-1DWQ6FL)RXQGDWLRQ6UL/DQND DOI: http://dx.doi.org/10.4038/jnsfsr.v47i3.9281 RESEARCH ARTICLE Distributed modelling of water resources and pollute transport in Malwathu Oya Basin, Sri Lanka A.C. Dahanayake * and R.L.H.L. Rajapakse 'HSDUWPHQWRI&LYLO(QJLQHHULQJ)DFXOW\RI(QJLQHHULQJ8QLYHUVLW\RI0RUDWXZD.DWXEHGGD0RUDWXZD Submitted: 04 April 2018; Revised: 11 April 2019; Accepted: 03 May 2019 Abstract: The Nachchaduwa sub-catchment (598.74 km 2) of the Malwathu Oya basin is seasonally stressed in the dry INTRODUCTION SHULRGVDQGLWVGRZQVWUHDPSDUWVXQGHUJRLQWHUPLWWHQWÀRRGV during monsoon seasons while the fate and behaviour of excess Water, being a vital natural resource to sustain all life QLWURJHQ 1 DQGSKRVSKRUXV 3 DGGHGWRWKHZDWHUZD\VGXH forms on earth, has now become a limited resource due to to agricultural fertilisers used in the upstream areas remain the adverse impacts of various natural and anthropogenic unresolved. This study incorporated the Water and Energy causes. Due to the increasing population and rapid 7UDQVIHU3URFHVVHV :(3 PRGHOWRDVVHVVWKHSUHVHQWVWDWXV urbanisation, the demand for water has been increasing of the catchment concerning water resources and pollutant drastically. Further, the quality of the available fresh transport. Results showed that the catchment response to the water resources has been deteriorating mainly due to UDLQIDOO LV KLJKO\ UHJXODWHG GXH WR UHVHUYRLU VWRUDJH H൵HFW pollution created by the anthropogenic activities in many XQJDXJHGEDVLQZLWKUHJXODWHGÀRZV 7KHDPRXQWVRI1DQG3 rivers in developing countries, which has become a threat in fertilisers applied in this catchment exceeded the actual plant to the ecosystem and to human health. requirement. In both wet [Maha cultivation season (October to March)] and dry [Yala cultivation season (April to September)] VHDVRQVWKHGL൵HUHQFHVEHWZHHQWKHXSVWUHDPDQGGRZQVWUHDP Malwathu Oya basin is the second largest catchment 2 YDOXHV RI PHDVXUHG ZDWHU TXDOLW\ SDUDPHWHUV 1 DQG 3 among the river basins in Sri Lanka (3284 km ), and one components - total, dissolved and particulate) in waterways of the most widely used sources of water for irrigation, ZHUHQRWVWDWLVWLFDOO\VLJQL¿FDQW2QDYHUDJHWKHZHWVHDVRQ water supply and other diversions in the North Central had about 5~7 timesି the dry season value of the total suspended 3URYLQFH )LJXUH ,W KDV QRZ EHFRPH D VHDVRQDOO\ ଷ solids (TSS) in the streams, and in both seasons, the modelled stressed river basin due to over exploitation and water TSS, and were within the ranges of the previously pollution. The basin experiences water scarcity during the publishedି results. Thisଷି study will be continued in the future to ଷ ସ dry periods and the downstream areas of the catchment analyse possible hydrological and material transport related DUHÀRRGHGGXULQJWKHZHWSHULRGVZKLOVWVHYHUHÀRRGLQJ scenarios to identify best water resources management practices was observed in 2011, 2014 and 2016 ( 'HSDUWPHQW RI and to pragmaticallyଷି cope with the excess fertiliser usage, an ସ issue commonly found in most of the similar catchments. ,UULJDWLRQXQSXEOLVKHGGDWD . Keywords: Nachchaduwa, nitrogen, nutrient concentrations, The basin is augmented by the adjacent Kala SKRVSKRUXV766:(3PRGHO Oya basin from Kala Wewa reservoir via Yoda Ela to * Corresponding author ([email protected]; https://orcid.org/0000-0002-4400-6979) This article is published under the Creative Commons CC-BY-ND License (http://creativecommons.org/licenses/by-nd/4.0/). This license permits use, distribution and reproduction, commercial and non-commercial, provided that the original work is properly cited and is not changed in anyway. $&'DKDQD\DNH 5/+/5DMDSDNVH )LJXUHV ORFDWLRQVGXULQJFXOWLYDWLRQVHDVRQV 3HUHUD et al ., 2014). The stream has been (mis) used as a site for waste disposal in the urban area and water becomes polluted easily (De Alwis, 2006). ି ଷ 6LJQL¿FDQWO\KLJKHUFRQFHQWUDWLRQVRI , and – Cl LQ VWUHDPV GXULQJ WKH ¿UVW LQWHUPRQVRRQି ଷି VHDVRQ (March-April) in the paddy growing areas ଷ have ସ been observed, along with the highest mean dissolved oxygen (DO) and turbidity values observed in the northeast ଷି monsoon season (December-February) inସ Malwathu Oya. The chemical fertiliser application with the paddy FXOWLYDWLRQLVFRQWULEXWLQJWRWKHVWDWLVWLFDOO\VLJQL¿FDQW seasonal variations observed in the water quality SDUDPHWHUV 3HUHUD et al ., 2014). In addition the outlet of the main stream from the Anuradhapura city and the ି FDQDOÀRZLQJDFURVVWKHFLW\KDYHVKRZQKLJKHUYDOXHV ଷ for total dissolved solids, pH, electrical conductivity, DO level, , and FRQ¿UPLQJ WKH LPSDFW RI urban land useି on waterଷି pollution in Malwathu Oya (Madushanka ଷet al ., 2015). ସ In addition, the fate and EHKDYLRXURIH[FHVV1DQG3DGGHGWRWKHZDWHUZD\VDV fertilisers applied in paddy lands