water Article Analyzing the Dependence of Major Tanks in the Headwaters of the Aruvi Aru Catchment on Precipitation. Applying Drought Indices to Meteorological and Hydrological Data Robin Saase *, Brigitta Schütt and Wiebke Bebermeier * Department of Earth Sciences, Physical Geography, Freie Universität Berlin, Malteserstraße 74-100, 12249 Berlin, Germany; [email protected] * Correspondence: [email protected] (R.S.); [email protected] (W.B.) Received: 30 July 2020; Accepted: 13 October 2020; Published: 21 October 2020 Abstract: This study aims to analyze the dependence of reservoirs (locally called tanks or wewas) in the headwaters of the Aruvi Aru catchment on precipitation and thus to evaluate their efficiency. The Aruvi Aru is located in the Dry Zone of Sri Lanka, and numerous human made reservoirs characterize the study area. The methodology is based on the application and correlation of climatic and hydrological drought indices. The Standardized Precipitation Index (SPI) is applied to precipitation data at different time scales and the Standardized Water-Level Index (SWLI) is applied to water-level data of five major tanks in the catchment. The results show that near normal present-day average precipitation is appropriate to fill the investigated tanks. The precipitation of the previous 6–12 months has the highest impact on water-level changes. A moderate to strong positive correlation between SWLI and SPI point to other factors besides precipitation affecting the water level of the tanks. These are: (i) catchment size together with the buffering capacity of the upstream catchment and (ii) management practices. As the overall conclusion of our study shows, the tanks functioned efficiently within their system boundaries. Keywords: Sri Lanka; water harvesting; Standardized Precipitation Index (SPI); Standardized Water-Level Index (SWLI); wewa 1. Introduction For more than 2000 years, agriculture in the Dry Zone of Sri Lanka has been based on a sophisticated water management system [1,2]. This system consists of thousands of human made reservoirs that collect and store surface water, so called tanks or wewas, which are constructed cascade-wise in shallow valleys. The reservoirs are connected with each other by channels and spillways and increase in size downstream. Depending on the size of the water surface area of the reservoir and the irrigated area, the irrigation works in Sri Lanka are classified as major, medium, minor, and micro tanks [3]. Major tanks are in general located in the downstream part of the cascade and are defined by a water surface of > 200 ha and a command area (irrigated area) of > 600 ha [3,4]. The administration of the major tanks is the responsibility of governmental institutions. In contrast, small tanks are situated in the upstream parts of the cascades and are managed by local village communities [4]. The hinterland of the ancient capital Anuradhapura was the center for the spread of this water harvesting system throughout the Dry Zone of Sri Lanka from the 3rd century BCE onwards [5] (Figure1). An initial expansion phase of the water harvesting system resulted in a distribution of tank cascade systems across the Dry Zone by the 2nd century CE. During the following centuries, the system experienced further expansion and refinement [5]. An essential measure to increase local Water 2020, 12, 2941; doi:10.3390/w12102941 www.mdpi.com/journal/water Water 2020, 12, 2941 2 of 20 Water 2020, 12, x FOR PEER REVIEW 2 of 20 watersystem availability experienced was further the construction expansion and of a supra-regionalrefinement [5]. An channel essential network measure in the to 5thincrease century local CE. Thiswater channel availability network was enabled the construction the routing of a of supra-regional water from the channel catchment network of thein the Mahaweli 5th century River CE. via theThis catchment channel network of the Kalaenabled Oya the River routing to theof water catchment from the of catchment the Aruvi of Aru the Mahaweli River in the River vicinity via the of Anuradhapuracatchment of [the5,6 ]Kala (Figure Oya1). River to the catchment of the Aruvi Aru River in the vicinity of AnuradhapuraWhen evaluating [5,6] (Figure this engineering 1). feat of supra-regional water transfer, the question arises as to theWhen reason evaluating for such costly this engineering and laborious feat activities.of supra-re Wasgional the water exploitation transfer, ofthe external question water arises sources as to necessarythe reason because for such the costly tanks inand the laborious immediate activities hinterland. Was of the Anuradhapura exploitation didof external not function water e ffisourcesciently? necessary because the tanks in the immediate hinterland of Anuradhapura