## 1 Introduction  
Droughts are a recurrent part of our climate and are considered one of the most complex and least understood phenomenon among all-natural hazards in terms of their impact on vegetation function [1; 2; 3; 4]. In recent years, droughts have become more frequent and severe across the world, with a noticeable increase in their spatial extent [5; 6; 7]. Droughts threaten societies at different economic, social, and environmental scales [8], and have been recognized as the principal cause of crop loss and severe food shortages, particularly in developing countries [7]. For the past few decades, droughts have constantly threatened the world's food security [7]. In addition to the economic damage and impacts on natural resources, droughts have severe societal consequences causing displacement and migration and even political unrest [7; 8; 9]. Droughts in the United Sates are the costliest weather-related disaster [10]. From 1980 to 2003, the cost of drought was estimated at more than $144 billion dollars [11]. In the southwestern United States, droughts are relatively a familiar phenomenon (Figure 1), due to land surface-ocean interactions [12]. Several studies have shown that both climate and human activities are the key driving forces of change in the American Southwest landscapes [13]. Assessment of vegetation response to droughts in the region remains difficult due to the human dimensions of this problem and the changing characteristics of drought [14]. In the southwestern United States, the severity of drought impacts on forested land have been well documented since the 1990s because of temperature increase recorded during this period [14; 15; 16]. Understanding the spatial and temporal dimensions of droughts in relation to climatological, oceanic, and atmospheric parameters is key for developing drought monitoring tools and to providing valuable information for better early warning and knowledge-based decision support systems to help mitigate their impacts [8; 17]. Therefore, effective drought monitoring systems would take into consideration past and present climatological conditions and changes [17]. Monitoring drought patterns and their impacts on vegetation activities and productivity at different spatial and temporal scales still presents some challenges, due in part to the complex relationship between vegetation, environmental, and biophysical factors [4; 18]. The Hopi Tribe and Navajo Nation lands, also known as the Four Corners Region, collectively cover over 77,699 km\({}^{2}\) of land area. The ongoing drought impacting the Colorado Plateau, where the Hopi and Navajo Nation are located, have appeared slowly and relatively unnoticed in conventional national drought monitoring efforts like the National Drought Monitor [19]. The most recent drought conditions (Figure 2) in the region have emerged rapidly and intensely, characterized in 2002 by peak drought conditions. They have remained for well over a decade [4; 14; 16; 19] and will have multiple impacts, not only on economic activity but also on the ecosystem services [12; 20]. One concern about the ongoing drought in the region is the storage capacity of the Colorado River and the overuse of the regional aquifers [21]. Assessment of the ongoing drought impacts on the Hopi and Navajo Nation area presents some difficulties due to lack of long-term hydro-climatological data and ecological monitoring observations. This has made it difficult to track, measure, and quantify the impacts through this most recent drought, creating a challenging situation for resource managers and decision makers. The numerous gaps and inconsistencies in the hydro-climatological data make the traditional climate-based drought monitoring strategies based on historical ground-based meteorological observations from the limited existing weather stations not ideal for this region.