Quantification of groundwater recharge from an ephemeral braided river using satellite photography

In the coastal plains of New Zealand, braided rivers lose a considerable amount of water to the groundwater system, and for many aquifers are the largest source of recharge. Quantifying the recharge rates/transmission losses and how they relate to river stage and flow is particularly challenging. A commonly used approach to estimating recharge rates/transmission losses is differential flow gauging, where river discharges are measured simultaneously at multiple locations along a given reach. However, differential flow gauging is labour-intensive and limited by the accuracy of river discharge measurements, particularly in braided river systems.

We have developed an alternative method for ephemeral rivers using river stage monitoring and widely available satellite photography. The method was applied to the upstream part of the Selwyn River (Canterbury, New Zealand), which is perennial in its mountainous environment, but becomes ephemeral once it crosses its alluvial plain. The river stage near the downstream boundary of the perennial reach was monitored for the period of March 2020 to May 2021 and a stage-discharge rating curve was developed. Downstream of the monitoring station, the river becomes ephemeral with the drying front location changing over time. On a number of 146 suitable satellite photographs, taken within the stage recording period, and retrieved from the Planet application program interface, we identified the position of the drying front, and used this to determine the length of the active (wet) river channel. This enabled us to calculate the average river transmission losses by dividing the river discharge at the monitoring station by the downstream active river length. The transmission losses estimated using the satellite photography correspond well with the losses estimated using seven sets of independent differential flow gauging surveys, given the respective uncertainties of both methods. The average estimated transmission losses range from 0.2 to 1 m³/s/km. Most of the estimated losses are below 0.4 m³/s/km and correspond to baseflow periods. The highest losses occur shortly after peak flows and decrease exponentially with time after the peak.

We hypothesize that the high losses, occurring shortly after peak flows, are due to the replenishment of the shallow braid plain aquifer associated with the river. Lower losses, occurring during baseflow periods, represent groundwater recharge to the deeper regional aquifer. Groundwater recharge to the deeper regional aquifer appears to be linearly correlated with the groundwater head in the shallow aquifer. This response is consistent with the presence of an unsaturated zone that has been identified between the shallow (riverine) and deeper (regional) aquifers. Furthermore, we have successfully trained a random forest regression model to reconstruct the transmission losses for every day of the study period. The daily transmission loss dataset can now be used to evaluate our physically-based groundwater – surface water interaction models, currently under development, as well as support water management in the Selwyn basin.