Simulation of rainfall runoff and pollutant load for Chikugo River basin in Japan using a GIS-based distributed parameter model

In watershed management, the determination of peak and total runoff due to rainfall and prediction of pollutant load are very important. Measurement of rainfall runoff and pollutant load is always the best approach but is not always possible at the desired time and location. In practice, diffuse pollution has a complex natural dependence on various land-use activities such as agriculture, livestock breeding, and forestry. Estimation of pollutant load is therefore essential for watershed management and water pollution control. In this study, a model of rainfall runoff and pollutant load, which uses a geographical information system (GIS) database, is a convenient and powerful tool for resolving the abovementioned complexities. This technology was applied in order to simulate the runoff discharge and the pollutant load of total nitrogen (TN) and total phosphorus (TP) in the Chikugo River basin of Kyushu Island, Japan. First, a hydrologic modeling system (HEC-HMS) and GIS software extension tool were used for simulations of elevation, drainage line definition, watershed delineation, drainage feature characterization, and geometric network generation. The spatial distributions of land cover, soil classes, rainfall, and evaporation were then analyzed in order to simulate the daily runoff discharge at the Chikugo Barrage from April 2005 to December 2007. An important point in this approach is that a new development for data input processing with HEC-HMS was introduced for optimizing parameters of the model. Next, the water quality indicators TN and TP were examined, and an efficient approach was investigated for estimating monthly pollutant loads directly from unit load and ground-observed hydrological data. Both nonpoint and point sources of pollutants were considered, including different land-cover categories, sewers, factories, and livestock farms. The observed and simulated results for the runoff discharges and pollutant loads were in good agreement and totally consistent, indicating that the proposed model is applicable to simulation of rainfall runoff and pollutant load in the Chikugo River basin. Further, this model will be able to provide managers with a useful tool for optimizing the water surface management of this river basin.     

A comparative study on grid-based storm runoff prediction using Thiessen and spatially distributed rainfall

The simulated streamflow from Thiessen average rainfall (T) and spatially distributed rainfall (R) may be significantly different from each other. To identify the hydrologic effects quantitatively, the grid-based kinematic wave storm runoff model was adopted. The model predicts temporal and spatial variations of surface and subsurface flow at each cell by calculating the water balance, and routes the streamflow to the outlet. The model was tested at the Yeoncheondam watershed (1,875 km²), one third of which belongs to North Korea. The watershed is elongated to north and south directions crossing the border. Four rain gauges cover the watershed within the territory of South Korea, while no records from North Korea are given. The simulated results showed the large differences in runoff volume and peak flow rates between T and R when rain moves in a north to south direction. The simulated results of east-to-west-direction storms showed little difference in the hydrographs. The hydrograph was strongly affected by the spatial variations of the rainfall moving along the stream of the watershed.     

Factors impacting yields in rain-fed paddies of the lower Mekong River Basin

The Mekong River Basin (MRB) is the biggest basin in Monsoon Asia. About 80% of the agricultural lands, which occupy about 40% of the basin are rain-fed paddy rice area. Therefore, it is assumed that changes in rain-fed paddy rice production affect the total agricultural production to a great degree in the Mekong River Basin. While there are many factors affecting the productivity of rain-fed paddies, such as climate, water use, rice varieties, applications of manure, fertilizer and agro-chemicals, sowing date and other agronomic practices, this paper focuses on the relation between rainfall and yields of rain-fed paddies. Agricultural statistics and rainfall data were collected and analyzed for all 24 provinces in Cambodia for the years 2001 and 2002. Factors such as soil fertility and other natural conditions were removed by comparing the yield and rainfall in one province for different years. Special attention was given to the relation between yields of paddy in the wet season and rainfall, considering factors such as rice varieties, soil fertility, irrigation ratio and the ratio of area damaged by flood, drought, and insect. Although it is not easy to assess those impact factors on yields because they are organically interactive, the following results were obtained: (1) The ratio of high yielding varieties (HYV), soil fertility, and irrigation ratio among many factors that affect yields individually, especially if they are combined, (2) Total rainfall did not have a significant influence on rice yields even for the rain-fed paddies if it was over 700 mm in wet season, and one of the reasons for this would be that there exist supplementary water uses through small ponds and water ponding in local land depressions in and around paddies.     

