The objective of this study is to develop a model which can analyze both runoff and flow movement in a large basin that includes mountainous, hilly, and low-flat tidal areas. In this paper, we sought some possibilities of developing such an integrated model. First, the tank model by Sugawara and the unsteady flow model based on the Preissmann scheme are outlined as typical runoff and unsteady flow models. Their structures and features are examined, and the tank model is shown to be imitated by the unsteady flow model. In the unsteady flow model, a tank is represented by a wide, shallow channel, and a hole is represented by a set of gate and weir. A hygrograph calculated by the imitated tank model is presented to show that it is essentially identical to that calculated by the tank model. Second, another model for a low and flat paddy field is also presented. In this model, surface and sub-surface of a paddy field are represented by some hydraulic structures, and some imaginary interior boundaries are also proposed, which are essential to describe water movement in an aquifer. As a conclusion, we confirm the feasibility that the unsteady flow model could develop into an integrated model.

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Fostering river flow and ground water by temporarily retention water to lighten the peak discharge of flood flow and slowing the effluent stream detention of water is called the water conservation function. The water conservation function of forests is fully understood, and it plays a role in efforts to conserve our forests. But awareness of the water conservation function of agricultural land is far lower than that of the same function served by forests. For this reason, it is necessary to preserve agricultural land and irrigation and drainage facilities with the assistance of people who are not farmers by scientifically evaluating the water conservation function of agricultural land and facilities based on data and to achieve a social consensus on the nature of this function. This report is an explanation of the water conservation function of agricultural land and of the irrigation and drainage facilities that help achieve such a consensus. We have introduced the “water buffering capacity” as a macro indicator of a water conservation function, and explaining water conservation functions based on the results of a water balance survey and of run-off analysis. These results reveal that agricultural land and agricultural facilities have substantial functions that play important role in water conservation.

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The creek network area along the lower reaches of the Chikugo River in Japan is facing serious water pollution problems. A model for water quality analysis is needed to prioritize possible countermeasures. The discharge load from paddy fields and changes in water quality in a creek seem to control the accuracy of the model in a study area which is bounded by the Ariake Sea, the Yabe, Okinohata, and Shiotsuka rivers. This paper focuses on the latter factor.
This paper introduces the investigation methods and results of “specific variation rates of substances” (SVRS) in a creek. Several kinds of investigations and measurements were carried out. The results of previous research were also reviewed for reference. The SVRS in a creek were estimated to be a -1, 000-2, 000 mg·m^-2·d-1 of COD, -200-100 mg·m^-2·d^-1 of T -N and -10-120mg·m^-2·d^-1 of T-P
This study, which arrived at SVRS by combining several methods, demonstrated the difficulty of obtaining reliable results. It is desirable to develop a more appropriate method.

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Recently, water quality of the creek network located along the lower reaches of Japan's Chikugo River has deteriorated remarkably. Conservation of the environment and improvement of water quality are needed to maintain good sanitary conditions and preserve a comfortable living environment as well as to promote sustainable farming.
This paper evaluates some countermeasures for water quality improvement in the study area bounded by the Ariake Sea and the Yabe, Okinohata, and Shiotsuka Rivers. An irrigation period and a non-irrigation period are discussed separately, becaue the type of farming, water use and temperature are different.
The present state of water quality in the main creek can be described by the measured average concentrations of certain substances--namely 6.16 mg·L^-1 of COD, 2.37mg·L^-1 of T-N, and 0.230 mg·L^-1 of T-P during the irrigation period. The tentative goal of water quality improvement is to reach concentrations of less than 6.0 mg·L^-1 of COD, less than 2.0 mg·L^-1 of T-N, and less than 0.2 mg·L^-1 of T-P The effects of countermeasures during theirrigation pedod can be highlighted as follows:
1) Realistic countermeasures at the farm level decreases COD concentration by 0.33 mg·L^-1. But significant improvement for T-N and T-P is difficult to obtain.
2) Wastewater treatment projects which cover the entire study area decreases concentrations of COD, T-N, and T-P by 0.84 mg·L^-1, 0.13 mg·L^-1, and 0.029 mg·L^-1, respectively, when advanced treatment processes are applied. Application of conventional treatment processes, however, increases T-N concentration because of its low removal capacity
3) The installation of 40 direct canal purification facilities whose removal ratios are between 22% and 24% decreases COD, T-N, and T-P concentrations by 0.29 mg·L^-1, 0.10 mg·L^-1, and 0.01 mg·L^-1, respectively.
4) The inflow of newly developed water resources from the Chikugo Barrage at the rate of 1.18 m3·s^-1 decrease concentrations of COD, T-N, and T-P by 0.34mg·L^-1, 0.11 mg·L^-1, and 0.015 mg·L^-1, respectively.
The tendency of the effects in the non-irrigation period was basically similar to that in the irrigation period. The effects of the wastewater treatment by the advanced process and of the direct canal purification were more significant in the non-irrigation period than in the irrigation period.

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The relative error arising in measurement of flow discharge by a level gauge in a gravity (open) canal is studied analytically. It is noted that resulting error is a combination of two types: fundamental (basic) and complementary. The sources of these errors are clarified. On the basis of Manning's uniform flow formula, the governing relations for both types of errors are derived and analyzed. The comparative effect of each error component on discharge measurement accuracy is estimated.

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