Geostatistical screening of flood events in the groundwater levels of the diverted inner delta of the Danube River: implications for river bed clogging

Abstract

The efficient operation of shallow groundwater (SGW) monitoring networks is crucial to water supply, in-land water protection, agriculture and nature conservation. In the present study, the spatial representativity of such a monitoring network in an area that has been thoroughly impacted by anthropogenic activity (river diversion/damming) is assessed, namely the Szigetköz adjacent to the River Danube. The main aims were to assess the spatial representativity of the SGW monitoring network in different discharge scenarios, and investigate the directional characteristics of this representativity, i.e. establish whether geostatistical anisotropy is present, and investigate how this changes with flooding. After the subtraction of a spatial trend from the time series of 85 shallow groundwater monitoring wells tracking flood events from 2006, 2009 and 2013, variography was conducted on the residuals, and the degree of anisotropy was assessed to explore the spatial autocorrelation structure of the network. Since the raw data proved to be insufficient, an interpolated grid was derived, and the final results were scaled to be representative of the original raw data. It was found that during floods the main direction of the spatial variance of the shallow groundwater monitoring wells alters, from perpendicular to the river to parallel with it for over a period of about two week. However, witht the passing of the flood, this returns to its original orientation in ~2 months. It is likely that this process is related first to the fast removal of clogged riverbed strata by the flood, then to their slower replacement. In addition, the study highlights the importance of assessing the direction of the spatial autocorrelation structure of shallow groundwater monitoring networks, especially if the aim is to derive interpolated maps for the further investigation or modeling of flow.

Assessment of the interaction between surface- and groundwater after the diversion of the inner delta of the River Danube (Hungary) using multivariate statistics

“…4). A similar process to this has been observed and discussed using geostatistical methods (Trásy et al 2018). There, it was pointed out that the floods in the Szigetköz (2006, 2009 and 2013) have changed the subsurface flow conditions by removing the clogged strata from the riverbed.…”

“…There, it was pointed out that the floods in the Szigetköz (2006, 2009 and 2013) have changed the subsurface flow conditions by removing the clogged strata from the riverbed. This was found as one of the -although indirect -main explanations of the change in representativity of the subsurface water level monitoring system in the area (Trásy et al 2018).…”

“…Due to the diversion the runoff in the old Danube decreased by ~80% (Jansky et al 2004, Smith et al 2000, thus radically changing the previously obtaining hydrological conditions. Consequently, water level in the riverbeds dropped several meters, and by 1993, some branches of the Danube had dried up (Trásy 2012, Trásy et al 2018. As the subsurface aquifer of the area is in close connection with the SW network, the level of shallow GW dropped significantly as well (Bárdossy and Molnár 2004).…”

Delineation of Hydraulic Flow Regime Areas Based on the Statistical Analysis of Semicentennial Shallow Groundwater Table Time Series

“…The set of methods were chosen based on their wide applicability in hydrogeology. Hierarchical cluster analysis combined with discriminant analysis has been used, for example, to group the SGW wells of a similar area in Austria [26] to try to delineate their common drivers [57], whereas variogram analysis was applied, for example, to determine and map the response of SGW time series to changes in recharge condition on the banks of the Danube [58]. However, this particular combination and order of application of these tools ( Figure 2) has not yet been used to delineate flow regime areas before.…”

Similarities among small watercourses based on multiparameter physico-chemical measurements

“…At the first design of a monitoring network, practical aspects are set against the goals of the network (Strobl and Robillard 2008;Telci et al 2009). Later, at any redesign / optimization of the system, previously produced monitoring data provide additional information.…”