SOIL MOISTURE AND ITS INFERENCE

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

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itoring surface moisture over large areas, especially in arid and semiarid regions" (19:2319). But, their data were taken from an altitude of 2.2 m (19:2320). The depths most R-S sensors penetrate are < 10 cm; the root zones of many plants are > 0.5 m; and (even) "the field measurements of soil moisture are seldom sampled in a manner . . . representative of the true emitting layer for a given wavelength and the existing soil conditions" (19:2321. Emphasis added). Measurements were taken at 1.67 Ghz (18 cm) and at 5 Ghz (6 cm) (19:2319). Engman and Chauhan incisively distinguished passive MW R-S (which measures the intensity of emission from the soil surface) from the active approach (which measures radar backscatter) (7:190). They found active "synthetic aperture radar" (SAR) to be guardedly promising (7:189, 197). "Existing and planned SARs can provide at least 20-30 m resolution over a swath width of 100 km [but] . . . [s]ome difficult problems [remain, including] soil moisture estimation from rocky soil, effects of discontinuous canopy or vegetation clumps" (7:197). They studied data across frequencies from 1.5 Ghz to 10 Ghz (7:194). Passive system resolutions (tens of km) "appear to be of little use" (7:197). Chen, Engma
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Some common words found in the essay are:
MW R-S, Engman Chauhan, Warner Civco, Lack TM, Simple SAR-soil, MEASURES UNITS, Owe Van, Shih Jordan, SENSING Techniques, Resources Bulletin, soil moisture, remote sensing, wat resources, wat resources bulletin, resources bulletin, shih jordan, puerto rico, et al, passive microwave, remotely sensed, surface soil, surface soil moisture, rosbjerg shuttleworth eds, eds hydrological interactions, shuttleworth eds hydrological,
Approximate Word count = 3089
Approximate Pages = 12 (250 words per page)