The Komesu underground dam is the first fullscale underground dam constructed to prevent saltwater intrusion in Japan. Although the cutoff wall of the dam effectively reduces the movement of saltwater into the reservoir area, saltwater masses remained behind the dam at the time of its completion, and saltwater can intrude beneath and diffuse through the wall, particularly when the reservoir level is below the sea level because of high pumping levels during the drought years. Therefore, it is necessary to estimate in advance whether the saltwater concentration in the pumped water is likely to exceed or not the permissible salinity level because of an increase in the residual saltwater mass as a result of saltwater intrusion and to take necessary measures to suitably manage the saltwater level behind the dam. To analyze saltwater intrusion, we first selected the optimal program suitable for the analysis of saltwater intrusion. Second, we examined the longitudinal dispersivity and the effect of the cone of depression around the pumping wells. Then we analyzed saltwater intrusion into the reservoir area in detail by using a two-dimensional convective-dispersive analysis. The results of the analysis make it possible to improve management of saltwater in the reservoir area behind the underground dam.
Water flows in irrigated paddy areas are complicated not only by the substantial volumes involved but also by repeated cycles of diversion and return flow. A spatially explicit hydrological model that simulates water diversion and return processes in irrigated paddy areas was used to evaluate the return ratio of diverted water to river basin. Assuming complete mixing of the rainfall and irrigated water over the entire irrigation period, the time-averaged return flow was calculated using the simulated net drainage from the irrigated areas. The return ratio, which is defined as the fraction of the total return flow volume to the total diverted water volume, was 48.5% (ranging from 42.3 to 52.1%) in our case study. These values suggest that most of the water diverted to the irrigated paddies will be discharged except that which is lost through evapotranspiration, because we consider the return ratio during a sufficiently long period and for large irrigation areas. While the largest and smallest return ratios were obtained in the driest and wettest years, respectively, the correlation between rainfall amount and return ratio was not significant, suggesting the need for a more appropriate averaging period that accounts for the short-term variabilities in return flow processes. RÉSUMÉLes flux d'eau dans les systèmes irrigués sont compliqués par les volumes mis en cause et les cycles alternés de détournement et de retour de flux d'eau. Un modèle hydrologique explicite spatialisé a été utilisé pour évaluer le taux de retour de l'eau détournée d'un bassin versant. En supposant un brassage complet des eaux de pluie et d'irrigation, nous avons calculé le drainage effectif pendant la période de calcul de la moyenne. Le taux de restitution, défini comme la part de l'écoulement restituée aux eaux détournées aux déversoirs, était de 48.5% (dans la gamme de 42.3 à 52.1%). Ces valeurs suggèrent que la plus grande partie des eaux prélevées retournent à la rivière modulo les pertes d'évapotranspiration, ce que nous permet de faire la longue période d'observation et la taille du système observé. Alors que les plus grands et les plus petits taux de restitution étaient observés en année sèche et humide, respectivement, la corrélation entre les volumes précipités et les taux de restitution n'était pas significative, suggérant un besoin de périodes plus appropriées pour le calcul des moyennes, qui comptent beaucoup dans la variabilité à court terme dans les processus de retours des flux.
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