The wettest place in Sri Lanka

: The average annual rainfall of Sri Lanka varies from 900 mm to 5,500 mm. A debate and misconception have continued on the “wettest place” or the rain gauge station that receives the highest rainfall in the country. Seven rain-gauge stations (seven locations) were selected considering the regions receiving with probable highest rainfall in Sri Lanka. The average annual total rainfall and average number of rainy days were analysed using daily rainfall data covering the most recent five years (2015 – 2019) to detect the rain gauge station receiving the highest rainfall. Among the locations considered, the newly established rain gauge station at the Welioya Estate (6.9288 N, 80.5582E, 1,258 m amsl) reported the highest rainfall, where the estate has received an average annual rainfall of 5,081 mm. Except during the South West Monsoon (SWM) season (May to September), the three rain-gauge stations located around Watawala Estate have recorded the lowest cumulative seasonal rainfall catch resulting in the same pattern of ranks in the average annual total rainfall. This study revealed that the Welioya Estate in Hatton receives the highest average annual rainfall in Sri Lanka. However, absence of standard rain-gauges installed in many locations in Sri Lanka to collect daily observations is a drawback in conducting detailed analysis rainfall across all agro-ecological regions in Sri Lanka.


Introduction
Despite its relatively small aerial extent, Sri Lanka exemplifies a variety of climatic conditions, especially the rainfall regime depending on the geographical settings of respective locations. Rainfall of Sri Lanka is of multiple origins, namely, monsoonal, convectional and weather systems formed in the Bay of Bengal account for major share of the annual rainfall (Punyawardena, 2020).
The four rainfall seasons that have been recognized in the rainfall calendar of Sri Lanka are based on the monsoons (Chandrapala, 2007). These are the First Inter-Monsoon (FIM; March-April), South West Monsoon (SWM; May-September), Second Inter-Monsoon (SIM; October-November) and North East Monsoon (NEM; December-February). The rainfall distribution during the FIM period shows that the entire southwestern sector and hill country receives over 250 mm of rainfall, with a localized area in the southwestern slopes experiencing rainfall in excess of 700 mm. Rainfall during the SWM period varies from about 100 mm to over 3,000 mm, and the highest rainfall is received in the mid elevations of the western slopes of Central highlands.
The SIM period is the most evenly balanced distribution of rainfall over Sri Lanka where almost the entire island receives in excess of 400 mm of rain during this season, with south western slopes receiving a higher rainfall in the range of 750 to 1,200 mm. During the NEM period, the highest rainfall values are recorded in the northeastern slopes of the Central hills with over 1,200 mm (Chandrapala, 2007). Being primarily convective in nature, rains during two inter-monsoon periods usually associates with thunder and lightning along with short-duration high intensity rains, especially during the FIM period (Punyawardena, 2020).
The southwestern region of Sri Lanka, including south-western slopes of the Central highlands, stands out clearly as the wettest region of the country and is named as the Wet zone. Hence, the wettest location of the country should lie within the Wet zone in an exposure of south-western slopes of the Central highland where altitude is below 1,000 m amsl (Chandrapala, 2007), the area where SWM wind stream is most active. Rainfall during SWM period rapidly decreases towards the higher elevations with Nuwara Eliya at 1,800 m amsl registering an annual rainfall of only 1,905 (Chandrapala, 2007).
Identifying the wettest place or the location having the rain gauge that received the highest rainfall within the Wet zone of Sri Lanka has been a debatable issue over the years. The text books of primary and secondary schools and some scholars in Sri Lanka have continued to rank Watawala location as the wettest place in Sri Lanka for decades and to-date, despite the fact that available local literature has clearly identified Ginigathhena (Kenilworth Estate;Silva 1988) and Yatiyantota (Wewaltalawa Estate; Chandrapala, 2007) as the wettest location of the country. These three locations under reference lie in the Wet zone within the mid slopes of Central highlands of Sri Lanka, with a possible exposure to high rainfall.
During extensive field studies carried out during the past decade in the effort to revisit agroecological boundaries of Sri Lanka by the Natural Resource Management Center of the Department of Agriculture, a location named Welioya in Hatton, within the same aforesaid exposure, was identified as relatively wetter compared to Watawala, Ginigathhena and Yatiyantota. Hence, a new standard rain gauge was established at the Welioya Estate in August 2014. The objective of this study was to compare the annual and seasonal rainfall catch in four different location to identify the wettest place in the Wet zone of Sri Lanka.

