The Hillslope-Stream Continuum

Wed 4/22/2009

"The El Nino-Southern Oscillation and Global Precipitation Patterns: AView from Space"

Dr. Scott CurtisAssistant Director

Center for Natural Hazard ResearchDepartment of Geography

East Carolina University

WEDNESDAY, APRIL 22nd at 3:30TOY LOUNGE, DEY HALL

How does rainfall become streamflow?

Bear Brook, ME

Is pH just an episodic depression?

Bear Brook, ME

If so, why do values stay low after the event? Why aren’t the variations in streamflow reflected in pH?

Two Hydrograph Components:Quickflow baseflow

Streamflow Components

Terms

Event water- water that enters the catchment during

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

Storage Reservoirs

Catchment components can be considered storage reservoirsFor example- Groundwater and Surface water

So for a long time it was assumed that quickflow was rain, slowflow was groundwater

Sources of water

Quantifying hydrograph separations using chemical or isotopic tracers

Assume: 1) old water is fairly constant in space and time, 2) Concentration in precipitation is constant over the

course of the event

Qt= Total StreamflowQo= Pre-event water Qn= Event water

Ct= Streamflow concentrationCo= Concentration in groundwaterCn=Concentration in rain

• Meaured chloride concentration during the peak of the rainstorm event

• Rainfall [Cl] =Cn= 4.5umol/L• Groundwater [Cl]= Co= 40.5 umol/L• Streamflow [Cl]= Ct = 36.0 umol/L

• What fraction of total streamflow is contributed by new and old water?

Rainfall [Cl] =Cn= 4.5umol/LGroundwater [Cl]= Co= 40.5 umol/LStreamflow [Cl]= Ct = 36.0 umol/L

Qn = (36-40.5)/ (4.5-40.5) * Qt

How does it get there?

Various mechanisms of Streamflow Generation

Baseflow streamflow maintained bygroundwater contributions

StormflowAugmented by direct precipationOverland flow

Infiltrating rainwater- return flowShallow subsurface stormflow

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

Streamflow Generation

• Direct precipitation onto stream channel• Overland Flow• Shallow Subsurface Stormflow• Groundwater Flow

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 geneation

mechanism- saturation excess overland flow

Hillslope Water Balance

Topographic (Wetness) Index

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

some equations

Soil moisture defecit=sDepth of water you’d need to addTo reach ground surface

Tmax = Transmissivity. Recall T = KBK=hydraulic conductivityB= soil (aquifer) depth

To simplify- assume soil is saturated

m=curve fitting parameter. If T decreases rapidly with depth, mWill be large. A small value of m means T decreases slowly with depth

TOPMODEL SIMULATIONS

• Alter Tmax and m• Effect of TI distribution

Remaining soil moisture utilized by vegetation- canopy growth

Balance of two major forces: gravity, ET

Links to water quality

Timing matters- implications for when DOC

Higher [DOC] in summer months- implications for water managers

Future Directions: Double Paradox

1) Rapid Mobilization of ‘old water’2) Variable chemistry of old water

Kirchner, 2003 HP