Uncertainties in Mean River Discharge Estimates Associated With Satellite Altimeter Temporal Sampling Intervals: A Case Study for the Annual Peak Flow in the Context of the Future SWOT Hydrology Mission

In the context of the Surface Water and Ocean Topography (SWOT) mission, investigations are needed to refine the error budget for discharge estimations. This letter proposes to evaluate the uncertainties in the estimation of mean river discharge around the seasonal peak flow due to the satellite temporal sampling intervals. The daily time series of in situ river discharge measurements for 11 large rivers are used to analyze the uncertainties associated with the sampling of four altimeter repeat cycles: the 35-, 22-, and 10-day repeat cycles in the nadir-looking configuration of current altimeters and the 22-day repeat cycle in the SWOT wide-swath configuration, where a given location is observed every cycle twice at the equator and six times in higher latitudes. Results show that, for boreal rivers, a sampling of 35 or 22 days from current nadir altimeters is too coarse to give an accurate estimate of the average discharge around the seasonal peak flow, whereas for all watersheds, the uncertainties associated with a 10-day repeat cycle or the 22-day repeat cycle in the SWOT wide-swath configuration are within the range of acceptable uncertainties (15%-20%). In addition, the absolute maximum mean discharge uncertainties associated with the SWOT time sampling have a strong relationship with the variance of the river discharge. This suggests that, rather than the commonly used basin area, the magnitude of the short-time-scale variance of the discharge could be used as a predictor of the uncertainties associated with temporal sampling intervals when estimating average discharge around the seasonal peak flow.