Paddy fields are manmade ecosystems. These fields undergo three water regimes namely, aquatic, semi aquatic and terrestrial within a cultivation season. Accordingly, the ecosystem also changes. In addition, agronomic practices can affect biodiversity and the overall ecology. A study was conducted to assess the water regime and respective ecosystem so that ecosystem can be enhanced as an adaptation for climate variability in paddy cultivation. The study was conducted in a command area of Bayawa

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... inor irrigation system located in Awlegama Agrarian service division in Kurunegala District, Sri Lanka. Six randomly selected locations in the field were used in the fauna survey at different growth stages in Yala season in 2015. A water management index (WMI) was developed to assess the water regime using physical factors of the field and social capacity of farmer. Results revealed that wet condition is expected in more than 69% of farmers since their WMI is more than 0.5. Water regime varied among head, middle and tail sections. Tail end of the command area has more reliability for water than the head and middle sections. The water fluctuation leads to higher biodiversity. The initial water regime and crop growth stages have provided more habitats for the both terrestrial and aquatic invertebrates.The abundance of aquatic invertebrates were significantly (p= 0.01) varied over the five growth stages anddragonflies' diversity was varied temporarily with respect to the species richness.Alternative wetting and drying have created more favorable habitats for aquatic fauna to increase the biodiversity of the fields. Increase of biodiversity promotes naturally enemies/predators for pests while aquatic fauna improve the nutrient availability in the fields. Therefore, water management in the fields could be used to enhance biodiversity in the rice field to achieve sustainable paddy cultivation.