In a special Soil Measurement & Methods section of Vadose Zone Journal, scientists review the state-of-the-art tools for measuring water content in soil
MADISON, WI, NOVEMBER 20, 2003 – Growing grapes for wine is tightly linked to soil moisture: too little, and the crop can be lost, but an oversupply of water tends to favor leaf development at the expense of fruit quality. It is often difficult to determine which portions of the vineyards require more or less irrigation due to California wine country’s natural geologic variations that control the moisture in soil. These natural variations, which appear over short distances, hamper the ability to map soil moisture of an entire field using conventional measurement techniques. Enter GPR, or ground penetrating radar.
In recent years, many researchers have made progress in the use of GPR as an alternative for TDR, time domain reflectometers, for determining field-scale variations of soil water content. These early TDR sensors came about in the 1980s and utilized the influence of water on the velocity of electromagnetic waves to obtain accurate measurements of soil water content; however, assessment of an entire field remained a tedious task because of the need to install a large number of TDR sensors to adequately cover the field. To overcome these difficulties, scientists have used GPR methods to map a field’s varied soil moisture, as in the case with the California vineyards.
GPR’s high-resolution information can be used to improve vineyard development and management, such as to develop vineyards within uniform "blocks" of soil that have optimal properties, or to improve precision irrigation approaches, says Susan Hubbard, long-time GPR researcher.
"Soil water content information obtained from GPR data can be extremely useful for guiding many environmental, engineering, and agricultural applications, such as in wine making. This is one example of a successful GPR application, but many others can be imagined," she adds.
Hubbard recently joined a group of scientists to review the current state-of-the-art methods for determining soil water content with GPR. This review is one of the contributions to a special section in the November issue of Vadose Zone Journal, published by the Soil Science Society of America (SSSA). Over 20 papers are published in this special section based on the symposium, "Soil Physical Measurements and Methods Symposium," held during the 2002 SSSA Annual Meeting. Fittingly, this symposium was dedicated to the research of C. Topp, one of the founding fathers of TDR.
Sander Huisman, an author of the review says, "After recognizing the dominating role of spatial variability at the field scale, it is now time to deal with it. Clearly, the traditional methods cannot detect these variabilities with sufficient resolution and, therefore, many scientists and practitioners are currently searching for alternative techniques. GPR is certainly one of the most promising methods currently under scrutiny".
Of course, there is still a gap between the advances made in the research community and the practical need for straightforward tools to accurately measure water content at high resolution and over large areas. The authors of the study, however, are confident that increased experience and application of GPR will eventually lead to the acceptance of GPR as one of the possible tools to measure field scale variation of soil water content.
American Society of Agronomy. November 2003.