Saturated and unsaturated flow in a hummocky landscape in relation to topography and soil morphology
Date
1988-03
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Doctoral
Abstract
Recent research on groundwater recharge in the Interior Plains has focused
primarily on saturated flow processes. The importance of unsaturated flow in groundwater
recharge and soil development is poorly understood. The purpose of the study was to
obtain quantitative estimates of saturated and unsaturated water flow and to relate the water
flow system to topography, soil morphology and the distribution of soluble salts.
The study was conducted in a hummocky upland area with a closed drainage pattern
in the Black soil zone in central Saskatchewan. The sites consisted of closed drainage
basins, 80 to 150 m across, surrounding individual temporary sloughs and were
instrumented with tensiometers, piezometers and neutron access tubes.
The sites were divided into lower slope, upper slope and saddle positions, which
accounted for 30, 50 and 20% of the cultivated areas of the basins, respectively, on the
basis of landscape surface form. The lower slope positions had soluble salts leached to
over 10 m depth, thick A horizons, thick zones of calcium carbonate depletion, high water
contents and relatively high unsaturated flux rates. The upper slope positions had limited
leaching of soluble salts, shallow A horizons, shallow zones of calcium carbonate
depletion, low water contents and relatively low unsaturated flux rates. The properties of
the saddle positions were intermediate between the upper and lower slope positions.
The groundwater recharge rates for the sloughs ranged from 250 to 300 mm yr-1
and the regional recharge rate was approximately 35 mm yr-1 or 10% of the annual
precipitation. Two main factors controlled the saturated flow regime. First, high hydraulic
conductivity deposits at the soil surface resulted in rapid horizontal flow at shallow depth at
the edge of the sloughs. Second, the presence of a low hydraulic conductivity layer at
approximately 12 m depth resulted in slow but uniform groundwater recharge. Less than
2% of the groundwater recharge could be attributed to unsaturated flow.
The magnitude of the unsaturated flux was commonly 10-8 m s-1 in the lower slope
positions, 10-9 to 10-8 m s-1 in the saddle positions, 10-10 to 10-9 m s-1 in the upper slope
positions following summer-fallowing, and 10-10 m s-1 or less in the upper slope positions
following three consecutive years of cropping. Downward flow was of similar magnitude
to upward flow in the lower slope positions and 2 to 5 fold higher than upward flow in the
upper slope and saddle positions. In a year with 30% above average rainfall, up to 25 mm
of downward flow and 5 mm upward flow occurred across the bottom of the root zone.
The unsaturated downward flux penetrated to greater than 2.5 m depth and was sufficiently
large to be of importance to soil development and crop growth. Deep drainage was greater
in saddle than upper slope positions, greater following summer-fallow than following three
years of cropping, and least under native vegetation.
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Degree
Doctor of Philosophy (Ph.D.)
Department
Soil Science
Program
Soil Science