, wetland storage, evaporation and transpiration, infiltration, soil water/groundwater storage Surface water budget - P = Q + I +Δ(M, G, S) o Precipitation (P), river discharge (Q), evapotranspiration (E), interception (I), soil water storage (M), groundwater storage (G), channel and surface water storage (S) Lecture 4 & 5 Hydrological system - Input: precipitation leads to interception or channel precipitation Precipitation - Saturation vapor pressure curve  o Water condenses at the saturation temperature or dew point o Relative humidity = (actual vapor pressure / saturation vapor pressure) * 100 ▪ Good indicator of rain (but says nothing about amount of water vapor in the air) o Absolute humidity is mass of water per unit air - Methods of cooling air o Convective uplift ▪ Warm air rises (from radiation on earth's surface) and cools to condense its water vapor into liquid and causes instant rainfall o Frontal/advective uplift ▪ When warm air meets dry air, warm air rises, and the water vapor condenses forming clouds and then it'll rain ▪ A warm front leads to long constant rain and a cold front causes more unstable weathering o Dry and wet adiabatic lapse rates ▪ Describe how temperature changes with altitude in the atmosphere, depending on the presence of moisture in the air o Orographic precipitation ▪ High moisture air is forced upwards due to a mountain and condenses into clouds which will cause rain ▪ The other side of the mountain is dry - Characteristics of precipitation: o Rainfall: magnitude and intensity ▪ Magnitude (amount of precipitation: depth in mm), duration (how long it rains: time), intensity (magnitude/duration: mm/time), frequency (how often it rains), combination of these (frequency-duration-return period)  ▪ A rain hyetograph is a graphical representation of the distribution of rainfall over time ▪ P and T Return period (average interval of time between occurrences of events): T = 100/P Probability of occurrence (likelihood of an event occurring within a time frame): P = 1/T *100 or 100/T In a sample of events (where y = # of events and n = rank of event), P can be calculated using different plotting positions: o Weibull plotting positions, Hazen plotting positions, arithmetic scale plot, transformation plot, normal probability paper Risk calculation (probability a T-year storm will occur at least once during the next n years): 1 - (1 - (1/T))^n Probable maximum precipitation (PMP) is the theoretical max amount of precip. that can occur over a specific time and area based on most extreme meteorological conditions possible o Snow: stored to be melted later - Rain is a flux into a system, but snow is a store (although snowmelt is a flux) - How rain forms o Raindrops form around condensation nuclei (salt crystals, dust, aerosols) o Updraughts moves drops so that they collide with each other and grow o When they are heavy enough, they fall as rain or snow - Types of precipitation o Rain: vary in size from 0.5mm to >5mm in diameter and fall from nimbostratus or altostratus clouds o Drizzle: smaller than 0.5mm and fall from stratus clouds or fog o Showers: precipitation that varies in intensity and generally falls from cumulonimbus or large cumulus clouds o Snow: forms when water vapor turns to ice without first condensing into a liquid ▪ Snowflakes are composed of microscopic water crystals that cluster in groups of 50+ ▪ When temp is low enough (40*), snow can fall from clear blue skies o Hail: forms when water droplets freeze in high, very dense clouds ▪ Hailstones grow bigger as they're coated with successive layers of ice ▪ Usually are only as big as a pea but can grow to be like a baseball - Rain gauges o Method/tool to measure rainfall  ▪ Rainfall height (mm) = height of measured water column * ration btwn cross- sectional area of water column and opening of the gauge o Impacted by: height, color, wind, methods, location, people, timing, technology and size o In cold climates, error can be 100% or more where frequent high-quality measurements are difficult, and as wind and snow drift are important factors o Should be equally distributed in space ▪ We don't have enough gauging stations to conduct proper assessments, so we average measurements from a network of gauges Arithmetic averages Thiessen polygon method (rainfall recorded at a gauge can be applied to any point at a distance halfway to the next station in any direction) Isohyets (contour lines of equal rainfall depth) Kriging (computerized isohyets) Remote