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Die Bodenkultur - Journal for Land Management, Food and Environment
Temporal changes in precipitation impacting groundwater and stream flow
In recent years there has been an apparent increase in the occurrence of extreme hydrological events in Poland. The central part of the country has the lowest precipitation with annual totals falling below 600 mm. The objective of this study was to investigate changes in precipitation in a lowland basin located in central Poland where severe frequent meteorological and hydrological droughts appear especially in summer. The focus was on the precipitation evolution that has occurred in the past six decades as well as on the possible shifts in the groundwater levels and stream flow regime induced by changes in precipitation. The Standardized Cumulative Annual Deviation was used to examine the sequences of dry and wet years. Trends detected in the long–term precipitation course were followed by the course of groundwater levels and stream flow. Significantly reduced summer precipitation has occurred during the last decade. Drought frequency evaluated by the Standardized Precipitation Index SPI has changed. As a signature of an intensification of the water cycle the number of 3-, 6- and 12-month droughts has significantly increased. Risk of occurrence of low groundwater levels and reduced stream flow has also increased as a response to tendencies in precipitation. As the raingauge network is too sparse to provide an adequate spatial coverage of precipitation, estimates at higher resolutions (1x1km) were acquired for the period 2004–2008 as a product derived from the weather radar. Monthly precipitation estimates from the rain gauge network correlate well with the radar estimate at a 1 km pixel, however they generally underestimate the areal precipitation in the basin. Spatial precipitation patterns derived from radar data show distinct gradients across the basin. From 2004 to 2008, the western part of the basin has received on average less precipitation than the eastern part. This concerns values for the summer half of the year as well as for the whole year. Differences reach over 100 mm/year. Extremely low precipitation has appeared in July 2006, however it was followed by extremely high precipitation in August 2006. Both are absolute values of monthly precipitation in the period 1956–2008. Key words: Rain gauge rainfall, weather radar rainfall, spatial patterns, temporal changes.