Soil-air Interface Research Articles

Cross-polarized radar backscatter from moist soil

Measurements of radar backscatter from bare soil at 9.0 GHz using a broad beam, with an effective beamwidth of product patterns and an incident angle of about 17° and 30°, respectively, have shown that the sensitivity of the cross-polarized ( HV) return to soil moisture content was much higher than that of the like-polarized ( HH) one. Analysis of the data shows that the observed HV back scattering power has a dependency of nearly Γ 2, where Γ is the power reflection coefficient at a plane soil-air interface. This fact suggests that multiple scattering on rough soil surface caused the soil moisture sensitivity of the cross-polarized return to be high.

Atmospheric pressure effects on222Rn transport across the Earth-air interface

The effect of large-scale atmospheric pressure changes on the 222Rn flux across the soil-air interface is investigated. Field data collected during 1972 and 1973 show that pressure changes of 1–2% associated with the passage of frontal systems produce changes in the 222Rn flux from 20 to 60%, depending upon the rate of change of pressure and its duration. A simple model of molecular diffusion combined with pressure-induced transport in the soil has been confirmed by laboratory experiments using a vertical column of 226Ra-bearing sand. On the basis of this model, pressure changes of 10–20 mbar occurring over a period of 1–2 days produce Darcy velocities of the order of 10−4 cm s−1 near the surface of a soil having a permeability of 10−8 cm2. The corresponding variations in the 222Rn flux predicted by the model are in agreement with those observed from valley alluvium in central New Mexico.

Soil electromagnetic parameters as functions of frequency, soil density, and soil moisture

Measurements are made to determine the conductivity and dielectric constants of a gray clay loam and a reddish-brown clay loam. The measurements are made as a function of soil density (from 1.2 g/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> to 1.8 g/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ), soil moisture (from 0 percent to 20 percent of the dry soil weight), and excitation frequency (from 30 MHz to 4 GHz), using standard transmission line techniques. A correction term is presented to allow the effective open-circuit and short-circuit connections to be placed at any convenient location with respect to the soil-air interface.

Radiation climate of New Delhi Part II : Longwave radiation and energy budget

Monthly budgets of long wave short wave, and net radiation, separate for air-space and air-soil interfaces, and also for the atmospheric column, are presented for the years 1964 and 1965 at New Delhi. A combination of direct radiation measurements at Lodi Rd. Observatory, New Delhi evaluations of satellite based scannings of the region, and climatonomical theories (as developed by H. Lettau), yields the monthly budgets of latent and sensible heat. Radiative shortwave heating and longwave cooling rates for the local atmospheric column are summarised for the seasons, resulting in a mean annual net cooling rate of 0. 4°C in both years.&#x0D; In this climate where the seasonal maximum of precipitation coincides closely with that of. insolation, the moisture budget at the air-soil Interface shows that about 80 per cent of the annual evaporation occurs from July to September In both years. Surface runoff appears to be restricted to the monsoon season and amounts to less than evapotranspiration even in the wettest month. Calculated values of exchangeable soil moisture were about twice as high in 1964 than in 1965 due to the intensive monsoon. The carry-over into the following dry season was considerable. Net moisture import (horizontal advection) into the atmospheric column over New Delhi was more than five times higher during the summer of 1964 than of 1965. The change over from moisture import to moisture export occurs abruptly with the greatest drop during October in both years.&#x0D; The calculation of a complete energy budget for the soil-air interface shows that due to the great demand of energy for evapotranspiration, monthly means of sensible heatflux, normally directed from the ground into the air during summer, can be reversed resulting in 'psychrometric cooling' of the ground. In 1964, such reversal occurred from July to September, but in 1965 it was weak and restricted to the month of September. Accordingly, the monthly mean surface temperature dropped from June to July by 10°Cin 1964, but only by 60Cin 1965. Flux of sensible heat from and into the soil reaches extremes in December and April, respectively, but turned out to be small in comparison with other constituents of the monthly heat budget.&#x0D; With the aid of the 'climatonomical' analysis method it is possible to establish quantitatively the constituent terms of the energy budget. For the atmospheric column, however, the energy supplied by advection cannot be separated from the contribution by subsidence. Considering the importance attributed to subsidence for maintaining arid conditions over northwestern India, a supplementary synoptic-aerological analysis of the atmospheric circulation over the region appears to be desirable in order to separate the effects of the two processes in the course of the year.

Determination of vertical variation in eddy diffusivity in lower layers of the atmosphere

A periodic convective heat flow solution for the lower layers of the atmosphere is presented in which the boundary temperature at the soil-air interface varies sinusoidally with time while the eddy diffusivity is postulated to vary sinusoidally with time and as a power law with height. The heat transfer differential equation and boundary values are simplified by trans-formations. Orthogonality properties of resulting eigenfunctions suggest the use of Stodola’s method with set boundary conditions to form an iteration device. This usage results in recurrence formulas programmed for numerical analysis. The height and the power law are varied in unit increments and a series solution of the mean temperature in the vertical is evolved.

Vertical Distribution and Persistence of Insecticidal Residues in Soils as Influenced by Mode of Application and a Cover Crop1

To determine, the effect of mode of application and a cover Crop on the persistence and vertical distribution of insecticides in soils, field tests were conducted over a 3-year period. Aldrin and heptachlor were applied either to the soil surface only or incorporated into the soil by rototilling to approximately 5 inches. Recoveries of insecticidal residues ranged from 2.7% to 5.3% of the applied dosages, 4 months after a soil-surface application. However, incorporation of insecticidal deposits into the upper soil layers increased the persistence of the residues by a factor of 10. One year after treatment of the upper soil layers, 90% of the recovered residues were located within the upper 3 inches of the soil. The highest concentration of the insecticides was found within the second in the of the soil layer. A deeper penetration and a more equal distribution of the insecticidal residues was noticed 3 year after soil treatment. Two to 3 times more insecticidal residues were recovered from alfalfa-covered plots than from fallowed ones. Volatilization appears to be the major factor in the loss of insecticidal residues. Their longer persistence after having been incorporated into the soil, may be extended by reducing the volatilization of the insecticides at the soil-air interface with a dense cover crop.