Soil water flow model with two‐dimensional automatic gamma ray attenuation scanner

Abstract

This paper describes a physical laboratory model to study two‐dimensional transient water and solute transport in unsaturated soil, including water uptake by roots. It features an automatic two‐dimensional gamma ray attenuation scanner for measuring soil water content. Automation is obtained with a simple closed loop control circuit. After a gamma count is transmitted to the teletypewriter, a signal is sent to the hydraulically moved scanner to search for the next grid point. Upon arrival the scanner sends a signal back to initiate another data acquisition sequence, and so on. In this way, synchronization between counter and scanner is assured independent of counting time, travel time, configuration of grid points, temporary slowdown, etc. It also eliminates dead time between grid points for any recording pattern. Grid points are established by an array of holes that trap the core of a solenoid actuator. The configuration of grid points can be changed easily by opening and closing the desired holes. This mechanical trapping was found to be simpler and more accurate than a potentiometric control. The soil water flow model is constructed in modules. It can be used as one unit of 3.15 by 1.07 by 0.178 m and inclined up to 30°, or it can be subdivided into up to eight compartments, each with its own drainage filter tubes. The front glass walls allow visual observations of soil packing, wetting fronts, root distribution, etc. The back aluminum walls allow installation of instruments, such as tensiometers, salinity sensors, and psychrometers.

Experimental evidence of lateral flow in unsaturated homogeneous isotropic sloping soil due to rainfall

“…Two frames belonging to the front wall of the two‐dimensional automatic gamma ray attenuation scanning apparatus described by Dirksen [1978] and Dirksen and Huber [1978] were mounted together. Each frame consisted of two 9.13‐mm‐thick glass plates mounted inside steel frames.…”

Water Uptake and Release by Alfalfa Roots1

“…The measurements were carried out with alfalfa grown in a section ofthe soil water flow model with two-dimensional automatic gamma-ray attenuation scanner described by Dirksen and Huber (1978). The section, 1.07 m high with a rectangular cross-section of 0.394 by 0.178 m, was packed with Pachappa very fine sandy loam (Mollie Haploxeral.fs) at a bulk density of about 1570 kg m -3 .…”

Water and salt transport in daily irrigated root zone.

“…Here, only a few particulars of the daily irrigation phase are mentioned. The experiments were carried out in six segments (0.394 m x 0.178 m x 1.07 m) of the soil water-flow model with two-dimensional automatic gamma ray attenuation scanner (Dirksen & Huber, 1978) to measure soil water contents. Soil water pressure and osmotic potentials were measured with automated tensiometer and salinity sensor equipment.…”