Transmission losses and groundwater recharge from ephemeral rivers in the Canterbury Plains (New Zealand): Historical reconstruction and environmental implications

Study region The Canterbury Plains (New Zealand), where gravel-bed rivers are a major source of groundwater recharge to underlying aquifers and some of these rivers are ephemeral. Study focus A recently developed method has been employed to estimate groundwater recharge from the Selwyn and Orari rivers using satellite images. We found that for both rivers the specific groundwater recharge is strongly correlated to the logarithm of the river discharge. These relationships were leveraged to predict the total groundwater recharge from flow data for the last 40 years. New hydrological insights for the region Our findings indicate an average annual groundwater recharge of 74 Mm3y-1 from the Selwyn River and 205 Mm3y-1 from the Orari River. Notably, there is a strong interannual variability in groundwater recharge of up to 100%, partly explained by El Niño events, but no distinct impact of longer-term climate change. Moreover, this study highlights the importance of groundwater recharge from ephemeral rivers of the Canterbury Plains for groundwater resources and groundwater-dependent ecosystems located in spring-fed streams. We show that to ensure flow in the studied spring-fed streams, the Selwyn and Orari monthly averaged recharge should be maintained above 2 and 8 m3s-1, respectively. We advocate for the broader application of our method to enhance the understanding and management of ephemeral rivers and their dependent systems in other regions. • Groundwater recharge from ephemeral rivers can be estimated using satellite images. • Groundwater recharge is linearly correlated to the logarithm of river discharge. • The parameters governing these relationships do not change significantly with time. • These relationships can be used to reconstruct past groundwater recharge. • Recharge from ephemeral rivers is important for water resources and ecosystems.