Water cycle: EPIC (E=Evaporation, P=Precipitation, I=Infiltration, C=Collection) β Evaporation, Precipitation, Infiltration, Collection (runoff). Energy from the sun drives it all.
The Hydrologic Cycle
The continuous movement of water through Earth's systems β the foundation of all hydrology
Sun drives evaporation from oceans, lakes, rivers (70% from oceans). Transpiration: water released by plants (evapotranspiration = combined). Condensation: water vapor cools β clouds form. Precipitation: rain, snow, sleet, hail. Infiltration: water soaks into soil β recharges groundwater. Surface runoff: water flows over land β streams β rivers β ocean. Interception: vegetation catches precipitation before it hits ground. The cycle has no start or end β water is continuously recycled. Average water molecule: ~9 days in atmosphere, ~3,200 years in ocean.
E
Evaporation β ocean to atmosphere
P
Precipitation β atmosphere to land/ocean
I
Infiltration β land to groundwater
C
Collection β runoff to streams/ocean
Watershed
Watershed (drainage basin): all land draining to a single outlet. Divides = ridgelines separating watersheds. 'Water sheds downhill.'
Watershed and Drainage Basin
The fundamental unit of hydrologic analysis β every drop of rain belongs to a watershed
Watershed: area of land that drains to a common outlet (stream, river, lake, ocean). Also called drainage basin or catchment. Drainage divide: ridge or high ground separating adjacent watersheds. Delineated from topographic maps β water flows perpendicular to contour lines. Mississippi River watershed: 3.2 million kmΒ² (40% of contiguous US). Watershed characteristics affecting runoff: size, slope, shape, land use, soil type, vegetation. Impervious surfaces (pavement, roofs) increase runoff, reduce infiltration β urban flooding. Order: first-order streams (no tributaries) β second, third β Mississippi = 10th order.
Stream Discharge
Discharge Q = A Γ V (cross-sectional area Γ velocity). Units: cubic feet per second (cfs) or mΒ³/s. Stage = water surface elevation.
Stream Discharge
The fundamental measurement of streamflow β how much water passes a point per unit time
Q = A Γ V. Area: width Γ mean depth (mΒ² or ftΒ²). Velocity: measured with current meter, ADCP (Acoustic Doppler Current Profiler), or float. Stage: height of water surface above a datum β measured at stream gauges. Stage-discharge rating curve: converts stage readings to discharge. USGS stream gauge network: 8,000+ stations across US β real-time data. Hydrograph: graph of discharge vs time β shows storm response. Peak flow lags behind peak rainfall (lag time). Baseflow: groundwater contribution to streamflow between storms. Flashy streams: rapid rise and fall (small urban watersheds). Regulated streams: dams flatten hydrograph.
Q
Discharge (cfs or mΒ³/s)
A
Cross-sectional area
V
Mean velocity
Stage
Water surface elevation β measured at gauges
Flood Frequency
100-year flood: 1% chance of occurring in ANY given year β NOT once per century. Recurrence interval = 1/probability.
Flood Frequency Analysis
The statistics of floods β and the critical misconception about return periods
Recurrence interval (return period): average time between floods of equal or greater magnitude. 100-year flood: 1% annual exceedance probability (AEP (Annual Exceedance Probability)) β NOT guaranteed to occur only once per century. In any 30-year mortgage: 26% chance of experiencing a 100-year flood. FEMA (Federal Emergency Management Agency) flood zones: Zone A (100-year floodplain, ~1% AEP), Zone X (500-year, 0.2% AEP). Flood frequency analysis: fit statistical distribution to historical peak flows β estimate return periods. Log-Pearson Type III: standard USGS method. Stationarity assumption: historical record representative of future β challenged by climate change.
