Geology tricks that make hydrology flow

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 — water flows perpendicular to contour lines. Mississippi River watershed: 3.2 million km² (40% of contiguous US). Watershed characteristics affecting runoff: size, slope, shape, land use, soil type, vegetation. Impervious surfaces (pavement, roofs) increase runoff, reduce infiltration → urban flooding. Order: first-order streams (no tributaries) → second, third → Mississippi = 10th order.

Q = A × V. Area: width × mean depth (m² or ft²). Velocity: measured with current meter, ADCP, or float. Stage: height of water surface above a datum — measured at stream gauges. Stage-discharge rating curve: converts stage readings to discharge. USGS stream gauge network: 8,000+ stations across US — real-time data. Hydrograph: graph of discharge vs time — 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.

Recurrence interval (return period): average time between floods of equal or greater magnitude. 100-year flood: 1% annual exceedance probability (AEP) — NOT guaranteed to occur only once per century. In any 30-year mortgage: 26% chance of experiencing a 100-year flood. FEMA 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 → estimate return periods. Log-Pearson Type III: standard USGS method. Stationarity assumption: historical record representative of future — challenged by climate change.

Infiltration excess (Hortonian) overland flow: rainfall intensity exceeds soil infiltration capacity → water ponds on surface → sheet flow. Common in arid regions, compacted soils, urban areas. Variable source area (saturation excess): near-stream areas saturate first → expand during storms → all rain on saturated area becomes runoff immediately. Common in humid regions, forests. Subsurface stormflow: water moves laterally through soil above water table — important in forested catchments. Groundwater ridging: rapid water table rise near streams. Which mechanism dominates depends on: soil type, antecedent moisture, rainfall intensity, topography.

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. Hjulström curve: velocity needed to erode vs deposit particles of different sizes — fine silt paradox (harder to erode than sand due to cohesion). Bankfull discharge: fills channel to top of banks — occurs ~1.5-year recurrence, does most geomorphic work.

Evaporation: from open water and soil surfaces. Transpiration: through plant stomata — plants pump water from roots to leaves → evaporates. ET combined = largest land-to-atmosphere flux. PET (potential ET): ET that would occur with unlimited water — function of temperature, humidity, wind, solar radiation. Actual ET ≤ PET (limited by water availability). Penman-Monteith equation: standard method for calculating PET — uses energy balance + aerodynamic resistance. In humid regions: actual ET ≈ PET. In arid regions: actual ET << PET. Forests: higher ET than crops or grass. Deforestation → more runoff, less ET.

P = ET + Q + ΔS. Precipitation (P): total input. Evapotranspiration (ET): loss to atmosphere. Streamflow/Runoff (Q): output through streams. Storage change (ΔS): groundwater, soil moisture, snowpack — near zero over long periods. Over annual timescale: ΔS ≈ 0, so P ≈ 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) — elegant global pattern. Climate change shifts: alters P, ET, and timing of snowmelt → changes Q magnitude and seasonality.

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 → more runoff → higher, faster floods → 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.

Rising limb: discharge increases as storm runoff reaches stream. Peak discharge: maximum flow — 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 — 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.

Three modes: Bedload — coarse particles roll, slide, saltate along channel bottom (10–20% of total load). Suspended load — fine sand, silt, clay carried in suspension (most of the mass). Dissolved load — ions in solution (invisible). Competence: largest particle a stream can move — varies as velocity². Capacity: total sediment a stream can carry — varies as velocity⁵. When velocity decreases: coarser particles deposit first (graded bedding). Hjulström curve: erosion requires higher velocity than deposition; fine silt hardest to erode (cohesion). Stream power: ω = ρgQS — controls erosion and transport. Dam effects: trap sediment → clear water below dam → channel incision downstream.

Snowpack: temporary storage of winter precipitation. SWE (snow water equivalent): depth of water if snowpack melted — measured at SNOTEL sites. Fresh snow: density ~100 kg/m³ (10% water). Settled/wet snow: ~400–600 kg/m³. Snowmelt: driven by solar radiation, air temperature, rain-on-snow events. Degree-day method: melt ≈ melt factor × (T_air - T_base). Energy balance method: more accurate, uses radiation, turbulence. Western US rivers (Colorado, Columbia, Sacramento): 60–80% of annual flow from snowmelt — critical for summer water supply. Climate change: less snow, earlier melt → reduced summer flows → water supply crisis. April 1 SWE: traditional measure of annual water supply outlook.