How Long And How Frequent Should We Measure Groundwater Heads?

Groundwater heads are measured in many observation wells across the globe. Longer head series may span a few decades. Older measurements may be taken a few times a year, while more recent measurements may apply modern pressure transducers to measure the head every day or more frequent. At the start of this project it was hypothesized that systems that respond quickly to a change in recharge can be modeled with a shorter series of measured heads while a system that responds slowly needs a longer series to obtain model outcomes with reasonable uncertainty. Furthermore, it was hypothesized that head series with a frequency of a few times per month would lead to results with smaller uncertainty than head series with a frequency of once per month or once per 3 months. Numerical experiments were carried out to simulate groundwater dynamics due to measured precipitation and potential evaporation. Both synthetic and real data were used. Estimates of model fit and uncertainty were related to the length and frequency of the measured heads and the memory of the system. Measured precipitation, evaporation, and heads were for the Netherlands, a deltaic region with predominantly shallow water tables and a temperate maritime climate. Surprisingly, it was found that relatively long observation periods (more than 10 years) were required to obtain results with reasonable uncertainty bands for both systems with a short and long memory. Furthermore, it was found that a smaller uncertainty was obtained with 20 years of observations once per month than 10 years of observations twice per month, even though the total number of observations is equal.