Comparing the Accuracy of Altimetry Data in a Mountainous Region with a Low Gradient River Reach: A Case Study of Indus River, Pakistan

Satellite radar altimetry technology is used to monitor oceans and inland waters. However, inland monitoring is still in the development phase. New technologies and advanced algorithms are introduced to improve data accuracies at broader scales. Today's challenge is to ascertain altimetry usefulness at least at a regional level. This study compares satellite radar altimetry-derived inland water heights with in-situ data in Pakistan. Tarbela reservoir (mountainous region) and Guddu barrage (flat topography) on Pakistan's Indus River were selected, having extended altimetry and in-situ data time series. Gauge measurements from irrigation departments were acquired to evaluate altimetry products from the multi-satellite mission Database for Hydrological Time Series of Inland Waters (DAHITI). DAHITI includes data from satellites Jason 2 and 3 at Tarbela, Envisat, Saral, and Sentinel 3A at downstream Guddu barrage. The evaluation periods are 2013–2020 for Tarbela reservoir and 2002–2020 for Guddu barrage, depending on the common availability of datasets. Statistically higher correlations were found for both cases ranging from 0.98 to 0.88 except for one dataset
$\mathrm{R}=0.45$
for Saral at Guddu). The root-mean-square error (RMSE) between in-situ and altimetry datasets at the Tarbela reservoir was much higher (340 to 531 cm) than the submeter errors (22–32 cm) at the Guddu barrage (Saral at Guddu gave a higher RMSE of 118 cm). Differences in satellite missions, data acquisition times, reference geoids, data processing methods, and topography may be the possible causes of these errors. While a more detailed investigation is required considering the inconsistencies of the datasets and the study sites (river vs. reservoir), it can be concluded that accuracies of altimetry derived Indus River water levels are better in flat areas than in the mountainous regions.