We investigated flood control capacity of 484 agricultural reservoirs with storage capacity of over 1 million m3 in South Korea. In general, agricultural reservoir secures flood control capacity by setting up limited water level during flood season from late June to mid-September. The flood control capacity of an agricultural reservoir during flood season can be divided into stable flood control capacity during non-flood season, stable flood control capacity associated with limited water level, and unstable flood control capacity associated with limited water level. In general, the flood control capacity significantly (P < 0.001) increased with reservoir capacity irrespective of type of spillway. The unstable flood control capacity accounted for about 20 % of reservoir capacity in the uncontrolled reservoirs. The study reservoirs showed flood control capacity of 0.60-65 billion (B) m3 and stable flood control capacity of 0.43-47 B m3, depending on the upper and lower limited water levels during the flood season. The stable flood control capacity of the controlled reservoirs (0.29-0.33 B m3) was about two times than that of uncontrolled reservoirs (0.14 B m3). Moreover, the reservoirs with over 100 mm ratio of flood control capacity to watershed area accounted for 38 % of total controlled reservoirs.
The flood control volume of 28 agricultural reservoirs with 20-year water storage data were examined for the 10 frequency wet year. The flood control volume significantly (P < 0.001) increased with effective reservoir capacity in the 10 frequency wet year. The flood control volume during flood season in 10 frequency wet year were estimated as 0.13 to 0.28 B m3. The ratios of flood control volume to effective reservoir capacity during flood season in 10 frequency wet year were 15 % for early season, 7 % for mid season, and 11 % for late flood season, suggesting that flood control operation of the reservoir is most important during mid season.
The results indicate that many agricultural reservoirs may contribute to flood attenuation in the small watersheds.