Two fundamentally different ways rainfall becomes streamflow β controls flood risk
Infiltration excess (Hortonian) overland flow: rainfall intensity exceeds soil infiltration capacity β water ponds on surface β sheet flow. Common in arid regions, compacted soils, urban areas. Variable source area (saturation excess): near-stream areas saturate first β expand during storms β all rain on saturated area becomes runoff immediately. Common in humid regions, forests. Subsurface stormflow: water moves laterally through soil above water table β important in forested catchments. Groundwater ridging: rapid water table rise near streams. Which mechanism dominates depends on: soil type, antecedent moisture, rainfall intensity, topography.
Channel Morphology
Stream channels: straight β meandering β braided. Meandering: high sinuosity, point bars (inside), cut banks (outside).
Stream Channel Types
How streams shape their channels β and what channel form reveals about hydrology
Straight channels: rare naturally, usually controlled. Meandering: sinuosity >1.5, dominant in low-gradient streams with fine sediment. Point bar: deposition on inside of bend (shallow, slower flow). Cut bank (outer bend): erosion on outside (faster flow, deeper). Braided: multiple channels, coarse sediment, high sediment load, steep gradient. Anastomosing: multiple stable channels, fine sediment, low gradient, stable banks. HjulstrΓΆm curve: velocity needed to erode vs deposit particles of different sizes β fine silt paradox (harder to erode than sand due to cohesion). Bankfull discharge: fills channel to top of banks β occurs ~1.5-year recurrence, does most geomorphic work.
Straight
Rare β usually controlled
Meandering
Sinuosity >1.5 β point bars, cut banks
Braided
Multiple channels β coarse sediment, steep
Bankfull
~1.5 yr flood β does most geomorphic work
Evapotranspiration
ET (Evapotranspiration) = evaporation + transpiration. PET (Potential ET): what would evaporate with unlimited water. Actual ET β€ PET always.
Evapotranspiration
The return of water from land to atmosphere β the largest component of the water budget in many regions
Evaporation: from open water and soil surfaces. Transpiration: through plant stomata β plants pump water from roots to leaves β evaporates. ET combined = largest land-to-atmosphere flux. PET (potential ET): ET that would occur with unlimited water β function of temperature, humidity, wind, solar radiation. Actual ET β€ PET (limited by water availability). Penman-Monteith equation: standard method for calculating PET β uses energy balance + aerodynamic resistance. In humid regions: actual ET β PET. In arid regions: actual ET << PET. Forests: higher ET than crops or grass. Deforestation β more runoff, less ET.
Water Balance
Water balance: P = ET + Q + delta S (P=Precipitation, ET=Evapotranspiration, Q=Streamflow, delta S=change in Storage). Precipitation = Evapotranspiration + Runoff + Change in storage.
Catchment Water Balance
The fundamental accounting equation for water in a watershed
P = ET + Q + ΞS. Precipitation (P): total input. Evapotranspiration (ET): loss to atmosphere. Streamflow/Runoff (Q): output through streams. Storage change (ΞS): groundwater, soil moisture, snowpack β near zero over long periods. Over annual timescale: ΞS β 0, so P β ET + Q. Runoff ratio (Q/P): fraction of precipitation that becomes streamflow. Arid regions: Q/P < 0.1 (most water evaporates). Humid regions: Q/P > 0.5. Budyko framework: relates runoff ratio to aridity index (PET/P) β elegant global pattern. Climate change shifts: alters P, ET, and timing of snowmelt β changes Q magnitude and seasonality.
P
Precipitation β input
ET
Evapotranspiration β atmospheric loss
Q
Streamflow β output
ΞS
Storage change β groundwater, snowpack
Flood Hazards
Floodplain: flat land adjacent to channel, regularly flooded. 100-year floodplain: FEMA mapped. Development increases flood risk.
