C.D. Johnston

Bio: C.D. Johnston is an academic researcher from Commonwealth Scientific and Industrial Research Organisation. The author has contributed to research in topics: Aquifer & Groundwater. The author has an hindex of 20, co-authored 41 publications receiving 1145 citations. Previous affiliations of C.D. Johnston include Kansas Department of Agriculture, Division of Water Resources & University of Western Australia.

Pumping Test Analysis for a Tidally Forced Aquifer

Abstract: The migration of pollutant plumes in ground water systems in the vicinity of surface water bodies is a subject of continuing interest. Modeling the behavior and fate of such plumes is complicated by the dynamics of the surface water bodies. The analysis of pumping test data for a shallow riverbank aquifer afforded the opportunity to compare standard methods for determining hydraulic conductivity (K) and storativity (S) with techniques based on the propagation of periodic fluctuations of head in the aquifer. Tidal fluctuations in water level at the riverbank were found to propagate approximately nondispersively for 22 m in the aquifer, allowing tidal phase lags and amplitude attenuations in the aquifer to be estimated from river levels by least-squares techniques. The least-squares attenuations and phases were used to correct measured pumping test drawdowns for tidal influences. Standard transient analyses of the corrected drawdowns were performed, yielding values of approximately 8 to 10 m/d for K and 0.002 for S. The low S value was taken to indicate semi-confined conditions. The tidal attenuation coefficients were used to determine the hydraulic diffusivity T/S, providing values that were integrated over the distance from the riverbank to the relevant monitoring wells. S values from themore » drawdown analyses then yielded K estimates ranging from 3 to 7 m/d. A three-dimensional finite-element simulation of the pumping tests showed that the estimated K and S values yielded simulated drawdowns in reasonable agreement with field observations.« less

Choosing appropriate techniques for quantifying groundwater recharge

Abstract: DOI 10.1007/s10040-0010176-2 Abstract Various techniques are available to quantify recharge; however, choosing appropriate techniques is often difficult. Important considerations in choosing a technique include space/time scales, range, and reliabili- ty of recharge estimates based on different techniques; other factors may limit the application of particular tech- niques. The goal of the recharge study is important be- cause it may dictate the required space/time scales of the recharge estimates. Typical study goals include water-re- source evaluation, which requires information on re- charge over large spatial scales and on decadal time scales; and evaluation of aquifer vulnerability to contam- ination, which requires detailed information on spatial variability and preferential flow. The range of recharge rates that can be estimated using different approaches should be matched to expected recharge rates at a site. The reliability of recharge estimates using different tech- niques is variable. Techniques based on surface-water and unsaturated-zone data provide estimates of potential recharge, whereas those based on groundwater data gen- erally provide estimates of actual recharge. Uncertainties in each approach to estimating recharge underscore the need for application of multiple techniques to increase

Global river hydrography and network routing: baseline data and new approaches to study the world's large river systems

Abstract: Despite significant recent advancements, global hydrological models and their input databases still show limited capabilities in supporting many spatially detailed research questions and integrated assessments, such as required in freshwater ecology or applied water resources management. In order to address these challenges, the scientific community needs to create improved large-scale datasets and more flexible data structures that enable the integration of information across and within spatial scales; develop new and advanced models that support the assessment of longitudinal and lateral hydrological connectivity; and provide an accessible modeling environment for researchers, decision makers, and practitioners. As a contribution, we here present a new modeling framework that integrates hydrographic baseline data at a global scale (enhanced HydroSHEDS layers and coupled datasets) with new modeling tools, specifically a river network routing model (HydroROUT) that is currently under development. The resulting ‘hydro-spatial fabric’ is designed to provide an avenue for advanced hydro-ecological applications at large scales in a consistent and highly versatile way. Preliminary results from case studies to assess human impacts on water quality and the effects of dams on river fragmentation and downstream flow regulation illustrate the potential of this combined data-and-modeling framework to conduct novel research in the fields of aquatic ecology, biogeochemistry, geo-statistical modeling, or pollution and health risk assessments. The global scale outcomes are at a previously unachieved spatial resolution of 500 m and can thus support local planning and decision making in many of the world's large river basins. Copyright © 2013 John Wiley & Sons, Ltd.