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GG2021: GEOMORPHOLOGY

2. Soil Texture and water movement. Sandy Loam: Large PoresGravity dominated flowMore saturated flow: Rapid, vertical. Clay: Finer pore textureMatric suction gradient > gravity in more cases => more unsaturated lateral flow. 3. INFILTRATION: THE HYDROLOGICAL CROSSROADS. The process of water movement into a soil through its surface.Infiltration Capacity: The maximum rate at which a soil can absorb water at its surface. Declines over time due to downward movement of water into the soil profi35660

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GG2021: GEOMORPHOLOGY

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    1. 1 GG2021: GEOMORPHOLOGY Session 2: Hillslope Hydrology & Water Harvesting

    2. 2 Soil Texture and water movement

    3. 3 INFILTRATION: THE HYDROLOGICAL CROSSROADS The process of water movement into a soil through its surface. Infiltration Capacity: The maximum rate at which a soil can absorb water at its surface. Declines over time due to downward movement of water into the soil profile. Increases with depth of standing water at the surface Due to cracks or fissues at surface Ground slope Cultivation Vegetation density – reduces compaction, increases porosity Decreases during rainfall due to: Surface sealing during rainfall Compaction of surface by raindrop impact Washing in of fines into surface pores

    4. 4 Soil Infiltration Infiltration Rate: Infiltration that is occurring at a rate less than the infiltration capacity Saturated Infiltration Capacity: relatively low and steady infiltration rate for soil which has been wetted to saturation for sufficiently long for clay swelling, compaction etc to have occurred. Infiltration occurs in TWO stages: Profile controlled rate- depends on soil wetting profile with depth and total potential differences in soil profile Saturated (Final rate)- See above. Related to Ksat.

    5. 5 Infiltration Capacity

    6. 6 Infiltration: Soil Texture factors

    7. 7 Infiltration- Profile control Moisture zones during ponded infiltration (Bodman & Coleman). As infiltration continues the transmission zone gets longer. Distribution of profile wetness controls potential gradients and velocity of water movement via Darcy’s Law.

    8. 8 Infiltration into slopes Redistribution of water in a soil column. Occurs after infiltration at the surface has ceased- controlled by matric potential gradients and gravity. Rate of redistribution slows down as matric potential gradient weakens as soil water content down the profile becomes more uniform and reducing water content causes hydraulic conductivity to fall.

    9. 9 Runoff generation Infiltration-Excess (Hortonian) overland flow Infiltration Excess Overland Flow occurs when : Rainfall intensity > soil infiltration capacity Rainfall Intensity = rainfall depth falling per unit time Soil Infiltration Capacity = maximum rate at which a soil can absorb water Horton assumed runoff occurred over WHOLE catchment

    10. 10 Response of streamflow to rainfall: Horton Hypothesis Betson: Infiltration capacities are spatially variable – so Horton overland flow will occur only on PART of a catchment = PARTIAL AREA MODEL OF RUNOFF

    11. 11 Some final infiltration rates measured with rainfall simulators (Dunne 1978) Region Soil Type Cover Final Rate I Vermont sandy loam pasture >8.0cm/hr Ohio sandy loan hardwood >7.6 Midwest US Silt loam bluegrass 0.4-1.55 Midwest US Silt loam old pasture 6.1 Midwest US Silt loam perm. Pasture 1.4 Midwest US Silt loam weeds/grass 1.0 Midwest US Silt loam 3-4 yr pasture 3.05 TOO HIGH FOR HORTON OVERLAND FLOW – AS RAINFALL INTENSITIES DO NOT REACH THESE VALUES…so is Horton model universally applicable??

    12. 12 Hewlett: Hillslope Throughflow Field method for monitoring flow through soil horizons. Gutters catch flow emerging from different soil horizons.

    13. 13 Hewlett: Throughflow Alternative names = Interflow, storm seepage Can distinguish QUICK (Sub-surface stormflow) and DELAYED throughflow. CAUSE: LATERAL hydraulic conductivity of shallow soil horizons exceeds vertical hydraulic conductivity through the soil profile (Ward & Robinson). During prolonged rainfall- Water enters the upper part of the soil profile more rapidly than it can drain to deeper zones. Perched (saturated) layer forms from which water flows laterally.

    14. 14 Evidence for ‘throughflow’ in slopes The diagram shows hydrographs derived from flow at different depths in a hillslope soil column.

