Jm. Reichert et Ld. Norton, AGGREGATE STABILITY AND RAIN-IMPACTED SHEET EROSION OF AIR-DRIED AND PREWETTED CLAYEY SURFACE SOILS UNDER INTENSE RAIN, Soil science, 158(3), 1994, pp. 159-169
Stability of aggregates at the immediate soil surface affects the deta
chment, transport, and deposition of soil particles. The aggregate sta
bility (expressed as mean weight diameter, MWD) and erosion rate were
tested for clayey surface soils at two different water contents. The s
oils consisted of six Oxisols, one Ultisol, three Vertisols, one Alfis
ol, and one Aridisol. The MWD was determined by wet sieving of 4.76 to
8.00 aggregates, which were either air-dried (MWD(d)) or capillary pr
ewetted at -0.5 kPa for 24 h (MWD(w)). Air-dried and prewetted (satura
ted from the bottom up for 2 h) soils were subjected to a 110-mm h-1 r
ain until a steady-state infiltration was reached. Runoff, sediment lo
ss, and infiltration (for prewetted soil only) were monitored througho
ut the rain. The MWD(d) ranged from 0.18 to 4.80 mm, and the MWD(w) ra
nged from 0.44 to 5.66 mm. Prewetting increased the MWD for the oxidic
or kaolinitic Oxisols and Ultisols and for one smectitc/kaolinitic Ve
rtisol. The MWD(w) was positively related to CEC for highly weathered
soils and negatively related for swelling soils, whereas base saturati
on was negatively related for both groups of soils. The DELTAMWD, whic
h expresses the increase in stability caused primarily by a decrease i
n slaking, was greatest for soils rich in kaolinite and Fe and Al oxyh
ydroxides and low in organic carbon. The steady-state infiltration rat
e (I(s)) ranged from 1.8 (illitic soil) to 54.8 mm h-1 (sandy loam kao
linitic soil). The I(s) was consistently less for swelling than for hi
ghly weathered soils, and it was related to the sand/clay ratio, sand,
and water retained between -33 and -1500 kPa. The increased aggregate
stability with prewetting was generally not reflected in smaller amou
nts of erosion and runoff, possibly because of the faster rate of wett
ing than for the stability test, a higher water table for the prewette
d soil than for the air-dried soil, and the inclusion of smaller aggre
gates in the erosion tests. Prewetting decreased the time to ponding f
or three highly weathered Oxisols and the illitic Alfisol, increased s
teady-state runoff rate (Q(w)) for five highly weathered soils, increa
sed sediment loss rate (Q(s)) for the illitic and for one highly weath
ered soil, and increased Q(s) on a stable Oxisol. Soil textural proper
ties (usually clay and sand/clay) and ''binding'' agents (organic carb
on and Fe and Al oxyhydroxides) were the significant independent varia
bles included in the stepwise multiple linear regressions for I(s), Q(
s), and Q(w).