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.
INVESTIGATION OF A METHOD FOR DERIVING UNSATURATED SOIL HYDRAULIC PROPERTIES FROM WATER CONTENT PROFILES
Hydraulic properties of unsaturated soil profiles are needed for quantitative analysis of water movement. A recently proposed method of estimating some of these properties from water content profiles after drainage was examined for a uniform soil satisfying certain constraints that allowed a new exact analytical solution to be obtained. A new analytical and numerical examination of the method, assuming correct property functional forms and boundary conditions, established that it can give good estimates of unsaturated hydraulic conductivities. If a matric potential value is available at one depth in the profile, matric potentials at other depths can also be estimated accurately. The method seemed to work reasonably well even when assumed property functions and boundary conditions had an incorrect form, although hydraulic conductivities were then in error up to 25 percent. This seemingly large error reflects the difficulty in measuring a highly variable dynamic property, like conductivity, in soils.
The status of water spreading for ground-water replenishment
Methods of spreading water on land surfaces and by injection are reviewed and some results given. Some localities utilizing or contemplating the me of water spreading to replenish ground-water supplies are mentioned. Included are: (1) developments in vegetative, chemical and mechanical treatments to increase infiltration rate; (2) shaft, trench, and pit injection methods which cut through less pervious soil and inject water into aquifers; and (3) operational procedures such as cultural methods, length of wetting and drying periods, and surface head. Soil-water movements are described and interpreted in the light of the Darcy equation. Limitations in the use of treatments based on the surface soil and stratigraphy are described. Solutions to some problems and research directed toward existing needs are mentioned. A strip system and rotational system of spreading water based on an infiltration rate-percolation rate ratio are described. The strip system uses portions of an area and it is conceived that as much water may infiltrate as using the entire area. Surface soil treatment to increase this ratio may result in additional benefits. Land between flooded portions might be farmed or used for other purposes. This system visualizes a ‘subsurface lake.’ In the rotation system some portions will be flooded while infiltration rates remain high and other portions will be drying for recovery in infiltration rate. Tools to determine the suitability and characteristics of a proposed spreading area, such as soil surveys, laboratory tests of water and soil, well logs, jetting, cable tool test holes and infiltrometers are discussed. Examples are given of water spreading grounds constructed on sites selected by the use of these tools.