Comparison of spectral water index methods
Figure 3 shows the results of different spectral water index methods. Through overlay analysis with the original image and detailed visual analysis, it was found that AWEIsh had the best extraction performance and could accurately identify the boundary of the water body. NDWI, MNDWI, and EWI had different degrees of leakage extraction; NDWI and EWI had an evident leakage extraction in the northwest corner of the image, and the water leakage extraction of MNDWI was mainly concentrated in the middle of the image. There was a lot of water body misidentified in WI, especially in the southeast corner of the image.
Based on the visual interpretation of the water boundary, 100 test samples were selected and the confusion matrix32 was calculated to obtain the extraction accuracy of the water body from three aspects: commission error, omission error, and overall accuracy (Table 1). As seen from the table, the overall accuracy of AWEIsh was the highest, attaining a value of 99.14%, with extremely low commission and omission errors. WI had the lowest overall accuracy and the highest commission error, and could not distinguish water bodies and low reflectivity features effectively. The overall accuracies of NDWI, MNDWI, and EWI were similar. Comparing the results of the visual interpretation and quantitative analysis, the rapid extraction model of surface water based on the Google Earth Engine utilizing AWEIsh index was used for assessing the spatiotemporal changes of water bodies.
Time series analysis of typical reservoir area
To understand the inter-annual variation trend and intra-annual variation of the reservoir area in the dry zone of Sri Lanka, time series analysis was conducted with the Maduru Oya Reservoir as the case study area. The Maduru Oya Reservoir is the second largest reservoir in Sri Lanka, located in the east-central region, which is the main water source for irrigation and drinking, and has a high incidence of chronic kidney disease of unknown aetiology (CKDu). Figure 4 shows the inter-annual and intra-annual variations of Maduru Oya Reservoir area.
Figure 4 shows that the inter-annual fluctuation of Maduru Oya Reservoir area is slight, while the intra-annual fluctuation is significant. From 1988 to 2018, the reservoir area showed an overall increasing trend with slight float; the smallest area was recorded in 1992 (27.43 km2) and the largest area in 2013 (42.97 km2) (Fig. 4a). The rainy season in the dry zone of Sri Lanka occurs from October to February, and the dry season occurs from March to September. In 2017, the maximum area of the Maduru Oya Reservoir was noted in February, and the minimum area was noted in September. The area in February was 2.24 times bigger than that of September, with a difference of 31.58 km2. The maximum area of reservoirs or lakes generally occurs at the end of the wet season (February), and the minimum area occurs at the end of the dry season (September)2, which is consistent with the occurrence of maximum and minimum area in the Maduru Oya Reservoir in 2017(Fig. 4b). The area of the reservoir increased significantly in May during the dry season. According to meteorological data33, there were persistent strong winds and torrential rains in Sri Lanka in May 2017, resulting in an abnormal increase in the reservoir area. Generally, the period in which the area increased was from October to February (rainy season), while March to September (dry season) was the period in which the area decreased regardless of the influence of abnormal weather factors. The intra-annual fluctuation of the reservoir was severe, and there was a risk of drought and flooding at the same time. This observation implied that the seasonal regulation of water resources must be focussed in the future.
Analysis of spatiotemporal change of inland lakes and reservoirs
To systematically analyze the spatiotemporal variation characteristics of inland water in Sri Lanka in recent years, and considering the cloud cover of Landsat-5/8 images, 1995, 2005 and 2015 were selected as the study year with an interval of 10 years. The distribution information of surface water in three stages was obtained by running the rapid extraction model of surface water in the Google Earth Engine. According to statistics, the surface water areas of Sri Lanka in 1995, 2005, and 2015 were 1654.18 km2, 1964.86 km2, and 2136.81 km2, respectively. In the past 20 years, the water area of Sri Lanka has increased significantly. To further analyse the spatiotemporal changes of inland lakes and reservoirs, a 5-m buffer data of rivers in 2015 were produced in ArcGIS10.3 software; further, the area corresponding to the river channels were removed from the three images and only the lagoon areas were preserved. Lagoons are ubiquitous in the coastal areas of Sri Lanka, with flood discharge, aquaculture, coastal protection, and other functions34. The results consisting of the extracted lakes, reservoirs, and lagoons are shown in Fig. 5.
The overall water area of lakes and reservoirs in Sri Lanka showed an increasing trend from 1995 to 2015, and the lagoon area increased over these 20 years (Fig. 5). Because the lagoon does not belong to inland freshwater sensu stricto, the corresponding statistical analysis was not included in the following step. According to statistics, the total area covered of lakes and reservoirs in Sri Lanka were 1020.41 km2, 1270.53 km2, and 1417.68 km2 in 1995, 2005, and 2015 respectively. In the past 20 years, the area of lakes and reservoirs in Sri Lanka has increased by a considerable margin, attaining a value of 397.27 km2. To further analyse the spatiotemporal variation of inland lakes and reservoirs, they were divided into four grades according to their area: I (< 0.1 km2), II (≥ 0.1–1 km2), III (≥ 1–5 km2), and IV (≥ 5 km2). The number and area of different types of lakes and reservoirs for each year are shown in Fig. 6.
In Fig. 6a represents the number of lakes and reservoirs in the four grades, which showed an increasing trend from 1995 to 2015; the lower the grade of lakes and reservoirs, the greater the increase in area was observed. The number of I-grade lakes and reservoirs increased most significantly, while that of the IV-grade only increased by 11. Among the newly added IV-grade lakes and reservoirs, seven were transformed from other lakes and reservoirs, and four were newly built large reservoirs, such as the Rambukkam Oya, the Weheragala, the Daduru Oya, and the Mau Ara reservoirs. From 1995 to 2015, the area of the four grades of lakes and reservoirs showed an increasing trend, and the area of IV-grade lakes and reservoirs increased significantly with a total increase of 197.36 km2 (Fig. 6b). The higher the grade of lakes and reservoirs, the larger the total area. In 2015, the total area of IV-grade lakes and reservoirs was 760.53 km2, accounting for 54% of the total area among the four grades.
Figure 7 shows the statistical results of the number and area of lakes and reservoirs in various provinces of Sri Lanka. From 1995 to 2015, the increase in the number and area of lakes and reservoirs in Sri Lanka were mainly concentrated in the dry zone, such as the Northern, North Central, Eastern, the Sabaragamuwa, the Uva, and Central provinces. The number and area of lakes and reservoirs in the Southern Province remained unchanged, whereas the number and area of lakes and reservoirs in the North Western and Western provinces decreased slightly. Nisansala et al. reported that the eastern, south eastern, northern, and north-central regions of the country experienced increasing rainfall trends from 1987 to 2017, while western regions and part of the northwestern and central regions of the country displayed a decreasing rainfall trend during the same period35. In recent years, Sri Lanka has built a large number of new water conservancy facilities to support agricultural irrigation, aquaculture, and local economic development, which can regulate the water distribution in the wet and dry seasons36. Therefore, in the provinces with the decrease of the number and area of lakes and reservoirs, the primary reason for the decrease was because of lesser amount of local rainfall. In the provinces with the increase of the number and area of lakes and reservoirs, the increase was mainly due to the increase in local rainfall and the construction of water conservancy facilities. In general, the number and area of lakes and reservoirs in the four grades differed, and the amount of available water resources in surface lakes and reservoirs in Sri Lanka showed an increasing trend.
Source: Ecology - nature.com