Groundwater recharge and time lag measurement through Vertosols using impulse response functions Mark Hocking a,b,c,⇑ , Bryce F.J. Kelly b,c a HGEC Pty Ltd, PO Box 369, Hampton, Victoria 3188, Australia b School of Biological, Earth and Environmental Sciences, UNSW Australia, UNSW Sydney, 2052 New South Wales, Australia c Connected Waters Initiative Research Centre, UNSW Australia, Australia article info Article history: Received 20 March 2015 Received in revised form 13 January 2016 Accepted 19 January 2016 Available online 29 January 2016 This manuscript was handled by Peter K. Kitanidis, Editor-in-Chief, with the assistance of J. Simunek, Associate Editor Keywords: Groundwater recharge Condamine Vertosol Impulse response Lag time Groundwater level summary Throughout the world there are many stressed aquifers used to support irrigated agriculture. The Condamine River catchment (southern Queensland, Australia) is one example of a globally significant agricultural region where groundwater use has exceeded recharge over the last 50 years. There is a high dependence on groundwater in this catchment, because yearly rainfall is highly variable, and actual evap- otranspiration often exceeds rainfall. To better manage the aquifer there is a need to correctly conceptu- alise the primary inputs and outputs of the system, and characterise the lags in system response to all forcings. In catchment models it is particularly important to correctly proportion diffuse (areal) rainfall recharge and to account for the lag between rainfall and recharge at the water table. Throughout large portions of the Condamine Catchment, groundwater levels are now 20 or more metres below the ground surface. This study aimed to better quantify the lag between rainfall and recharge at the water table using the predefined impulse response function in continuous time method (PIRFICT; von Asmuth et al., 2002; von Asmuth, 2012). The PIRFICT method was applied to 255 multi-decadal groundwater level data sets throughout the catchment. Inputs into the modelling include rainfall, irrigation deep drainage, stream water level, evap- otranspiration, and groundwater extractions. As an independent check the PIRFICT model derived diffuse recharge estimates are compared to point lysimeter and geochemical recharge estimates in the Vertosol soils within this catchment. It is estimated using the PIRFICT method that in the Condamine Catchment between 1990 and 2012, the mean rain-derived groundwater recharge is 4.4 mm/year. Mean groundwater response from rainfall was determined to be 5.3 years: range 188 days to 48 years. The recharge estimates are consistent with both geochemical and lysimeter point measurements of recharge. It is concluded that where extensive groundwater level and climatic data sets are available the PIRFICT method provides an independent assessment of recharge. Importantly, the PIRFICT method highlights the need to consider the lag between rainfall and recharge at the water table. This lag is often overlooked when calibrating spatial distributed water balance models used to guide groundwater allocations. Ó 2016 Elsevier B.V. All rights reserved. 1. Introduction For the past 50 years to guide groundwater allocation decisions considerable scientific attention has been placed on understanding the catchment water balance in the Murray–Darling Basin in Aus- tralia. In recent years there has been an additional focus to more accurately quantifying the water balance due to: (1) a prolonged dry period in eastern Australia (BOM, 2014), which has caused a reduction of groundwater recharge, (2) allocation reductions, which are forcing irrigators to increase water use efficiency (MDBA, 2012), (3) a goal to better protect groundwater dependent ecosystems, and (4) the development of coal seam gas fields adja- cent to the irrigated agricultural districts throughout much of southern Queensland and northern New South Wales (Hiller, 2010), and the need to assess cumulative impacts on groundwater systems within the region. With the increased focus on understand- ing the catchment water balance, there is increased scrutiny on the processes and metrics of these dynamic systems, such as ground- water recharge. The interpretation of groundwater monitoring well hydrographs is fundamental for understanding and quantifying http://dx.doi.org/10.1016/j.jhydrol.2016.01.042 0022-1694/Ó 2016 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: HGEC Pty Ltd, PO Box 369, Hampton, Victoria 3188, Australia. E-mail address: mark.hocking@hgec.com.au (M. Hocking). Journal of Hydrology 535 (2016) 22–35 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol