Assessing the Potential of Agricultural Reservoirs as the Source of Environmental Flow

Young-Jun Jo,Minhyuk Jeung,Jung-Hun Song,Seung-Hwan Yoo,Young-Joo Kim,Younggu Her,Giorgio Provolo,Kwang-Sik Yoon,Jina Beom

doi:10.3390/w13040508

Abstract

Excessive nutrient loadings from drainage areas and resulting water quality degradation in rivers are the major environmental issues around the world. The water quality further deteriorates for the large seasonal variation of precipitation and water flow. Environmental decision makers have been exploring affordable and effective ways of securing environmental flow (EF) to improve the water quality, especially in dry seasons, and agricultural reservoirs have attracted the attention of policymakers as an alternative source of EF. This study proposed an analysis framework for assessing the EF supply potential of agricultural reservoirs as alternative sources of EF. A reservoir water balance model was prepared to mathematically represent the reservoir water balance and quantify temporal variations of the amount of water available for the EF supply. The simulation model was designed to explicitly consider inflow from the upstream drainage areas, irrigation water requirement, and hydrological processes happening in the reservoirs. The proposed framework was applied to four agricultural reservoirs located in South Korea to evaluate its efficiency. Results showed that the additional storage capacity added by the dam reinforcement enabled the study reservoirs to satisfy both needs, EF and irrigation water supply. The surplus capacity turned out to be enough to satisfy various EF supply scenarios at the annual time scale. However, the current operation plans do not consider the seasonal variations of reservoir hydrology and thus cannot supply EF without violating the original operational goal, irrigation water, especially in dry months. The results demonstrate that it is necessary to consider the temporal variations of EF when developing reservoir operation rules and plans to secure EF. This study also highlights the unconventional roles of agricultural reservoirs as resources for improved environmental quality. The methods presented in this study are expected to be a useful tool for the assessment of agricultural reservoirs’ EF supply potential.

Highlights

Reservoirs play an important role for water resources management by helping store rainwater received from their drainage areas and regulate downstream streamflow
The amount of streamflow including reservoir inflow and the Youngsan river flow is substantially low in the dry seasons, compared to that of the wet season; it is necessary to account for the seasonal variations of the reservoir hydrology when determining the timing of environmental flow (EF) supply
This paper proposed an analysis framework to evaluate the capacity of an agricultural reservoir as the source of EF and identify effective reservoir operation scenarios that can meet both reservoir operation goals, irrigation water, and EF supplies

Summary

Introduction

Reservoirs play an important role for water resources management by helping store rainwater received from their drainage areas and regulate downstream streamflow. Four large agricultural reservoirs were expected to be the immediate option to help supply EF of the Yeongsan River because of their relatively large storage capacities It is not clear how much water can be taken from the reservoirs to provide EF while meeting their original operational goals of supplying irrigation water to its downstream agricultural areas [1,4]. This study proposed an analysis framework to evaluate the capacities of reservoirs as the source of EF and explore advanced reservoir operation options that can balance the trade-off between irrigation water and environmental flow. We explored efficient ways to release reservoir water to the river to meet both EF and irrigation requirements, considering the temporal variations of watershed hydrology and EF. This study assumed that the reservoir-groundwater interaction is negligible because the reservoir groundwater outflow is known to have little effect on water balance in Korean reservoirs [6,36,37]

Reservoir Inflow

Study Reservoirs

Development of Alternative EF Supply Scenarios

Evaluation of the Reservoir Storage and Historical EF Supply Practices

Evaluation of the EF Supply Plans

Conclusions