Sources of streamflow from hillslopes

Baseflow streamflow maintained bygroundwater contributions

StormflowAugmented by direct precipitation on

saturated areaOverland flowReturn flowShallow subsurface stormflow

Streamflow ComponentsTerms

Event water- water that enters the stream network during a storm event– Stormwater: typically surface runoff– Throughflow: typically shallow soil water flow

Pre-Event water- any water that resided in the catchment prior to event

Base-flow – Subsurface water discharging into streams from groundwater source

Two Hydrograph Components:Quickflow baseflow

Runoff mechanisms

Various mechanisms of Streamflow Generation

Urban sourcesRural sources

Runoff production terms

• Overland flow– Infiltration-excess overland flow- runoff generated where

infiltration capacity is exceeded by rainfall intensity– Saturation-excess overland flow- runoff generated where

shallow water table intersects ground surface• Return flow- groundwater reemerges from the soil

at a saturated area and flows downslope as overland flow

• Variable source areas as saturated areas expand and contract

Dominant Runoff Processes

Horton Overland Flow DominatesSubsurface flow less important

Direct Precipitation &Return Flow Dominate

Subsurface stormflow dominatesPeaks produced by return flow & Direct precipitation

Variable Source Area

Climate, Vegetation, & Land Use

Topography and Soils

Thin SoilsGentle slopesWide Valley Bottoms

Steep straight slopesNarrow Valley Bottoms

Humid ClimateDense VegetationArid to Semi-Arid

Sparse VegetationUrbanizing

Factors controlling variable source area

TOPMODEL

• Numerical model for routing water through a catchment to predict hydrographs

• Based on catchment characteristics• Fundamental streamflow generation

mechanism- saturation excess overland flow

Hillslope Water Balance

et

R recharge from unsaturated to saturated zone

Water conservation equation for saturation zone – quantified as saturation deficit_ _S(t) = S(t-1) - R + qsubsurface + qreturn + evap from saturated zone

Topographic (Wetness) Index

TI= ln( a/ Tan B)Where:a= upslope contributing areaB= local slope

Flux and water conservation equations and saturation deficit redistribution

Saturation deficit=sWater equivalent depth to water table

S catchment mean saturation deficit

m model parameter representing depth decline of soil conductivity and redistribution of saturation zone water

λ mean wetness indexβ topographic slope

_

=

For each time step:

• For each wetness index interval:– Calculate infiltration to unsat zone, et from root

zone, recharge to sat zone, return flow, overland flow from direct precipitation

• For whole catchment (or hillslope)– Compute subsurface drainage– Update catchment mean saturation deficit– Redistribute local saturation deficit