A simulation model for investigating the effects of rice paddy fields on the runoff system

Rice production in Taiwan is likely to decrease because of two major threats: an increase in the industrial demand on water resources and cheaper imported rice resulting from the free trade policy which will be enforced when Taiwan joins the World Trade Organization. A shrinkage in rice paddy acreage raises environmental concerns, especially in relation to water conservation. Besides rice production, the existence of paddy fields promotes groundwater recharge and remediates floods. Unlike rice production, the value of the environmental conservation is difficult to quantify. In this study, the effects of paddy fields on groundwater recharge and flood remediation have been investigated. To assess the long-term significance of these effects, the study team developed a model based on hydrologic equilibrium. This model describes the performance of water recharge and detention. According to a case study, the amount of runoff from a rice paddy field is about 27% of the amount of the precipitation. This percentage is significantly lower than that of the runoff from a dry farming field, which is normally as high as 55% of the precipitation. In this case, the amount of evapotranspiration from a rice paddy field and that from a dry farming field are 56% and 31%, respectively. In this case, the amount of recharge from a rice paddy field, 17%, is higher than that from a dry farming field, which is 14%. Clearly, a significant increase in runoff and a decrease in groundwater recharge can be expected when a rice paddy field is converted to other uses. The second analysis includes an event-based simulation study regarding the effects of rice paddy fields on the flood peak. Attributed to its superior detention capacity, a paddy field typically has a much lower flood peak, about one third of a dry farm field. These positive effects of rice paddy fields on runoff should be considered in making decisions about the reduction of rice cultivation.