Sampling period intervals for monitoring water table dynamics in perched water table systems

Accurate water table measurement in the near-surface zone is important for siting various land uses, soil taxonomic classification, and transport of c ontaminants, such as sediment and nutrients, to streams. Because of cost an d logistical constraints, many water table studies measure water tables wee kly or less frequently and use interpolation methods to simulate daily moni toring, The objective of this study was to evaluate the reliability of week ly measurements in a perched water table system in soils containing fragipa ns, Daily measured water table levels were compared with daily simulated wa ter table levels derived from weekly measurements using two simulation meth ods: the linear and the constant interpolation methods. Water table data fo r 1 year at four landscape positions on a hillslope underlain by fragipan s oils were used for the comparison. Both simulated methods underestimated si gnificantly the presence of water tables in the upper part of the soil prof ile, Differences decreased with depth in the soil profile. The constant sim ulated method was more accurate than the linear method for all hillslope po sitions. Up to 89% of the actual days of saturation were not predicted by t he simulated water table data in the upper part of the soil profile. The la rgest differences were found where the average saturation event duration wa s less than the sampling interval. Most of the saturation events were of le ss than 3 days duration at those depths (e.g., 20 cm), The scale of samplin g interval should be matched to the scale of the water table fluctuations. In many hydrological settings where short-term saturation events are preval ent, such as perched water table systems, accuracy in quantifying water tab les with weekly or bimonthly measurements depends on previous knowledge of the frequency and duration of short-duration saturation events.