Author's Reply

We do not disagree with anything in the Comment by Cuthbert et al. (this issue) on our article (Fritz and Mackley 2010). However, there are a few points that we would like to clarify and expand upon. First, we stand by our statement that “There did not appear to be any fundamental difference in the methods; if the instruments could measure the same point in space and time, the results would likely have been nearly identical,” with the caveat that we were addressing our data at our field site. It is certainly possible that at other field sites with different conditions, a difference between methods might be observed; in particular, a site with a much lower VHG, where variations in the head measurements caused by dynamic head variations become more significant than at our site. However, we believe that it is generally understood by researchers that each field site should be evaluated with respect to the methods being used and that measurement techniques need to be selected such that the value of the measurement is sufficiently larger than the sources of error associated with the method. The Comment by Cuthbert et al. provides a theoretical quantification of some of these sources of variability for researchers implementing wet/wet differential pressure sensors for VHG measurement in the future. Given the space limitation of the Methods Note format of our article, all the details regarding the field set-up and supplementary testing were not included. In the field set-up, we attempted to minimize potential issues associated with dynamic head variations by placing the river end of the wet/wet differential pressure sensor very close to the riverbed and by using a small diameter tube. During installation, we also observed the data at a high temporal frequency (1 Hz) after installation to assess stability and sensitivity of the instrument response. We observed no significant variations, although that was likely due to the VHG being so much larger than any potential dynamic head variability. However, this step would be recommended during installation for any one using a wet/wet differential pressure sensor for VHG measurement in the future; it also provides an opportunity to verify that the instrument is providing correct readings prior to a lengthy deployment. We agree with Cuthbert et al. (this issue) that dynamic head might need to be considered for specific field conditions. For cases where VHG C v2 2g (as in our field study), then the dynamic head should not have a significant impact on the measurement. For cases where VHG is not significantly larger than the dynamic head, then either the contribution of dynamic head to the measurement may need to be accounted for or the VHG measurement must be magnified. In these situations, we recommend installing the subsurface measurement point deeper below the riverbed surface to amplify the VHG signal. If VHG is relatively low, then either the formation has a high hydraulic conductivity or there is not much water flow through the formation. If the hydraulic conductivity is high, then most likely deeper installation will not be overly difficult. We appreciate the additional comments and perspective Cuthbert et al. (this issue) bring to our article and believe that they will prove beneficial to future applications of wet/wet differential pressure sensors for VHG measurement. In particular, we believe that the most important aspects of these discussions are the importance of understanding the methods being applied in the field and the sources of variation or error associated with those methods. All potential sources of error associated with measurement methods should be considered at the beginning of a field campaign to ensure that the data will be of good quality.