Integrated assessment of the effects of dams on irrigation sustainability in a data scarce watershed

Abstract:

Several development projects are currently under way in developing countries to meet growing demand for water and energy. However, due to the lack of the hydro-meteorological data, some projects were conducted without rigorous check of water balance and the potential changes in the flow regime likely to be induced by reservoirs, and their implications for irrigation projects and ecosystems. To cope with this issues, we carried out analysis by using a hydrological model and quasi-observed rainfall data.

A distributed water circulation model was introduced as a tool to implement the analysis. Given daily meteorological data, the model calculates spatial distribution of surface runoff, evapotranspiration, river flow and water demand. In addition, it represents operation of water use facilities, and return flow from irrigated areas.

We performed a case study in the Pursat River Basin in Cambodia, where multiple projects are ongoing. We first calculated river discharge with observed rain data and calibrated it.

Next, we performed a water balance analysis of the basin using the compiled model with 7 years of rainfall data. Because 20-30 years of data is generally required for water resources planning, we thus prepared 25 years of data by using a climate model with a statistically corrected bias. We determined a reference year for irrigation planning from the long-term data such that annual precipitation of 5-year return period.

We selected a scenario for irrigated areas from the Water Balance Study Report (JICA, 2013) to project the future water demand, and checked the water balance under no-dam conditions. The results revealed that water supply was more than adequate to meet water demand in the reference year.

We finally incorporated the future dam operations into the calculations and evaluated the impact of the dams on river flows and irrigation projects. Even under the changed flow regimes, the water balance was satisfied in the reference year. However, river flows highly decreased in the beginning of rainy seasons, whereas the river flows increased notably in the dry season due to a hydropower dam.

Our analysis is not intended to the precise ‘prediction’ of the future, but to show stakeholders what will happen if we manage water resources based on scenarios and to indicate what actions may be needed to prevent undesirable outcomes.