The water year 2017 approached its end. Welcome a new year 2018!
Whilst the common New Year’s Eve celebrations are still two months away, a group of hydrologists and water specialists commonly start with a new year celebration much sooner. The last day of October brought not only Halloween trick-or-treating, but also the end of a so-called water year, also known as a hydrological year, discharge year or flow year. Therefore on 1st November we gave farewell to the previous water year and welcomed a new water year 2018!
The end of something old is the beginning of something new and the beginning of something new always deserves a celebration, and so many hydrological and water institutions in the Czech Republic celebrate 31st October as ´Hysil´ or ´Hydrological Silvestr´.
Why water year ends on 31st October and begins on 1st November?
The water year defines a 12 month period during which precipitation totals are measured. The period of the water year has been determined so that all precipitation are drained within the same annual period. Accordingly, during the months of November and December, precipitation in the mountains are likely to accumulate in the form of snow and may not drain until the spring time of the following calendar year (although this cannot be said about the recent winters in the Czech Republic). As a result, hydrologists would have struggled to produce adequate hydrological equations that would express an actual and equal balance between an inflow and an outflow in the hydrological year. Hydrologists and water specialists therefore had to find a successive twelve month period that most consistently, year after year, gives the highest correlation between precipitation and streamflow and negligible changes in storage (i.e., soil water and snow). It had to be a period without any accumulated snow and at the same time, with the lowest risk of floods. Consequently, the overlap between October and November was selected as the most appropriate period to meet these criteria, at least in Central Europe. The hydrological year, however, may differ in other parts of the world. For instance, in the western North America, the hydrological year ends a month earlier because snowfalls in the Rocky Mountains (commonly known as the Rockies) may start sooner, in October.
In accordance to the climate conditions in Central Europe, the water year runs from 1st November to 31st October. Nevertheless, it is worth mentioning that one hundred years ago, the water year used to commence on 1st December but as the climate has been prominently changing, the snow spells are usually expected earlier.
Snowmelt and spring precipitation replenish water in the ground and reservoir dams
The volume of water generated from snowmelt and spring precipitation, which enable water to be supplemented into the ground, soils and water dams, is essential for sufficient water supplies throughout the entire hydrological year. In the event of an extremely dry year, flows could remain dry and below the average level even during the winter period. Notably mild winters can cause groundwater falling below the threshold level and influence the outlook for water resources for the summer season, alternatively, even for the rest of the hydrological year.
This year’s dry summer consequently influenced the Dyje River Basin (Czech Republic)
Following the extremely dry summer in the region of Znojmo, Southern Moravia in the Czech Republic, the water level of the very famous dam Vranov has been continuously falling below the threshold level since May of this year. As a result, after more than 20 years, the dam bottom has revealed house foundations of a flooded village, once called Bítov, as well as ruins of the village Church, which had been exploded. In the middle of this October, the water level has been recorded as low as nine metres below the maximum filling level.
The water level of the dam Vranov in the Southern Moravia of the Czech Republic, has fallen below its threshold level. The bottom of the dam reveals ruins of the flooded village, Bítov. For more video images, please click here.
Last but not least, our Team would like to wish you joy, happiness and reasonably wet conditions in the new Water Year 2018!
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