Agricultural Drought Indices - US Department of Agriculture

Conclusions and Discussion
Several authors have applied methodologies based on remote sensing and climate data for
monitoring drought conditions at the regional scale, generally showing that indicators based only
on meteorological data are not enough to describe the spatial patterns of droughts (Bhuiyan et al.
2006).
The time monitoring of spatial distributions of indicators correlated with soil moisture and
vegetation water stress, such as VTCI, TDVI, and WDI, given the strong theoretical base on which
they are founded, has been shown to provide a good approximation for conditions of water stress
or drought at the regional level.
In the present work, it has been demonstrated that during dry conditions (less availability of soil
moisture), the increase in surface temperature (involving temperature of vegetation as well as soil
temperature, according to the cell) serves as a good indicator of vegetation water stress preceding
the onset of drought. The response of vegetation to conditions of high temperature can occur even
when the vegetation is green, because of stomatal regulation to minimize water loss through
transpiration (Wan et al. 2004). This leads to a reduction in latent heat flux and, consequently, to a
noticeable increase in heat flux, resulting in a rise in leaf temperature with respect to the air
temperature. This increase in leaf temperature is often used to identify stress in crops.
The increasingly widespread availability of satellite images allows evaluation studies to address
drought conditions and impacts from remote sensing, either based on indicators correlated with
soil moisture, or directly on estimates of actual evapotranspiration.
Acknowledgments
This work was developed in the framework of the agreement between the UPCT and the CHS,
“Development of indicators of water scarcity and droughts, in the context of climate change”, and
the agreement “Development of early warning system for Jucar River and Segura River Basins”,
between UPCT and the General Deputy of Water, of the Ministry for the Environment, Marine and
Rural Affairs. The financial support, as well as the support of Project CGL, are acknowledged.
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