L3. Water balance equation, systems approach, and drainage basins Flashcards
What are the elements of a system?
Boundaries (can be arbitrarily defined)
- ex. hydraulic divides, catchment boundaries
Fluxes (aka boundary conditions)
- movement in and out of a system
- usually measured in volume per time (sometimes height per time)
- ex. river flowing in and out of a lake
Storage
- in steady state system storage won’t change
Basic systems equation
Flux in(I) - flux out(Q) = change in storage for a given period of time (∆ S)
- Steady state: I = Q
- Transient state: I ≠ Q
(usual case)
What is turnover/residence time or (τ)
The time it takes to replace all the water stored in a system
What is the residence time for steady state systems
τ = S / Q
What are some common fluxes in hydrology?
Flux in: rainfall, snowmelt, groundwater inflow
Flux out: evapotranspiration, runoff from catchment, groundwater outflow
Stores in hydrology
- Surface water (lakes, reservoirs, wetlands)
- Soil water (aerated zone, wetlands)
- Groundwater (below groundwater table)
- Glaciers
- Biomass
What is a watershed (catchement)?
- All the land surface area that drains to the same outlet point
- Water that flows to the same place (everywhere on land can be described as a watershed)
- Typically defined as surface drainage but water can also flow underground, flowing different than the surface topography (tricky!)
Catchment Outlet points
- Can be defined anywhere, thus any number of (nested) sub-catchments can be defined within a larger catchment
- The ultimate outlet point is the coast
Catchment delineation strategies
- defines drainage divides through the use of contour lines
- Traditionally done on a map, today catchment boundaries are derived from digital elevation models in GIS (ex. HydroSHEDS)
What is the water balance equation?
Precipitation (P) + Groundwater inflow (Gin) - Evapotranspiration (ET) - Discharge or runoff (Q or R) - Groundwater outflow (Gout) = Change in storage (∆S)
Link between water balance and energy balance
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