Flood Hazards and Floodplains
How floods shape landscapes β and why floodplain development is inherently risky
Floodplain: formed by lateral migration and overbank deposition over thousands of years. Naturally functional: stores floodwater, recharges groundwater, supports riparian ecosystems, filters pollutants. Development on floodplains: 41 million Americans live in flood zones. FEMA National Flood Insurance Program (NFIP): required for federally backed mortgages in Zone A. Urbanization effects: impervious surfaces β more runoff β higher, faster floods β channel incision. Levees: protect specific areas but increase flood magnitude elsewhere (constrict floodplain). Flash floods: rapid onset (<6 hours), most deadly (half of all flood fatalities). Turn Around Don't Drown: 6 inches of water can knock you down, 12 inches can sweep a car.
Hydrograph Analysis
Hydrograph: discharge vs time. Rising limb β peak β recession limb β baseflow. Unit hydrograph: response to 1 inch of rain.
Hydrograph Analysis
Reading the flood response of a watershed from its hydrograph shape
Rising limb: discharge increases as storm runoff reaches stream. Peak discharge: maximum flow β lags behind rainfall peak (lag time). Recession limb: discharge decreases as runoff drains. Baseflow recession: slow groundwater drainage after storm. Hydrograph shape reflects: watershed size (larger = longer response), shape (elongated vs compact), slope, land use, storm characteristics. Unit hydrograph: dimensionless response to 1 inch (or 1 cm) of excess rainfall over the watershed β used to predict floods from any storm. S-curve method for deriving unit hydrograph. SCS Curve Number method: widely used empirical approach for estimating runoff from rainfall.
Sediment Transport
Sediment transport: bedload (rolling/saltating), suspended load, dissolved load. HjulstrΓΆm curve: erosion vs deposition by velocity.
Sediment Transport
How streams move particles β the link between hydrology and geomorphology
Three modes: Bedload β coarse particles roll, slide, saltate along channel bottom (10β20% of total load). Suspended load β fine sand, silt, clay carried in suspension (most of the mass). Dissolved load β ions in solution (invisible). Competence: largest particle a stream can move β varies as velocityΒ². Capacity: total sediment a stream can carry β varies as velocityβ΅. When velocity decreases: coarser particles deposit first (graded bedding). HjulstrΓΆm curve: erosion requires higher velocity than deposition; fine silt hardest to erode (cohesion). Stream power: Ο = ΟgQS β controls erosion and transport. Dam effects: trap sediment β clear water below dam β channel incision downstream.
Bedload
Rolling/saltating along bottom β coarse
Suspended
Fine particles in water column β most mass
Dissolved
Ions in solution β invisible
Competence
Largest movable particle β β velocityΒ²
Snow Hydrology
Snowpack: stores winter precipitation, releases in spring melt. SWE (Snow Water Equivalent) = depth Γ density. Critical in western US.
Snow Hydrology
How snow transforms seasonal water storage and the timing of river flow
Snowpack: temporary storage of winter precipitation. SWE (snow water equivalent): depth of water if snowpack melted β measured at SNOTEL (SNOwpack TELemetry network) sites. Fresh snow: density ~100 kg/mΒ³ (10% water). Settled/wet snow: ~400β600 kg/mΒ³. Snowmelt: driven by solar radiation, air temperature, rain-on-snow events. Degree-day method: melt β melt factor Γ (T_air - T_base). Energy balance method: more accurate, uses radiation, turbulence. Western US rivers (Colorado, Columbia, Sacramento): 60β80% of annual flow from snowmelt β critical for summer water supply. Climate change: less snow, earlier melt β reduced summer flows β water supply crisis. April 1 SWE: traditional measure of annual water supply outlook.
Mnemonic
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🎓 Common Exam Questions
Q: What does EPIC stand for and explain the complete hydrologic cycle.
A: EPIC = Evaporation, Precipitation, Infiltration, Collection β the four key process categories. Complete cycle: Solar energy evaporates water from oceans (70%), lakes, rivers, and land. Plants transpire water from the soil through their leaves (ET = evaporation + transpiration, together called evapotranspiration). Water vapor rises, cools, condenses around dust particles forming clouds (condensation nuclei). Precipitation falls as rain, snow, sleet, hail. On land: some is intercepted by vegetation (evaporates back). Some infiltrates into soil and eventually reaches the water table (groundwater recharge). Some runs off over the surface to streams. Streams flow to oceans. Groundwater discharges to springs, streams, and the ocean. Snow accumulates in snowpack, melts in spring. The cycle is continuous and driven by solar energy. Water balance: P = ET + Q + delta S (storage).
