9:30 AM - 9:45 AM
[HTT16-03] Quantitative assessment of the impact of rice paddy agriculture on hydrological cycle using water stable isotope ratios
Keywords:Rice paddy agriculture, Basin hydrological cycle, Water stable isotope ratios, Gonfiantini equation, Hydrological model, Quantitative assessment
In many areas where rice paddies are widely distributed, a large amount of agricultural water is required. For example, the total amount of water used annually in Japan is approximately 79.7 billion cubic meters based on water intake, of which agricultural water accounts for 67%. This is significantly higher compared to domestic water (17%) and industrial water (16%). Most rice paddy areas rely on rivers as the main water source, and a high proportion of river water is taken during the irrigation period. As an example, in the Kino River in the Kinki region, flow management is conducted to ensure a stable supply of agricultural water to downstream farmland in the Kii Plain. In this river, the flow is controlled so that the river flow at Suda point in the middle reaches does not fall below the specified flow of 13 cubic meters per second. On the other hand, the total water rights at the four headworks downstream amount to 33 cubic meters per second, which is more than 2.5 times the specified flow rate at the Suda point and much larger than the river flow, even considering the inflow from the tributaries downstream from the point. The large water intake is possible because much of the water introduced into the rice paddies is returned to the river and reused.
In addition, during the rice cultivation period, the paddy field is designed to store a certain depth of water, which helps disperse runoff during heavy rain and contributes to reducing the peak flow of rivers.
Therefore, rice paddies have a significant impact on the basin hydrological cycle. Although the relationship between rice paddy agriculture and the hydrological cycle has been widely recognized, the actual situation has not been quantitatively clarified. One of the most important reasons for this is due to the lack of established methods to accurately identify and evaluate the runoff components from the paddy fields in river water. In order to perform such an evaluation on a basin scale, an approach using a hydrological model is effective. However, even if the model structure can adequately represent the hydrological cycle structure of a basin, it is difficult to obtain valid data for the above reasons, and it has been considered difficult to obtain model parameters that adequately reflect the actual conditions.
The oxygen and hydrogen stable isotope ratios of water (water stable isotope ratios) are expected to be effective indicators to overcome this issue. Water stable isotope ratios clearly reflect the effects of dynamic fractionation caused by evaporation and exhibit high conservation unless fractionated, showing almost no changes in soil. This characteristic allows for clear differentiation of rice paddy water from other surface waters. Incorporating this characteristic into hydrological models is expected to enable unprecedentedly accurate evaluations of the impact of rice paddy agriculture on the basin hydrological cycle.
However, rice paddies experience irregular and non-stationary inflows of precipitation and irrigation water with different isotope characteristics, and the isotope ratios in rice paddy water fluctuates widely due to dynamic fractionation. Developing a hydrological model that considers water stable isotope ratios requires appropriately representing the dynamics of water isotope ratios within the rice paddies.
In this study, we developed a model that can track isotope ratios within rice paddies by applying the Gonfiantini equation to a rice paddy water balance model that considers water management operations. Using isotope ratio data from rice paddies in the Chikusa River basin in Hyogo Prefecture, we identified model parameters and achieved a reproduction accuracy of 11-17% relative error for δ2H and δ18O. The developed model faces the challenge of enormous computational capacity when applied to raster-type models targeting basins of several hundred square kilometers, which remains a future challenge.
In addition, during the rice cultivation period, the paddy field is designed to store a certain depth of water, which helps disperse runoff during heavy rain and contributes to reducing the peak flow of rivers.
Therefore, rice paddies have a significant impact on the basin hydrological cycle. Although the relationship between rice paddy agriculture and the hydrological cycle has been widely recognized, the actual situation has not been quantitatively clarified. One of the most important reasons for this is due to the lack of established methods to accurately identify and evaluate the runoff components from the paddy fields in river water. In order to perform such an evaluation on a basin scale, an approach using a hydrological model is effective. However, even if the model structure can adequately represent the hydrological cycle structure of a basin, it is difficult to obtain valid data for the above reasons, and it has been considered difficult to obtain model parameters that adequately reflect the actual conditions.
The oxygen and hydrogen stable isotope ratios of water (water stable isotope ratios) are expected to be effective indicators to overcome this issue. Water stable isotope ratios clearly reflect the effects of dynamic fractionation caused by evaporation and exhibit high conservation unless fractionated, showing almost no changes in soil. This characteristic allows for clear differentiation of rice paddy water from other surface waters. Incorporating this characteristic into hydrological models is expected to enable unprecedentedly accurate evaluations of the impact of rice paddy agriculture on the basin hydrological cycle.
However, rice paddies experience irregular and non-stationary inflows of precipitation and irrigation water with different isotope characteristics, and the isotope ratios in rice paddy water fluctuates widely due to dynamic fractionation. Developing a hydrological model that considers water stable isotope ratios requires appropriately representing the dynamics of water isotope ratios within the rice paddies.
In this study, we developed a model that can track isotope ratios within rice paddies by applying the Gonfiantini equation to a rice paddy water balance model that considers water management operations. Using isotope ratio data from rice paddies in the Chikusa River basin in Hyogo Prefecture, we identified model parameters and achieved a reproduction accuracy of 11-17% relative error for δ2H and δ18O. The developed model faces the challenge of enormous computational capacity when applied to raster-type models targeting basins of several hundred square kilometers, which remains a future challenge.