Spatial and Temporal Assessment of Rainfall Variability using GIS Approach in Trichy District of Tamil Nadu

Precipitation is one of the transportation components in hydrological cycle. The magnitude of precipitation swings with time and space. Majorly India receives precipitation in the form of rainfall. Precipitation plays a key role in the rainfed agriculture. The present study deals with the rainfall characteristics of Tiruchirappalli district, Tamil Nadu. Seasonal rainfall data from eighteen rain gauge stations (1971-2012) have been taken for analysis of seasonal and annual rainfall pattern of Tiruchirappalli district. Mean rainfall of the district is about 696 mm. The highest rainfall of 1247 mm recorded in the year 2005 and the lowest precipitation of 227 mm recorded in the year 1976. About 48 percent and 35 percent of the rainfall received in North East and South West Monsoon, respectively. Spatial rainfall distribution was studied with the help of Kriging interpolation technique and respective maps were prepared with Geographical Information System. The percentage departure of annual rainfall is classified under the category of excess, normal and large excess category. South East and central part of Tiruchirappalli receives moderate to low rainfall. North East parts of Tiruchirappalli district such as pullambadi, Lalgudi and nearby areas received maximum rainfall during North East Monsoon and South West Monsoon. In winter season Manapparai and Vaiyampatti region received more rainfall while in summer season Thottiam and Mayanur area received more rainfall. The two major highlighted crops in Trichy district are Banana and Onion. Tiruchirappalli district is one of the Banana growing belts in Tamil Nadu. Spatial distribution of rainfall maps will be helpful to form a crop plan for different crops to increase the agricultural productivity of Tiruchirappalli district and to ensure the food security. CONTACT A. Raviraj Rraj@Tnau.ac.in Tamil Nadu Agricultural University, Coimbatore-3 © 2021 The Author(s). Published by Enviro Research Publishers. This is an Open Access article licensed under a Creative Commons license: Attribution 4.0 International (CC-BY). Doi: http://dx.doi.org/10.12944/CWE.16.3.19 Article History Received: 17 May 2021 Accepted: 30 November 2021


Introduction
Precipitation is the important component in a Hydrologic cycle and Precipitation plays an impactful role in maintaining the sustainability of resources in the earth. Precipitation generally deals with the Rainfall events only. Based on the intensity of rainfall, it is classified into three types viz., heavy ->7.5 mm/h, moderate -between 2.5 and 7.5 mm/h and light rain -trace to 2.5 mm/h. 1 The rainfall is one of the paramount factors affecting vegetation and accessibility of water resources. Rainfall is the major source of water to the agriculture especially in rainfed agriculture. Crop productivity mainly depends upon the characteristics of rainfall such as rainfall intensity, amount of rainfall received and distribution of rainfall over an area. Characteristics of rainfall change over time and from place to place. Analysis of the rainfall characteristics is important for planning of water harvesting structures and other conservation measures for ensuring sustainability. Rainfall is a pivotal agro-climatological factor in the seasonally parched region of the world and its assessment an important perquisite for agricultural management in India. 2 Many studies have been conducted in different parts of India to predict the rainfall variability. 3 The crop plan can be carried out through the weekly rainfall pattern to take the extension activities and it will help to increase the agricultural productivity. 4 The climate of Tamil Nadu is generally wet subtropical climate and features fairly hot temperature over the year except during the monsoon season. Rainfall, groundwater, reservoir levels and crop conditions are the major factors to determine the nature and extent of drought conditions in Tamil Nadu.5 Contemporary studies have stated that each and every analysis of hydroclimatic elements should be done at the local scale rather than at a large or global scale.
In Tiruchirappalli district Banana and Onion are the top two ranked crops due to high water requirement for other crops. Banana is a succulent, evergreen and shallow rooted crop. It require more water for its higher production. Tiruchirappalli district is one of the Banana growing belts in Tamil Nadu. The district has an area of 7390 km2 (40.6 % of net sown area) underneath rainfed situation. Spatial distribution of rainfall maps will be helpful to prepare a crop plan for different crops to increase the agricultural productivity. Based on the availability of water resources, the crop selection like drought tolerant crop and management practices such as drip irrigation, mulching etc., can be followed to ensure the food security. Keeping in view the importance of rainfall in rainfed agriculture, the present study was made to understand the Tiruchirappalli district's rainfall characteristics.

