1. Sources of streamflow from hillslopes Baseflow streamflow maintained by groundwater contributions Stormflow Augmented by direct precipitation on saturated area Overland flow Return flow Shallow subsurface stormflow

2. Streamflow Components Terms 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

3. Two Hydrograph Components: Quickflow baseflow

4. Runoff mechanisms Various mechanisms of Streamflow Generation Urban sourcesRural sources

5. 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

6. Dominant Runoff Processes Horton Overland Flow Dominates Subsurface flow less important Direct Precipitation & Return Flow Dominate Subsurface stormflow dominates Peaks produced by return flow & Direct precipitation Variable Source Area Climate, Vegetation, & Land Use Topography and Soils Thin Soils Gentle slopes Wide Valley Bottoms Steep straight slopes Narrow Valley Bottoms Humid Climate Dense Vegetation Arid to Semi-Arid Sparse Vegetation Urbanizing

7. Factors controlling variable source area

10. TOPMODEL Numerical model for routing water through a catchment to predict hydrographs Based on catchment characteristics Fundamental streamflow generation mechanism- saturation excess overland flow

11. 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 + q subsurface + q return + evap from saturated zone

12. Topographic (Wetness) Index TI= ln( a/ Tan B) Where: a= upslope contributing area B= local slope

13. Flux and water conservation equations and saturation deficit redistribution Saturation deficit=s Water 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 index β topographic slope _

14. 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