Lateral Flow Component Research Articles

Estimating Recharge Rates Through Unsaturated Glacial Till by Tritium Tracing

AbstractA tritium profile obtained from a core located on the Tipton Till Plain near West Lafayette, Indiana, showed a “tritium reversal” (corresponding to a low‐high‐low activity of tritium relative to depth) at 7 m, indicating the present position of recharge water derived from 1963/64 precipitation. The average recharge rate calculated using the tritium data is 3.5 cm/yr for the “mass‐balance” method and 4.7 cm/yr for the “transit‐time” method. The average of 4.1 cm/yr is in reasonable agreement with Arihood's (1982) estimate of 5.1 cm/yr for tills, based on numerical modeling of data from a water‐budget study in the White River basin of Indiana. Two other cores, located on a nearby topographically lower slope, showed no definite reversal in tritium activity, probably because of lateral flow components at these locations.To be applied, the tritium technique should assess the contribution of summer and winter precipitation to recharge because tritium activities tend to peak in spring and summer precipitation. Contrary to the usual assumptions that recharge to aquifers in temperate zones of the northern hemisphere occurs between October and April, stable isotopic data of well water in the study area indicate that an appreciable amount of annual recharge (about 34%) actually occurs between April and October.

Soilwater distribution in a nonuniformly irrigated field with root extraction

A steady-state solution is developed for two-dimensional distributions of irrigation water flux at the soil surface and water extraction by plant roots. The solution of the linear equation is based on the small perturbations method, in which a small parameter is introduced into the governing differential equation. For the sprinkler irrigation method, this parameter is determined by scaling the soil characteristic length, α, by the distance between the sprinklers. Linearization is attained by assuming that the unsaturated hydraulic conductivity is exponentially related to the pressure head. Vertical distribution of the two-dimensional plant-root uptake is based on the one-dimensional model, which meets the approximation that the extraction pattern of plant roots is 40, 30, 20, and 10% of the total transpiration requirements in each successively deeper quarter of the root zone. Water extraction by plant roots increases lateral unsaturated flow beyond that which occurs as a result of irrigation nonuniformity. However, for a homogeneous soil profile, this lateral flow component increases water availability to those parts of the field where water shortages occur because of irrigation nonuniformity. Indeed, actual irrigation uniformity increases as measured by the decrease in the amplitude of the vertical flux around the mean. Younger plants suffer more from spatial fluctuations of irrigation water than do older plants, which have deeper roots. Actual irrigation uniformity, as measured by water availability to roots, is also affected by the distance between sprinklers and by soil type.

Two‐dimensional steady state unsaturated water flow in heterogeneous soils with autocorrelated soil hydraulic properties

The stochastic nature of soil water pressure head and vertical and horizontal flux density under two‐dimensional unsaturated steady state flow conditions was examined by using a Monte Carlo technique. Random autocorrelated scale factors were generated by a first‐order nearest neighbor model and were used to describe a stationary random field of hydraulic conductivity functions. Scale factor values were assumed to vary only in the horizontal direction, perpendicular to the mean flow. The water flow equation was numerically solved for steady state infiltration and transpiration in a two‐dimensional domain, of which the lower boundary was described by a groundwater level. Differences in results between a saturated and unsaturated soil water system were most likely caused by the direction in which the hydraulic properties are randomly autocorrelated and differences in the applied boundary conditions. In contrast to earlier findings with respect to the saturated system, the variability in fluxes for the unsaturated system decreased with an increase in the autocorrelation length, and the variation in soil water pressure head for one‐ and two‐dimensional systems tended to be equal with increasing autocorrelation lengths. When considering a two‐layer system it was found that the variability in the lateral flow component at the layer boundary was severely increased as compared to the single‐layer simulations.

Measuring Hydrologic Properties of Soil with a Double‐ring Infiltrometer and Multiple‐depth Tensiometers

AbstractA 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.