Rangeland: Soil and Water Effects
157
the period of structural deterioration, the magnitude of
deterioration, and the time for recovery in a widely
distributed soil of the region. The mean weight diameter of
wet-sieved aggregates (MWD) was determined in grazed
and ungrazed (exclosure) plots. Aggregate MWD was often
lower in the soil under grazing (from 4.4 to 5.1 mm) than in
that of the exclosed area (from 4.7 to 5.4 mm). This
reduction in aggregate size was attributable to the
mechanical shearing action of trampling. Soil water content
accounted for 74% of the variation in aggregate MWD
under grazing. At low soil water contents, the structure of
the grazed soil became less stable. Grazing effects on soil
structural stability are significant only in periods when the
soil dries. Stocking rates must be regulated at those dry
periods.
© CSA
547. Water quality of surface runoff from grazed fescue
grassland watershed in Alberta.
Mapfumo, E.; Willms, W. D.; and Chanasyk, D. S.
Water Quality Research Journal of Canada 37(3): 543-562.
(2002); ISSN: 1201-3080
Descriptors: nutrients (mineral)/ carbon/ pathogens/
parasites/ environmental impact/ agricultural runoff/ Giardia/
Cryptosporidium/ Canada, Alberta
Abstract: A study was conducted at Stavely Research
Station, Alberta, to determine the quantity and quality of
surface runoff from small grassland watersheds under three
grazing intensities, viz. ungrazed, heavy grazing and very
heavy grazing. The volume of surface runoff varied each
year (1998, 1999 and 2000) and also differed across
watersheds, with lower runoff in the ungrazed compared
with the heavy and very heavy grazed watersheds. Total
dissolved solids in surface runoff water ranged between 34
to 360 mg L super(-1), and that for runoff from the very
heavy grazed watershed was greater than that from other
watersheds. Electrical conductivity increased with
increased grazing intensity on the watershed. In two of
three years the very heavy grazed watershed had greater
nitr