An analysis of groundwater drought in combination with meteorological droughts. Case study of the Gwda River catchment (northern Poland)

impact of meteorological droughts on groundwater droughts in the
Gwda River catchment (northern Poland). The analysis was based
on the Standardised Precipitation Index (SPI) and Standardised
Groundwater Index (SGI) in various cumulation periods (1, 6, 12,
18, 24, 30 and 36 months) from 1986–2015. Monthly groundwater
levels measured in wells and monthly sums of precipitation from
meteorological stations in the vicinity of those wells were used to
assess the relationships between droughts. During the study period
from 1986–2015, three to 43 meteorological droughts and one to
five groundwater droughts were identified. Meteorological droughts
were most numerous for the shortest cumulation period (1 month),
while droughts for the longer accumulation of 24 to 36 months were
less numerous. The SPI and SGI indices were most strongly
correlated over the annual cycle in the upper part of the catchment
(between the Sępólno Wielkie station and well I-33_1). The
correlation coefficient r was highest (0.69) between SPI-18 and
SGI-1. Correlations were much lower in the middle of the
catchment, where the maximum annual r coefficient was 0.39.
There was no correlation between droughts in the lower catchment
(r=0.14). The correlations presented for the Gwda catchment
indicate that the relationship between droughts is not clear.