Q: What is stream discharge (Q = A x V) and how is it measured?
A: Discharge (Q) = cross-sectional area (A) x mean velocity (V). Units: m3/s (cumecs) or ft3/s (cfs). Q represents the volume of water passing a cross-section per unit time. Measurement methods: Wading measurement: field crew wades across stream, measures width, depth, and velocity (with current meter) at multiple verticals β sum of (width x depth x velocity) for each section. ADCP (Acoustic Doppler Current Profiler): mounted on a boat or remote-controlled device, uses Doppler shift of acoustic signals to measure velocity profiles continuously β much faster for large rivers. Stage-discharge rating curve: install a stream gauge measuring water level (stage), then develop a rating curve relating stage to discharge from multiple Q measurements β allows continuous discharge record from continuous stage record. USGS maintains thousands of stream gauges across the US providing real-time data.
Q: What is the 100-year flood and how does AEP (Annual Exceedance Probability) work?
A: AEP (Annual Exceedance Probability): the probability that a flood of a given size will be equaled or exceeded in any single year. A 100-year flood has 1% AEP β a 1 in 100 chance in any given year. Important misconceptions: A 100-year flood does NOT happen exactly every 100 years β it is a probability, not a schedule. After experiencing a 100-year flood, the probability in the next year is STILL 1%. In any 100-year period, the probability of experiencing at least one 100-year flood is about 63%. Flood frequency analysis: plot historical peak flows vs return period (or AEP) on probability paper, fit a distribution (Log-Pearson Type III is standard in the US), extrapolate to estimate rare events. Limitations: assumes stationarity (climate and land use are not changing) β increasingly violated as climate changes. FEMA uses AEP to map regulatory floodplains and set flood insurance requirements.
Q: What is ET (Evapotranspiration) and how does it affect water balance?
A: ET (Evapotranspiration) = evaporation from soil and water surfaces + transpiration through plant stomata. ET is the largest loss of water from the land surface β in humid temperate climates about 60-70% of annual precipitation is returned to the atmosphere as ET. PET (Potential ET): the ET that would occur if water were unlimited β driven by temperature, solar radiation, humidity, and wind. Actual ET is constrained by available water. Methods to estimate PET: Penman-Monteith (physically based β requires radiation, temperature, humidity, wind data), Hargreaves (temperature-based, simpler), Priestley-Taylor. Water balance: P = ET + Q + delta S. If P minus ET is positive (humid regions), the surplus drives streamflow and groundwater recharge. If P minus ET is negative (arid regions), there is a moisture deficit and irrigation is needed. Climate change increases PET due to warming, intensifying water stress in already-dry regions.
Q: What is SWE (Snow Water Equivalent) and why is snowpack critical for water resources?
A: SWE (Snow Water Equivalent): the equivalent depth of liquid water if the snowpack were melted. Determined by: SWE = snow depth x snow density. Fresh snow density: about 0.05-0.1 (5-10% water). Settled pack: 0.2-0.4. Wet spring snow: up to 0.5+. Why snowpack matters: In the western US, snowpack in mountain ranges (Sierra Nevada, Rocky Mountains, Cascades) is the primary water storage system. Snow accumulates all winter and melts in spring and early summer β when demand is highest and natural precipitation is lowest. About 70% of western US water supply comes from snowmelt. Monitoring: SNOTEL (SNOwpack TELemetry) β automated network of about 800 stations measuring SWE, precipitation, and temperature throughout the western US. SWE forecasts drive water supply forecasts for irrigation, municipal water, and hydropower. Climate warming is reducing snowpack and shifting melt timing earlier β a major water security concern.