did not function Or was the exploitation of external water sources necessary to increase local water availability in efficiently? Or was the exploitation of external water sources necessary to increase local water drought periods? In this paper, we attempt to answer these questions by assessing the dependence of availability in drought periods? In this paper, we attempt to answer these questions by assessing the major tanks on precipitation. We apply meteorological and hydrological drought indices to modern dependence of major tanks on precipitation. We apply meteorological and hydrological drought meteorological data resulting in an evaluation of how efficiently and reliably the tanks are filled by indices to modern meteorological data resulting in an evaluation of how efficiently and reliably the local precipitation. Due to the lack of runoff or discharge data, the water-level data of major tanks tanks are filled by local precipitation. Due to the lack of runoff or discharge data, the water-level data serves as a proxy for water availability. of major tanks serves as a proxy for water availability. FigureFigure 1. 1.( A(A)) Digital Digital elevationelevation modelmodel (DEM)(DEM) of Sri Lanka, river river network, network, location location of of the the Aruvi Aruvi Aru Aru andand Mahaweli Mahaweli Ganga Ganga catchmentscatchments andand study area; ( B)) DEM DEM of of the the study study area: area: the the headwaters headwaters of of the the AruviAruvi Aru Aru catchment catchment and and thethe tankstanks ofof interestinterest within:within: (1) (1) Nachchaduwa Nachchaduwa wewa, wewa, (2) (2) Mahakanadarawa Mahakanadarawa wewa,wewa, (3) (3) Nuwara Nuwara wewa, wewa, (4)(4) TissaTissa wewa,wewa, (5)(5) BasawakkulamaBasawakkulama wewa. wewa. Kala Kala wewa wewa (6), (6), situated situated in in the the KalaKala Oya Oya catchment, catchment, receivesreceives waterwater via a channelchannel from the the Mahaweli Mahaweli Ganga Ganga and and water water from from Kala Kala wewawewa is is diverted diverted through through thethe channelchannel of Yoda Ela to different different tanks tanks within within the the Aruvi Aruvi Aru Aru catchment. catchment. Data source: DEM (Shuttle Radar Topography Mission (SRTM) 1 Arc-Second Global 30 m × 30 m, Data source: DEM (Shuttle Radar Topography Mission (SRTM) 1 Arc-Second Global 30 m 30 m, 2015); 2015); catchment, tanks, rivers, channel, Anuradhapura [7]. × catchment, tanks, rivers, channel, Anuradhapura [7]. WaterWater2020 2020, 12, ,12 2941, x FOR PEER REVIEW 3 of3 of 20 20 2. Study Area 2. Study Area The Aruvi Aru River and its tributaries drain a catchment area of about 3170 km2 in north- westernThe Aruvi central Aru Sri River Lanka. and The its tributaries local relief drain is a aslightly catchment rolling area plain of about corresponding 3170 km2 in to north-western the expanded centralpeneplain. Sri Lanka. This Theplain local is reliefintersected is a slightly by shallow rolling plainvalleys corresponding and sporadic to inselbergs. the expanded Altitudes peneplain. vary Thisbetween plain is 33 intersected and 755 m by a.s.l. shallow Bedrock valleys consists and sporadic of Precambrian inselbergs. crystalline Altitudes basement vary between rocks, 33 mainly and 755 of mgranitic a.s.l. Bedrock gneisses consists and quartzites of Precambrian [7,8]. Local crystalline soils are basement dominated rocks, by mainly reddish of graniticbrown earths gneisses and and low quartziteshumic gley [7,8 [9],]. Local whereby soils reddish are dominated brown earths by reddish are the brown most earths frequent and soil low type humic [10]. gley Paddy [9], cultivation whereby reddishis the brownmain crop earths inare the the north most central frequent part soil of type Sri [Lanka,10]. Paddy and cultivationin the areas is of the plateau-like main crop in divides the north this centralalternates part ofwith Sri Lanka,chena agriculture and in the areas (shifting of plateau-like cultivation) divides [11]. The this main alternates cultivation with chena period agriculture for rice is (shiftinglocally cultivation)called Maha [and11]. lasts The mainfrom October cultivation to March. period The for riceAruvi is locallyAru catchment called Maha is situated and lasts in the from Dry OctoberZone of to March.Sri Lanka The north Aruvi of Aru the catchment Central Highlands. is situated inAccording the Dry Zone to the of SriKöppen-Geiger Lanka north ofclimate the Centralclassification, Highlands. the Accordingstudy area tois thecategorized Köppen-Geiger as an As-climate climate classification, [12], with an the average study areaannual is categorized temperature asof an 28.4 As-climate °C and [mean12], with annual an average precipitation annual of temperature 1341 mm (Figure of 28.4 2).◦C The