ため池周辺の地下水流入・流出量の評価―広島中央の谷ため池タイプを例として―

Recently, irrigation ponds have come to be viewed not only as an agricultural facility for storing water for irrigation but also as a precious regional resource with multiple functions. One function of an irrigation pond is groundwater recharge, in which water infiltrates into the bottom sediments of the pond and recharges the groundwater downstream of the pond, thus contributing to the hydrologic cycle. In order to discuss the effectiveness of the groundwater recharge function of irrigation ponds as a regional resource, groundwater recharge by irrigation ponds in diverse locations must be verified, because very few studies on the groundwater recharge function of irrigation ponds are available.
Toyosaka Town in Hiroshima Prefecture, western Japan, has 177 irrigation ponds. Each pond irrigates an area of at least 5 ha. Most of these ponds are in valleys, impounded behind earthen dams. Ponds located 30-40 m above the Mukunashi River, elevation 370-360 m, account for abut 90% of the irrigation ponds in Houai Town. Many of these are located where the mountain slope adjoins the flood plain. Five ponds were selected to evaluate groundwater recharge to and groundwater discharge from flow-through lakes: Utetobi 1 and Utetobi 2 ponds are located one above the other in the same drainage, with the former upstream of the latter, and Koujin, Rokudou and Okinote ponds are on the 400-m contour line on the footslope of Mount Nishihara (elevation, 733.6 m) where it adjoins the flood plain of the Mukunashi River. Surface water inflow and outflow of these five ponds were measured, and pond water was sampled for geochemical and radon analyses. Water level in the Utetobi 2 pond and the groundwater level in a well downstream of the pond were monitored for one year, from October 2002 to October 2003. Two unknown quantities, groundwater inflow into the five ponds and outflow from the ponds, were determined by solving simultaneous equations, namely, the flow balance equation and the radon mass balance equation. The results are as follows:
1. The monitoring results showed a high correlation (r2 = 0.71) between the continuous records of the Utetobi 2 pond level and the groundwater level, suggesting that water from the pond recharges the groundwater downstream of the pond. This evidence supports the existence of a high degree of hydraulic connection between the pond and the groundwater in the downstream aquifer. Groundwater flowed into the Utetobi 1 pond at the rate of 179 m3/day, and water from Utetobi 1 recharged the Utetobi 2 pond at the rate of 25 m3/day. In addition to the flow from the Utetobi 1 pond, groundwater flows into the Utetobi 2 pond at the rate of 310 m3/day. Water from Utetobi 2 recharged groundwater in the downstream aquifer at the rate of 307 m3/day.
2. Groundwater flows into the Kougami pond at the rate of 135 m3/day, and the water in the pond dose not recharges the groundwater of the downstream aquifer.
3. Groundwater flows into the Rokudou pond at the rate of 219 m3/day, and the water in the pond dose not recharges the groundwater of the downstream aquifer.
4. Groundwater flows into the Okinote pond at the rate of 455 m3/day, and the water in the pond recharges the groundwater of downstream aquifers at the rate of 366 m3/day. This value is concordant with the measured discharge of 346 m3/day from a spring that emerges from an escarpment below the pond.
5. The relationship between groundwater residence time, calculated from the radioactive equilibrium equation of Rn-222, and groundwater discharge suggests that groundwater recharge of the Okinote pond may occur over a wide area of the bottom of pond.
Evalution of groundwater flow around irrigation ponds reveals that the Utetobi and Okinote ponds are referred to as flow-through ponds and the Kougami and Rokudou pond are referred to as discharge ponds. Therefore, the Utetobi and Okinote ponds may have recharge function of groundwater recharge.

言語

書誌情報

ja : 農業工学研究所技報
en : Technical report of the National Research Institute of Agricultural Engineering

巻 204, p. 157-174, 発行日 2006-03-31

出版者

独立行政法人農業工学研究所

言語

出版者

NATIONAL INSTITUTE FOR RURAL ENGINEERING INDEPENDENT ADMINISTRATIVE INSTITUTION

言語

ISSN

収録物識別子タイプ

PISSN

収録物識別子

0915-3314

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2024-05-08 05:08:14.418738

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今泉, 眞之, 石田, 聡, 土原, 健雄, 吉迫, 宏, 2006, ため池周辺の地下水流入・流出量の評価―広島中央の谷ため池タイプを例として―: 独立行政法人農業工学研究所, p. 157–174.

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