1.72, Groundwat er Hydrology Prof. Charles Harvey
Le ct u r e Pa ck e t # 1 3 : St a ge s of D e sa t u r a t ion
St a ge s of D e sa t u r a t ion
1. Com plet e sat urat ion 2. Air ent ry
3. Funicular sat urat ion
a. Pores dewat er from large t o sm all b. Air and wat er phases are cont inuous
c. Average suct ion ( over REV) dom inat ed by direct suct ion t ransm ission t hrough t he cont inuous phase, alt hough film s of sorbed wat er exist at very large negat ive pot ent ials.
4. Pendular sat urat ion
a. Wat er phase discont inuous – no direct pressure t ransm ission b. Air phase is cont inuous.
5. Hydroscopic wat er a. Sorbed wat er b. Large suct ions
c. May or m ay not be cont inuous
1 2
4
3 2
3
4
4 4
4
D a r cy’s La w for ve r t ica l u n sa t ur a t e d flow
Use Mass Balance equat ion:
Advant ages
1. D does not vary over as m any orders of m agnit ude as K 2. D is not as hyst eret ic as K
3. I f gravit y can be neglect ed ( horizont al flow ) equat ion is nonlinear diffusion equat ion
Disadvant age
∂
θ
Does not w ork for ψ < ψae
C
(
θ
)
=
=
0
I n filt r a t ion ( “pe r cola t ion ”)
- The process by which wat er arriving at t he soil surface ent ers t he soil.
The rat e of infilt rat ion changes in a syst em at ic w ay during rainfall, even t hough t he rainfall rat e m ay be const ant .
Consider t hree different condit ions:
1. No ponding. I nfilt rat ion rat e is less t han or equal t o t he infilt rat ion capacit y, t he m axim um possible rat e.
2. Sat urat ion from above. Wat er input rat e exceeds t he infilt rat ion rat e, so ponding develops. The infilt rat ion rat e is equal t o t he infilt rat ion capacit y. 3. Sat urat ion from below. The wat er t able has risen t o or above t he surface so
t he soil is sat urat ed. Ponding occurs and t he infilt rat ion rat e m ay be zero. Fact ors affect ing infilt rat ion rat e:
1. Rat e at which wat er arrives. 2. Conduct ivit y at t he surface
a. Plant s
i. Large pores
ii. Shingling ( leaves spread on t he ground) b. Frost
i. “ concret e” effect ii. Polygonal cracks c. Sw elling- Drying
d. I nw ashing of fines e. Modificat ion of soils 3. Wat er Cont ent
a. Sat urat ion
b. Ant ecedent wat er cont ent ( how wet is t he soil before t he rainfall?) 4. Surface slope and “ roughness” cont rols ponding
θ
θ0 t1 t2 φ
tp tw
0 Khsat
w
t
zf( t5)
t5
t6 t4
t3= tp
Wat er cont ent , θ
0 0
t2 t1 t0
’
t2 t1 t0 t3
D
e
p
th
,
z
Profiles show a t ypical w at er dist ribut ion aft er deep soil infilt rat ion in t he absence of evaporat ion. t0 is defined as t he t im e w hen redist ribut ion begins.
Ra in fa ll- Ru n off M e ch a n ism s rem ainder is rainfall. Unclear w hy but w e know t his based on chem ical st udies
T Hort onian Overland Flow
• Wat er input rat e exceeds t he sat urat ion hydraulic conduct ivit y, so t he t op becom es sat urat ed and ponding and runoff occurs
Su bsu r fa ce Pr oce sse s
• Large scale groundwat er flow
• Groundwat er m ounds near st ream s I nfilt rat ion
quick
• Pressurizat ion of capillary fringe
st rong suct ion Hydrost at ic
o Moist ure dependent anisot ropy – wat er want s t o flow lat erally because
of higher conduct ivit ies due t o higher m oist ure cont ent
o Response m ight be quick because wat er does not have t o penet rat e
very far downward.
• Sat urat ion from below can also occur when:
o Groundw at er flow lines converge in surface concavit ies o Slope breaks
Decreasing conduct ivit y
sat urat ed
Ch e m ica l a n d I sot opic I n dica t or s
Mass balance for dissolved const it uent s:
