Groundwater recharge from ephemeral rivers of the Canterbury Plains (New-Zealand): historical reconstruction using satellite imagery and environmental implications

In numerous regions, surface water courses provide essential recharge to aquifers, particularly in times of increased aridity. In the Canterbury Plains (New Zealand), gravel-bed rivers are a major source of groundwater recharge to the aquifers, which are intensively pumped for irrigation but also sustain a number of groundwater-dependent ecosystems. Some of these rivers lose so much of their water to the underlying aquifers that they are ephemeral.

In this study, a recently developed method has been employed to estimate transmission losses in the upper Selwyn and Orari Rivers using the extensive Planet Monitoring satellite image collection (2010-2023). Using a simple linear modelling approach, we have converted the transmission losses to total groundwater recharge and extended the record to the 1980s. This dataset unveils historical variations in groundwater recharge from these rivers.

The findings indicate an average annual groundwater recharge of approximately 50 million m³/y from the upper Selwyn and about 183 million m³/y from the upper Orari. Notably, the influence of climate is evident through significant interannual fluctuations (up to 100%) correlated with precipitation, tied to broader climatic phenomena such as El Niño/La Niña and the Interdecadal Pacific Oscillation. However, no distinct impact of longer-term climate change has been observed in this context.

Moreover, this study delves into the environmental implications of these recharge sources. Notably, the upper Selwyn River sustains a critical groundwater-dependent ecosystem, supporting the endangered Kōwaro (Canterbury mudfish) in springs downstream of the studied losing reach. The analysis reveals that recharge from the upper Selwyn maintains adequate water levels and flows in these springs. However, during dry periods, when the seasonal average recharge rate drops below 2 m³/s, groundwater levels are insufficient to reach the springs, potentially causing harmful effects on this ecosystem.

Ongoing research focuses on further exploring the environmental implications of groundwater recharge from the upper Selwyn and Orari Rivers and explaining the differences observed between the two river systems.