Drought is one of the complex natural phenomena. In extreme cases, drought causes great damage to the economy and human life. The economic loss caused by drought is high compared to all other weather disasters. (Wilhite, 2000). Globally, drought causes an economic loss at an average of 6 to 8 billion USD (Yagci et al., 2013). Drought is one of the least mention natural calamities (Kao and Govindaraju, 2010). Droughts can be classified as agricultural, hydrological, meteorological, and socio-economic droughts (Shiau et al., 2012). The climatic parameters such as rainfall and temperature play a crucial role in defining drought. Among the climatic parameters, rainfall is an important factor and is a readily available weather measurement. Using rainfall data from thirty or more years is better for effective growth planning than using short-term ones (Dennett et al., 1985). It also affects agriculture, drainage, human settlements, and politically and its impact varies from place to place. The main factor leading to the impact of drought depends on the size, frequency, duration of the drought, and extent. (Degefu and Bewket, 2015; Zargaret al., 2011; Surendranet al., 2017).Many drought indices have been developed for drought forecastingsuch asPalmer Drought Severity Index, Rainfall Anomaly Index, Departure Index, Drought area Index, Drought Severity Index, Gumbel Recurrence Interval, and Standardized Precipitation Index,etc. There are drought indices that use rainfall alone to define drought. Consecutively,some indices combine other parameters such as potential evapotranspiration (PET), temperature,and soil moisture along with rainfall to understand drought. Here, an attempt has been made in this study considering rainfall as a prime factor to understand the drought characteristics of the study area using the Rainfall anomaly index (RAI) and departure index.

The base map of the Nambiyar river basin was digitized from the river basin map of Tamil Nadu. The in and around rainfall data for the Nambiyar basin from the year 1981 to 2017 were used in this study to understand the drought. The basin covers 13 gridded points collected from the ECMWF reanalysis Interim (ERA-1) data set having a horizontal resolution of 0.75⁰. This gridded data was downscaled to 17X17 km resolutions using the WRF re-initialization method were adopted. The gridded points are considered as rainfall locations such as Valliure, Elankulam, Sirappur, Chidambarapuram, Vadakankulam, Nainaputhoor, Boothapandi, Keeriparai, Manjolai, Venkatarngapuram, Nellaiyappapuram, Veppenkulam, and Ammanpuram. The graphs were prepared using Excel software and spatial maps were prepared using the Arc GIS platform.

The Meteorological Department of India and the National Agriculture Authority provide the percentage of normal rainfall and associated weather drought intensity rainfall departures (Table 1). During this study period, the study area comes under two categories such as moderate and mild drought. Most of the location in this study area is under mild drought. The stations which have a maximum number of years fall under moderate drought is Vallioor, followed by the stations are Veppankulam, Elankulam, Sirappur, Vadakkankulam, Venkatarengapuram, Nellaiyappapuram, Manjolai and Ammanpuram. Minimum drought years are noted in Chidambarapuram, Nainaputhoor and Boothapandi stations during the study period (Figure 2). Chidambarapuram and Nainaputhoor are the location have high number of years with mild droughts and minimum year are found in Elankulam station followed by Boothapandi, Keeriparai, Manjolai, Venkatarengapuram and Nellaiyappapuram station (Table 2).

Table 2

Sl. No	Station	No drought	Mild drought	Moderate drought
1	Vallioor	15	16	6
2	Elankulam	19	15	3
3	Sirappur	15	18	3
4	Chidambarapuram	12	24	1
5	Vadakkankulam	17	17	3
6	Nainaputhoor	12	24	1
7	Boothapandi	14	22	1
8	Keeriparai	15	22	0
9	Manjolai	13	21	3
10	Venkatarengapuram	15	20	2
11	Nellaiyappapuram	14	21	2
12	Veppenkulam	17	16	4
13	Ammanpuram	16	19	2

Figure 2

Spatially, the moderate drought years found above three years are noted in the western part, the northeastern part of the study area, and below 3 drought years found in the central and southeastern part of the basin. The northern, central, and southeastern parts of the study area have a maximum number of mild drought category i.e., above 20 years and below 20 years were found in the western and northeastern parts. The years which have greater than 15 years of no droughts were observed in the western and eastern part of the study area. Below 15 drought years found in the central part of the study period (Figure 3).

The main aim of the study is to analyze the drought character using DI and RAI index. The results obtained in this study give almost identical results while comparing the drought years. The two-drought index reveals more or less similar results in the drought years. The common drought years between two indexes are notes as 1986, 2003, 2013, and 2016 during the study period. So, both the index is useful to identify the drought condition in the study area. Even though the study area is not affected by severe and extreme drought but the recent years indicate most of the stations are under moderate drought conditions. It shows that the basin slowly experiencing drought conditions. So, a proper drought management plan is required to mitigate before it goes to extreme conditions.