Eutrophication in Lake Rotorua. 1. Using OVERSEER to estimate historic nitrogen loads

Abstract

ABSTRACTNitrogen concentrations are increasing and causing eutrophication in many New Zealand waterways, including Lake Rotorua. Groundwater (mean age 35 to 170 years) dominates lake inflows, and nitrogen losses from farmland may take several decades to reach the lake. This study collates published agricultural statistics and estimates farm losses from 1900–2015 using the OVERSEER model. Changes in survey methods, reporting only at district scale, and uncertainty in OVERSEER mean that uncertainty in losses averages ±50%. Nevertheless, there is a clear trend of increasing nitrogen losses arising from the intensification of farming which mimics the trend in measured stream loads. From 1958 to 2015 farm losses doubled and now comprise c. 80% of lake load. OVERSEER provides a quantitative index of the changes in nitrogen losses, which in a companion paper are routed to the lake accounting for groundwater lags and attenuation.