Changes in the catchment scale water balance have important social implications for usable water now and in the future. Stream discharge is also directly related to radionuclides flux in the river water system. The aim of this study was to clarify the water balance in the Chernobyl Exclusion Zone (CEZ) under current and future climate conditions. A catchment scale hydrological model was used with long-term discharge data to project the future trend of radionuclides wash-off from the contaminated catchment at the CEZ in Ukraine. The Sakhan river catchment at the CEZ (51.41°N, 30.00°E) in Ukraine is one of the Pripyat river systems, and has a total surface area of 186.9 km2. We found that under the current climate, 84% of annual input (sum of rainfall and snowmelt) was consumed as evapotranspiration, and discharge was estimated to be 16%. In future climates, annual precipitation is expected to increase. However, a projected increase in the vapor pressure deficit led the consumption of precipitation as evapotranspiration and no significant increase in discharge. The study found that warmer winter and spring temperatures will decrease the snowfall, and increase the rainfall, but it was not enough to increase evapotranspiration. As a result, the peak of discharge shifted from April to March. The increase of future average discharge during the winter and spring came from a combination of (1) increasing rainfall in the winter and spring, and (2) relatively small levels of evapotranspiration, which enhanced the catchment scale water recharge in soil moisture and gave rise to greater discharge during winter and spring. The reduction of extreme river discharge from the hydrological projections could reduce the probability of high radionuclides concentration in the river water system in the future, owing to the reduction of surface runoff water from the contaminated surface soil and/or top layer of floodplain soils in the CEZ.
Populations experiencing varying levels of ionising radiation provide an excellent opportunity to study the fundamental drivers of evolution. Radiation can cause mutations, and thus supply genetic variation; it can also select against individuals that are unable to cope with the physiological stresses associated with radiation exposure. Since the nuclear power plant explosion in 1986, the Chernobyl area has experienced a spatially heterogeneous exposure to varying levels of ionising radiation. We sampled Daphnia pulex (a freshwater crustacean) from lakes across the Chernobyl area, genotyped them at eleven microsatellite loci, and also calculated the current radiation dose rates. We then investigated whether the pattern of genetic diversity was shaped primarily by radiation-mediated supply of variation consistent with increased supply of de novo mutations, or by greater radiation-mediated selection at higher dose rates. We found that measures of genetic diversity, including expected heterozygosity (an unbiased indicator of diversity) were significantly higher in lakes that experienced higher radiation dose rates; this is consistent with mutation outweighing selection as the key evolutionary force in populations experiencing high radiation dose rates. We also found significant but weak population structure, and clear evidence for isolation by distance between populations. This evidence suggests that gene flow between nearby populations is eroding population structure, and that mutational input in high radiation lakes could, ultimately, supply genetic variation to lower radiation sites.