and other crop areas in ଷି the upstream ସcatchment areas remain unresolved, and suspected to be a cause of the high prevalence of chronic kidney disease of unknown aetiology (CKDu) in the area (Munasinghe et al ., 2015). )LJXUH1DFKFKDGXZDFDWFKPHQWDQGWKHGHOLQHDWLRQRIVXEFDWFKPHQWV Figure 1: Nachchaduwa catchment and the delineation of Therefore, water resources management alternatives sub-catchments (to overcome issues occurring due to water scarcity, climate change impact, etc.) and measures to address feed Nachchaduwa, Tissa wewa and Basawakkulama issues related to degrading water quality (caused due to reservoirs. The basin is distinct as it possesses a large excessive use of fertilisers and agro-chemicals etc.) in number of tanks, from small size village tanks to the basin should be studied, while the fate of pollutants comparatively fair-sized tanks, from which the rainfall including their conveyance and spatial and temporal UXQR൵ SURFHVV LV ODUJHO\ LQÀXHQFHG 8S WR D FHUWDLQ accumulation patterns should also be investigated by WKUHVKROG WKH ULYHU ÀRZ LV PRUH RU OHVV UHJXODWHG DQG studying the dispersal and accumulation behaviour of ÀRRG SHDNV DUH UHGXFHG 'HSDUWPHQW RI ,UULJDWLRQ these elements after they have been added to the crop XQSXEOLVKHGGDWD ). However, the water holding capacity ¿HOGVDQGZDWHUZD\V RIPRVWRIWKHWDQNVKDVEHHQUHGXFHGVLJQL¿FDQWO\GXH to aquatic weeds, eutrophication and heavy siltation. 3K\VLFVEDVHG GLVWULEXWHG K\GURORJLFDO PRGHOV Hence, most of the tanks are facilitating only for Maha can take account of temporal and spatial variations season and lack of irrigation water has become a problem of all variables and parameters involved in the basic in the Yala season (Gunarathna & Kumari, 2014). PDWKHPDWLFDO HTXDWLRQV RI WKH ZDWHU ÀRZV IRU D watershed. In addition, the used parameters are physically Anthropogenic activities cause stresses on Malwathu measurable. Therefore, they give a detailed and 2\DE\LPSDLULQJWKHTXDOLW\RILWVZDWHU 3HUHUD et al ., potentially more correct descriptionϮϮ of the hydrological 2014). With respect to irrigation-related water quality of processes in the watershed than empirical and conceptual the reservoirs, Nachchaduwa has exceeded the threshold hydrological models. Today, there exist several popular value of electrical conductivity (0.750 mScm -1 ) and models of this type, like SHE, IHDM, SWAT, MIKE salinity (0.500 gL -1 ) (Silva, 2004). Water of the nearby SHE (Jia et al D DQG :(3 :DWHU DQG (QHUJ\ SDGG\ ODQGV LV GLYHUWHG WR WKH ULYHU IURP GL൵HUHQW WUDQVIHU3URFHVVHV PRGHO -LD et al ., 2001b). September 2019 Journal of the National Science Foundation of Sri Lanka 47(3) :DWHUUHVRXUFHVDQGSROOXWHWUDQVSRUWPRGHOOLQJ 7KH :(3 PRGHO KDV EHHQ GHYHORSHG E\ DGGLQJ PRUH The entire catchment has been delineated into three detailed energy balance analysis in hydrological modelling sub-catchments according to the terrain and stream path -LD 7DPDL ,WV PDLQ GL൵HUHQFHV IURP RWKHU distribution (Figure 1). physics-based models involve detailed consideration of energy transfer processes, use of sub- grid heterogeneity Collection and pre-processing of data of land use, application of generalised Green-Ampt model to save computation time and its potential use for The model input data required for the hydrological various scenario analyses. It has been further improved component and the material transport component were by coupling a soil erosion-transport model to introduce FROOHFWHG DQG SURFHVVHG 6WUHDPÀRZ UHODWHG GDWD DQG a particle-bound pollutant component (Rajapakse Nachchaduwa reservoir operation data (sluice release, et al ., 2010), and by adding simulation of multi-layered spill release, water issues, irrigation issues, etc.) were DTXLIHUVGLUHFWFRPSXWDWLRQRIJURXQGZDWHURXWÀRZWR collected from the Department of Irrigation. Land use ULYHUVDQGVLPXODWLRQRILQ¿OWUDWLRQWUHQFKHV,WKDVEHHQ details and soil types of the catchment were extracted successfully applied to river basins in Japan, Korea and from the spatial maps prepared by the Survey Department China (Jia et al ., 2001a; 2001b; 2007; Rajapakse et al ., in year 2001. Geological and soil layer details were 2010; Cunwen et al ., 2011). Detailed descriptions of obtained from the borehole data of the construction model development, hydrologic and material transport projects undertaken in the close proximity. modelling procedures and input/output data, are given in the literature [Jia et al . (2001a; 2001b; 2005) and Meteorological data including rainfall, temperature, Rajapakse et al. (2010)]. wind velocity, relative humidity and sunshine hours were collected from the Meteorological Department 7KHUHIRUH WKH SUHVHQW VWXG\ LQFRUSRUDWHG WKH :(3 and checked using the hydrological and statistical data model to the Nachchaduwa