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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：TSINGHUA UNIV  

It is well known that paddy irrigation contributes to groundwater recharge. Field researches have been carried out to identify which part of groundwater originates from irrigation water, by comparing the groundwater levels during the irrigation and non-irrigation periods. However, quantitative evaluation of water flux from paddy fields to an aquifer is very difficult in the context of Monsoon Asia, where the mean precipitation is higher during the irrigation periods while the annual precipitation differs year to year. Numerical analysis using a physically based model is a promising approach to clarify the effect of irrigation water on variation in groundwater levels, when comparisons are made between different irrigation regimes under the same climate conditions. This study focuses on the unconfined aquifer of Dogo Plain, Ehime Prefecture, Japan, with a total area of 63 km(2), out of which the paddy fields cover about 26 km(2). To numerically reproduce spatio-temporal distribution of groundwater in the aquifer, a quasi-three-dimensional unconfined groundwater finite element model is employed. Time series data of river water levels and pumping discharges from 47 wells for municipal and industrial purposes are used for setting inner boundary conditions. Two cases, where the paddy fields are irrigated and rainfed, are considered for a two years period from January 1st 1993 through December 31st 1994. These two years include both wet (1993) and dry (1994) extremes. In the irrigated case, infiltration at a rate of 2 mm/day is taken into account to represent groundwater recharge from the paddy fields during the 4 months long irrigation periods. The spatially averaged groundwater level is 120 mm and 176 mm higher during the irrigation periods in 1993 and 1994, respectively, when irrigated. The difference of groundwater levels is the largest in the south-western part of Dogo Plain, attaining to 900 mm in the drought year of 1994. This indicates the significant contribution of irrigation water to a rise in the groundwater table in this part of Dogo Plain, where paddy fields are the dominant land use and the river system is distant.

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Language：English   Publisher：The Japanese Society of Irrigation, Drainage and Rural Engineering  

An inverse modeling to reproduce a variably saturated water flow in non-isothermal soil based on field observation is proposed. Since the water movement in the surface soil is significantly affected by the soil temperature, the governing equations system is composed of the mixed form Richards equation for the water movement and heat conduction equation for the thermal transport. To complete the water flow model of interest, unknown model parameters are determined with inverse technique. The major unknown parameter is the relative hydraulic conductivity (RHC) described as a free-form parameterized function which is a sequential piecewise cubic spline function and therefore can express the flexible functional form of the parameter. The inverse problem is defined as the minimization of errors between the observed and computed pressure heads to determine the coefficient values of the free-form function, and solved through a simulation-optimization method. To validate the water flow model developed, its practical application to <i>in-situ</i> soil is implemented. The results show that the functional form of RHC is successfully identified, and that both water movement and thermal transport models can produce the forward solutions which are good agreement with observed data for desorption period.

DOI： 10.11408/jsidre.80.471

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Language：English   Publisher：日本水産工学会  

Hydraulic structure in vertical slot fishways with 12 designs is numerically investigated and the effect of baffles of the fishways on the hydraulic structure is discussed. The numerical model employed is based on the finite volume method of the shallow water flow equation discretized on quadtree grids with a Godunov-type scheme. The vertical slot fishway models consist of 9 pools with width and length of 2.0m and 2.2m, respectively. The width of the slot at each pool is 0.25m and the bottom gradient of the fishway is 1/20. Each pool is framed by at most three types of baffles: main, short and guide baffles. Based on the numerical investigations, it is concluded that the short baffles decrease the main stream velocity by enhancing meandering of main stream and the guide baffles suppress excessive meandering to create calm recirculation areas in the pools. Hence, both kinds of baffles are indispensable for vertical slot fishways.

DOI： 10.18903/fisheng.49.2_85

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Language：English   Publisher：Japan Rainwater Catchment Systems Association  

Water quality characterization is important, and often, the first step in watershed water quality management. The characterization activity centers on the quantification of water quality parameters relevant to the monitoring objective and the analyses of the impact of the parameters on the state of water body. This study characterizes the general water quality of a network of agricultural rivers in Southern Ehime, Japan and conducted Factor Analysis (FA) and Cluster Analysis (CA) to determine the causal factors and spatial variation of pollution. The Water Quality Index (WQI), based on nine water quality parameters, was computed and optimized-determining the parameters that could best predict the overall water quality. Results show that the river sampling sites have good overall water quality and that physico-chemical parameters pH, DO, and BOD_5 best correlated and predicted WQI. Based on the FA, the primary factors that influence pollution in the sites are the solids or sediment transport (turbidity and sediment), inorganic pollution (NO_3-N, PO_4-P) and trophicity (total N, total P, fecal bacteria). CA, on the other hand, somehow provides information on the effect of the tributaries to the receiving rivers.

