Cause of rapid decrease of discharge of minor streams in Central Bohemia during summer 2019, and extremely low specific runoff: effect of evapotranspiration from riparian zone and areas with shallow groundwater table
Abstract
In the years 2018 and 2019, an extremely low specific discharge in several streams in Central Bohemia was observed. For example, the Brzina stream with the catchment area of 133 km2was dry for part of the summer in both years (Figs 1, 2; Table 1).
To determine the cause of very low discharge of streams, the following methods were applied: (i) streamflow measurements in high summer, (ii) hydrograph analysis during spring and summer, (iii) calculation of potential evapotranspiration (PET), and its comparison to the stream discharge and antecedent precipitation index (API), (iv) comparison of discharge and baseflow of springs. Field measurements confirmed extremely low specific discharge of some streams in the peak summer in Central Bohemia indicated by a long term monitoring of the Czech Hydrometeorological Institute.
The specific discharge of some streams (within the catchment area exceeding 100 km2) dropped below 0.1 l/s/km2. Alluvium transmissivity is too low to explain such a low stream discharge by groundwater flow parallel with the stream course.
The water abstraction is also too low to play important role. Based on various approaches, the intense evapotranspiration in the summer period from that part of alluvium, where the groundwater level and stream water are within the reach of plant roots (PET alluvium), is probably the major cause for a decrease of discharge to very low values (Fig. 1).
This assumption is supported by the times of the lowest discharges corresponding to the highest summer temperatures (Fig. 4), extremely high PET during the summer, and a rapid drop in baseflow of streams observed in June 2019 (Fig. 2). In agreement with this assumption, the yield of springs unaffected by evapotranspiration from the PET alluvium has not decreased considerably (Fig. 3).
Based on the obtained data, and providing that the PET alluvium occupies about 2-5 % of the catchment, this can result in the total loss of stream discharge in the hot summer period in Central Bohemia (mean specific base flow 1-2 l/s/km2). Providing that some, recently favored, remediation measures will be taken in the future, such as streambeds return to near- natural state or plugging the underground artificial drainages to increase the water table in adjacent alluvial plain and to get larger storage capacity and consequently to increase the streamflow during dry periods, an opposite result is likely to occur.
The rise of water table will boost up the evapotranspiration in summer, and thus decrease the stream discharge. This clearly shows that intense evapotranspiration from the PET alluvium must be considered an important part of the hydrologic balance, and its effect should be taken into account when planning the remediation measures and actions.
The evapotranspiration from the PET alluvium should also be considered when calculating the upper limit for groundwater abstraction and for minimum streamflow to limit surface water withdrawal. The effect of evapotranspiration from alluvium on streamflow in hot summer seems to be underestimated or even ignored issue in the Czech Republic. (C) 2020 Czech Geological Survey.
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