Abstract

Strategies for Conserving Water and Effecting Mosquito Vector Control in Rice Ecosystems: A Case Study from Tamil Nadu, India

by S. Krishnasamy, F.P. Amerasinghe, R. Sakthivadivel, G. Ravi, S.C. Tewari and W. van der Hoek

A case study on the impacts of water saving irrigation techniques on land and water productivity and mosquito vector breeding was carried out in farmer-managed rice fields under the command of the Andaman Tank in Madurai District, Tamil Nadu State, India, during October 1999 - January 2000.

The tank received water from its own catchment as well as from a sluice of the main canal of a nearby major irrigation system. The tank had three outlets, at which different water management regimes were implemented as follows: outlet-1 for continuous submergence irrigation (CSI); outlet-2 for irrigation to 5 cm depth one day after disappearance of ponded water in fields (alternate wet/dry irrigation - AWDI); and outlet-3 for rotational water supply (RWS), i.e., 4 days "on" and 3 days "off".

Five rice fields were selected for detailed data collection under each water management regime. Water accounting was based on measurements at tank outlet, field and drainage levels. Agronomic evaluations included rice plant growth (root length and width, leaf area index [LAI]), yield attributes (no. of panicles/m2, panicle length, grains/panicle, sterility percentage), fertilizer and pesticide applied, weed density and de-weeding effort. Mosquito immature stages breeding in the rice fields, together with associated arthropod fauna, were sampled.

Rice yield, LAI and % grain sterility were significantly influenced by rice variety and water regime. When water management regimes were compared for the same rice variety (ASD 19), 6-7% higher yields and root length, 4% greater panicle length and 29% lower grain sterility were recorded under AWDI than CSI. Intermediate values were recorded for RWS. Overall, therefore, AWDI resulted in greater land productivity in rice, compared to CSI and RWS.

Water accounting studies showed that local irrigation efficiency (consumed water / supplied water) and depleted fraction at field and outlet levels were highest, and ground water level change lowest, under AWDI. This practice locally retained about 22% of water compared to CSI, the saving occurring mainly from reduced recharge loss and return flow.

Five Culex and five Anopheles mosquito species occurred in the rice fields, the dominant species being two vectors of Japanese encephalitis, viz., Culex tritaeniorhynchus and Cx. vishnui. Statistically equivalent mosquito immature densities occurred under all three water regimes, but both Anopheles and Culex occurred significantly more frequently in AWDI and RWS than CSI fields. Clearly, the AWDI technique was not effective in suppressing mosquito breeding under the conditions encountered in the field where frequent rainfall and inadequate field leveling confounded the intended drying effects of the AWDI and RWS water management regimes.

Overall, this case study under farmer managed conditions showed that alternate wet/dry irrigation resulted in greater land and water productivity, saving water locally whilst maintaining yields on par with, or greater than, the other management regimes tested. However, the results on mosquito breeding indicate that caution needs to be exercised in promoting the technique as a method of mosquito vector control.text.