Methodology Area of Interest
Tiruchirappalli is one of the districts of Tamil Nadu state of India is sited in the central portion of Tamil Nadu (Fig 1) and the geographical extent of the Tiruchirappalli is 4403.83 Sq.km. For administrative purpose it was divided into 4 revenue Divisions, 11 Taluks and 507 Revenue Villages. Tiruchirappalli district experiences a tropical savanna climate and the average temperature is 28.8°C which varies from 25.3°C in winter season to 31.7°C in summer season. River Cauvery is the significant source for drinking and irrigation water requirement in the district. One twelth of the area of Trichy district is enclosed by means of hills and revenue forest. Pachamalai hill near Thuraiyur is the significant hill in this district. Main crops grown in the district are Ragi, Paddy, Onion, Groundnut, and pulses. The high priority cash crops in the district are Banana and Sugarcane. interpolation is the technique of utilizing points with well-known values to assessment the values at other points. Kriging produced better results than other interpolation methods such as Inverse Distance Weighted (IDW) and spline. 6 [Kriging tool in GIS was used to interpolate a raster surface from points using Kriging an advanced geostatistical procedure (Syntax: ArcToolbox -Spatial Analyst Tools-Interpolation -Kriging).] In Kriging, the seasonal rainfall data of each rain gauge stations are given as an input point feature. Where ordinary kriging method and linear semivariogram method was used for interpolation (Fig 3). Percentage Departure of annual rainfall in the district was calculated using normal annual rainfall. The category of annual departures from normal with range and colour code ( Table 1) done for 10 years period (2003-2012).

Fig. 2: Locus of rain gauge stations in Tiruchirapalli district
Methodology of the study Seasonal and annual rainfall data was taken for analysis of rainfall pattern of Tiruchirappalli district. Eighteen rain gauge stations under the study are located in different parts of the district to observe the rainfall event (Fig 2).

Fig. 3: Methodology for preparing spatial distribution of rainfall map
Geographical information system (GIS) of ArcGIS software version 10.3 was used for preparation of temporal and spatial rainfall distribution maps.
Microsoft excel was used for the preparation of rainfall data base and used as GIS input. Spatial

Results and Discussion
Normally the climate of the district is cold and wet. Spatial variability of climate can be large due to the precipitation ranging from <1000 mm up to >4000 mm. 8,9,10 The Mean annual rainfall, Mean North East Monsoon (NEM) rainfall, Mean South West Monsoon (SWM) rainfall, Mean summer and winter rainfall, Mean rainfall distribution, spatial distribution and Percentage Departure of annual rainfall for Tiruchirappalli district are discussed below.

Mean Annual Rainfall
Tiruchirappalli district has the mean annual rainfall of 695 mm. From 1980 to 2012, the rainfall received was 24 % less than the mean rainfall for almost 19 years. During this period, the following years  5).

Mean Summer Rainfall
The mean summer rainfall is 104 mm.  (Fig 7).

Mean Winter Rainfall
The annual and mean winter rainfall of Tiruchirappalli district is illustrated in Fig 8. The mean winter rainfall is 23 mm. From 1980 to 2012, the rainfall received less than the mean winter rainfall for 31 years and 11 years received more rainfall than the mean winter rainfall.

Season Wise Distribution of Mean Rainfall (1971-2010)
Tiruchirappalli district received 48% rainfall during NEM; 35% during SWM; 14% during summer and 3% in the course of winter season out of annual rainfall (Fig 9). North East Monsoon contributed more in rainfed agricultural areas of the district followed by SWM.

Spatial Spread of Rainfall
The spatial spread of rainfall map was generated using Kriging interpolation technique of ArcGIS software. The ordinary Kriging method is one of the interpolation technique widely used, which plays an important role in interpolation and mapping precipitation data in any region. 12 Spatial distribution of mean annual rainfall map clearly indicated that the North East part of Tiruchirappalli district received maximum rainfall compared to South West parts of Tiruchirappalli. While South East and central part of Trichy received moderate to low rainfall during the period 1971-2012 (Fig 10). Spatial map of distribution of rainfall (season wise) is illustrated in Fig 11. During NEM and SWM, North East part of Tiruchirappalli district such as pullambadi, Lalgudi and nearby area received maximum rainfall. In winter season Manappaarai and Vaiyampatti region received more rainfall where as Thottiam and Mayanur area received more rainfall in summer season.

Conclusion
Rainfall characteristics of Tiruchirappalli district were analyzed in the present study with the help of spatial and temporal data of GIS. It has the maximum rainfall of 1247 mm recorded in the year of 2005 while minimum rainfall of 227 mm recorded in the year of 1976. Tiruchirappalli district has the mean areal precipitation of 695 mm. The mean north east monsoon rainfall was 358 mm, which is 26% more than the mean south west monsoon rainfall (264 mm). Tiruchirappalli district secured with 48%, 35%, 14% and 3% of rainfall in the four seasons like NEM, SWM, summer and winter season respectively out of annual rainfall. The north east parts of Tiruchirappalli received maximum rainfall compared to south west parts of Tiruchirappalli. The south east and central part of Tiruchirappalli receives only moderate to low rainfall. The percentage departure of annual rainfall of the district is falling under the categories of normal, excess and large excess. The developed spatial distribution rainfall maps will be helpful to form a crop plan to increase the agricultural productivity of Tiruchirappalli district.