Abstract
While providing water for irrigation, the reservoirs have been a source of fresh water for the rural communities, and, more recently, they have become the focus for expanding the inland fishery industry. This paper assessed nutrient enrichment in minor seasonal irrigation tanks in the Kurunegala District in relation to different stages in the paddy cultivation cycle. Findings of the study will be useful to determine their compatibility with safe levels for fisheries, among other uses. The study was extended to cover other selected parameters relating to water quality. Surface water from 50 minor tanks were sampled over pre-cultivation, cultivation and post-cultivation stages of the paddy cultivation cycle in the Yala (n= 27) and Maha (n=23) seasons and analyzed for NO3-N, PO4- P, dissolved oxygen, turbidity, conductivity and pH, using standard methods and portable instruments. Results showed considerable spatial variation in nutrients between the 50 tanks and marked temporal variation through the three sampling stages. For NO3-N, there was a shift in the proportion of tanks containing low and high nutrient levels through the pre-cultivation, cultivation, and post cultivation stages. The mean levels were 0.01- 1 mg l-1 during pre-cultivation and post-cultivation and 1.0 - 5.0 mg l-1 during cultivation, with distinct peaks observed in the Yala and Maha seasons. The variation of PO4- P was different, with the majority of the tanks having levels of 1 – 5 mg l-1 PO4- P during all sampling stages, and with no pronounced peaks. Nutrient levels indicate that a majority of the minor tanks become eutrophic during the two cultivation seasons. The other tested parameters also depicted spatial and temporal variation, and water quality indices revealed that the quality of water deteriorates during paddy cultivation. Based on these findings, particularly for nutrients, we conclude that many of the minor tanks become eutrophic during the cultivation season and may not offer optimal conditions for the intended uses, one of them being rearing of fish and shellfish. Mitigation measures must therefore be implemented to reap maximum benefits associated with these minor irrigation tanks.
Highlights
The dry and intermediate climate zones of Sri Lanka are dotted with irrigational tanks
The overall values recorded in the present study for NO3-N are consistent with values reported previously for irrigation tanks in Sri Lanka, but those for PO4-P far exceed previously reported values (Young et al, 2010; Wijesundara et al, 2012)
Nutrient loads in tanks would increase considerably when fertilizerladen water stocked in paddy fields is discharged through outlet canals or streams and enter the irrigation tanks at lower elevations in the cascade during the post-reproductive phase of the crop, or when surface run off enters the water body during the rainy season
Summary
The dry and intermediate climate zones of Sri Lanka are dotted with irrigational tanks These tanks, whose primary function is to supply irrigation water for rice cultivation, were originally constructed many centuries back, mostly in the form of cascades (Sakthivadivel et al, 1997; Shinogi, 2001). Nutrient accumulation is aggravated during the prolonged dry season, which is characteristic of the dry zone, and to a lesser degree of the intermediate zone. During this period, water levels in the tanks recede considerably, with many of the smaller tanks (or minor tanks) turning into mere water holes and termed ‘seasonal tanks’.
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