Data for a comparison of national water model retrospective analysis snow outputs at SNOTEL sites across the Western U.S.

The HydroShare resources in this collection contain the data and scripts used for: Garousi-Nejad, I. and Tarboton, D. (2022), \"A comparison of National Water Model retrospective analysis snow outputs at snow telemetry sites across the Western United States\", Hydrological Processes, https://doi.org/10.1002/hyp.14469.

Abstract from the paper:
This study compares the US National Water Model (NWM) reanalysis snow outputs to observed snow water equivalent (SWE) and snow‐covered area fraction (SCAF) at snow telemetry (SNOTEL) sites across the Western United States SWE was obtained from SNOTEL sites, while SCAF was obtained from moderate resolution imaging spectroradiometer (MODIS) observations at a nominal 500 m grid scale. Retrospective NWM results were at a 1000 m grid scale. We compared results for SNOTEL sites to gridded NWM and MODIS outputs for the grid cells encompassing each SNOTEL site. Differences between modelled and observed SWE were attributed to both model errors, as well as errors in inputs, notably precipitation and temperature. The NWM generally under‐predicted SWE, partly due to precipitation input differences. There was also a slight general bias for model input temperature to be cooler than observed, counter to the direction expected to lead to under‐modelling of SWE. There was also under‐modelling of SWE for a subset of sites where precipitation inputs were good. Furthermore, the NWM generally tends to melt snow early. There was considerable variability between modelled and observed SCAF as well as the binary comparison of snow cover presence that hampered useful interpretation of SCAF comparisons. This is in part due to the shortcomings associated with both model SCAF parameterization and MODIS observations, particularly in vegetated regions. However, when SCAF was aggregated across all sites and years, modelled SCAF tended to be more than observed using MODIS. These differences are regional with generally better SWE and SCAF results in the Central Basin and Range and differences tending to become larger the further away regions are from this region. These findings identify areas where predictions from the NWM involving snow may be better or worse, and suggest opportunities for research directed towards model improvements.

Order to follow the developed scripts:
1. Notebook to get the indices of National Water Model grid cells containing SNOTEL sites
2. Notebook for retrieval of National Water Model Retrospective run results at SNOTEL sites
3. Notebooks for post-processing the retrieved National Water Model Retrospective run results and inputs at SNOTEL sites
4. Notebook for retrieval of precipitation, air temperature, and snow water equivalent measurements at SNOTEL sites
5. JavaScript code for retrieval of MODIS Collection 6 NDSI snow cover at SNOTEL sites to be run using Google Earth Engine
6. Notebooks for combining the National Water Model results/inputs with observations from SNOTEL and MODIS at SNOTEL sites
7. Notebooks for visualizations reported at A Comparison of National Water Model Retrospective Analysis Snow Outputs at SNOTEL Sites Across the Western U.S.

本合集内的HydroShare资源包含用于以下研究的数据与脚本：Garousi-Nejad, I. 与 Tarboton, D.（2022）发表于《Hydrological Processes》的《美国西部雪遥测站点的国家水文模型回溯分析积雪输出对比》，DOI: https://doi.org/10.1002/hyp.14469。

本论文摘要如下：
本研究将美国国家水文模型（National Water Model, NWM）再分析积雪输出结果，与美国西部雪遥测（SNOTEL）站点观测得到的雪水当量（snow water equivalent, SWE）及积雪覆盖面积占比（snow-covered area fraction, SCAF）进行对比。其中SWE数据取自SNOTEL站点，而SCAF数据则来自标称分辨率为500米网格尺度的中分辨率成像光谱仪（moderate resolution imaging spectroradiometer, MODIS）观测结果。NWM回溯分析结果的网格尺度为1000米。本研究将各SNOTEL站点的对比结果，与包含该站点的网格单元内的NWM与MODIS格网化输出结果进行匹配比对。
模拟与观测SWE之间的差异，既源于模型自身误差，也源自输入数据的误差——尤以降水与气温输入误差最为显著。NWM总体上对SWE存在低估现象，部分原因在于降水输入的偏差。此外，模型输入的气温整体略低于观测值，这与“导致SWE模拟低估”的预期影响方向相悖。在部分降水输入精度较高的站点中，SWE同样存在低估情况。此外，NWM整体存在积雪提前消融的问题。
SCAF的模拟结果与观测值，以及积雪覆盖存在性的二分类对比结果均存在较大波动，这阻碍了对SCAF对比结果的有效解读。这在一定程度上源于模型SCAF参数化方案与MODIS观测本身的缺陷，尤其是在植被覆盖区域。不过，当对所有站点与年份的SCAF进行聚合分析时，模拟得到的SCAF整体高于MODIS观测结果。这些差异具有区域特征：中央盆地与山脉区域的SWE与SCAF模拟效果整体更佳，且模拟与观测的偏差随区域远离该区域而逐渐增大。本研究明确了NWM积雪相关预测表现优劣的区域范围，并为模型改进相关的研究方向提供了参考。

需按照以下脚本开发顺序执行：
1. 用于提取包含SNOTEL站点的NWM网格单元索引的Notebook
2. 用于在SNOTEL站点处获取NWM回溯分析运行结果的Notebook
3. 用于对SNOTEL站点处获取的NWM回溯分析结果与输入数据进行后处理的Notebook
4. 用于提取SNOTEL站点处的降水、气温与雪水当量观测数据的Notebook
5. 用于在SNOTEL站点处提取MODIS Collection 6 归一化差分积雪指数（NDSI）积雪覆盖信息的JavaScript代码，需通过Google Earth Engine运行
6. 用于将NWM结果/输入数据与SNOTEL及MODIS在SNOTEL站点处的观测数据进行整合的Notebook
7. 用于生成《美国西部雪遥测站点的国家水文模型回溯分析积雪输出对比》一文中可视化成果的Notebook