Combining rainfall–runoff and hydrodynamic models for simulating flow under the impact of climate change to the lower Sai Gon-Dong Nai River basin

This study presented simulation results of runoff discharge combining the Mike NAM and Mike SHE models for lower Sai Gon-Dong Nai River basin (SG-DNRB). Sai Gon-Dong Nai River basin is Vietnam’s largest population center and main industrial area in Ho Chi Minh City. In recent years, the area faces flooding problems because of heavy rainfalls and high tides, as part of the impacts of climate change and sea level rise. The lower of SG-DNRB was selected as a case study to highlight the necessity to investigate combination of the rainfall–runoff model and the hydrodynamic model for long-term strategies to resource water in the future. The models were calibrated using water level and runoff discharge data during low and flood seasons in 2014. The calibrated results showed satisfactory coefficients (NASH index, R² up to 0.70 and RMSE is smaller than 0.20). The results confirmed that the combination of Mike NAM and Mike SHE models is well suitable to simulate runoff discharge in the lower SG-DNRB. Combination of the models can be considered a useful tool to help professional agency operator water resources management projects in other areas in the context of climate change and sea level rise.     

Evaluation of HEC-HMS and WEPP for simulating watershed runoff using remote sensing and geographical information system

Although a variety of rainfall-runoff models are available, selection of a suitable rainfall-runoff model for a given watershed is essential to ensure efficient planning and management of watersheds. Such studies are relatively limited in developing nations, including India. In this study, rainfall-runoff modeling was carried out using HEC-HMS and WEPP hydrologic models, and remote sensing and GIS (geographical information system) techniques in the Upper Baitarani River basin of Eastern India using daily monsoon season (June–October) rainfall and the corresponding streamflow data of 6 years (1999–2005). Other input data such as soil map, land use/land cover map, and slope map were prepared using remote sensing and GIS techniques. The modeling results revealed that both the models under predict streamflow for 1999, 2002, 2004, and 2005 and over predict for 2001 and 2003, whereas HEC-HMS under predicts and WEPP over predicts streamflow for the year 2000. The percent deviation of total runoff volume simulated by HEC-HMS ranges between −2.55 and 31%, while it varies from −13.96 to 13.05% for the WEPP model which suggests that the WEPP model simulates annual flow volumes more accurately than the HEC-HMS model for most years. However, the lower values of root mean square error (RMSE) and RMSE-observation standard deviation ratio coupled with the higher values of Nash–Sutcliffe efficiency, percent deviation and coefficient of determination for HEC-HMS during calibration and validation periods indicated that the streamflow simulated by HEC-HMS is more reliable than that simulated by WEPP. Overall, it is concluded that the HEC-HMS model is superior to the WEPP model for simulating daily streamflow in the Baitarani River basin of Eastern India.     

Water quality monitoring and multivariate statistical analysis for rural streams in South Korea

South Korea is located in the Asian monsoon region, and paddy rice farming is one of the important agricultural activities, which may contribute to the non-point source pollution of inland water bodies along with rainfall runoff. The status of water quality in rural streams located throughout South Korea was examined in this study by water quality monitoring and statistical analysis. Totally six surveys were conducted in 2003 and 2005 to monitor 300 streams located in rural subwatersheds; these streams are affected by agricultural activities and water supply for agricultural practices. The monitoring was performed at the terminal point of each subwatershed. In each study year, the streams were monitored in the three hydrological periods (April, July, and October) to observe differences in the impacts of agricultural activity and rainfall pattern. During the surveys, 15 water quality parameters were measured and interpreted using multivariate statistical methods including factor analysis and cluster analysis. Results show that the water quality of the rural streams monitored in this study appeared to meet the Korean water quality criteria for agricultural use, which are 8.0 and 100 mg/L for biochemical oxygen demand and suspended solids, respectively. In terms of organic contamination and suspended solids, the best stream water quality was observed in October compared to other periods. This can be attributed to the fact that October follows the rice-harvesting period and has low rainfall; thus the streams are probably less affected by agricultural activities and surface runoff. The three hydrological periods did not show much variation in the nitrogen and phosphorus parameters related to stream water nutrient conditions. Factor analysis indicates that the first five factors for April explained about 67% of the total sample variance. In July, the first four factors explained about 60% of the total variance, while the first four factors for October explained about 65%. Cluster analysis reveals that the streams could be divided into four groups in April and October and five groups in July. The box-and-whisker plots of the physicochemical variables indicate that Group A had the best water quality among the groups. This study demonstrates that the rural stream water quality of South Korea in the Asian monsoon region can be greatly affected by agricultural activities such as paddy rice farming and rainfall patterns.     

Nitrogen and phosphorus runoff modeling in a flat low-lying paddy cultivated area

Chiyoda basin is located in Saga Prefecture in Kyushu Island, Japan, and lies next to the tidal compartment of the Chikugo River to which the excess water in the basin is drained away. Chiyoda basin has a total area of about 1,100 ha and is a typical flat and low-lying paddy-cultivated area. The main environmental issue in this basin is total nitrogen (TN) and total phosphorus (TP) load management because TN and TP, which loaded from farmlands, degrade surface water as a result of anthropogenic eutrophication. This paper presents a mathematical model of TN and TP runoff during an irrigation period in Chiyoda basin in order to elucidate the pollutant fluxes that accompany water transportation in paddy fields and drainage canals, and to evaluate pollutant removal from the study area to the Chikugo River. First, the water flow and the algorithm of gate operation were simulated by a continuous tank model and the accuracy of the model was then evaluated by comparing the simulated water levels with observed ones during an irrigation period. The observed and simulated water levels were in good agreement, indicating that the proposed model is applicable for drainage and water supply analyses in flat, low-lying paddy-cultivated areas. Second, the TN and TP runoff during an irrigation period was simulated based on the TN and TP loads that were determined by observed data in paddy fields. For TN runoff, the simulated results and observed data were in good agreement whereas for TP runoff, the simulated results were higher than the observed data. However, if the settled TP within the paddy tank was calculated as 6%, then the simulated results and the observed data were in good agreement. We concluded that TN runoff from paddy field to the drainage canal system was not affected much by the sediment related process. The present study could provide farmers and managers with a useful tool for controlling the water distribution in an irrigation period, and the TN and TP loads in the downstream area as well as the Chikugo River.     

Terraced paddy field rainfall-runoff mechanism and simulation using a revised tank model

Terraced paddy fields play important roles in water and soil conservation because their water storage effect reduces and delays flood peaks. This study applies the terraced paddy field rainfall-runoff mechanism to the tank model. Though the traditional four-section tank model can easily simulate rainfall-runoff in a terraced paddy field, it has many parameters that are difficult to calibrate. To address the shortcomings of the traditional four-section tank model, this study develops a revised tank model to simulate rainfall-runoff. This study selects a terraced paddy field located in Hsuing-Pu village in Hsiuing-Chu County as the experimental field. The field under investigation was equipped with automatic monitoring stations, water-stage, and rain gauges. These stations collected data on rainfall and water flow to simulate the rainfall-runoff model in that region. To simulate the runoff behavior of the experimental terraced paddy field, two rainfall events were selected from the gathered data and five normal evaluation indexes based on static and hydrological theory were applied to calculate the results of simulation simultaneously. The revised tank model performed better than expected, and precisely predicted the variations and trends in flow charge. Comparison with representation indexes proved that the revised tank model is an appropriate and valuable tool for rainfall-runoff simulation.     

Comparison of efficiency between differential evolution and evolution strategy: application of the LST model to the Be River catchment in Vietnam

Parameter calibration is an important step in the development of rainfall–runoff models. Recently, there has been a significant focus on automatic calibration. In this paper, two evolutionary optimization algorithms were applied to calibration of the long- and short-term runoff model (LST model) to simulate the daily rainfall–runoff process in the Be River catchment located in southern Vietnam. The differential evolution (DE) and evolution strategy (ES) algorithms were employed to optimize three objective functions: the Nash–Sutcliffe efficiency coefficient, root mean square error, and mean absolute error, which are indices for evaluating the simulation accuracy of the LST model. Hydrometeorological data for the periods 1985–1989 and 1990–1991 were used for calibration and validation, respectively. The LST model was calibrated for each objective function using five different parent and offspring population conditions. The results show that both the DE and ES algorithms are efficient methods for automatic calibration of the LST model. After 1000 generations, the best values of the fitness indices found by the DE technique were slightly better and more stable than those found by the ES technique in both calibration and validation. The average computation time for each generation using the DE algorithm was approximately two-thirds as long as that using the ES algorithm.     

