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<br />001221 <br /> <br />VIII. 5 <br /> <br />If bulk density of the soil is known, water content of the SSlD.ples <br />can be expresseo either as a percentage by volume or as equivBlent depth. <br />However, the volume of the soil sample before being disturbed is difficult <br />to determine accurately, so that the apparent bulk density ~y not be <br />reliable. <br /> <br />If water content as a percentage by volume is essential, a sample of <br />little~disturbed soil of known iolume is obtained by a special device that <br />embodies a standard metal can or a collector ring. The 'Ii'll.ter ratio is then <br />determined by drying this knOlill bulk volume of sample. Particull!1r disad- <br />vantages are that the sampling equipment is especially cumbersome and <br />commonly cannot be operated easily at depths of more than a few feet. The <br />vo11illletric method is also destructive in that the samples IJRlSt be removed <br />for weighing ano drying. . <br /> <br />The equivalent depth of water ina sample;..depth lOne is found by <br />multiplying thickness of the zone by mean water content E!!:xpl'Ei'sSEi'd a.s a <br />percentage by volume. This procedure is convenient bEi'cause rainfall and <br />irrigation wate:r usually are expressed also in terms of equivalent depth <br />of water. <br /> <br />Neutron-scattering $oil-water meter <br /> <br />Nuclear equipment for measuring jater eontertt an'd, density. of "soil was <br />developed in the early 19501 s in connection;with airfield cohstruction; <br />subsequently it has been adapted to' agricultural and hydrologic invest;i~ <br />gation. In principle, soil-water content is determined by measuring the <br />absorption of energy emitted by a neutron source; density is dEi'termined <br />by measuring the scatter of gamma rays. Probes are available for deter~ <br />mining moisture and density at depth or at the land surface. <br /> <br />In depth-moisture measurement a probe containing a neutron Sollrce <br />is lowered into prepared access holes in the ground. Fast neutrons <br />emitted by the source are moderated by hydrogen atoms that are present <br />principally in the water of the soil. A portion of the slowed (thermal) <br />neutrons are detected by a component of the probe and the resultant <br />impulses are transmitted to an indicating unit. Inst:rument readings lire <br />related to volumetric water content of the soil according to calibration <br />curves determined experimentally. <br /> <br />The neutron technique is non-destructive and permits continUous, <br />repetitive or periodic observations as the same soil profile is involved <br />each time. The neutron meter's "sphere of influence" is from 1 foot to <br />3 feet in diameter, with the diameter larger as the water oontent is <br />smaller. Owing to successive observation of the sllJlle soil mass ano the <br />relatively large volume of soil involved in each reading,expEi'rimental <br />errors are fairly small and results are reproduciblEi' in a high degree. <br /> <br />