Repeated Observations, or Time-Data Series

Repetition of field measurements and laboratory analyses is an essential part of hydrochemical studies. Sources of recharge vary over the year—summer rains, winter rains, and snowmelt. These are distinguishable by their isotopic (and occasionally also chemical) composition (Chapter 9), and their seasonal inputs can be established provided the relevant water systems are monitored over at least 1 year. Water table and discharge responses to precipitation or flood events provide information on hydraulic interconnec- 

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Having praised repeated measurements, we now discuss their incorporation into the planning of hydrochemical studies. A common rule of the thumb is that each water source should be studied four times to cover at least one hydrological cycle, including the four major seasons. This may be regarded as a minimum, because time-data series with only four points suffer from poor resolution. 

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