Materials and Methods
Daily rainfall data from four different locations covering seven rain-gauge stations (Table 1; Figure  1) were collected for a period of five years from 2015 to 2019. As the general public and some scholars perceive Watawala as the location with the highest rainfall in Sri Lanka, three rain-gauge stations were selected from this area for the study. All these rain-gauge stations are located in areas where the highest annual rainfall is most likely, due to the full-swing of SWM, FIM and SIM rains.. Daily rainfall data collected from all seven rain-gauge stations were analyzed for the average annual rainfall and its standard deviation and the average number of rainy days and its standard deviation.  (Punyawardena, 2007), amsl = above mean sea level; * locations within parenthesis are the nearest town areas.

Results and Discussion
Comparison of average annual total rainfall (2015-2019) The newly established rain-gauge at Welioya Estate in Hatton has recorded the highest average annual total rainfall (5,081 mm) with the lowest variability during the period from 2015-2019, compared to other six study locations (Table 2). Moreover, this rain-gauge station is located at the highest elevation position among the other stations of the study (Table 1). The results of the present study did not endorse the previous findings on exposure of the wettest region of the country, i.e., western slopes of Central highlands, below an altitude of about 1,000 m (Chandrapala, 2007). It is further interesting to note that all three locations (Lonach Estate, Mount Jean Estate, and Carolina Estate) selected from Watawala area, which is commonly perceived as the wettest location of the country, has recorded the lowest average annual rainfall catch (Table 2). Even though the Carolina Estate at Watawala has recorded the lowest average annual rainfall among the seven rain-gauge stations, it recorded the highest number of rainy days compared to the rest (Table 3) with a fair distribution of rainfall throughout the year in the said geographical area.  (Table 1), where the topography is less complex than that of lower and upper slopes of the Central highlands. All three rain-gauge stations selected from the Watawala area recorded a comparatively lower rainfall during the FIM season along with lower standard deviations (Table 4). Generally, relatively simple topographic features is a conducive setting to have strong land-heating resulting in enhanced convection during daytime when the sun is at its directly overhead position (April and September). This leads to more FIM rains at lower elevations than mid-and upper-slopes of the Central highlands (Punyawardena, 2008).
The number of rainy days during the FIM season ranged from 27 to 34 with a high variability at every station except at Kenilworth Estate, Ginigathhena (Table 5). The high temporal variability shown in the FIM rains, being convective in nature, is inevitable and a common feature of the rainfall climatology of the country.  (Table 6), accounting for 63% of the average annual rainfall of the location. The lowest annual rainfall (2,605.2 mm) was recorded at Carolina Estate at Watawala. Other two rain gauge stations that recorded lower annual rainfall, namely, Weweltalawa Estate and Maliboda Estate, are located at the lower elevations in the southwestern slopes of the Central highlands. The moisture laden SWM wind enters Sri Lanka from its southwestern coast. It is forced to ascend after hitting the southwestern slopes of the Central highlands, which acts as an orographic barrier for approaching heavily moisture laden wind, and produce heavy rains upon condensation. Therefore, southwestern slopes in the Central highlands normally receives heavy rains during the SWM season (Punyawardena, 2008). The period with the effectiveness of SWM is the strongest rainy season of Sri Lanka. The relative contribution from SWM is the most decisive factor in identifying any location as the wettest place in the country along with complimentary contribution from other three rainfall seasons, i.e., FIM, SIM and NEM seasons, to the total annual rainfall.
