Concepts Flashcards
Catchment Water Balance
Inputs: precipitation P + groundwater inflow IGW
Outputs: evapotranspiration E + groundwater outflow OGW + streamflow Q
When to use velocity-area?
- calmer, predictable flows, weak mixing
- can be made by wading, cable spanning the channel, hydro-acoustic
When to use dilution gaging?
- highly turbulent, cross-sectional area difficult to measure, hazardous conditions, mobile bed, rapid mixing
- tracer must be conservation, found naturally in low concentrations, and not pose a significant risk to aquatic life
Constant rate injection
best suited to smaller streams/lower flows (<1 m3/s)
Slug injection
“relative concentration method” (pre-mixed)
limited to discharges up to about 10 m3/s due to logistical challenge of mixing and handling larges volumes of salt solution
Slug injection of dry salt
“mass balance method”
has been used to measure discharges of 100s of m3/s
best if salt is at least partially dissolved in brine to avoid dry salt setting on bed of stream
Rating Curve
concurrent measurements of stage and discharge plotted
rating curves are not always stable
monitoring sites should be stable, low turbulence, easy to read
Hydraulic Structures
Geometrically defined controls constricting flow to generate a consistent stage-discharge relation
- Weirs: designed to minimize approach velocity, trap sediment
- Flumes: velocity approach, allows sediment to flush through, self-cleaning
Gauge under catch causes
Mainly wind, also:
- wetting: initial rain held on funnel surface by adhesion and to recorded
- measurement height: lower height = decrease wind speed, but increased possibility of splash-in from ground
- changes in vegetation or land use near gauge can introduce spurious change sin measured precipitation
Orographic processes
Upward deflection: air forced over topographic barriers
Orographic augmentation of other lifting mechanisms
Fictional effects: increased duration of storms
Topographic convergence
Seeder-feeder mechanism
Characteristics of convective precipitation
Summer, esp. in afternoons on sunny days.
Scattered cell each 1-10 km in scale
Minutes to an hour
High intensity
Characteristics of frontal precipitation
Dominantly autumn and winter Spatially expensive (10s-100s of km), warm fronts more extensive than cold fronts Hours to days, warm fronts longer than cold fronts Low to moderate intensity (warm fronts), cold fronts be can more intense
Elevation and Continentality
Variations with elevation Rain shadow effect Interactions between topography and weather systems Lake effect Seasonal variation
Extreme value assumptions
- events are independent
- the precipitation events are generated by the same meteorological mechanism
- the event probabilities do not vary through time
Interpretation of extreme value graphs
- if all assumptions are valid, data points should fall roughly along a straight line
- it is not uncommon to have one or two outliers at the upper end due to sampling
- “dog-legs” indicate possibility that more than one type of event is represented: one responsible for lower intensity values and another for higher intensities
Functions of interception
Reduces amount of water reaching the ground
Modifies spatial pattern of water reaching the ground
Can reduce kinetic energy of water reaching the ground
Modifies chemistry of water reaching the ground
Controls on interception
Canopy characteristics: species (shape and orientation of foliage and branches), canopy density or leaf area index
Weather/climate: storm intensity (shaking), storm duration (does the canopy become saturated), wind speed (shaking), air temperature, humidity, solar radiation
Empirical vs process based-models
Empirical: require less input data, need to be calibrated to data, and may not be transferable to other situations. Based on a linear relation between IL and P
Process-based: in principle require little or no calibration and are transferable in time and space, typically require extensive input data that are frequently unavailable especially in operational situations
Tensiometer
Moisture content in an unsaturated soil is related to water pressure (“soil moisture retention curve”)
Measures water pressure and provides an indirect measure of soil moisture changes
Complicated by “hysteresis” in the relation between soil water potential and water content
Well
Water level inside indicates depth of the water table
Piezometer
Water inside has the same hydraulic head as water in soils surrounding the screen at equilibrium
Hysteresis
Differences in contact angle between wetting and drying phases
Entrapped air bubbles on wetting phase
Ink bottle effect
Causes of anisotropy
Orientation of clay minerals in sedimentary deposits and soils, layering (e.g. fluvial deposits)
Factors causing the decline of hydraulic conductivity with decreasing water content
Volume of soil that is conducting water decreases
Velocity of flow decrease as water films get thinner (increased friction)
“Tortuosity” of flow paths increases (longer flow path = longer travel time)
Green and Ampt
- assume uniform soil properties and initial water content, no confining layers, water table far below surface
- invoking the water balance for the assumed condition, the infiltration rate
- early in event, hydraulic gradient is dominated by pressure head gradient, leading to high infiltration rate
- later in event, pressure head gradient declines, and hydraulic gradient becomes dominated by gravity
Hyporheic exchange
- involves the infiltration of stream water into the bed or banks, after which it flows some distance downstream before discharging back into the stream
- depends on morphology of stream and riparian zone
Evaporation pans
E = P - h h = change in water level heat exchange across sides and bottoms small water bodies tend to have higher evaporation than larger water bodies due to their reduce "fetch" heat storage changes seasonal timing
Potential Evapotranspiration Models
Thornthwaite’s: empirical, based on measurements of E from well-watered pyrometer, requires only air temperature and day length
Hergreave’s: empirical, calibrated using well watered lysimeters
Penman’s: combines energy balance with aerodynamic formulae for Qh and Qe
Priestly-Taylor:
Models of actual evapotranspiration
Penman-Monteith: assumes canopy can be treated as a big leaf
Soil-moisture balance: assumes plants act like passive wicks, coming approach in operational hydrological models
Modified Priestley-Taylor: uses empirically defined coefficient 𝛼’ which varies with soil moisture (due to the effect of soil moisture on hydraulic conductivity and ability of water to move in soil)
Snow storage controls
Elevation, wind, vegetation, avalanches, interactions
Snow albedo depends on
snow grain size and shape, dry versus wet, fractions of direct and diffuse radiation, incidence angle for direct radiation
Lysimeters (snow)
Measures vertical water drainage
Drain hole with stone placed to minimize clogging by leaves and needles
Time of concentration
The time taken for a molecule of water to travel from the most remote part of a catchment to the catchment outlet
Infiltration-excess
“Hortonian overland flow”
Occurs when rainfall intensity exceeds infiltration capacity of soil
“fill and spill” in depressions– controls connectivity between upslope areas and stream channel
stormflow is “direct runoff”
Saturation-excess
Rising water table
Total “saturated-excess overland flow” = direct precipitation on saturated source areas + return flow
Source area concept
Areas contributing water to streamflow vary intimate and space, primarily controlled by soil moisture conditions
Partial-Duration curves
Analyze all peak flows above some threshold
Statistical theory less well developed
Annual extreme flood series
Analyze only the highest flow each year
Statistical theory well developed
Bank storage
Channel water infiltrates banks on rising limb, reducing streamflow in channel; water discharges back into the channel on the falling limb
Role of forests
Transpiration, increased interception with age, high infiltration capacity
