AIH Professional Hydrologist - Surface Water Examination flashcards
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32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Hydrologic cycle
- Continuous movement of water between atmosphere, land, and oceans through evaporation, condensation, precipitation, infiltration, runoff, and groundwater flow, driven by solar energy and gravity.
- Water balance equation
- For a catchment over a time period: precipitation minus evapotranspiration minus runoff equals change in storage; all terms in same units (e.g., mm).
- Potential evapotranspiration
- Maximum water loss to atmosphere from a well-watered surface, limited only by meteorological conditions; estimated by Penman-Monteith or Thornthwaite methods.
- Actual evapotranspiration
- Real water loss from surface and plants, limited by available soil moisture; cannot exceed potential evapotranspiration and decreases as soil dries.
- Return period
- Average recurrence interval in years for a given event magnitude; reciprocal of annual exceedance probability (e.g., 100-year flood has 1% chance each year).
- Unit hydrograph
- Direct runoff hydrograph resulting from one unit (e.g., 1 mm) of effective rainfall uniformly distributed over a catchment in a specified duration.
- Time of concentration
- Time required for water to travel from the most hydraulically remote point of a catchment to the outlet; used to select storm duration for design.
- Infiltration capacity
- Maximum rate at which soil can absorb water; decreases from initial rate to a constant final rate as soil saturates, often modeled by Horton equation.
- Rational method
- Peak discharge formula Q = C i A, where C is runoff coefficient, i is rainfall intensity for duration equal to time of concentration, and A is drainage area.
- Curve number
- SCS parameter (30-100) representing runoff potential of soil-cover complex; higher values indicate greater runoff; used to estimate effective rainfall from total rainfall.
- SCS curve number equation
- Effective rainfall Q = (P - 0.2S)²/(P + 0.8S) for P > 0.2S, where S = 25400/CN - 254 (mm); Q and P in mm.
- Muskingum method
- Flood routing technique using storage equation S = K[XI + (1-X)Q], where K is travel time and X is weighting factor between 0 and 0.5.
- Flow duration curve
- Plot of discharge versus percentage of time exceeded; used to characterize streamflow variability and determine water availability (e.g., Q90).
- Hydrograph
- Graph of discharge versus time at a point on a stream; shows baseflow, rising limb, peak, and recession limb in response to precipitation.
- Baseflow
- Portion of streamflow derived from groundwater discharge and delayed drainage; sustains flow during dry periods and appears as the slow recession component.
- Log-Pearson III distribution
- Probability distribution fitted to logarithms of annual peak flows for flood frequency analysis; requires mean, standard deviation, and skew coefficient of log values.
- Manning's equation
- Open channel flow formula Q = (1/n) A R^(2/3) S^(1/2) in SI units, where n is roughness, A is cross-sectional area, R is hydraulic radius, S is slope.
- Runoff coefficient
- Ratio of runoff to rainfall in rational method; depends on land use, soil, and slope; ranges from 0.05 for forest to 0.95 for paved areas.
- Direct runoff
- Portion of rainfall that reaches stream quickly as overland flow and interflow; represented by unit hydrograph and separated from baseflow.
- Design storm
- Synthetic rainfall event with specified depth, duration, and temporal pattern used for hydraulic design; often based on IDF curves for selected return period.
- Darcy's law
- Groundwater flow rate Q = -K A (dh/dL), where K is hydraulic conductivity, A is cross-sectional area, and dh/dL is hydraulic gradient; valid for laminar flow.
- Hydraulic conductivity
- Property of porous medium and fluid describing ease of water flow; units of velocity (m/s); depends on permeability and fluid viscosity and density.
- Transmissivity
- Rate of groundwater flow through a vertical strip of aquifer of unit width and full saturated thickness under unit hydraulic gradient; equals K times saturated thickness.
- Storativity
- Volume of water released from storage per unit surface area per unit decline in hydraulic head; for confined aquifers equals Ss b; for unconfined approximates specific yield.
- Specific yield
- Volume of water that drains by gravity from a saturated rock or soil divided by total volume; typical values: sand 0.20-0.30, clay 0.01-0.05.
- Theis equation
- Non-equilibrium well equation: s = (Q/(4πT)) W(u), with u = r²S/(4Tt); describes drawdown s around a pumping well in confined aquifer.
- Cooper-Jacob approximation
- Simplified Theis method valid for u < 0.01: s = (Q/(4πT)) ln(2.25Tt/(r²S)); used for pumping test analysis by plotting drawdown versus log time.
- Pumping test
- Field test where a well is pumped at constant rate and water levels in observation wells are measured to determine aquifer transmissivity and storativity.
- Equipotential line
- Line connecting points of equal hydraulic head in an aquifer; groundwater flow is perpendicular to these lines, from higher to lower head.
- Specific capacity
- Pumping rate divided by drawdown in a well (Q/s); indicates well productivity; decreases with increasing pumping rate due to well losses.
- Well efficiency
- Ratio of theoretical drawdown (aquifer loss) to actual drawdown (aquifer plus well loss) expressed as percentage; decreases as pumping rate increases.
- Cone of depression
- Drawdown pattern around a pumping well shaped like an inverted cone; its radius of influence depends on pumping rate, aquifer properties, and time.