DOI： 10.7132/jrcsa.KJ00008229263

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Language：English   Publisher：Japan Rainwater Catchment Systems Association  

This paper investigates applicability of the method determining blanket design length based on the analytical solution proposed by Yoshitake et al. (2011) to smaller embankment dams, and demonstrates its validity to the application of half-size of the dams employed in the previous study through the comparison with results computed by BEM. Then, the reason why the resultant design blanket length of smaller dams is always larger than that of larger ones is investigated by introducing the idea of effective seepage path length. This investigation reveals that the effective seepage path lengths of the half-sized dams are almost the same as those of the original dams, keeping the high accuracy of analysis compared to the BEM solution.

DOI： 10.7132/jrcsa.KJ00008229264

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Language：English   Publisher：The Japanese Society of Irrigation, Drainage and Rural Engineering  

This paper presents a model using the Dupuit approximation applicable to the case in which a small homogeneous earth dam constructed on inclined foundation in order to estimate the phreatic surface location and the seepage discharge through the dam. Analytical results obtained from the model are validated by comparison with the results obtained from a BEM model of the Laplacian field. Main results are as follows: i) The phreatic surface location of the analytical result of the model tends to be higher than that computed by the BEM, ii) The height of the seepage-out point obtained from the analytical result is almost identical with that by the BEM, and iii) The seepage discharge obtained from the analytical result is larger than that by the BEM; their ratio is between 1.113 and 1.243. From all results of the test cases, it is shown that these analytical results are useful for analyzing seepage through a homogeneous earth dam on inclined foundation.

DOI： 10.11408/jsidre.80.261

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Language：Japanese   Publisher：Japan Society of Civil Engineers  

Annual SS (Suspended Solids) load from the Hiromi river to the Shimanto river was estimated based on the 33-months continuous daily observation data. The water quality of the Shimanto river, which is often called "the last clear stream of Japan", is observably deteriorated especially during rice transplanting period. Suspended load prediction models were established and the effect,of rice transplanting activities to the rivers' suspended load was also estimated. Results showed that the amount of annual SS load to the Shimanto river from the Hiromi river was 9.8x10<sup>6</sup> kg and if drainage from paddy fields during rice transplanting is completely stopped, the amount could be reduced by 8.0-12x10<sup>4</sup> kg SS.

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Language：English   Publisher：Japan Society on Water Environment  

Suspended sediment load in relatively smaller rivers draining agricultural areas was monitored and the suspended sediment rating curve was established using two regression analysis approaches—applying data stratification to ameliorate the prediction model equations. The sediment load data were observed for forty-five months in three rivers in an agricultural area in southern Ehime Prefecture, Japan. The data were analyzed using the power function and detransformed logarithmic function regression methods, while testing and elucidating the appropriateness and efficiency of these regression methods. Results showed that data stratification significantly improved the discharge-sediment load correlation and reduced curve-fitting errors, thereby, improving the efficiency of the derived model equation. Moreover, data stratification was found necessary in the analysis to account for nil sediment concentration observed during low flow periods. Between the two regression analysis methods, power function regression appears to have better predictive capability and, thus, more appropriate to smaller rivers. Specifically, as compared to the detransformed logarithmic function regression, power function yields models with significantly higher correlation and efficiency coefficients, as well as predicted sediment load closer to the observed sediment load.

DOI： 10.2965/jwet.2012.387

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IWA PUBLISHING  

Suspended sediment, which is an important water quality characteristic concerning effluents from agricultural areas, was studied in relatively small rivers that drain agricultural watersheds with considerable rice paddy areas. Suspended sediment load (SL) was observed daily for thirty three months and analysed - applying data stratification. Suspended sediment prediction models were established and the effect of rice transplanting activities on the rivers' SL was estimated. Results showed that data stratification improved the discharge-SL correlation and reduced regression and curve-fitting errors, thereby improving the efficiency of the derived model equations. Clustering the months into the rice- and non-rice transplanting seasons also improved the resulting regression equations, although not statistically significantly. Suspended SL was found to be higher during the rice transplanting season and the activities contributed a considerable amount of suspended sediment during the period, supporting the conjecture that sediments come from sources other than natural soil erosion.

