Saturated and unsaturated flow in a hummocky landscape in relation to topography and soil morphology

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.