Dissolved oxygen dynamics in the Lower Waihou River
Report: TR 2014/53
Author: Paul Franklin and Joshua Smith (NIWA)
The Waikato Regional Council (WRC) is responsible for managing water resources in the Waikato Region. WRC’s approach to the protection, allocation and use of water resources is set out in the Waikato Regional Plan: Variation No. 6 – Water Allocation, which became operative on 10 April 2012 (Waikato Regional Council 2012). As required by the National Policy Statement for Freshwater Management (MfE 2011), the Plan defines minimum flows and allocation limits for all catchments in the region (Table 3-5; Waikato Regional Council 2012).
WRC is currently undertaking a scheduled review of flow and allocation limits in the Waihou catchment. Work contributing to the review has included monitoring of instream ecological values (Franklin & Booker 2009, Franklin et al. 2011, Franklin et al. 2013), evaluation of minimum flow requirements for fish habitat (Jowett 2008), characterisation of flow regime variability (Franklin & Booker 2009) and analyses of dissolved oxygen dynamics (Franklin 2010a, 2010b, Vant 2011). The assimilative capacity of the lower Waihou River (i.e. the ability of the river to absorb and process contaminants) has subsequently been identified as a potential bottleneck for water resource use. The Kerepehi water intake is at times already compromised by reduced water quality (Ed Brown, WRC, personal communication) and preliminary monitoring undertaken by WRC indicates the presence of a natural dissolved oxygen sag in the lower river (Vant 2011). It is hypothesised that the location, duration and magnitude of the dissolved oxygen sag will be influenced by flow in the Waihou and tide height. WRC has requested that NIWA undertakes a targeted study of dissolved oxygen dynamics in lower Waihou River to try and establish the nature of dissolved oxygen dynamics and their dependence on freshwater flows and tidal fluctuations.