DOI： 10.2166/wst.2012.256

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DOI： 10.11408/jsidre.79.417

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DOI： 10.11408/jsidre.79.411

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Language：English   Publisher：Japan Rainwater Catchment Systems Association (JRCSA)  

An inverse method for identifying the unsaturated hydraulic conductivity in variably saturated seepage flow model is proposed. As the basic equation which governs the forward problem, the mixed-form Richards equation is considered which is conservative in mass balance describing the flow in terms of pressure head and moisture content. For functional representation of the relative hydraulic conductivity (RHC) which is a major unknown parameter to be identified, the free-form parameterized function (or sequential piecewise cubic spline function) which provides high flexibility in identifying the functional form of the parameter is employed. To determine values of the coefficients of the function in the manner that errors between the observed and computed pressure heads are minimized, a simulation-optimization algorithm with the aid of the Levenberg-Marqurdt method is constructed. The method proposed is applied to in-situ soil column in an upland crop field, and its validity is examined in terms of reproducibiliry of desorption process in the soil. The results show that the simulation-optimization algorithm is successful in finding RHC of functional form, and that the forward solution model with the RHC parameter so identified could well reproduce actual desorption process when air-temperature dependency of the observed pressure head variation is appropriately eliminated.

DOI： 10.7132/jrcsa.17.1_11

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

A distributed hydro-environmental model is developed that achieves detailed analysis of the movement of water at a field-plot-scale resolution in a mesoscale watershed including lowland areas where, especially for agricultures, it is an essential need to get rid of redundant groundwater by drainage facilities such as rivers, canals and/or underdrains. For this, the problem geometry is meshed with unstructured cells of triangular shape. Profile of a column cell is zoned into two: surface zone and groundwater zone in which water movement is represented by combined tank and soil moisture sub-models, and well-defined two-dimensional unconfined shallow groundwater flow sub-model, respectively. The top-two sub-models serve to evaluate evapotranspiration, infiltration, soil water content, lateral surface water flow, and vertical percolation. The vertical percolation so evaluated is given as longitudinal recharge to the bottom sub-model for computing groundwater flow. Surface water-groundwater interactions through beds and stream-banks of perennial and ephemeral canals are considered by treating the canal courses as internal boundaries in the groundwater flow model. The finite volume method (FVM) that allows of unstructured mesh and produces conservative solutions is employed for groundwater flow computation. The model developed is applied to an actual watershed which includes a low-lying paddy area to quantify the hydrological impact of land-use management practices over a period of 29 years in which the farmland consolidation project was implemented and part of the paddy fields were converted to upland crop fields and housing lands. From the results obtained, it is concluded that the model presently developed lends itself to water-as well as land-use management practices.

DOI： 10.1007/s10333-010-0196-4

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Language：English   Publisher：Japan Rainwater Catchment Systems Association  

Soils of rainfed rice fields have unique characteristics. Inland valleys are extensively found in West African savannas, and their hydromorphic valley bottoms are intensively used for rainfed rice-culture during rainy seasons when become wetlands. Data of rainfall and soil moisture are perennially collected in a study site located in Northern Region of Ghana. The soil of the site is classified as Dystric Planosols. The soil moisture retention curve at a depth of the soil is identified from observed volumetric water content and matric head. Since the soil is variably saturated, it is important to clarify abruptly changing hydraulic properties of the soil near saturation. A bimodal model including eight empirical parameters turns out to be suitable for this purpose. The vertically one-dimensional Richards equation is considered to reproduce infiltration process of rainwater into the soil. The saturated water content, one of the empirical parameters, and saturated hydraulic conductivity are assumed to be functions of the depth of soil. Then, the finite element method is applied for solving the Richards equation to simulate the process. The simulation results are in good accordance with actually observed data. The physically-based model serves as a tool for comprehending formulation process of the wetlands at early stages of rainy seasons.

DOI： 10.7132/jrcsa.KJ00006069058

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Language：English   Publisher：農業農村工学会  

DOI： 10.11408/jsidre.77.623

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

A fuzzy optimization model is developed to allocate allowable total nitrogen (T-N) loads to distributed nonpoint sources (NPSs) and point sources (PSs) in a watershed for river water quality management using the linear programing technique. The watershed is divided into uniform grid cells on which T-N loads issuing from NPSs such as paddy fields, upland crop fields and cities are controlled. A geographic information system integrated with the digital elevation model facilitates computation of route lengths of surface and subsurface flows from cells to a river running through the watershed. The T-N loads discharged from their sources are assumed to decay, subject to distance-related first-order kinetics. As management goals, maximizations of total allowable NPS loads, total allowable PS loads and total yield of rice are considered from environmental and economic viewpoints. A prime constraint is an effluent limitation standard for the aggregate amount of loads that arrive at the downstream end of the river. The fuzzy sets theory helps appropriately describe vague attitudes of decision-makers (i.e., stake-holders and management authorities) in terms of constraints and conflicting goals. An application of the fuzzy optimization model, developed as an improvement over our last nonfuzzy model, to a real watershed in Shiga prefecture, Japan, demonstrates that the fuzzy model embodies our last model, and is capable of creating management alternatives for T-N load allocation in a more practical and flexible manner.