Runoff simulation considering the effects of hydraulic structures on agricultural water uses

For the efficient management of water resources in the target basin, this study proposed a method to improve the reliability of a long-term hydrological simulation model by applying to the model agricultural water more approximate to actual water uses (than planned water demands) through their adjustment based on the effects of small-scale hydraulic structures. To verify agricultural water uses estimated using the proposed method, they were applied to a basin management model. And then, simulated runoff at main station points was compared with measured runoff. As a result, there occurred errors with large differences from measured data, mainly, at station points where their dependency on river water was high. To verify simulated return rate, return rate for a test zone was estimated, and then compared with the simulated return rate. Correlations between annual rainfall and runoff errors were analyzed. As a result, it was found that those errors were enlarged in dry years. Long-term runoff simulation analysis showed that simulated runoff came to be negative when a farming season began. This could be significantly improved using water uses adjusted to consider the effects of small-scale hydraulic structures. Also, correlation analysis quantitatively confirmed that simulated runoff after adjustment was more correlated with measured runoff than before adjustment. Finally, fitness tests for runoff simulations before and after adjustment were carried out through a residual analysis to analyze residual normality and independence. As a result, the fitness of runoff simulation after adjustment was significantly improved.     

Estimation of supplementary water to paddy fields in the lower Mekong River basin during the dry season

Efficient management of water resources in paddy fields requires an understanding of the volume of supplementary water used. However, quantifying the volume is laborious due to the large amount of data that must be collected and analyzed. The purpose of our study was to estimate the volume of supplementary water used in paddy fields, based on several years of available statistical data, and to provide information on how much water can be supplied to paddy fields in each target area. In this study, the lower Mekong River basin of northeast Thailand, Laos, and Cambodia was selected as the study area. In the first step, we used agricultural statistics for each country, rainfall data acquired from the Mekong River Commission Secretariat (MRCS), and the value of virtual water required per unit of rice production. Because several years of data were used for dry season harvested areas and rice production in each country, the supplementary water to paddy fields in each province was calculated using virtual water and rainfall. This method made it possible to estimate changes in supplementary water in each province. Through this study, the supplementary water to paddy fields during the dry season in three countries was approximated from the minimum number of data sets. Moreover, for cases in which it is not possible to procure agricultural water use data for a hydrological model simulation, an alternative solution is proposed.     

Application of shallow water equations to analyze runoff processes in hilly farmlands

Analysis of runoff processes in farmlands during storm events in particular is an important engineering topic. The shallow water equations describing the physical conservation laws are attractive tools for understanding motion of surface water. A numerical model is developed to reproduce surface water flows during storm events, using the finite element method and the finite volume method applied to the one-dimensional shallow water equations. The numerical model has the advantage of dealing with transcritical flows, wet, and dry processes, irregular channel bed slopes, and channel junctions involving multiply connected networks. Standard numerical tests demonstrated some of these features. Runoff processes from farmlands in a Japanese hilly area is analyzed with the numerical model coupled with another runoff model of black-box type. Parameters of the models are calibrated and validated using observed series of rainfall and runoff discharge data. Simulated results included submergence of hydraulic jumps and flood storage in farming plots, which are very informative in terms of assessing diverse functions of the farmlands.     

A physically based FVM watershed model fully coupling surface and subsurface water flows

