sign in sign up
<<
Home , Infiltration (hydrology), Water content

Land processes: vegetation atmosphere transfer ② 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 – 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 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], … ⤷ 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 (overland flow) by modulating the partitioning of into the three runoff components RAINFALL

INTERCEPTION – surface runoff – interflow RAINFALL EXCESS INFILTRATION EVAPO(TRANSPI)RATION (sub-surface runoff ) /GROUND – baseflow SURFACE SUB- RUNOFF ( 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.: – à impermeable (unless fractured)

– bare soil àpermeability depends on soil material (/lime/), 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: ⤷ ------> ⤷ texture ⤷

!

T = Clay % Vp U = S = Sand Vp/V G = 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 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 • dry weighing ê weight of water expelled at 105°C dry weight fraction M = dry overdry weight of 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 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 • 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 = [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 Its shape reflects essentially the experimental breakthrough curve Can be obtained integrating Richards’ equation for z= surface and K, D = constant ê df − = k( f − fc ) dt

ê rate

t = 0 à f=f0 à the infiltration rate f(t) is: infiltration

−kt f (t) = fc + ( f0 − fc )e time

-1 where k = decay constant [T] f0 = infiltration rate at t=0

fc = (saturated) infiltration rate at t=∞ ⤷≈ Ks, saturated hydraulic conductivity f − f Cumulative infiltration (volume) à ( 0 c ) −kt F(t) = fct + 1− e k ( )

Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 20 Infiltration models – Service Curve Number (SCS-CN)

Conceptual model combining infiltration and saturation excess Hypothesis: the ratio between actual, F, and potential, S, soil water content is equal

to the ratio between actual, Pe , and potential, P, runoff

ê • if no infiltration is possible à runoff = gross rainfall F Pe • with infiltration à runoff = net rainfall = (•) S P − Ia

where P = gross rainfall, Pe = rainfall excess à fraction of rainfall not infiltrated à net rainfall Ia = initial abstraction à water stored in depression storages

Obeying continuity P = Pe + Ia + F (••)

P = Pe + Ia + F ê (•) + (••) rate 2 P − I Pe ( a ) P = cumulative net rainfall Pe = (•••) e P − Ia + S precipitation If Ia = α S, (•••) is only function of the parameter S Ia F

S is parameterised on the basis of hydrologic time land cover CN Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 21 Infiltration models – Soil Conservation Service Curve Number (SCS-CN)

⎛ 100 ⎞ S is parameterised through the Curve Number, CN S = S −1 ① So = 254 à S [mm] 0 ⎝⎜ CN ⎠⎟ CN is function of • hydrologic soil type CN tables [Chow et al., 1988] • land cover

① takes minimum value = 0 for CN = 100 takes max. theoretical value = ∞ for CN = 0

Example: … … … … … ….. • Land use – meadow CN = 71 • Hydrologic Soil Type – moderatly permeable à C

Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 22

!

! Infiltration models – SCS-CN hydrologic soil types and AMC

Hydrologic Soil Type Description Permeability A Deep sand, deep , aggregated very high B Shallow loess, sandy high C Clay , shallow sandy loam, low in organic moderate content, and soils usually high in clay D Soils that swell significantly when wet, heavy plastic low clays, and certain saline soils

Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 23 Infiltration models – SCS-CN and AMC AMC = Conditions To account for the temporal variability of the storage due to the temporal dynamics of the soil water content (effect of and ET), the table values of CN can be converted to two ANTECEDENT MOISTURE CONDITIONS normal conditions (AMC II) à tables

dry conditions (AMC I) à 4.2CN II CN I = 10 − 0.058CN II

wet conditions (AMC III) à 23CN II CN III = 10 + 0.13CN II AMC conditions are classified on the basis of the vegetation activity ☞ TOTAL 5-DAY ANTECEDENT RAINFALL, P [mm] AMC GROUP dormant season growing season I < 12.7 < 35.6 II 12.7 ≤ P ≤ 27.9 35.6 ≤ P ≤ 53.3 III > 27.9 > 53.3 Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 24 Infiltration models – Pe vs P template for Ia = 0.1 S and AMC II 100" 100

