CONTRÔLE DES COTES HYDROMÉTRIQUES JOURNALIÈRES—TEST DU RAPPORT DES COTES RÉDUITES

Abstract

Summary A test is presented to reveal automatically doubtful water levels and to detect sudden or gradual modifications of the stream flow. Each hydrometric station in a catchment area, homogeneous as to climatic conditions, is compared with a reference station. For this purpose we consider the ratios of the daily “reduced” water levels of the station under examination to the corresponding levels of the reference station. The “reduction” of the daily water stages is achieved by a fictitious shifting of the scale so that the mean yearly water level is placed at a predetermined level (the same everywhere) and by a dilatation (or a shrinking) of this scale so that the range Δ (the difference between the largest and the smallest monthly water levels) is equal to the range Δ0 of the reference station. The daily ratios of the reduced water levels are consequently approximately 1 either with high or low water. When a daily ratio deviates too much (more than 10 % for instance) from the mean monthly ratio, the corresponding water level is considered as doubtful and must be verified. If the running mean monthly ratios do not remain approximately uniform but change suddenly or slowly, it is because the stream flow is disturbed and the reason for this disturbance must be investigated. The test, used in a converse way, can also be satisfactorily used for filling in gaps in the observational data. Some examples enable the value of the method to be assessed.