Multi-scale analysis of snow dynamics at the southern margin of the North American continental snow distribution

Snow provides a key water source for stream flow and agricultural production across western North America and drinking water for large populations in the Southwest. Accurate estimates of snow cover spatial distribution and temporal dynamics are important at regional and local scales as snow cover is projected to decrease due to global climate change. We examined regional-scale temporal trends in snow distribution across central and northern Arizona using two tiles of 2928 daily images of MOD10 snow product. The analysis included the entire MODIS archive time period, October 1, 2003-June 1, 2014, and a 245,041 km2 area of 51 HUC8 watersheds. We also examined the effects of a regional forest restoration effort, known as the Four Forest Restoration Initiative (4FRI), aimed at enhancing snow accumulation and retention for increased groundwater recharge through forest thinning and burning treatments. We analyzed 66 Landsat TM/ETM + images spanning 26 years between 1988 and 2014 at five sites and one hyperspectral image from 2014 at two sites. The MOD10 snow product performs well in estimating Arizona's thin and discontinuous snow distribution. Mann-Kendall time-series analysis indicate significantly increasing trends in the annual number of snow cover days (SCD) over the 12-year period in 1.6% of the region at elevation transitions such as the Mogollon Rim in central Arizona, while significantly decreasing trends are observed at a few locations of lower elevations leading to the desert margins in eastern Arizona. The observed temporal trends are mostly consistent with ground-based SNOTEL snow measurements. An Arizona specific, Landsat sensor-derived binary classification model, similar to the MOD10 product, was developed at a local scale. It performs better than commonly-used simple threshold-based approaches, but demonstrates the continued challenges associated with Landsat sensor-derived snow classification in Arizona likely due to its coarse temporal resolution. Landsat-derived multi-temporal Normalized Difference Snow Index (NDSI) analysis indicate that treated (thinned and thinned-and-burned) forest sites had significantly greater NDSI values than untreated control sites. Snowpack at treated sites also appeared to persist longer into the spring season with potentially greater contributions to groundwater recharge in this semi-arid region. The high-resolution hyperspectral data analysis indicate that sites treated to approximately 24% forest canopy cover appear to have an optimum threshold for accumulating and maintaining snowpack. It balances canopy cover versus canopy gap, which reduces snow interception and sublimation by canopy, while providing enough shade. These results are encouraging for the 4FRI, the first and largest forest restoration effort in the US history, aimed at improving watershed health and function in the face of changing climate.