Hiroaki Somura

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.

Relationship between farming activities and water quality variations of drainage water from paddy fields area

We paid attention to relationship between farming activities and changes in drainage water quality from a paddy fields area, especially in land preparation period for transplanting of paddy. Drainage water quality during land preparation period became highest concentration of all items through a year. In 2006, average water qualities in the period were 7.9 mg/L in T-N, 1.6 mg/L in T-P, 15.3 mg/L in TOC, and 372 mg/L in SS. In addition, maximum values in the period were 36.2 mg/L in T-N, 12.3 mg/L in T-P, 83.2 mg/L in TOC, and 3020 mg/L in SS. The changes in water quality moved on with land preparation by tractor and drainage gate operation on each paddy field by farmers. Moreover, relatively high concentrations of drainage water were also observed during non-irrigation period. As no agricultural activities were done during the period, it is considered that residual nutrient in soil will infiltrate by rain water and flow to the drainage channel.

Spatial Distribution of Water Quality and Load Units in the Iinashi River Basin, Shimane Prefecture, Japan

The spatial distributions of water quality and load units were determined for each small sub basin of the Iinashi River basin as a first step in water environment management. In addition, a GIS database was established to comprehensively resolve the issue of water quality degradation. It was found the concentrations of water quality parameters in the Iinashi River basin were relatively low at almost all sampling points. The average concentrations of each parameter are 1.86 mg·L -1 of chemical oxygen demand, 0.73 mg·L -1 of dissolved organic carbon, 18.5 % of biodegradation ability, 6.58 mg·L -1 of silica, 519.8 μgN·L -1 of total nitrogen, and 27.4 μgP·L -1 of total phospho- rus. In addition, the load units discharged from the small watersheds were obtained. The load units varied greatly regardless of similar land use in the basin. Moreover, discharged loads from observation point 1-31 in the Yamasa River basin were higher than those from observation point 1-5 in the upstream of the Iinashi River basin regardless of high percentage of forest area (91.51%), low percentage of agricultural land (2.3%) and low population density (24 people·km -2 ). Finally, the discharged loads from observation point 1-3 were de- termined as 53.2 g·ha -1 ·day -1 of suspended solid, 45.4 g·ha -1 ·day -1 of chemical oxygen demand, 20.9 g·ha -1 ·day -1 of dissolved organic car- bon, 8.1 g·ha -1 ·day -1 of particulate organic carbon, 16.3 g·ha -1 ·day -1 of total nitrogen, 5.0 g·ha -1 ·day -1 of dissolved organic nitrogen, 1.3 g·ha -1 ·day -1 of particulate organic nitrogen, and 0.3 g·ha -1 ·day -1 of total phosphorus.