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.
Stable Isotopic Compositions across Central Sri Lanka the baseflow from a streamflow hydrograph (Jones et al. 2006, Tweed et al. 2016), estimating the influx of groundwater into lakes (Sacks et al. 2014, Ala-aho et al. 2015), and identifying the origin of groundwater and flow processes (Kamtchueng et al. 2015). Rainfall in Sri Lanka is controlled by the tropical monsoon system. Monsoon rains are closely related to the regional livelihood and social water use, making stable water supply and flood/drought management a necessity. Accordingly, various studies have used stable isotopes to investigate the meteorological and hydrological settings in Sri Lanka. Dharmasiri & Athuluwage (1991) reported the monthly stable isotopic variations in rainwater at seven sites to constrain the isotopic input into groundwater. Jayasena et al. (2008) characterized the isotopic compositions of rainwater in the dry and intermediate
The Distributed Water Circulation Model incorporating Agricultural Water Use (DWCM‐AgWU) was modified and the revised model was applied to assess water use in the Chao Phraya River basin in Thailand for the years 2008–2011, during which there were droughts and floods. A reservoir management model was introduced to incorporate two large dams (Bhumibol and Sirikit dams), with several remote irrigated areas in water allocation/management, and performed special treatments of flood peaks to account for agricultural practices. In the system, the two dams are the main sources of irrigation water during the dry season and they are used to control floods during the rainy season. Moreover, for the lower Chao Phraya River from Nakhon Sawan to the sea, another special water management was applied on rainy days to keep the release at the Chao Phraya Diversion Dam at or below a certain threshold. That is, when discharge exceeded the threshold, water was diverted to main irrigation canals on the western and eastern sides of the Greater Chao Phraya Irrigation Project. According to the simulation, the average relative error between the calculated and observed daily discharge was 21% at the Nakhon Sawan station, which is located below the confluences of the four main tributaries of the Chao Phraya River. The modified model will enable continuous calculations for mitigating the impacts of extreme events, such as the 2011 flood in Thailand, managing the use of irrigation water, and proposing adaptive countermeasures to climate change. Copyright © 2016 John Wiley & Sons, Ltd.
The aim of this study was to detect changes in heavy rainfall patterns and to evaluate the impact of these changes on drainage systems in low-lying areas. Our study area was in the Kaga three-lagoon basin in Ishikawa Prefecture, Japan. First, heavy rainfall events were extracted from observed rainfall data from 1940 through 2008. Comparison of the hourly data showed that the average maximum 6-, 12-, 24-, and 48-h rainfall increased slightly over the 69 years and that the distribution of the maximum amount of 6-h rainfall also changed. Next, rainfall data predicted by MIROC were examined. The results implied that the amount of rain in 3-day rainfall events with a 10-yr return period would reach a peak of 1.23 times the present amount in the near future. At the same time, a drainage analysis model was developed consisting of kinematic and diffusive tank models. The rainfall predicted by MIROC was used in the models to assess the impact of climate change on drainage systems. As a result, the duration of paddy inundation to a depth of more than 30 cm was estimated to increase in the future. Countermeasures, such as the improvement of pumping capacity, are suggested. Copyright © 2013 John Wiley & Sons, Ltd. Une comparaison des données horaires montre que la moyenne maximale des précipitations sur 6, 12, 24 et 48 h a légèrement augmenté durant les 69 années de données et que la répartition des volumes maximaux des précipitations sur 6 h a également évolué. Ensuite, nous avons étudié les données de précipitations prédites par MIROC. Les résultats indiquent que, dans l'avenir, le volume des précipitations des cas de 3 jours de pluie dans une période de 10 années en arrière va atteindre 1.23 le volume actuel. Simultanément, un modèle d'analyse de drainage consistant en une maquette de réservoir cinématique et diffusive a été développée. Nous avons utilisé ces modèles ainsi que les données de précipitations prédites par MIROC pour évaluer l'effet du changement climatique sur le système de drainage. Nous avons conclu que la durée des inondations de 30 cm des rizières de plus allait augmenter dans l'avenir. Des contre-mesures telles que l'amélioration de la capacité de pompage sont suggérées.
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