Impact of evapotranspiration on diurnal discharge fluctuation determined by the Fourier series model in dry periods

Precise measurements of discharges at the outlet of a small catchment, using high resolution sensing equipment, can currently be done without difficulty. In particular, measurements can take place even during dry periods, when high temperatures increase actual evapotranspiration on the catchment and diurnal streamflow fluctuation changes occur in a harmonic wave at any time of the day. Some 10-15 years ago, a current runoff measurement record based on a high resolution equipment clearly recognizing a diurnal wave-shape fluctuation could hardly be available. The measurement of discharge ordinates from the catchment, and from free water pan evaporation, showed an undulating fluctuation tendency. However, the discharge minima appeared at day time and their maxima at night. The measured discharge data are represented not only by a fluctuating form, but also by a mild form, an even straight line, or by a flat depletion curve. For the purpose of analyzing the wave shape of discharge we implemented the Fourier series model, simulating the measured data through the Fourier input, output, and transformation coefficients. The purpose of this analysis was to use the Fourier equations in order to substitute the missing data (when the discharge or evaporation measurements collapsed). Due to very sensitive data, when the measured discharge series are jagged, the equation can be smoothed by the harmonic approximation or by the polynomial approximation. Our study was carried out on the small experimental catchment of the Starosuchdolsky Brook, in the vicinity of the campus of the Czech University of Life Sciences Prague. The harmonic analysis provided an interesting outcome, as well as innovative methodology.