90" 90 8080"

[mm] 70" e 70

P CN'='100'

60" 95' 60'[mm]' Pe 90' 50" 50 85' 80' 40" 75' 40Rainfall'Excess,' 70' 65' 30" rainfall excess, 30 60' 55' 20" 50' 20 45' 40' 1010"

0" 0 0" 20" 40" 60" 80" 100" 120" 0 20 40 Rainfall'Depth,'P'[mm]'60 80 100 120 rainfall depth, P [mm] Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 25 Infiltration models – Runoff coefficient Φ for Ia = 0.1 S and AMC II P CNII 1.0 Φ = e 10 P 15 0.9 20 0.8 25 30 0.7 35 40 0.6 45 50 0.5 55 0.4 60 65 0.3 70 Runoff [-] Coefficient Runoff

, 75 0.2 Φ 80 0.1 85 90 0.0 95 0 100 200 300 400 500 600 P, Rainfall Volume [mm] Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 26 Infiltration models – Φ-index method

Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 27 Infiltration models – Percentage method

Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 28 Ponding time (1)

The infiltration rate provided by Horton’s equation (and by some other equations derived from Richards’ eq.) is a potential infiltration rate à i.e. considering that water is continuously available at the surface à all water available for infiltration infiltrates. We have to account for this condition when applying the model to determine rainfall excess (net rainfall) à ponding time, tp = elapsed time between the beginning of rainfall and the instant that water begins to pond on the soil surface, i.e. ponding begins

⤷ t < tp à i < f(t) à no rainfall excess;t = tp à i = f(t) à ponding; t > tp à i > f(t) à rainfall excess

à Horton’s infiltration curve needs to be corrected for ponding

The correction is obtained by shifting the (potential) infiltration curve by a time t0 which is computed such that

t p t p P = i t dt = f t − t dt = F ∫ ( ) ∫ ( 0 ) 0 t0

P = cum. rainfall, Fp = cum. infiltration

Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 29 Ponding time (2)

To shift the curve by t0 : 1)compute ponding under potential infiltration conditions

⤷ i = f(ts) à compute ts

f0 ⤷ compute F(ts) = cumulated amount of infiltrated water that would lead to ponding under potential infiltration rainfall excess conditions i

2)compute the actual ponding time by imposing i⋅tp = Fp

⤷ F(ts) = Fp = i⋅tp à tp = …

⤷ tp > ts if i < f0

fc 3)shift the curve to account for tp > ts using the condition ⤷ t – t = t – 0 à t = t – t with t computed from step 1) p 0 s 0 p s s t=0 t0 ts tp For constant rainfall rate:

⤷ Fp = F(ts) Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 30 Horton‘s equation (summary + ponding)

Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 31 Infiltration example of application of knowledge

Engineering Problem: ⤷ Design of protection structure à estimate the amount of surface runoff generated by a given area/catchment to determine the flood volume to be stored by an artificial Solution ⤷ Compute the rainfall excess volume for the design rainfall event (e.g. obtained from a DDF for the design and rainfall duration)

100" 100

90" Method 90 8080"

P P I F [mm] = e + a + 70" e 70 rate ⤷ e.g. SCS-CN P CN'='100' 60" 95' '[mm]' 60 Pe 90' 50" 50 85' 80' 40" 75' Rainfall'Excess,' 40 70' 65' 30" rainfall excess, 30 60' 55' 20" 50' precipitation 20 45' 40' 10" 10

0" 0 0" 20" 40" 60" 80" 100" 120" 0 20 40 Rainfall'Depth,'P'[mm]'60 80 100 120 time rainfall depth, P [mm] Hydrology – Land Processes (Infiltration) – Autumn Semester 2017 32