Geoscience Reference
In-Depth Information
LOAMY SOILS
SANDY SOILS
INITIAL STAGE
Seedbed
Seedbed
STRUCTURAL CRUST
Slaking
(or Coalescing or Infilling)
Sieving
2 - layer 1-3 mm
5-15 mm h
-1
3 - layer 1-3 mm
0-5 mm h
-1
coarse pavement 2-30 mm
0-2 mm h
-1
1-3 mm
5-20 mm h
-1
2-15 mm
3-9 mm h
-1
DEPOSITIONAL CRUST
Still
Runoff
2-50 mm
0-2 mm h
-1
2-50 mm
1-5 mm h
-1
EROSION CRUST
< 1mm
0-2 mm h
-1
Figure 11.4
Development of crust types and characteristics based on observations in West Africa by Valentin and Bresson (1992).
maximize photosynthesis, thus affecting flow pathways
on low-angled slopes in semi-arid regions. Kidron (1999)
also found a distinct difference between runoff on north-
facing slopes of longitudinal dunes in the Negev compared
to the south-facing slopes.
The latter typically had half the bacterial biomass and
produced 3.2 times less runoff. Maestre
et al
. (2002) also
found biological crusts in the interspace between
Stipa
tenacissima
grass clumps in southeast Spain and sug-
gested that they were important controls, increasing runoff
in these locations and making it available downslope for
the grasses, in a similar way as has been suggested for
semi-arid sites in the US and elsewhere (Belnap, 2006).
Hyper-arid areas can often produce significant biologi-
cal crusts, sustained by the interception of fog and dew
as a moisture source (Kidron, 2005; Lange
et al.
, 2007;
Shanyengana
et al.
, 2002; Warren-Rhodes
et al.
, 2007).
Sealing of the soil surface can occur by slaking of ag-
gregates of fine particles following wetting, so that clays
in particular collect in surface pores, producing highly im-
permeable surfaces (Figure 11.6). Where swelling clays
are present, the surface may become essentially totally im-
permeable (Sposito, 1972). The extent to which surfaces
are vulnerable to slaking can be measured by evaluat-
ing the aggregate stability, e.g. by observing the number
of drops of water required for the aggregate to start to
