Evaluating L-band InSAR snow water equivalent retrievals with repeat ground-penetrating radar and terrestrial lidar surveys in northern Colorado

• Published in: The Cryosphere
• Main Authors: R. Bonnell, D. McGrath, J. Tarricone, H.-P. Marshall, E. Bump, C. Duncan, S. Kampf, Y. Lou, A. Olsen-Mikitowicz, M. Sears, K. Williams, L. Zeller, Y. Zheng
• Format: Article
• Language: English
• Published: Copernicus Publications 2024-08-01
• Online Access: https://tc.copernicus.org/articles/18/3765/2024/tc-18-3765-2024.pdf
• ISSN: 1994-0416; 1994-0424

<p>Snow provides critical water resources for billions of people, making the remote sensing of snow water equivalent (SWE) a highly prioritized endeavor, particularly given ongoing climate change impacts. Synthetic aperture radar (SAR) is a promising method for remote sensing of SWE because radar penetrates snow, and SAR interferometry (InSAR) can be used to estimate changes in SWE (<span class="inline-formula">Δ</span>SWE) between SAR acquisitions. We calculated <span class="inline-formula">Δ</span>SWE retrievals from 10 NASA L-band (1–2 GHz, <span class="inline-formula">∼25</span> cm wavelength) uninhabited aerial vehicle SAR (UAVSAR) acquisitions covering a <span class="inline-formula">∼640</span> km<span class="inline-formula">²</span> swath in northern Colorado during the winters of 2020 and 2021. UAVSAR acquisitions coincided with <span class="inline-formula">∼117</span> mm of accumulation in 2020 and <span class="inline-formula">∼282</span> mm of accumulation in 2021. <span class="inline-formula">Δ</span>SWE retrievals were evaluated against measurements of SWE from repeat ground-penetrating radar (GPR) and terrestrial lidar scans (TLSs) collected during the NASA SnowEx time series campaigns at two field sites (total area <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mo>∼</mo><mn mathvariant="normal">0.2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="35pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="6f849f4e58925f8bb1bba366b8bd7c0c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tc-18-3765-2024-ie00001.svg" width="35pt" height="10pt" src="tc-18-3765-2024-ie00001.png"/></svg:svg></span></span> km<span class="inline-formula">²</span>) as well as SWE measurements from seven automated stations distributed throughout the UAVSAR swath. For single InSAR pairs, UAVSAR <span class="inline-formula">Δ</span>SWE retrievals yielded an overall <span class="inline-formula"><i>r</i></span> of 0.72–0.79 and an RMSE of 19–22 mm when compared with TLS and GPR <span class="inline-formula">Δ</span>SWE retrievals. UAVSAR <span class="inline-formula">Δ</span>SWE showed some scatter with <span class="inline-formula">Δ</span>SWE measured at automated stations for both study years, but cumulative UAVSAR SWE yielded a <span class="inline-formula"><i>r</i></span> of 0.92 and an RMSE of 42 mm when compared to total SWE measured by the stations. Further, UAVSAR <span class="inline-formula">Δ</span>SWE RMSEs differed by <span class="inline-formula">&lt;10</span> mm for coherences (i.e., the complex interferometric coherence) of 0.10 to 0.90, suggesting that coherence has only a small influence on the <span class="inline-formula">Δ</span>SWE retrieval accuracy. Given the evaluations presented here and in other recent studies, the upcoming NASA-ISRO SAR (NISAR) satellite mission, with a 12 d revisit period, offers an exciting opportunity to apply this methodology globally.</p>