Seasonal variability of unconfined groundwater resources in the Pilica catchment

Abstract

The paper attempts to identify seasonal variability of groundwater table in the Pilica catchment. Procedure created by Ch. G. Markham was used. It allows to compare results with analogical characteristics of other aquifers from central Poland. Sixteen piezometers were used for the research.

They represent shallow aquifers located in the Pilica catchment and its surrounding and they are part of a measurement network of PIG-PIB (years from 2003 till 2014 were used). All of the aquifers with 10 years data series were chosen for the analyzed group. They represent the whole spectrum of occurrence of groundwaters in the Pilica catchment.

Before the analysis homogeneity of time series was checked by two statistical tests: parametric F-Snedecor test (exams null hypothesis about variance equality) and non-parametric series test (exams null hypothesis about origin of two samples from the same general population). All of numeric strings were statistically homogeneous.

Time series of mean monthly depth to groundwater table in all piezometers were changed into groundwater ordinates. In that way, maximum values are compared with the maximum level of groundwater table. Next, for all the aquifers the Markham characteristics of seasonal variability were calculated. First of them is seasonality index (IS) and it ranges from 0 to 100%. The second characteristic is time of concentration (WPK). During the interpretation of WPK values it is important to remember that this characteristic shows only time of concentration (it must not be associated with terms of maximum level of the examined phenomenon).

Aquifers in the Pilica catchment are characterized by differential seasonality index raning from 1 to 30%. It means that rhythm of groundwater table changes is not seasonal in part of aquifers. Despite that, the seasonal variability of groundwater level is stronger in the Pilica catchment than in other aquifers in central Poland (IS is on average almost double). Time of concentration coefficients are differential too. They occur from February to July in the Pilica catchment, while in the whole central Poland they are concentrated in two terms (late winter and summer). Unlike other parts of central Poland, changes of groundwater resources are characterized by poor correlation with hydrogeological parameters of aquifers or mean depth to the groundwater level in piezometers in the Pilica catchment.