Field determination of unsaturated hydraulic conductivity of forest soils
This study introduces a field method to determine unsaturated hydraulic conductivity which is applicable to sloping terrain with a limited water supply. A single steel ring infiltrometer and an artificial rainfall simulator are used in this method to reduce the amount of water required to attain a steady-state flux condition. Six tensiometers and a time domain reflectometry (TDR) are employed as the soil capillary pressure head and the volumetric soil water content measurement devices, respectively. Water contents measured by the TDR are corrected using a simple calibration method suggested by Hook and Livingston (1996). Unsaturated hydraulic conductivities are computed based on the instantaneous profile method using capillary pressure head and water content changes measured during a drainage process. The proposed method was applied to a forest soil profile in Rokko Mountain range. Results showed that the relationships between the unsaturated hydraulic conductivity and capillary pressure head developed by the proposed method coincide well with those measured by the conventional steady-state laboratory experiment. The proposedin-situ method is the effective simple means to determine unsaturated hydraulic conductivities of forest soils, since this method is enough accurate and consuming less amount of water and time.
Measuring Hydrologic Properties of Soil with a Double-ring Infiltrometer and Multiple-depth Tensiometers1
A ring infiltrometer with varying width of buffer zone in combination with multiple-depth tensiometers was tested for determining the hydrologic properties of a Typic Dystrandept (Tantalus silty clay loam) soil profile on a Hawaii forested watershed. With location of one multiple-depth tensiometer at the vertical axis of the axi-symmetric flow system and one or more at a given radius outside the inner ring, both vertical and radial hydraulic gradients could be simultaneously measured. It was thus possible to estimate the lateral flow components and determine vertical infiltration rates and hydraulic conductivities (at field saturation) of the different soil horizons. Lateral flow decreased with time during infiltration. Under the moist to wet conditions of the soil under study, lateral flow was not appreciable. From an inner ring of 30-cm diameter, the latter was practically eliminated when a buffer ring of 90-cm diameter was employed. Its effect on the final infiltration rate was negligible even when a buffer ring of 60-cm diameter was used.
Effect of Tillage Practices Upon Intake Rates, Runoff and Soil Losses of Dry-Farm Land Soils1
During the growing season, serious losses of water and soil occur on extensive areas of dry-farm land in New Mexico, Colorado, and Utah due to poor surface soil condition and to the presence of tillage pans, usually at depths of from 3 to 8 inches. Studies were made of runoff and soil losses from plots containing 30½ square feet. Simulated 10-year storms were applied to these plots to evaluate tillage practices. The following conclusions were made during these studies. Chiseling through the tillage pan every 12 inches reduced both soil and water loss more than cultivation with either sweeps or one way disks on deep, medium textured soils. Excessive runoff and erosion occurred where the chisel spacing was 5 feet, or where chiseling, sweeps, and oneway did not penetrate through the tillage pan. Runoff from snow on frozen ground causes serious losses of soil and water in northern Utah. A frozen watershed that had been chiseled lost 0.61 inch less runoff than a similar watershed that had been cultivated with a oneway disk. Medium textured soils high in silt, when frozen at high moisture contents, had slow intake rates, irrespective of tillage practices.