Acqua e Suolo - Infiltrazione

Acqua e Suolo - Infiltrazione

Obiettivi formativi:

-  conoscere le principali proprietà idrauliche del suolo -  Conoscere i meccanismi che controllano l’infiltrazione -  Saper distinguere infiltrazione potenziale ed effettiva -  Saper utilizzare i principali metodi per il calcolo

dell’infiltrazione

Riferimenti:

•  Tarboton, Rainfall-Runoff Processes workbook chapter 4

•  Maione: Le Piene Fluviali, cap 2

•  Versace: Scheda didattica

1

Suolo:

-  mezzo trifase

-  sede di fenomeni di trasporto d’acqua in forma non satura

2

0 10 20 30 40 50 60 70 80 90 100

0.001 0.01 0.1 1 10 100

Diameter (mm)

Percentage finer by weight Clay Silt Sand Gravel

Figure 22. Illustrative grain-size distribution curves. The boundaries between size classes designated as clay, silt, sand and gravel are shown as vertical lines.

Uniform ly gra

ded sand

Well graded silt, sand, gravel mixture

Curva granulometrica con i limiti di appartenenza alle classi tessiturali

3

Soil texture triangle, showing the textural terms applied to soils with various fractions of sand, silt and clay. (from Dingman, 2002)

B A

Triangolo tessiturale

4

Cilindro di Darcy

(From Freeze and Cherry)

1

2

!1

!2

K=flusso (saturo) specifico a gradiente unitario [cm/s]

q = !K "h

"z

5

Moisture Content "

Suction head |!| Elevation z

"s

Quota parte dei pori occupati da acqua alla quota z

100%

! = !2" cos(#)

$gR

Risalita capillare (profilo di equilibrio).

(from Freeze and Cherry, 1979)

Relazione tra suzione e contenuto d’acqua

La forma della curva di ritenzione idrica è

funzione della distribuzione dei pori

7

(from Freeze and Cherry, 1979)

Differenza di potenziale -> flusso

8

From Brutsaert, 2005 9

From Brutsaert, 2005 ¹⁰

Curve caratteristiche (ritenzione idrica) θ-ψ

!"#$%&'()*+'ψ

(from Freeze and Cherry, 1979).

Residual moisture content "_r =5%

Curve caratteristiche (ritenzione idrica) θ-ψ

!"#$%&'()*+'ψ

(from Freeze and Cherry, 1979).

Curve caratteristiche (ritenzione idrica) K-ψ

(from Freeze and Cherry, 1979). 13

Variation of soil suction head, |!|, and hydraulic conductivity, K, with moisture content^. (from Chow et al, 1988) ¹⁴

"r

soil water characteristic functions (funzioni di Pedo-trasferimento)

r r

e

n

S " !

!

"

= !

c e sat e

b e a e

S K ) S ( K

S

|

|

| ) S (

|

=

!

=

!

Brooks and Corey (1966)

( )

2 m m / 1 e 2

/ 1 e sat e

m m 1 / 1 e e

) ) S 1 ( 1 ( S K ) S ( K

1 1 S

| ) S (

|

!

!

=

" !

=

#

van Genuchten (1980)

sat

b a

K n ) ( K

| n

|

| ) (

|

! "

$ #

%

& '

= '

! "

$ #

%

& ' (

= ' (

)

Clapp and Hornberger (1978)

15

Grado di saturazione effettivo

Soil Texture Porosity n

K

(cm/hr) |!

| (cm) b Sand 0.395 (0.056) 63.36 12.1 (14.3) 4.05 (1.78) Loamy sand 0.410 (0.068) 56.16 9 (12.4) 4.38 (1.47) Sandy loam 0.435 (0.086) 12.49 21.8 (31.0) 4.9 (1.75) Silt loam 0.485 (0.059) 2.59 78.6 (51.2) 5.3 (1.96) Loam 0.451 (0.078) 2.50 47.8 (51.2) 5.39 (1.87) Sandy clay loam 0.420 (0.059) 2.27 29.9 (37.8) 7.12 (2.43) Silty clay loam 0.477 (0.057) 0.612 35.6 (37.8) 7.75 (2.77) Clay loam 0.476 (0.053) 0.882 63 (51.0) 8.52 (3.44) Sandy clay 0.426 (0.057) 0.781 15.3 (17.3) 10.4 (1.64) Silty clay 0.492 (0.064) 0.371 49 (62.0) 10.4 (4.45) Clay 0.482 (0.050) 0.461 40.5 (39.7) 11.4 (3.7)

Clapp and Hornberger (1978) parameters for Soil Moisture Characteristic functions. Values in parentheses are standard deviations.

based on analysis of 1845 soils. ¹⁶

Potenziale di suzione |!| per differenti tessiture dei suoli

[Clapp and Hornberger (1978)]

1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10 1.0E+11 1.0E+12 1.0E+13 1.0E+14

0 0.2 0.4 0.6 0.8 1

!/n

|"

| (cm)

sand loamy sand sandy loam silt loam loam

sandy clay loam silty clay loam clay loam sandy clay silty clay clay

17

1.0E-26 1.0E-24 1.0E-22 1.0E-20 1.0E-18 1.0E-16 1.0E-14 1.0E-12 1.0E-10 1.0E-08 1.0E-06 1.0E-04 1.0E-02 1.0E+00 1.0E+02

0 0.2 0.4 0.6 0.8 1

!/n K(!) (cm/hr)

sand loamy sand sandy loam silt loam loam

sandy clay loam silty clay loam clay loam sandy clay silty clay clay

Conducibilità idraulica K(") per differenti tessiture dei suoli

[Clapp and Hornberger (1978)]

Soil-water status as a function of pressure (tension).

Natural soils do not have tensions exceeding about -31000 cm;

in this range water is absorbed from the air. [from Dingman, 1994]

19

Ranges of porosities, field capacities, and permanent wilting points for soils of various textures.

(from Dunne and Leopold, 1978)

20