Land processes: soil vegetation atmosphere transfer ② Infiltration and Runoff Generation Mechanisms Interception Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 1 Land processes: soil vegetation atmosphere transfer ② Infiltration and Runoff Generation Mechanisms Mass conservation: P = I + E + ET + F + R Infiltration à Runoff Generation Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 2 Land processes: soil vegetation atmosphere transfer ② Infiltration and Runoff Generation Mechanisms Lecture content Skript: Ch. IV.5 Ch. VI §2.3 • Infiltration 2.3.2 – relevance of the process 2.3.4 (including sub-sections) – influencing factors (soil characteristics) – measurement □ infiltration vs soil water content – infiltration and runoff generation mechanisms – mathematical models of infiltration □ physically based vs empirical/conceptual Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 3 Infiltration Definition: Infiltration is the process of water penetrating from the ground surface into and through the soil it depends on properties of the medium where it takes place, which are • static/intrinsic ⤷ land/ground cover ⤷ soil properties • or time variable Infiltration is a flux (per unit area) à [L][T]-1 à [mm/h], [cm/s], … ⤷ soil water content infiltration rate ßà infiltration capacity Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 4 Relevance of infiltration in the transformation of rainfall into runoff infiltration controls ⤷ the generation of surface runoff (overland flow) by modulating the partitioning of precipitation into the three runoff components RAINFALL INTERCEPTION – surface runoff – interflow RAINFALL EXCESS INFILTRATION EVAPO(TRANSPI)RATION (sub-surface runoff ) BASEFLOW/GROUND – baseflow SURFACE SUB-SURFACE WATER RUNOFF (groundwater runoff) RUNOFF RUNOFF TOTAL RUNOFF Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 5 Infiltration controlling factors land/ground cover controls the permeability at the ground surface and below (effects of roots) ⤷ i.e. controls the ability of the soil to let water infiltrate e.g.: – rock à impermeable (unless fractured) – bare soil àpermeability depends on soil material (sand/lime/clay), generally does NOT favour infiltration – vegetated cover (grassland, forest,…) à permeable, favours infiltration Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 6 Infiltration controlling factors soil properties control the permeability and the hydraulic properties of the porous medium, through which water flows: ⤷ porosity --------------------------------------> ⤷ texture ⤷ ! T = Clay % Vp U = Silt S = Sand Vp/V G = Gravel Vc/V X = Pebbles Hydrology – Land Processes (Infiltration) ! – Autumn Semester 2017 ! 7 ! Infiltration – process characteristics infiltration is variable at a point along the soil profile in space in space saturation WATER zone CONTENT at a point in space transmission zone transition in time zone INFILTRATION RATE DEPTH wetting front wetting zone TIME Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 8 Measurement vs estimation of infiltration Direct measurement of ring infiltrometer infiltration rate is carried out by means of ⤷ infiltrometers ⤷ lysimeters (water bal.) ⤷ experimental plot (water bal.) Infiltration rate is estimated by equations, which express its temporal dynamics http://en.wikipedia.org/wiki/File:Double_ring.JPG ⤷ soil water content measurement (gravimetric sampling, tensiometers, neutron probes, time domain reflectometers) Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 9 Measurement of infiltration - Infiltrometers Measuring principle: lowering of head in infiltrometer or tube ring water refill to keep constant head infiltrometer infiltrometer ê breakthrough curve Problems: • soil disturbance • lateral flow INFILTRATION RATE TIME • boundary effects • poor rainfall similitude Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 10 Measurement of infiltration - plots shelter enclosure Measuring principle: mass conservation at plot scale water supply ê water balance equation I = P − R Rainfall intensity runoff cm/h Runoff collected Infiltration rate wetted • constant rainfall border • E, ET =0 Hydrology time,– Land Processes (Infiltration) t – Autumn Semester 2017 11 Measurement of soil water content – gravimetric sampling Measuring technique: • extraction of soil carrot in the field • lab analysis • wet weighing • drying in oven • dry weighing ê weight of water expelled at 105°C dry weight fraction M = dry overdry weight of soil moisture volume volume of water expelled at 105°C MVF = fraction volume of soil before drying Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 12 Measurement of soil water content – tensiometer