DOI： 10.1007/s10333-009-0160-3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

A cell-based distributed watershed model is developed which enables us to simulate the hydrological and hydraulic aspects of the watershed in a refined fashion. With three-zoned cell profiling, the model is composed of three sub-models; tank model for a surface water zone, soil moisture model for a surface soil zone, and unconfined shallow groundwater flow model for a subsurface zone. Inclusion of the soil moisture sub-model modified to reroute the infiltration, routed from the tank sub-model, into the return flow and the groundwater recharge features the model. The groundwater flow sub-model, numerically approximated by use of the finite volume method and the implicit time-marching scheme, considers a network of on-farm drainage canals as internal boundaries, which is an essential need for modeling the watershed including farmlands. Cascade-linking of the three sub-models in a cell and assembling of all the cells over the entire watershed domain provides the global equations system to be solved. Applicability of the model is demonstrated with its practical application to a real watershed in that paddy and upland crop fields take great part of the land-use practice. It is then indicated in a quantified manner that rice farming significantly contribute as a major groundwater recharger in an irrigation period to fostering and conservation of regional water resources. Along with appropriately profiling a cell, the model is so versatile and tough that it can be applied without difficulty to a watershed of diverse terrains and land-uses and the computations can stably be carried out. It is thus concluded that the model presently developed could be a powerful "watershed simulator'' to investigate and assess the time-varying hydro-environmental properties of a watershed while separating and integrating the hydrological and hydraulic components of particular importance.

DOI： 10.1007/s10333-008-0142-x

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Language：English   Publisher：Japan Rainwater Catchment Systems Association  

An alternative approach to the inverse problems encountered prior to the computations of saturated-unsaturated groundwater flow is addressed. The soil hydraulic properties (i.e., the relative hydraulic conductivity K_r and the volumetric water content θ) which are the a priori unknown parameters in the Richards equation governing the flow are interpolated by an assembly of the piecewise cubic spline functions expressed in terms of the pressure head. This free-form parameterization approach provides high flexibility and degrees of freedom in identifying the functional form of the parameters, compared with the conventional approaches in that the functions are of specific shape as in the van Genuchten model of a fixed functional form. In addition, the approach is field-oriented in a point that the solution of the inverse problem can be achieved having only recourse to the observed time-series data of pressure head which are relatively easily in situ available. To find the shapes of such spline functions best expressing head-dependency of the soil hydraulic properties, a simulation-optimization algorithm with the aid of the Levenberg-Marquardt method is developed which serves to iteratively solve an optimization problem of minimizing errors between the observed (or measured) and computed values of the pressure head, in combination with the embedded simulation module for solving forward problems. Validity of the approach is examined with its application to different two soil types which are characterized by less and more sensitive changes of the relative hydraulic conductivity near saturation, respectively. The results show that the approach developed could be a viable alternative to the conventional fixed functional form approaches.

DOI： 10.7132/jrcsa.KJ00004871197

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Language：English   Publisher：Japan Rainwater Catchment Systems Association  

In Japan, more and more paddy fields, left fallow mainly by nationwide rice production control policy, have recently been changed to upland crop fields or artificial wetlands. Since such change or diversification of land use probably has an impact on the water quality environment of the whole neighborhood, it must strategically be implemented in the context of land use arrangement for environmentally sound watershed management. Of great importance is then to make effective use of self-purification functions fulfilled by paddy fields or wetlands. The present study is associated with development of a simulation-optimization model for designing an optimal land use on a watershed scale. The model is composed of a hydro-environmental subsurface watershed simulation model, represented by coupled 3-D Richards and advection-dispersion equations, and a GA (Genetic Algorithm)-aided optimization part. An optimization problem is formulated which, with the most use of self-purification, minimizes the objective function expressed in terms of a weighted total of discharged nutrient from a watershed and residual nutrient within the watershed, taking the location of paddy fields in fallow, changed to upland crop fields or wetlands, as a decision variable. An example problem for a small watershed of an irrigation tank is solved that requires deciding the optimal locations of paddy fields that can be changed to other land use categories. The results show that the model developed could be a powerful tool for making tactics to combat the environmental impact resulting from land use diversification.

DOI： 10.7132/jrcsa.KJ00004871198

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