precipitation measurements (radar, etc., difficult to calibrate) - Lysimeters o Device originally designed to measure amount of actual ET released by plants o Records the amount of precipitation that an area receives to calculate the amount of water lost to ET Interception - Gross and net precipitation o Interception: The process of rainfall and snowfall being stored above ground, mostly on vegetation ▪ Interception flow = gross precipitation (P) - net rainfall (N) ▪ Gross rainfall: amount of precipitation measured above the canopy or in opening in a forest ▪ Net rainfall = throughfall (T) + stemflow (S) - Throughfall and stemflow o Throughfall: water that falls between plants (and drippage: water that drips from the plants to the ground) o Stemflow: flow of water down stems and trunks - Interception storage o Precipitation doesn’t become part of flow system and results in net loss of water available to hydrologic cycle o Interception can significantly reduce magnitude/intensity of precipitation arriving at the ground  o Interception loss is part of precipitation on canopy that doesn't reach ground but evaporates from canopy o Source of water for evaporation (net loss of system is typically small) - Controls of interception o Interception capacity is a function of vegetation characteristics and growth form (eg. coniferous trees vs. deciduous trees, trees vs. grasses and shrubs) o Precipitation intensity and duration o Wind speed (promotes interception loss by evaporation) o Type of precipitation: rain vs. Snow - Horizontal interception and fog drip o Occur where water is filtered out of fog as it passes through vegetation ▪ Precipitation then generated as drippage (fog drip) and stemflow o Horizontal interception can cause an additional input of water (observed in coastal and cloud forests) Lecture 6 Snow metamorphosis - Snow (fresh, 50-300kg/m^3), névé (max 1yr, 500kg/m^3), firn (more than 1yr, 500kg/m^3), ice (multiple/many years, 850kg/m^3) Snow - Occurs mostly in northern hemisphere (but in other regions as well) - Many roles: modifies energy and moisture fluxes (evaporation), represents dynamic store in hydrological system, excellent insulator, reduces energy exchange between earth surface and atmosphere - High albedo of 0.8-0.9 (reflects 80-90% of incoming solar radiation) o Positive feedback (more snow = more reflected solar radiation = colder air) - Snowpack acts like soil (has grains of different sizes and pockets of air that can fill with liquid water) - Changes its structure and properties quite quickly (snow metamorphosis) Typical snow measurements - Snowfall = depth of fresh snow o Measured with ruler, error due to snow drift and setting o Solid precipitation = water equivalent of snowfall ▪ Measured with standard precipitation gauge, harder to measure - Snow depth = depth of snow cover that accumulated on the ground  o Not good property to measure at it can change easily (due to changes in density) o Measured with ruler, ultrasound, radar, etc. o Influenced by location of measurement o Density of fresh snow is almost less than half of older snow cover (but highly heterogenous of space and time) - Snow water equivalent (SWE, of snow cover) o Defined as depth of water if the snow cover is completely melted o Measured with gravimetric method that involves taking vertical core through the snowpack and weighing or melting it (or by using ‘snow pillows’) o SWE (m) = 0.001 * snow depth (m) * density of snow (kg/m^3) Snow and energy - Heat capacity and phase change o Snow has low heat capacity meaning it needs less energy to phase change than water o When snow melts it absorbs it absorbs a lot of energy from the environment leading to a cooling effect (this energy is important to phase change) o During winter, snowcover energy balance (EB) shows an outward flux due to radiation loss but as snow begins to melt, the energy flux shifts inward, contributing to snow ripening and melting - Snow melt rates o Measuring energy balance is difficult o Estimates of snow melt are often based on simple temperature measurements o Mass melt for the day (mm) = degree-day factor * (avg. air temp – base temp) Rain-on-snow events  - Major drivers of snowmelt - Condensation of water = main (energy) process that melts snow o During rain period, humidity is high and its relatively warm, so water vapor condenses at cold snow surface releasing high amounts of latent heat o Condensed water infiltrates into snowpack and distributes energy by