A Methodology for the Fast Comparison of Streamwater Diurnal Cycles at Two Monitoring Points

Abstract

There are numerous streamwater parameters that exhibit a diurnal cycle. However, the shape of this cycle has a huge variation from one parameter to another and from one monitoring point to another on the same river. Important variations also occur at the same point during some events, such as high waters. Water level, specific conductivity, dissolved oxygen, oxidation reduction potential, and pH of the Suceava River were monitored for 365 days (2018–2019, hourly sampling frequency) in order to assess the upstream-downstream changes in the diurnal cycle of these parameters, some of these changes being caused by the impact of Suceava city, which is located between the selected monitoring points. The multiresolution analysis of the maximal overlap discrete wavelet transform and the wavelet coherence analysis were combined in a flexible methodology that helped in comparing the upstream and downstream shapes of the diurnal cycle. The methodology allowed for a fast comparison of diurnal profiles during periods of high waters or baseflow. Notable changes were observed in the moments of diurnal maxima and minima.

Water Quality Index of Suceava River in Suceava City Metropolitan Area

“…Consecutive values smaller than 0.5 are frequent during summer at both stations. The hourly distribution of the minimum and maximum values is similar to those of SC and ORP at the upstream station and to those of DO and pH at the downstream station (according to the diurnal cycles of the water quality parameters described in our previous study [35]). The DO oscillation downstream is bound to the water temperature; water temperature downstream is influenced by the Suceava city urban heat island and wastewaters [36]-the higher thermal amplitude might explain the higher influence of the DO diurnal profile on the shape of the WQI diurnal profile at the downstream station.…”

“…Therefore, we prefer to use the first variant of WQI. Supplementary arguments to this preference is the fact that (1) the diurnal profiles of various parameters have very labile positions of the minimum and maximum values, suggesting that the river undergoes substantial changes inside the city, even if its discharge increases only by ≈0.84 m 3 /s between the upstream and downstream stations [35]-for example, the diurnal cycles of DO can be in antiphase at the selected stations; (2) the annual cycle that creates low WQI in the late summer and in autumn is a fact with real consequences in the aquatic environment and riparian area.…”

“…We applied the wavelet coherence method of Grinsted [42] for creating WTC scalograms of the selected parameters, which indicate the time when periodicities in two time series are coherent. The wavelet analysis used Morlet as mother wavelet because this nonorthogonal wavelet function has proven to provide good results in wavelet coherence analyses of geophysical time series [35,36]. The cone of influence on scalograms is created by the edge effects specific to finite time series.…”

“…The WTC scalograms contain arrows with various orientations: right means that the selected time series are in-phase; left means anti-phase; down means that series 1 leads series 2 by 90 • . For equations of the wavelet analysis, see our complementary study [35].…”

“…It has an average flow rate of 16.87 m 3 /s at Iţcani station [34]. It collects urban tributaries that transfer the impact of the urban areas into their collector (chemical and thermal pollution [35,36]). Downstream of Suceava city there is the wastewater treatment plant (WWTP) of Suceava city, which collects most wastewaters of the metropolitan area.…”