Comment on Dong and Ochsner (2018): “Soil Texture Often Exerts Stronger Influence Than Precipitation on Mesoscale Soil Moisture Patterns”

Abstract

The Cosmic-Ray Neutron (CRN) rover is a mobile application of the CRN sensing method to measure fieldscale soil moisture noninvasively by surveying large regions (Schrön et al., 2018). Dong and Ochsner (2018) used an extensive data set of 18 CRN rover surveys along an approximately 150 km long transect to assess the influence of precipitation and soil texture on mesoscale soil moisture patterns. To this end, they used sand content to represent soil texture and the Antecedent Precipitation Index (API) to represent the influence of precipitation. Based on autocorrelation and Pearson correlation analysis, Dong and Ochsner (2018) concluded that soil texture exerted a stronger influence on mesoscale soil moisture variability than precipitation on 17 out of 18 survey days. We attempted to reproduce the results of Dong and Ochsner (2018) and found an error in the calculation of volumetric soil moisture from neutron count rates in their analysis (data were retrieved from https://osf. io/59j6c/). Dong and Ochsner (2018) wrongly derived volumetric soil moisture content from gravimetric soil moisture content (θ g [g/g]) by dividing with the soil bulk density (ϱ bd [g/cm 3 ]). Obviously, the correct approach to obtain the volumetric soil moisture content (θ v [m 3 /m 3 ]) would be the multiplication of θ g with ϱ bd : • g bd v w      (1) where ϱ w (=1 g/cm 3) is the density of liquid water (e.g., Marshall, Holmes, & Rose, 1996). Figure 1 exemplary shows the wrong volumetric soil moisture content as published by Dong and Ochsner (2018) in comparison to our own calculation of volumetric soil moisture with Equation 1 for one measurement day. We found a considerably higher soil moisture content for all survey days after correction, which is not surprising because bulk density was always higher than 1.36 g/cm 3. After correction of the originally published soil moisture content values of Dong and Ochsner (2018), some differences with the soil moisture content values we obtained from the neutron count rates were still present (Figure 2). These differences are most pronounced between ∼35 and ∼75 km, where a distinct drop in soil bulk density that was used by Dong and Ochsner (2018, lower panel of Figure 3 in the original publication) is visible. The soil data we extracted from the same database as used by Dong and Ochsner (2018) (SSURGO, https://websoilsurvey.sc.egov.usda.gov/, retrieved on 13 April 2020) did not feature this decrease in soil bulk density (not shown), which explains most of the remaining differences in water content estimates shown in Figure 2.

Reply to Comment by Jakobi et al. (2020) on “Soil Texture Often Exerts a Stronger Influence Than Precipitation on Mesoscale Soil Moisture Patterns”

“…We grant that the dominance of soil texture is somewhat reduced in the corrected data set compared to the original data set. The average correlation coefficient between corrected volumetric water content and sand content was −0.48 across all transect dates, similar to the value of −0.51 computed by Jakobi et al (2020) and of lower magnitude than the −0.65 value for the uncorrected data set. In contrast, the average correlation coefficient between the corrected volumetric water content and API was only 0.25, approximately one-half the magnitude of the sand content correlation.…”

“…The code was modified to account for this problem and all output files and figures were updated on the OSF page. Some remaining minor differences between our results and those of Jakobi et al (2020) are probably due to version differences in the SSURGO database. NRCS actively updates SSURGO, and the current version for Oklahoma was updated in September and October 2019.…”