AIH Fundamentals of Hydrology Examination flashcards

31 cards. Read the term and the definition here. The deck at the top of this page is optional.

Water balance equation

For a catchment over a period, precipitation equals evapotranspiration plus streamflow plus change in storage plus net groundwater outflow. It is the core accounting identity of hydrology.

Potential evapotranspiration

The maximum evapotranspiration possible when water supply is unlimited, governed by atmospheric demand such as radiation, temperature, wind, and humidity. Actual ET is limited by available water.

Time of concentration

The travel time for water to flow from the hydraulically most distant point of a catchment to its outlet. It determines the critical storm duration for peak discharge estimation.

Unit hydrograph

The direct runoff hydrograph resulting from one unit of rainfall excess occurring uniformly over a catchment in a specified duration. It assumes linear response and superposition.

Return period

The average interval between exceedances of a given flow or rainfall magnitude. It equals the reciprocal of annual exceedance probability, so a 50-year event has 2 percent annual probability.

Rational method

Estimates peak discharge as Q equals the runoff coefficient times rainfall intensity times drainage area. The coefficient represents the fraction of rainfall becoming runoff.

Residence time

The average time a water molecule spends in a given reservoir of the hydrologic cycle. It ranges from days in the atmosphere to thousands of years in deep groundwater.

Hydrograph recession limb

The declining portion of a streamflow hydrograph after the peak, dominated by drainage of water stored in the catchment and by baseflow contributions from groundwater.

Manning's equation

An empirical formula for open-channel velocity: velocity equals roughness factor one over n times hydraulic radius to the two-thirds power times slope to the one-half power.

Hydraulic radius

The cross-sectional area of flow divided by the wetted perimeter. It appears in Manning's and Chezy equations and reflects channel conveyance efficiency.

Froude number

The ratio of flow velocity to the square root of gravitational acceleration times hydraulic depth. Values below one indicate subcritical flow, above one supercritical flow.

Critical flow

A flow condition where the Froude number equals one and specific energy is minimum for a given discharge. It often occurs at channel controls such as weirs and drops.

Specific energy

The sum of flow depth and velocity head, measured relative to the channel bottom. It is used to analyze transitions, drops, and hydraulic jumps in open channels.

Hydraulic jump

A rapid transition from supercritical to subcritical flow, accompanied by turbulence and energy loss. It is used to dissipate energy below spillways and at stilling basins.

Rating curve

A relationship between water surface stage and discharge at a gauging station, often fitted as a power function. Extrapolation beyond measured stages carries large uncertainty.

Baseflow separation

A procedure that divides a streamflow hydrograph into groundwater-fed baseflow and direct storm runoff, using graphical or digital filtering methods.

Muskingum routing

A flood routing method using continuity and a storage relation combining inflow and outflow with weighting factor x and travel-time parameter K.

Critical depth

The flow depth at which specific energy is minimum for a given discharge. In a rectangular channel it equals the cube root of discharge squared per unit width divided by gravity.

Wetted perimeter

The length of the channel cross-section in contact with flowing water. It is used with area to compute hydraulic radius and estimate friction losses.

Darcy's law

Groundwater discharge equals the negative product of hydraulic conductivity, cross-sectional area, and hydraulic gradient. It describes laminar flow through porous media.

Hydraulic conductivity

A measure of a porous medium's ability to transmit water, depending on both the medium's properties and the fluid's density and viscosity. Units are length per time.

Specific yield

The volume of water released from storage per unit area of an unconfined aquifer per unit decline in water table. It is always less than total porosity.

Transmissivity

The rate at which water flows through a unit width of aquifer under a unit hydraulic gradient. It equals hydraulic conductivity multiplied by saturated thickness.

Storativity

The volume of water released from aquifer storage per unit surface area per unit decline in hydraulic head. Confined aquifers have much smaller storativity than unconfined ones.

Theis equation

An analytical solution for transient drawdown around a pumping well in a confined aquifer, using an exponential integral function of time, distance, transmissivity, and storativity.

Cooper-Jacob method

A straight-line approximation of the Theis solution valid at large times. Plotting drawdown versus log time lets slope and intercept yield transmissivity and storativity.

Pumping test

A field test in which a well is pumped at controlled rate while water levels are monitored in observation wells, allowing estimation of aquifer transmissivity and storativity.

Effective porosity

The fraction of total void space that contributes to fluid flow through interconnected pores. Average linear groundwater velocity equals specific discharge divided by effective porosity.

Flow net

A graphical representation of steady groundwater flow using equipotential lines and flow lines. Flow lines cross equipotentials at right angles and indicate flow direction.

Ghyben-Herzberg relation

A principle stating that freshwater lens thickness below sea level is roughly forty times the freshwater head above sea level, due to the density contrast with saltwater.

Cone of depression

The roughly conical lowering of the water table or potentiometric surface around a pumping well. Its shape and extent depend on pumping rate, aquifer properties, and recharge.