# Assessment of Drought Severity and Vulnerability in the Lam Phaniang River Basin, Thailand
## 2 Materials and Methods
### Assessment of Drought Hazard and Vulnerability
#### 2.6.4 Calculation of Drought Vulnerability Index  
The magnitude of a drought's impact depends mainly on the vulnerability of exposed elements to a drought event, which is an imbalance among exposure, sensitivity, and adaptive capacity [30]. Therefore, the vulnerability to drought of each sub-district within the Lam Phaniang River Basin, which refers to a relative measure of levels of susceptibility to damage caused by occurrence of drought with an incapability to cope with its adverse consequences, was assessed and eventually aggregated into a single Drought Vulnerability Index (DVI) (see the overall framework in Figure 4). To define the effective set of indicators reflecting the vulnerability in each drought category and sub-district, three individual physical and socio-economic vulnerability indicators related to vulnerability components were considered and used in this study, with details as follows.  
* _Sensitivity Indicator (SI)_ was used to describe the degree to which the sub-districts within the Lam Phaniang River Basin were adversely affected by drought with either positive or negative impacts. In this study, the WEAP simulated water demand for domestic use, agriculture, and industry, was used to measure the sensitivity to the drought-associated complications, as higher water demand can lead to more vulnerability to the adverse impacts of drought.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**DHI Value** & **Classification** \\ \hline
0.00 to 0.25 & Low \\
0.25 to 0.50 & Moderate \\
0.50 to 0.75 & High \\
0.75 to 1.00 & Very High \\ \hline \hline \end{tabular}
\end{table}
Table 2: The classification of DHI for drought hazard assessment.  
Figure 3: Steps used for estimating the Drought Hazard Index (DHI).  
* _Exposure Indicator (EI)_ to drought was captured by water shortage for domestic use, agriculture, and industry, under climate change impacts for affected sub-districts within the Lam Phaniang River Basin.
* _Adaptive Capacity Indicator (ACI)_, which is a function of three determinants, including socio-economic, technological, and institutional capabilities, was considered since the greater the adaptive capacity of the Lam Phaniang River Basin to a specific climate event, the less vulnerable to drought due to climate change. In this study, two indicators, i.e., Gross Provincial Product (GPP) and surface runoff simulated by WEAP, were used to quantify adaptive capacity to climate change, because the higher the GPP and more sufficient the water supply are, the higher the adaptive capacity.  
It can be noted that the indicators (SI, EI, and ACI) were expressed in different units of measurement, their values were then normalized as dimensionless values within a range of 0-1 in order to overcome the different unit effects. Each indicator was also divided into four classes using the Jenks Natural Breaks method, with a rating on a scale of 0-1 for each class. Thereafter, by assigning equal weightage for each indicator, the aggregate DVI was calculated using a simple average of individual indicators (Equation (2)). However, before aggregating the DVI value, the ACI value would need to be modified by deducting from 1 in order to adjust the direction of ACI to be the same as the SI and EI. That is to say, for the SI and EI the higher the value, the higher the vulnerability, while the higher the ACI, the lower the vulnerability. Subsequently, the DVI was again re-scaled to a 0-1 range, and evenly classified into four classes using the Jenks Natural Breaks method based upon the criteria described in Table 2. The DVI was then used for integrating the sub-district level thematic maps for the Lam Phaniang River Basin, in which the sub-districts were also classified into four classes according to the DVI values.  
\[\text{DVI}=\frac{\text{SI}+\text{EI}+(1-\text{ACI})}{3} \tag{2}\]