A sophisticated modeling approach for simulating-coupled surface and subsurface flows in a watershed is presented. The watershed model developed is a spatially distributed physically based model of composite dimension, consisting of 3-D variably saturated groundwater flow submodel, 2-D overland flow submodel and 1-D river flow submodel. The 3-D subsurface flow is represented by the complete Richards equation, while the 2-D and 1-D surface flows by the diffusive approximations of their complete dynamic equations. For piecewise integration of these equations, the finite volume method (FVM) is employed assuming unknown variables such as the water depth and the pressure head to be volume-averaged state ones. Problem plane geometry is meshed with the unstructured cells of triangular shape which conforms to external as well as internal irregular boundaries such as those between 1-D and 2-D flows. A cell size controlling scheme, referred to as quasi-adaptive meshing scheme, is introduced to keep the local discretization errors caused by topographic elevation gradient even over the entire-meshed geometry. Performance of the model is tested through its practical application to a rugged intermountain watershed. Tuning the values of the three key parameters ensures successful calibration of the model. Once the model is so calibrated, it could reproduce satisfactory runoff response to any rainfall event. Expansion and shrinkage of the contributing area importantly affecting the direct runoff, caused by the vicissitude of rainfall during its total duration, are well reproduced, like what the commonly accepted runoff theory argues. It is thus concluded that the model developed could serve as a powerful watershed simulator usable for investigating and assessing the hydrological aspect of a watershed.     

SWAT modeling of best management practices for Chungju dam watershed in South Korea under future climate change scenarios

Suitable and practicable best management practices (BMPs) need to be developed due to steadily increasing agricultural land development, intensified fertilization practices, and increased soil erosion and pollutant loads from cultivated areas. The soil and water assessment tool model was used to evaluate the present and future proper BMP scenarios for Chungju dam watershed (6,642 km²) of South Korea, which includes rice paddy and upland crop areas. The present (1981–2010) and future (2040s and 2080s) BMPs of streambank stabilization, building recharge structures, conservation tillage, and terrace and contour farming were examined individually in terms of reducing nonpoint source pollution loads by applying MIROC3.2 HiRes A1B and B1 scenarios. Streambank stabilization achieved the highest reductions in sediment and T-N, and slope terracing was a highly effective BMP for sediment and T-P removal in both present and future climate conditions.     

Seasonal fluctuation of groundwater in an evergreen forest, central Cambodia: experiments and two-dimensional numerical analysis

This study of a water cycle was conducted in an evergreen forest located in the Mekong River Basin in central Cambodia. At the observation site, we measured the dynamics of the spatial distribution of groundwater levels. The groundwater movement was analyzed two-dimensionally using boundary conditions and parameters that had been observed in the field. The climate in the research area is dominated by two seasons, which occur annually: a rainy and a dry season. The groundwater levels are generally high during the rainy season and low during the dry season. Groundwater levels were measured along a stream, which flowed through the study site. The streambed was visible at the head of the stream in January. At the next downriver well point, the streambed appeared in March. Finally, it became visible at all well points in April, meaning that surface runoff had disappeared temporarily and instead flowed underground during the ensuing dry period. Groundwater levels of the studied lateral flow perpendicular to the stream that seeped and infiltrated into the stream were 1.2–2.5 m deep (in April), which was the lowest level recorded for the year. During that period, the depth of the groundwater of the studied lateral flow fell by as much as 56 mm per month. In addition, the lateral flow groundwater infiltrated into groundwater of the stream during that period. The groundwater level fluctuation was estimated based on a two-dimensional analysis of lateral flow perpendicular to the stream using a numerical simulation model with soil physical parameters and observed boundary conditions. The observations of ground water fluctuations were well reproduced. Deep seepage of groundwater was estimated using a uniform boundary condition that allowed efflux through the bottom, estimated as being approximately 30 mm per year. The simulated deep seepage rate was considered plausible considering other hydrological components such as soil water storage fluctuation.     

The role of paddy rice in recharging urban groundwater in the Shira River Basin

Agricultural fields in the middle Shira River basin play an important role as a source of groundwater recharge; however, the water balance between the agricultural water and river water is unclear. This study was conducted to investigate the water balance in the fields by measuring the stream flow of agricultural water channels, which draw water from the Shira River. The flow rate of water channels was found to increase in the beginning of May, which corresponded to the cultivation of paddy rice fields. During summer, the total agricultural intake was comparable to the river flow observed in the middle Shira River Basin. Determination of the water budget for the targeted area revealed that most of the recharged water was dependent on agricultural irrigation from the river. The annual recharge of the overall target area was estimated to be as high as 15,300 mm. In addition, the infiltration rate was as high as 170 mm/day in the paddy fields during summer, and as high as 30 mm/day in the upland fields during winter. In order to recover the groundwater recharge in this region, it is necessary to extend the submerged period to include periods in which the stream water in the Shira River is not subject to heavy rainfall as well.     