Measuring technique: air safe cover • water content equilibrium ê capillary flows through a porous ceramic tip from tensiometer to soil proportional to the water content gradient vacuum range: 0.35÷0.85 capillary flow generates a vacuum in the air safe tensiometer tube filled with water [Manning, 1977] Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 13 Measurement of soil water content – neutron probe Measuring technique: • attenuation of flux (or velocity) of radioactive neutrons ê collision with hydrogen nuclei contained in soil water molecules root zone • non disturbing measurements • more accurate than tensiometers [Manning, 1977] Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 14 SOIL MOISTURE MEASUREMENTS BY TDR TDR = Time Domain Reflectometer Working Principle Measurement of soil water content – TDR The soil bulk dielectric constant (Kab) is determined by TDR = Time Domain Reflectometry measuring the time it takes for an electromagnetic pulse (wave) Measuring principle: to propagate along a velocity of propagation of a high-frequency signal transmission line (TL) that is surrounded by the soil. Since the ê propagation velocity (v) is a function of Kab, the latter is computation of the dielectric constant of the soil, therefore proportional to the SOwhich is related to its water contentIL MOISTURE MEASUREMENTS BY TDR square of the transit time (t, in Twater dielectric constant ≈80, solid soil components ≈2DR = Time Domain Reflectometer ÷7 seconds) down and back along the TL. 2 2 Kab = (c v) = (c⋅t / 2 ⋅ L) a Kab = soil bulk dielectric constant c = velocity of electromagnetic L waves in vacuum (3·108 m/s) t = time required by the wave to travel from a to b and return b (distance 2L) ©Maja Krzic, University of British Columbia, Vancouver BC Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 15 Working Principle The soil bulk dielectric constant (Kab) is determined by measuring the time it takes for an electromagnetic pulse (wave) to propagate along a transmission line (TL) that is surrounded by the soil. Since the propagation velocity (v) is a function of Kab, the latter is therefore proportional to the square of the transit time (t, in seconds) down and back along the TL. Infiltration models – theoretical frameworK θ = soil water content q = flow SUBSURFACE FLOW WATER TABLE • 1D flow SUBSURFACE OUTFLOW • continuity equation zoom GROUNDWATER FLOW GROUNDWATER ∂θ ∂q OUTFLOW + = 0 ∂t ∂z voids • momentum equation ⎛ ∂θ⎞ q = −⎜ K + D ⎟ ⎝ ∂z ⎠ control ê soil volume Richards’ equation water particles where: ∂θ ∂ ⎛ ∂θ ⎞ = ⎜ D + K⎟ • K = hydraulic conductivity [L][T]-1 • Ψ = suction head [L] ∂t ∂z ⎝ ∂z ⎠ • D = soil water diffusivity = [L]K ⋅dΨ dθ 2[T]-1 ⤷ governing equation for 1D unsteady unsaturated flow in a porous medium [Richards, 1931] Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 16 Infiltration models – runoff generation mechanisms • Hortonian mechanism à infiltration excess • runoff generation occurs when the rainfall rate exceeds the infiltration capacity (rate) more frequent INFILTRATION – in arid and semiarid environments [Horton, 1933] – where vegetation coverage is limited • Dunnian mechanism WATER à saturation excess TABLE • runoff generation occurs when the unsaturated zone becomes saturated and the soil does not allow any infiltration more frequent – in humid environments SATURATION [Dunne, 1933] – where vegetation coverage is rich Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 17 Infiltration models – infiltration excess vs saturation excess Dunnian mechanism i(t) q(t) i(t) Dunnian mechanism q(t) t t t t 1 1 2 2 3 3 4 4 1 2 infiltration excess saturation excess 1 2 runoff runoff saturated saturated rate rate area area 3 4 3 4 runoff runoff saturated saturated area area rate rate ! Hydrology – Land Processes (Infiltration) ! – Autumn Semester 2017 18 Infiltration models – physically based vs empirical/conceptual models Physically based models ∂θ ∂ ⎛ ∂θ ⎞ Richards’ equation = ⎜ D + K⎟ numerical solution only ∂t ∂z ⎝ ∂z ⎠ where: • K = K(θ), D = D(θ) simplified physically based models à K , D = constant à Green-Ampt model (1911) (analytical solutions) à K = constant, D = D(θ) à Philip’s model (1957, 1969) Empirical / conceptual models • derived from empirical investigations (e.g. Horton equation) • based on conceptualisation • e.g. storage + infiltration excess à CN method • e.g. constant infiltration rate à Φ index • e.g. infiltration rate variable and proportional to rainfall rate à percentage method Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 19 Infiltration models – Horton‘s equation Horton’s equation was developed in the 1930’s from repeated observation of infiltrometer tests