the hillslope -stream continuum
DESCRIPTION
The Hillslope -Stream Continuum. Wed 4/22/2009. "The El Nino-Southern Oscillation and Global Precipitation Patterns: A View from Space" Dr. Scott Curtis Assistant Director Center for Natural Hazard Research Department of Geography East Carolina University WEDNESDAY, APRIL 22nd at 3:30 - PowerPoint PPT PresentationTRANSCRIPT
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