Reduction of runoff and soil loss over steep slopes by using vetiver hedgerow systems

Vetiver hedgerow system has potential for reducing runoff and soil loss especially on steep slope areas, but the dynamics of these reductions are not fully understood. This research was conducted to determine reduction in runoff and soil loss by vetiver hedgerow system. Vetiver hedgerow systems with three vertical intervals of hedgerow were tested on three land slopes and compared with the case without hedgerow for six simulated rainfall amounts. The vetiver hedgerows reduce runoff volume and soil loss by 31–69 and 62–86 %, respectively compared to the case without vetiver hedgerow. Runoff volume increases with rainfall amount, and hence increases soil loss. Therefore soil loss increases with land slope, runoff volume, rainfall amount, and vertical hedge interval. Two final equations for estimating soil loss are presented in this study. The first equation contains parameters of runoff volume, land slope, and vertical hedge interval, while the second equation contains rainfall amount instead of runoff volume. The correlation coefficients between estimated soil losses and the experimental data in this study and in the literatures were found to be 0.94 and 0.90 for the first and second equations, respectively.     

Modification of SWAT auto-calibration for accurate flow estimation at all flow regimes

To secure accuracy in the Soil and Water Assessment Tool (SWAT) simulation for various hydrology and water quality studies, calibration and validation should be performed. When calibrating and validating the SWAT model with measured data, the Nash–Sutcliffe efficiency (NSE) is widely used, and is also used as a goal function of auto-calibration in the current SWAT model (SWAT ver. 2009). However, the NSE value has been known to be influenced by high values within a given dataset, at the cost of the accuracy in estimated lower flow values. Furthermore, the NSE is unable to consider direct runoff and baseflow separately. In this study, the existing SWAT auto-calibration was modified with direct runoff separation and flow clustering calibration, and current and modified SWAT auto-calibration were applied to the Soyanggang-dam watershed in South Korea. As a result, the NSE values for total streamflow, high flow, and low flow groups in direct runoff, and baseflow estimated through modified SWAT auto-calibration were 0.84, 0.34, 0.09, and 0.90, respectively. The NSE values of current SWAT auto-calibration were 0.83, 0.47, ?0.14, and 0.90, respectively. As shown in this study, the modified SWAT auto-calibration shows better calibration results than current SWAT auto-calibration. With these capabilities, the SWAT-estimated flow matched the measured flow data well for the entire flow regime. The modified SWAT auto-calibration module developed in this study will provide a very efficient tool for the accurate simulation of hydrology, sediment transport, and water quality with no additional input datasets.     

辽河平原区玉米旱田氮磷流失特征研究

采用田间试验方法,2016～2017年原位监测不同试验处理下地表径流与地下淋溶水分和氮磷流失过程.结果表明,径流和淋溶流失均集中发生于6～8月,且流失水量与降雨密切相关.氮磷在径流和淋溶水中均以可溶态为主(径流水中磷除外),且淋溶水氮浓度高于径流,径流水磷浓度高于淋溶.氮流失负荷在两年监测中均表现为淋溶高于径流,磷流失...     

Impacts of baseflow contribution on the streamflow variability of major river systems in Korea

High streamflow variability is a potential risk factor in river management in Korea because the use of water resources in Korea depends primarily on surface water. In this regard, analysis of streamflow variability is critical for efficient water resources management. Because streamflow variability is mainly influenced by the contributions of direct runoff and baseflow, the relationship between baseflow and streamflow is an important hydrological indicator that reflects river characteristics. Accordingly, this study was conducted to estimate the effect of baseflow on streamflow variability. For this purpose, a number of streamflow variability indices (SVIs), such as the Richard–Baker flashiness index, the coefficient of variation, the ratio of high flow to low flow (Q5:Q95), and the coefficient of flow regime, were calculated for Korea’s major river systems to determine which SVI best reflects the characteristics of Korean rivers. In addition, baseflow separation was performed to calculate the relationship between SVIs and the baseflow index. The results of this study show that the baseflow index is inversely proportional to streamflow variability. In particular, the impact of baseflow on streamflow variability was highest in the Yeongsan–Sumjin River system. These results are valuable information expected to be used in river management to better secure water resources.