A new modified spatial approach for monitoring non-perennial river water availability using remote sensing in the Tankwa Karoo, Western Cape, South Africa

Abstract

Non-perennial rivers (N-PRs) make up two thirds of all rivers in South Africa, yet many are ungauged. Traditionally, it has been assumed that when a flow is recorded, there is water throughout that river. These assumptions have led to incorrect estimations of available water resources. This work thus aimed at developing a new spatially explicit framework, for monitoring river water availability in a N-PR system. The Tankwa River in South Africa was used for testing this approach. The length of the river reach with water was determined using the Sentinel-1 and Sentinel-2 data derived indices. Image thresholding was applied to Sentinel-1, and the normalised difference water index (NDWI) to Sentinel-2. Sentinel-2 yielded an overall accuracy (OA) of 85%, whereas Sentinel-1 yielded an OA of 38%. The analysed reach of the Tankwa River had an actual length of 9 244 m. Based on the performance of Sentinel-2 data, further analysis was undertaken using Sentinel images acquired during the months of February, May and July of 2016. The results indicated that the lengths of the reaches of inundated Tankwa River were 2 809 m, 3 202 m and 2 890 m, respectively. Overall, the findings of this study show that an estimated length of a river inundated by water can be determined using new-generation Sentinel data and these results provide new insights on the dynamics of N-PRs – a previously challenging task with broadband multispectral satellite datasets.