Error Analysis of Wind Effects on Natural Flow Estimation

When a large reservoir is fed by many tributaries, the measurement of reservoir inflow is a difficult task. For a hydroelectric reservoir, the estimation of natural inflow is required to optimize hydropower production without compromising environmental and infrastructure safety. This can be accomplished using the water balance equation (WBE). An important input of this equation is water levels measured from the reservoir of interest. However, the water-level measurements obtained from limnimetric gauges are affected by wind. Water-surface fluctuations caused by this meteorological forcing are carried over in the WBE. Consequently, natural inflow’s signal may become noisy. On an hourly or daily basis, the wind-induced errors in natural inflow may become significant enough to make a reasonable estimation of real-time values impossible. This paper uses unsteady- and steady-state hydrodynamic modeling to estimate the WBE short-term errors originating from wind effect in the natural inflow of the reservoir. A hydrodynamic model calibrated with in situ observations has been used for this purpose in the Gouin Reservoir case study in Quebec, Canada. These results show that using a steady-state approximation can underestimate the effect of wind on daily estimates of the natural inflow because of the neglected seiches and inertial forces related to wind effect.