Evaluation of the OhmMapper Instrument for Soil Moisture Measurement

of the spatial variability in soil moisture within the root zone. An instrument for making rapid measurements of the soil moisture Studies have shown that soil moisture content can content in the root zone is an essential tool for many applications, be inferred from measurements of the soil electrical including understanding of soil water dynamics, evaluation of agriculresistance (e.g., Seyfried, 1993; Amer et al., 1994; Hymer ture water stress, and validation of soil moisture modeling. Studies et al., 2000) by means of point measurement techniques have shown that electrical resistance measurements may be used to infer soil moisture content under special circumstances. In this paper, with a coefficient of determination greater than 0.8, electrical resistivity (resistance multiplied by a geometric factor) meawhen the pore water conductivity is low. A new instrusurements of the soil by the Geometrics Inc. OhmMapper instrument ment for mapping the spatial variability of electrical are compared with point measurements of soil moisture to a depth resistivity (resistance multiplied by a geometric factor) of 70 cm. It was found that the OhmMapper resistivity measurements over the top several meters of the earth’s surface is the could be used to infer soil moisture content with a coefficient of OhmMapper by Geometrics Inc. (San Jose, CA). In this determination as high as 0.34 when a simple power law relationship paper, we evaluate the OhmMapper’s potential to map was used. A more sophisticated analysis of the resistivity measureroot zone soil moisture distribution over large areas in ments could potentially lead to a greater coefficient of determination. a timely fashion. MATERIALS AND METHODS K of the spatial distribution in soil moisture content for the top 1 to 2 m of the earth’s Study Area surface, known as the root zone, is important for underEvaluation of the OhmMapper was undertaken for a single standing meteorologic, hydrologic, and agronomic pro250-m transect running from top to bottom of a small watercesses. However, soil moisture is highly variable in space shed within the United States Department of Agriculture, and time because of soil, topography, land cover, evapoAgricultural Research Center at Beltsville, MD, USA. The transpiration, and precipitation heterogeneity (Engman, transect had an average terrain slope of 2% and consisted of 1991; Wood et al., 1993). Furthermore, soil moisture a sandy loam soil (22% silt, 16% clay and 62% sand) with an prediction models are subject to representation, forcaverage organic content of 3 g/cm. Observations were made on a total of 8-d during a 30-d ing, and parameterization error (Wood et al., 1993), and period immediately following corn (Zea mays L.) planting in adequate spatial coverage by point measurements is June 2000 to capture a wide variation of soil moisture contents. prohibitive (Giacomelli et al., 1995) and soil moisture The transect had very little plant biomass during the study measurement from remote sensing observations is limperiod, ranging from initially bare fallow soil to a corn height ited to the top few centimeters (Schmugge, 1985). As a of 15 cm. Soil moisture was monitored every 25 m along the result, research is being directed toward a combination 250-m transect to a depth of 70 cm by means of the widely of these three approaches for estimating the space and accepted capacitance and time domain reflectometry point soil time variation of soil moisture content; assimilation of moisture measurement techniques. Continuous OhmMapper resistivity measurements were made along the entire length remote sensing observations into the soil moisture preof the transect for 12 different configurations. diction model (e.g., Houser et al., 1998) and calibration– evaluation of the soil moisture model from point meaSoil Moisture Measurements surements (e.g., Walker, 1999). However, evaluation of such a system over even moderately sized areas is diffiPoint measurements of soil moisture content were made cult because of a lack of knowledge about spatial variwith the commercially available Theta Probe capacitance soil moisture instrument from Delta-T Devices Ltd. (Delta-T Deability in soil moisture and an inability to measure it at vices Ltd., 1996) and the TRASE connector-type Time Doan appropriate scale within an adequate coverage. This main Reflectometry (TDR) soil moisture instrument from Soil necessitates a dependence on sparse point measureMoisture Equipment Corp. (Soil Moisture Equipment Corp., ments of soil moisture and measurements of surrogate 1989). By inserting the portable soil moisture sensors vertically variables, such as surface temperature and evapotransfrom the soil surface, the Theta Probe provided an average piration, for evaluation of the soil moisture estimates. soil moisture measurement over the top 6 cm of soil, while To overcome such limitations, there is a demonstrated the TDR instrument provided average soil moisture measureneed for an instrument that allows rapid measurement ments over the top 15, 30, 45, 60 and 70 cm of soil, depending on the waveguide length used. The TDR waveguides with lengths greater than 15 cm were left in place between monitorP.R. Houser, Hydrological Sciences Branch, Lab. for Hydrospheric ing times, while shorter waveguides were reinserted for each Processes, NASA’s Goddard Space Flight Center, Greenbelt, MD measurement date. This was because the balun (connector 20771; J.P. Walker, Dep. of Civil and Environmental Engineering, between the waveguides and measurement device) could not Univ. of Melbourne, Parkville, Victoria 3010, Australia. Received 15 Nov. 2000. *Corresponding author ([email protected]). 1 The mention of trade and company names is for the benefit of the reader and does not imply an endorsement of the product. Published in Soil Sci. Soc. Am. J. 66:728–734 (2002).