SOIL MOISTURE AND ITS INFERENCE VIA REMOTE SENSING
There is wide interest for use of satellites to measure environmental factors globally--with respect to alleged global warming and to address feeding the world. Among the most fundamental bases of agriculture is the water content of soil during crop growth. So, inference of soil moisture through remote sensing techniques has been researched for two decades.
Soil moisture is water held--Marshall uses "retention" (16:7)--within earthen pore spaces. Ogrosky and Mockus tabulated "Moisture Holding Capacities," from 0.4 in./ft. of soil for sand to 2.00-3.00 in./ft. for peats (18:21-83). These are maximum amounts soils will retain; often soils are drier than this. Childs indicates 10-30% soil moisture near the ground surface and 35-50% at some depth is typical for wet soils (4:224-29).
To measure water in soil at any time directly, "a weighed sample is dried at some chosen temperature (usually 105 ?C) and the mass of water per unit mass of dry soil determined" (16:7). This gives soil moisture content (SMC) in grams of water per gram of dry soil. (Marshall called this quantity, cw.) Multiplied by 100, this becomes the percent of soil moisture on a weight basis. Data reported by Owen-Joyce (20) were reported in this unit.
However, a "better" measure of soil moisture for many applications is a depth, or percent by volume (16:7). "If the apparent (or bulk) density of the soil is ?a in g/cm3, then the volume of water per unit volume of soil is given by c = cw?a/? cm3/cm3 and ?, the density of water, may be taken as unity" (16:7). "Here the water content, c, represents also the equivalent depth of water in a unit depth of soil" (16:7). Data of Lindsey, Gunderson, and Riley (14:872-73) were reported in cm; and this is, in a meter of depth, equivalent to percent by volume, as Engman and Chauhan reported their data (7:194).
For years it has been obvious that gravimetric soil ...