Trends in the long-term variability of groundwater discharge

Groundwater recharge is irregular both seasonally and during long-term periods. An analysis of long-term observations of the groundwater level regime and groundwater discharge (baseflow) made it possible to establish tendencies to grouping of dry and wet year series, ie to the so-called cyclicity, with frequent periods of 2–3 years, 5–6 years, and 21–22 years. A longer duration is possible of series (60–80 and even 100 years and longer), found only in the longest observation series as trends. The statistical significance of trends is not high. However, the average directed variability in groundwater recharge (increase or decrease) may be as large as 0.02–0.1 litre per second per square kilometre that may be considered in practical computations and predictions. The different direction of trends depends both on the direct human impact on groundwater (water withdrawal, irrigation and drainage, etc.) and on the indirect effect (the effect of man-induced climatic transformations). The predictions showed that normal groundwater recharge in the USSR area may increase under the effect of climatic changes by 10–40% at the beginning of the next century. This will result in improvement of water supply conditions, on the one hand, and in some negative ecological after-effects related to waterlogging and flooding, particularly in cities and towns.