Within the 153 days of the season starting from May to September, the average number of Rainy days recorded from the Carolina Estate was 116 during the SWM season. This was the highest recorded among the seven rain-gauge stations studied, despite having the lowest cumulative seasonal rainfall catch at Carolina Estate during the same season. Thus, as in the case of average annual rainfall, the rainfall distribution during the SWM period was well spread at the Carolina Estate located at Watawala (Table 7). Comparison of rainfall during the Second Inter-Monsoon (2015-2019) Similar to the FIM season, the second Inter-Monsoon (SIM) has also brought in a substantial amount of rains due to the convectional activity of the atmosphere, especially during the month of October (Punyawardena, 2008). The rainfall during the SIM season usually get enhanced with the passage of the Inter Tropical Convergence Zone (ITCZ) on or near Sri Lanka resulting in the formation of Weather Systems in the Bay of Bengal.
This would result in widespread rains with less spatial variability (Punyawardena, 2008). Further, the rain-gauge station at Wewaltalawa Estate at Yatiyantota has been identified as the station registering the highest rainfall during SIM season (Chandrapala, 2007). The present study has confirms that the same Estate recorded the highest amount of rainfall during the SIM season (1,268 mm), a 25% of its average annual rainfall (Table 8).
Interestingly, as in the case of average annual rainfall (Table 2) and the average SWM rainfall (Table 6), and to a certain extent on the average FIM rainfall amount (Table 4), the Carolina Estate at Watawala recorded the lowest cumulative seasonal rainfall during the SIM season. Despite recording the lowest rainfall (866.1 mm) among the seven stations used in the study, the Carolina Estate at Watawala recorded 44 rainy days (average) during the season, and was as the highest among seven stations (Table 9). This again revealed a scenario of well distributed rainfall within the season despite the cumulative catch is relatively low. During the NEM season, the highest rainfall was recorded at Welioya Estate at Hatton (382.1 mm) in the leeward side (Table 10). The ranking order of the average seasonal rainfall during NEM season apparently follows the order of the average total annual rainfall where in both cases the highest was at the Welioya Estate and the lowest at the Carolina Estate. In all locations, temporal variability was relatively high as shown by the standard deviation of cumulative seasonal rainfall at respective raingauge stations. If any rain-gauge station in a given area possess a higher standard deviation during a particular rainy season, it means that the said rainy season is not effective over that geographical area. This study further confirms that the NEM rains are not much effective over the southwestern slopes of the Central highlands, but has only a very weak leverage on deciding the wettest place of Sri Lanka.
Similar to the average total seasonal rainfall (Table  2), the average SWM rainfall (Table 6), and the average SIM rainfall (Table 9), a comparatively higher number of rainy days was observed at the Carolina Estate at Watawala during the NEM season (Table 11). Although Carolina Estate has recorded the highest number of rainy days during the NEM season, it was not a considerable deviation from the average values of other stations considered in the present study (Table 11). However, comparatively higher standard deviation values of the number of rainy days confirmed that the NEM is not a strong rainfall season in the southwestern slopes of the Central highlands of Sri Lanka, but accounts for a higher temporal variability.

Conclusion
This study used the most recent daily rainfall data for the period 2015 -2019 collected from seven rain-gauge stations located in the southwestern slopes of Central highlands in Sri Lanka, without any missing values. The results clearly indicated that the common belief of Watawala in Hatton as the highest rainfall receiving or the "wettest place" of the country is no longer true and acceptable. This study revealed that the Welioya Estate in Hatton receives the highest average annual rainfall in Sri Lanka. The most appropriate scientific term for the "wettest region" in Sri Lanka should read as "the highest rainfall receiving geographical exposure of the country, which lies across an area in the southwestern slopes of the Central highlands with an attitudinal range of 300 to 1,300 m, where the SWM wind stream is most active". It is important to note that within the foregoing exposure, there may be more locations where rain-gauges have not been installed still to collect daily observations using a standard rain-gauge.