Abstract
A variety of vegetation indices derived from remotely sensed data have been used to assess vegetation conditions, enabling the identification of drought occurrences as well as the evaluation of drought impacts. Moderate Resolution Imaging Spectroradiometer (MODIS) Terra 8-day composite data were used to compute the Modified Soil Adjusted Vegetation Index II (MSAVI 2 ) of four dominant vegetation types over a 13-year period (2002 – 2014) on the San Carlos Apache Reservation in Arizona, US. MSAVI 2 anomalies were used to identify adverse impacts of drought on vegetation, characterized as mean MSAVI 2 below the 13-year average. In terms of interannual variability, we found similar responses between grassland and shrubland, and between woodland and forest vegetation types. We compared MSAVI 2 for specific vegetation types with precipitation data at the same time step, and found a lag time of roughly two months for the peak MSAVI 2 values following precipitation in a given year. All vegetation types responded to summer monsoon rainfall, while shrubland and annual herbaceous vegetation also displayed a brief spring growing season following winter snowmelt. MSAVI 2 values of shrublands corresponded well with precipitation variability both for summer rainfall and winter snowfall, and can be potentially used as a drought indicator on the San Carlos Apache Reservation given its wide geographic distribution. We demonstrated that moderate temporal frequency satellite-based MSAVI 2 can provide drought monitoring to inform land management decisions, especially on vegetated tribal land areas where in situ precipitation data are limited.
Highlights
Drought cycles are complex natural climate phenomena
When compared to other vegetation types, shrubland was the dominant land cover type in the medium-high drought impact category, and it became more predominant as the drought impact increased to the severe level, suggesting that shrubland can be used as a drought indicator vegetation type for the San Carlos Apache Reservation
Moderate Resolution Imaging Spectroradiometer (MODIS)-derived MSAVI2 can serve as an effective drought monitoring tool in regions where in situ precipitation data are sparse or not available
Summary
Drought cycles are complex natural climate phenomena. The Southwestern United States has experienced persistent drought conditions since the 1990s, the severity and duration of which has not been seen in over 800 years (Cook et al, 2004; Meko et al, 2007). Climate change is projected to increase the severity and frequency of droughts in the Southwestern United States (Schwalm et al, 2012; Seager and Vecchi, 2010). Climate change is expected to disrupt this existing weather patterns and hydrological cycle. Vegetation stressed by drought can result in a decrease in net primary production (NPP) – the transfer of carbon from the atmosphere to terrestrial biomass, which reduces the overall amount of carbon stored by vegetation. Understanding drought impacts on vegetation can help inform natural resources management decisions, and predict carbon cycle feedback to climate change (Friedlingstein et al, 2006)
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