Abstract
Monitoring water storage in lakes and reservoirs is critical to water resource management, especially in a changing climate. Satellite microwave remote sensing offers a weather and light-independent solution for mapping water cover over large scales. We have used 13 years of synthetic aperture radar (SAR) data from three different sensors (Sentinel-1, RADARSAT-2, and Envisat advanced synthetic aperture radar (ASAR)) to develop a method for mapping surface water cover and thereby estimating the lake water extent (LWE). The method uses the unsupervised K-means clustering algorithm together with specific post-processing techniques to create binary maps of the water area. We have specifically tested and validated the method at Altevatn, a medium-sized arctic lake in Northern Norway, by using in-situ measurements of the water level. The multi-sensor SAR LWE time series were used in conjunction with the water level measurements to derive the lake hypsometry while at the same time quantifying the accuracy of our method. For Altevatn lake we estimated LWE with a root mean squared error (RMSE) of 0.89 km2 or 1.4% of the mean LWE, while the inferred lake water level (LWL) was associated with an RMSE of 0.40 m, or 2.5% of the maximum annual variation. We foresee that there is potential to further develop the algorithm by generalizing its use to other lakes worldwide and automating the process such that near real-time monitoring of LWE may be possible.
Highlights
Inland lakes and reservoirs are a vital source of freshwater to both human and ecological systems in order to sustain life
In this work we developed and demonstrated the use of a simple processing chain for estimating lake water extent using synthetic aperture radar (SAR) images acquired by the Envisat advanced synthetic aperture radar (ASAR), RADARSAT-2, and Sentinel-1 missions for the Altevatn reservoir in Northern Norway
We have shown that a consistent time series of the lake water extent (LWE) can be derived from SAR data on a decadal scale for a medium-sized arctic lake using a variety of historical sensors and taking care of biases caused by variable sensor capabilities
Summary
Inland lakes and reservoirs are a vital source of freshwater to both human and ecological systems in order to sustain life. Human activity is dependent on sources of water through agriculture, industry, and electricity. How sensitive a lake is to these changes is to a large extent dependent on the morphology of the lake (e.g., larger lakes tend to be more affected by wind at the surface due to greater fetch which in turn results in greater mixing). Studying and monitoring variations and trends in lake area, or lake water extent (LWE) can be an important tool in identifying climatic variations over time since this physical parameter is regulated by changes in climate. Changes in LWE can be indicators of climate variations since they are sensitive to changes in water and heat balance
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