The Devon Karst Research Society, Plymouth, England,
in partnership with
Speleološko Društvo "Zelena Brda" - Спелеолошко Друштво "Зелена Брда", Trebinje, RS-Bosna i Hercegovina.
Part 3.  Eastern Hercegovina.
F-BiH Neum Canton - Ravno Municipality.
RS-BiH Trebinje Region - Trebinje Municipality.
Updated 02 July 2017.
Historical Information :
Extensive hydrotechnical and hydromelioration works under the Hidroelektrane na Trebišnjici scheme of the 1950's., 1960's and 1970's have destroyed the natural hydrography of the Rijeka Trebišnjica. Before this scheme was begun, there was an annual flooding event in Lower Popovo Polje. 
Although not as famous as the Cerknica Intermittent Lake (Cerkniško Jezero) in Slovenia, the Popovo Polje Lake was one of the largest such 
intermittent or ephemeral lakes in the Dinaric Karst. 
According to the Austrian Groller, in 1883 there was a flood with an estimated accumulation of 356.5 million m³ of water in Popovo Polje, which completely flowed out within a 56 day period! 
The maximum depth of water has been recorded as being 40m. (131 feet) at the Dobri Do Water Measuring Station in the Lower Polje.
Such periodic lakes occur in closed karst polja during such times as the combined water input is greater than the total drainage or absorption capacity 
of the ponors. In the case of Popovo Polje, we are considering potentially vast quantities of water.  The maximum level of the lake in the polje has been the primary controlling factor for many human activities, for example, such as being the deciding factor for the chosen elevation for the construction of the main settlements and also that of the now defunct Austro-Hungarian Railway running between Hum and Metković or the positioning of the landing quays 
for the fishing boats and of the precursors to the roads that we use today on either side of the polje.
The levels of the lake over many years has been recorded and we provide some details below. In some cases where the flood was exceptional, it has been marked by the placing of a "bench mark", such as that shown below in Images.PP5-2 and PP5-3 and which is located in the village of D. Sedlari.
The construction of the Hidroelektrane na Trebišnjici scheme of the 1960's and 1970's was supposed to have eliminated flooding. In recent years, notably 2010., 2012 and 2013, flooding has returned in a spectacular fashion. 
The almost reliable periodicity of the Popovo Polje Lake gave rise to two principal industries; firstly, a fishing industry reliant on catching the nutritionally-rich stygophilic salmonoid fish Popovska gaovica or Popovo Polje Minnow. Complementing a similar activity being undertaken at many of the estavelle fishing mills before the main floods arrived, this peculiar landlocked fishing fleet was comparatively small in number. There are the extant remains of 2 wharves where the fishing boats landed their catch; one near Galićići and the other at Ravno on the opposite side of the polje.
The other industry was the planting, rearing and harvesting of the famous Popovo Polje "100-day Maize".  The maize or corn crop provided the grain which was processed to make flour in the many Ponor Mills lining the riverbanks of the Trebišnjica.
So, both these environmentally friendly and ancient industries were destroyed by the Hidroelektrane na Trebišnjici Co. in the mid 1950's onwards, when it changed the whole regional karst hydrography for the sake of producing "cheap" hydroelectric-energy. The livlihoods and way of life for thousands of people were destroyed in a very short space of time, without their permission and with no financial compensation from that Company or from the Yugoslav State. This, together with the dismantling of the Popovo Polje narrow-guage Railway, further isolating many villages, probably contributed significantly to the exodus of the population towards industrial centres or even to emigrate to other countries.
Duration of Flooding Events :
The periodicity and duration of the presence of the lake has varied considerably, since recordings of the event were first made. The longest durations of floods in Popovo Polje have been recorded as being :
Year 1915 - for 303 days......Year 1937 - for 271 days......Year 1930 - for 204 days.

Popovo Polje contains the karst geomorphological sub-units of Trebinjsko Polje and Mokro Polje. Historically, it is only the flooding of Popovo Polje proper (ie. the Lower Polje) and the formation of Popovopoljsko jezero that has ever been reported on. However, Mokro Polje also has an intermittent lake, the Mokropoljsko jezero, which is reported on separately within these webpages. The area of Trebinjsko Polje itself rarely floods, although in the area of Trebinje, Rijeka Trebišnjica does break its riverbanks causing localised flooding only during exceptionally wet winters.
Here we report upon the periodic lake of Popovo Polje.

Archived records show that during the period between 1923-1940., the Polje was underwater for an average of 253 days per year and was dry for only 112 days. During the maximum flood, 7,500 hectares were inundated.
A return to more serious flooding events in recent years has not yet given rise to any data. The years 2010-2013 witnessed considerable flooding events.

A view of Popovopoljsko jezero during its formation,16 January 2010. looking towards the south side of the polje from the vicinity of Galičići, recorded by Dubravko Kurtović.

Images PP5-2 and PP5-3 Right :
A former water-level marker at Donje Sedlari.
The stone has been purposely engraved in Cyrillic; transliterated into the Latin Form as follows : 


Translated into English, this reads :

6th April 1941.

Water level.
Importance of the Periodic Lake Functioning as a Natural Migration Corridor and as a Food Supply for Hypogean Species :
Popovo Polje forms is contained within the Trebišnjica River Basin in which the geographic distribution of many hypogean species has been assisted by the annual flooding event under natural hydrographic conditions. When these natural conditions were largely removed by the Hidroelektrane na Trebišnjici scheme of the 1960's and 1970's., this natural migration corridor also disappeared. Many locations which were the habitats of various hypogean species have become depleted of hypogean populations, probably due to the non-renewal of nutrients normally provided by the floods. Many of these habitat locations are not fed by ponors but by percolation water and received most of their nutrients from the Popovo Polje Lake. With the periodic lake no longer inundating these locations, they have become hydrologically "stranded" or isolated and have a much reduced nutrient supply, with the knock-on effect of not being able to support either the biodiversity or the surviving population numbers that were formerly recorded.
Some such locations which are estavelles or ponors, fare slightly better in both biodiversity and population numbers than those which are not, insofar as there is a limited replenishment of nutrients from the localised surface waters which are drawn down into the underground.

A view of Popovopoljsko jezero during its formation, 24 November 2010.