conduction of sensible heat - After the water is cooled to 0° C it freezes and releases latent heat resulting in fast ripening of the snowpack - Important for the mass balance as it produces lots of runoff o Rain + condensation + snowmelt Fast and slow melting under sunny/warm spring conditions - Advection is main component o After the first patches of snow disappear, bare ground heats up o These warm spots deliver heat and humid air to remaining snow patches o Condensation leads to accelerated ripening of remaining snow o Once the bare ground dries up, the effect stops Lecture 7 and 8 Evapotranspiration - ET is an output of the hydrological system o Completes the hydrological cycle ▪ All water in the atmosphere comes from ET ▪ Effect on runoff generation and groundwater recharge o Important to climate system (cooling system) and impacts natural vegetation growth (in turn, agricultural yield with irrigation) - Units of evapotranspiration o Amount of ET (depth): mm, etc. o Rate of ET (depth per time): mm/s, etc. - Basic components o Available soil water, transpiration from leaves, evaporation from soil, humidity and temperature o Evaporation (open water, soil, vegetation surfaces) and transpiration (plants) - Controls o Temperature: warmer temp = more E and T o Relative humidity (VPD): easier for water to evaporate into dryer air o Solar radiation: insolation is main energy supply for ET  o Wind and air movement: increased movement of air around plant = higher transpiration rate o Type of plant: diff plants transpire at diff rates o Soil moisture availability: at lacking moisture, plants transpire less Evaporation - Thermal agitation within molecule provides energy for it to escape the internal bonds of the liquid and be absorbed into the air - Evaporation process o Water molecules move permanently o Some break away from water surface and into atmosphere while some comeback to water surface o Net evaporation occurs if more water molecules leave the surface than return o Eventually an equilibrium will occur where there is no more net evaporation (air has reached saturation vapor pressure) - 2 main controls of evaporation o Energy (provided by water pressure driven by energy budget) ▪ Latent heat of vaporization (energy that a molecule needs to escape water surface) ▪ Energy comes mostly from solar radiation (insolation) Air temperature is a good proxy for average energy availability o Water vapor deficit (the ability of air to hold more water) ▪ Drives (or limits) evaporation ▪ Vapor pressure deficit (VPD): difference between the amount of moisture in the air and how much moisture air can hold when saturated VPD = actual vapor pressure at air temp – saturation vapor pressure at air temp Actual vapor pressure (ea): actual amount of moisture that air holds at air temperature o Must know air temperature or dew point temperature to derive ea Saturation vapor pressure (es): max amount of moisture air can hold at air temperature o Is a property of air, only a function of air temperature ▪ Evaporation is a function of the ability of air to hold water, based on air temp and relative humidity (RH) - Constant evaporation o 2 factors needed  ▪ Constant energy supply to provide latent heat of vaporization Otherwise, water would keep cooling until evaporation stops ▪ Mechanism that efficiently removes added water vapor from air right above water surface to keep concentration gradient (VPD) constant Mechanism is provided by air movement due to wind - Measuring o Relative humidity ▪ Psychrometer: measures temp at which air starts to condensate (or is saturated) Air is moved at a constant speed over two bulbs and evaporation at the wet bulb causes cooling (lower the humidity = stronger cooling) Based on temp difference, relative humidity can be looked up in psychometric chart o Evaporation ▪ Evaporation pan (normally overestimates by 20-40%) o Dalton’s Law ▪ Evaporation = coefficient * wind-speed correction function * (actual vapor pressure – saturation vapor pressure at air temp) o Estimating E from open water ▪ Equation from Meyer (based on Dalton’s law) Transpiration - Plant transpiration o Process of water loss from plants through stomata o Critical to plants functioning b/c plant obtains nutrients and minerals through water solution o Plants actively regulate transpiration through opening and closing stomata - Factors affecting transpiration o Plant type ▪ Taller plants transpire more, trees transpire more than other vegetation o Roots ▪ Different root systems, deeper roots need more energy, not all roots have same capacity to absorb water o Wind ▪ Wind blows water vapor away from air-water interface allowing for more room to absorb water ▪ Wind increases transpiration rate (but too much closes stomata) ▪ Important part of air movement is that in vertical direction  o Plant available water (PAW) ▪ PAW = available soil water between field capacity and wilting point Field capacity is max amount of water that soil can hold before percolating downwards (at field capacity ET = 100%) At wilting point, no more water can be extracted by plant (at wilting point ET = 0) 2 types of ET and E (evapotranspiration and evaporation) - Actual evapotranspiration (AET) and actual evaporation (Ea) o The actual rate of E or Et at the place of interest o Ea and AET are the values we’re really interested in (but Ep and PET are easier to derive) - Potential evaporation (Ep) o Evaporation from a surface when all surface-atmosphere interfaces are wet (no restriction on the rate of evaporation from the surface) o Magnitude of Ep depends on atmospheric conditions and surface albedo (but also varies with surface geometry characteristics) o Ex. evaporation from open water: Ea = Ep (unlimited water supply) - Potential evapotranspiration (PET) o Originally defined as the amount of water transpired in unit time by a short green crop, completely shading the ground, of uniform height and never short of water ▪ Crop assumed to be short, uniform and completely shading the ground so that no soil is exposed ▪ Never short of water so that water content is no longer a variable, and stomata is always fully open ▪ In agriculture, we use typical crops (ie. Alfalfa) to define a ‘reference crop evapotranspiration’ - PET vs. AET o PET is used as proxy (estimates potential ET under ideal conditions) o AET is influenced by local water availability and is typically lower than PEt under dry or normal conditions o In wetlands, AET can exceed PET due to plant structures increasing surface area and modifying wind exposures, enhancing ET o PET is highest in Sahara (VPD is lower so E occurs faster) o AET is highest in tropical forest/wetland (most soil saturation, hotter, more solar radiation) - Bare soil (example) o If care soil is saturated: Ea = Ep (like open water)  o If bare soil isn’t saturated: Ea ≤ Ep ▪ Stage 1: Ea ≈ Ep Soil surface is close to or at saturation ▪ Stage 2: Ea < Ep Soil surface starts to dry, evaporation occurs below soil surface and water vapor diffuses to soil surface - Measuring o Ea and AET are estimated or calculated from Ep or PET by multiplying with crop and stress coefficients o Measuring Ep ▪ Evaporation pan (class A pan) Good surrogate for lake evaporation which in turn is good proxy for Ep and PET o Measuring PET ▪ PET gauge o Calculating AET ▪ Catchment water balance Normally done for a watershed over a long period of time Measure precipitation (P) and discharge (Q) AET = P – Q [don't forget to include ± ∆G (groundwater flow) and ± ∆θ (soil water change) for short periods of time] o Measuring AET ▪ Soil water depletion Measure the change sin soil water over a period time as the change is approximately AET (if there’s no rain input or runoff output) AET = ΔSM / Δt (Δt = time change between sampling, ΔSM = change in soil water content) ▪ Lysimeter Device designed to measure amount of AET released by plants Records the amount pf precipitation that an area receives and the amount of water that drains the soil ▪ Energy balance and mass transfer Eddy-Covariance tower o Mainly used in research settings o Based on change sin vertical wind speed and water vapor density o Dalton’s Law  ▪ E = K * f(u) * (es - ea) ▪ Many ET estimation formulas are based on the Dalton-type equation o Estimating ET with equations ▪ Many methods with different data requirements (also for different regions) Results can differ significantly (more data intensive methods deliver more reliable results) ▪ Most ET estimation methods predict ET for a well-watered short green crop ▪ Convert reference crop ET to AET by using crop coefficient (empirically derived) o Converting PET to AET ▪ Use crop coefficients ▪ AET = Kc * PET (Kc = crop coefficient) This parameter is normally developed based on lysimeter measurements Aridity Index - Relationship between seasonal P (precipitation) and PET o PET highest with lowest P - Aridity index = P/PET o Average annual PET is often compared to average annual precipitation (ratio of the two)