    15. 15 Favourable conditions for Throughflow 1. thin permeable soil overlies impermeable bedrock. 2. stratified soil profile 3. ploughpan or ironpan below soil surface 4. presence of macropores and fissures in soil

    16. 16 Factors affecting onset of Throughflow The diagram shows penetration resistance changes with depth in a soil profile. Areas of high resistance indicate more compacted horizons where hydraulic conductivity may be less. This may lead to ponding of water and eventual drainage laterally downslope rather than vertically.

    17. 17 Sub-surface water movement on slopes Downslope water movement most marked in layered soils with permeability differences. Parallel flow also occurs in relatively uniform soils if the slope is steep. a) Dry conditions- gravity drainage b) Rain – surface layers wet so suction reduced c) Percolation wets deeper layers and saturated layer grows upslope. d) After rain stops. Drainage upper-lower layers continues & upper layer dries.

    18. 18 Response of streamflow to rainfall: Hewlett Hypothesis

    19. 19 Types of Flow Overland Flow: Water that flows over ground surface to stream channels as sheet flow or anastomosing trickles and minor rivulets. (Ward & Robinson) Hortonian Overland Flow: Occurs when rainfall intensity exceeds soil infiltration capacity. Saturation Overland Flow (SOF): When shallow groundwater tables rise to the surface during rainfall or throughflow. Hence infiltration capacity of ground surface falls to zero and overland flow results. Throughflow: Water that infiltrates the soil surface and then moves laterally through the upper soil horizons towards stream channels, either as unsaturated flows, or shallow perched saturated flow above the main groundwater level. (Ward & Robinson)

    20. 20 Variable Source Area Runoff Soil Moisture Potential at 60cm depth in one storm (Anderson & Burt 1978)

    21. 21 Saturated Overland Flow

    22. 22 Dunne (1978): Dynamic Contributing areas for saturated overland flow (Hewlett Model).

    23. 23 Hillslope Hydrology: Zones of saturation Topography: Contributing area (a) & Slope (S) combine to define potential for SOF. Concave Slopes/Hollows: Large contributing area & reducing slope so water accumulates and SOF may be generated. Convex Slopes/Spurs: Smaller contributing area & increasing slope – low soil moisture accumulation.

    24. 24 Hillslope Hydrology: Disjunct source areas Locations for flow convergence in catchments. Do these areas have effective hydrological links to valley bottom/channels?

    25. 25 Throughflow - Hewlett Throughflow & groundwater flow were thought to be too SLOW to contribute to QUICKFLOW in rivers. BUT…… ‘Thatched Roof Analogy’: On slopes there is a preferential hydraulic conductivity though upper soil as textures are more ‘open’. Hence throughflow rather than overland or groundwater flow occurs. HYPOTHESIS: Throughflow is the MAIN contributor to storm flow in humid-deep regolith environments. BUT: Measured through flow rates: 5-6m/day So how does it reach channel fast enough to contribute to Stormflow (Quickflow)? What’s the MECHANISM??

    26. 26 Hillslope runoff- Macropores & pipes Top- Variable source areas (Hewlett). Contiguous with channels. Other areas also may preferentially produce SOF: DISJUNCT CONTRIBUTING AREAS. If they have effective hydrological connections to rivers, they can contribute Quickflow. Jones – Soil Pipes can be effective hydrological links of topslope – bottom-slope

    27. 27 How does throughflow contribute to Quickflow?? 1. PISTON DISPLACEMENT: Most throughflow is in lower slopes. As saturated horizons move upslope, flowpath for throughflow from distant locations shortens. Piston displacement occurs as ‘new’ rainfall increments displace ‘old’ water from base of slope Problem …. This mechanism only works if soil is saturated. In unsaturated conditions rainfall inputs/displacements would increment the soil storage.

    28. 28 Problem of quick throughflow OR sub-surface stormflow ROLE OF GROUNDWATER Can groundwater make a contribution to QUICKFLOW? Idea of ‘Groundwater Ridge’ – ‘an ephemeral rise in groundwater table near the stream channel’ Hypothesis: Formation of high moisture potential at slope base reinforced by convergence of water in concave topography

    29. 29 Problem of sub-surface stormflow - resolution 1.Convergence & infiltration in lower slope area leads to surface saturation & groundwater recharge. 2. Thus overland flow + groundwater contribution to storm hydrograph. 3. groundwater ridge eventually merges into a wider riparian saturated area. With further rainfall this might extend to the lower slope.

