Runoff Pathways Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001

(Grip and Rodhe, 1994) Southern Sweden—much like NE US

A different form of overland flow

Overland flow (infiltration excess+ saturation excess) emerging from a sugar cane paddock over Kasnozem (Oxisol) soils (originating from Basalt), South Johnstone near Innisfail during a monsoon event, March Photo courtesy of Brian Prove

Experimental Design of Dunne and Black (1970)

Seasonal Variations in VSA Dunne, 1969; 78

The link to flow From Dunne and Leopold, 1978

From the original diagram by Hewlett, 1982 Direct Precipitation onto Saturated Areas and Return Flow Expands and contracts during events Expands and contracts seasonally Key zone for partitioning fast and slow runoff Key non-point source hot spot! Brooks et al., Fig 4.11

Where Saturation Occurs Relation to live streams Ward, 1970

Saturated areas: We can sometimes estimate based on topography Dave Tarboton, Utah State U.

Generalised dependence of Runoff Coefficient and Style of Overland Flow on Arid-Humid scale and on Storm Rainfall Intensities Seasonal or storm period fluctuations Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001 HOF vs SOF

Runoff Pathways Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001

The British Invasion Benchmark papers by Burt, 1970s and early 1980s and Weyman, Anderson, Kirkby, Chorley………. From Kirkby, 1978

Topographic Convergence Anderson and Burt, 1978 Hornberger et al text

Topographic Controls on Saturation Development Ruhe and Walker, 1968

Subsurface Stormflow At the start of an event, percolation occurs vertically Soil moisture increases & some water bypasses to depth Where percolation reaches a less permeable layer that will not accept the wetting front, saturation will develop Saturation development controlled by permeability & available storage The saturated “wedge” or perched water table contributes significantly during peak runoff Weyman 1973

Whipkey’s work

Whipkey, 1965 Data:

Highly preferential Tarboton web course Sidle et al 2001 HP

What are the conditions necessary for lateral flow regardless of process?

Gradient Hydraulic Conductivity Contrast

Hydraulic Conductivity Contrasts Where do they occur? – Soil surface IF Ksat< rainfall rate HOF

Hydraulic Conductivity Contrasts Where do they occur? – Soil surface – Wetting front Even in uniform texture, character curves for a soil can be responsible for generating saturated layers under the right circumstances…HOW?

Hydraulic Conductivity Contrasts Where do they occur? – Soil surface – Wetting front – Grain anisotropy K x >> K y Can lead to ponding

Hydraulic Conductivity Contrasts Where do they occur? – Soil surface – Wetting front – Grain anisotropy – Capillary barrier Pic is of snow, can happen in soil under what conditions?

Hydraulic Conductivity Contrasts Where do they occur? – Soil surface – Wetting front – Grain anisotropy – Capillary barrier – Layering in saturated soils High K over low K can lead to ponding ON low K layer – Perched aquifers – Impermeable basement

Hydraulic Conductivity Contrasts Where do they occur? – Soil surface – Wetting front – Grain anisotropy – Capillary barrier – Layering in saturated soils High K over low K can lead to ponding ON low K layer Low K over high K

Lateral Gradients Where do lateral gradients occur? – Unsaturated soil? When K contrasts lead to ponding on sloped surfaces 3D perspective – Water balance in convergent zones

Flow pathways Must somehow mobilize stored water

Not a new idea

Pinder and Jones 1969 WRR

Two component mixing model  Solve two simultaneous mass-balance equations for Q old and Q new Q stream = Q old + Q new Q stream = Q old + Q new C stream Q stream = C old Q old C stream Q stream = C old Q old + C new Q new  To yield the proportion of old water Hooper (2001)

Q pe /Q s = (C s -C e )/(C pe -C e ) Weiler et al. 2004, WRR

Variations in stream discharge,  D, and electrical conductivity at M8 (Sklash et al., 1986 WRR) Groundwater Surface Water Interactions “Groundwater” is the main component of flood hydrographs

Runoff Pathways Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001

How is old water mobilized?  Many theories including  Groundwater ridging  Pressure wave translation  Transmissivity feedback

Groundwater Ridging

The Soil-Water Interface and the Effect of Suction Abdul and Gillham, 1984

Groundwater Ridging Flow Lines Precipitation Seepage face Equipotential lines Capillary Fringe

...a Swedish view on the subject From Grip and Rodhe; Seibert et al HP Rodhe, 1987 Transmissivity feedback

Runoff Pathways Putting it all together Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001

Storm Precipitation Soil Mantle Storage Baseflow Channel Precip. + Overland Flow Overland Flow Interflow Subsurface Stormflow Saturation Overland FlowHortonian Overland Flow Basin Hydrograph Re-drawn from Hewlett and Troendle, 1975

Dominant processes of hillslope response to rainfall Horton overland flow dominates hydrograph; contributions from subsurface stormflow are less important Direct precipitation and return flow dominate hydrograph; subsurface stormflow less important Subsurface stormflow dominates hydrograph volumetrically; peaks produced by return flow and direct precipitation Arid to sub-humid climate; thin vegetation or disturbed by humans Humid climate; dense vegetation Steep, straight hillslopes; deep,very permeable soils; narrow valley bottoms Thin soils; gentle concave footslopes; wide valley bottoms; soils of high to low permeability Climate, vegetation and land use Topography and soils Variable source concept (Dunne and Leopold, 1978)