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ARS Home » Plains Area » Bushland, Texas » Conservation and Production Research Laboratory » Soil and Water Management Research » Research » Publications at this Location » Publication #208573

Title: Direct and surrogate measures of soil water content

item Evett, Steven - Steve

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 2/6/2007
Publication Date: 2/1/2008
Citation: Hignett, C., Evett, S.R. 2008. Direct and surrogate measures of soil water content. In: Evett, S.R., Heng, L.K., Moutonnet, P., Nguyen, M.L., editors. Field Estimation of Soil Water Content: A Practical Guide to Methods, Instrumentation, and Sensor Technology. IAEA-TCS-30. International Atomic Energy Agency, Vienna, Austria. ISSN 1018-5518. p. 1-22.

Interpretive Summary: A major user of soil water estimation methods for agricultural research and development is the joint division of the United Nations Food and Agriculture Organization and the International Atomic Energy Agency. At the invitation of the joint division, the Soil and Water Management Research Unit of the USDA-ARS at Bushland, Texas, guided research in Texas, California and Austria aimed at comparing and quantifying the performance of currently available soil water sensing methods. This chapter gives an overview of soil water content sensing that includes discussion of both direct and indirect measurements, variability and accuracy of each method, and recommendations for calibration. Most of the indirect measurement methods for soil water content involve surrogate measurements of soil electrical properties that are influenced by water content. Because these properties can be influenced by other factors, such as salinity or clay content, that are common in soils, the authors found that the indirect measurements were not as accurate as direct measurements and were too variable. The chapter is a useful introduction to soil water estimation both for irrigation managers and for researchers aiming to improve the efficiency with which water is used in agriculture. The authors show conclusively that the newer sensor technologies based on electrical properties of soils are as yet too inaccurate to be used for irrigation scheduling and research in general. Guidance is given as to which methods are sufficiently accurate; and conclusions are drawn that point the way to development of new sensors that avoid the problems that are delineated in the book.

Technical Abstract: An expert group from five nations and three continents did comparative trials of soil water sensing methods under laboratory and field conditions at the behest of the joint division of the International Atomic Energy Agency and the United Nations Food and Agricultural Organization (FAO), resulting in a nine-chapter guide to field estimation of soil water content. Chapter 1 gives an overview of soil water content that includes discussion of both direct and indirect measurements, of variability caused by instrumental noise and by spatial variability of soil properties, and of important interferences to the main methods and how calibration may be done to minimize problems of interference and variability. The field variability of soil profile water content measurements is discussed with examples and statistical theory for both direct and indirect measurements. The indirect methods that rely on the electromagnetic (EM) properties of soils are shown to be much more variable in the field than either direct measurements using soil coring or the neutron moisture meter, which has been the standard for more than 35 years. Reasons for the increased variability of EM methods include their relatively smaller measurement volumes, which may be smaller than the representative elemental volume for soil water content, and the nature of the interaction of the electromagnetic field with soil peds or structural elements, which can bias the field towards more conductive volume elements in the soil. The authors conclude that only the neutron moisture meter or direct measurements are sufficiently accurate for scientific work.