    30. 30 Sklash & Farvolden: Role of Groundwater in Quickflow Water isotope analysis to study sources of QUICKFLOW or STORMFLOW in the field. Concentrations of Oxygen 18 isotope in slope water & rainfall Before Storm: Baseflow -12ppt Rainwater (Event water) - 6.6ppt Peak Discharge (Quickflow) -11.4ppt

    31. 31 Groundwater & Stormflow: Sklash & Farvolden Flow hydrographs and Oxygen 18 Isotope changes. Change of 018 concentrations for two quickflow events. This allowed calculation of % contributions to the Quickflow peak by the three types of water.

    32. 32 Sklash & Farvolden (1979): Role of Groundwater in runoff production in streams Result: 80% Groundwater in some event quickflow peaks

    33. 33 Exceptions to Hewlett’s Hypothesis? Semi-Arid & Arid Landscapes: Low vegetation density, crusted soils would produce Infiltration-Excess Overland Flow (Hortonian). Some semi-arid/arid areas however DO produce Quickflow via the profile-controlled SOF mechanism (Scoging & Thornes in Spain) Tropical Rainforest Soils – May produce Infiltration-Excess overland flow due to the exceptional rainfall intensities found in these locations. However, this may be actually a function of the very wet antecedent conditions found ALL OVER these catchments, and may in fact be SOF in effect. Ward & Robinson conclude that Hewlett’s Hypothesis applies widely unless Slope Materials, Slope Vegetation, Slope angle have been modified by human activity or where ‘catastrophic’ rainfall events occur.

    34. 34 Hillslope Runoff Processes

    35. 35 Hillslope runoff: Summary (after Dunne, 1978)

    36. 36 Negev – Air Photos

    37. 37 Avdat (Israel): Old City & Runoff Farm

    38. 38 Negev Desert: Wadi systems

    39. 39 Avdat (Negev): Conduits on runoff farm

    40. 40 Runoff-Farming: External Catchment System

    41. 41 Runoff- Farming: Micro-Catchment System

    42. 42 Avdat

    43. 43 Runoff Farming Evanari, Shanan & Tadmor – Relationship between watershed area and %runoff harvested. Note: Larger catchment, smaller % overland flow harvested. Due to: Roughness of surface Infiltration loss

    44. 44 Typical ratios of catchment-cultivated areas Av. Rainfall(mm) Within Field Systems External Catchments 300-600 Kenya-Baringo 1:1, 2:1 5:1, 20:1 Kenya-Turkana 2:1 10:1, 20:1 150-300 Tunisia 2:1 10:1 India, Rajasthan - 11:1, 15:1 80-120 Israel – Negev Microcatchments 10:1 Runoff Farms 17:1, 30:1 Contour Strips 4:1, 20:1

    45. 45 Runoff Farming- Avdat: Orchard

    46. 46 Runoff Farm at Nitzana, Negev Desert (Israel)

    47. 47 Yair & Lavee (1983): Runoff continuity experiments, Avdat

    48. 48 Lavee, Poesen & Yair (1992): Infiltration curves for cleared and uncleared plots at Avdat. Plots with embedded stones (1 & 2) - Cleared Natural, uncleared plots (3) Runoff Yield for 5mm rainfall: Natural: 0.275 Treated: 0.95 Runoff Yield (mm) for 20mm Rainfall Natural: 8.2 Treated: 12.1

    49. 49 Overland flow infiltration & surface particle size a- Large stones over fines surface. The large stones are impermeable and produce large volumes of runoff which may exceed ability of lower layer to infiltrate. HIGH RUNOFF b- Smaller stones – produce less runoff to percolate BETWEEN them. Fines may thus absorb MORE water. LOWER RUNOFF

    50. 50 Conditions & crop character suitable for runoff farming Climate: Winter Rainfall Max: Min MAR 100mm, 200mm better Tropical Summer Max 500-600 MAR Soil Conditions: Within-field catchments: Deep soils, clay content, crust forming capability. External Catchments: Crust forming soils, impermeable ground Cultivated Areas: deep soils (1.5-2.5m), high water-store capacity, low salinity Earthworks: Stable soil types, not subject to piping Crop Characteristics: Perennials, trees: able to withstand long drought Annuals: short growing season (quick maturing millets); deep rooting (sorghum); Tolerance of occasional waterlogging & drought.

    51. 51 Tabor et al: Effect of erosion on soil infiltration rates. Narawalle: less eroded soil. Karan-Karan: Eroded, crusty soil. Infiltration rates measured in desertified areas of Senegal. Final rates in brackets.

    52. 52 Tabor et al: Niger systems Micro-catchment design used in Niger study. Runoff fills the basin and excess flows around ends. Height of dyke- 20-30cm. Arms – 2-3m.

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