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Four gauging stations were established to measure ground water levels in holes close to rivers. Comparison basis for measuring ground water level is based on data from  2000-2007 before the power plant and the data after the power plant, from 2008, the years 2010 - 2011 og 2015-2016 (Icelandic only).

For Valþjófsstaðanes, two pictures are shown; individual weekly measurements over three periods; two before and two after activation (Figure 1) compared to the continuous registration of a water level in the relevant waterfall. On the other hand, the classic presentation is the average of measurements in each hole (Figure 2) and for Bessastaðanes and Hól (Figures 3 and 4). Those pictures have not been updated, nor have pictures of measurements at Hólmatunga (pictures 5 and 6).


Mynd 1.  Grunnvatnsstaða í hólum í Valþjófsstaðanesi FLJ1 (bláir punktar), FLJ2 (rauðir punktar) og FLJ3 (grænir punktar).

Figure 1. Groundwater level in holes in Valþjófsstaðanes FLJ1 (blue dots), FLJ2 (red dots) and FLJ3 (green dots).

Mynd 2.  Grunnvatnsborð við Valþjófsstaðanes fyrir virkjun (2001 - 2007) og eftir virkjun (2010 - 2011)

Figure 2. Groundwater at Valþjófsstaðanes before power plant (2001 - 2007) and after power plant (2010 - 2011)
Labels: Y-axis: Groundwater level (m a.s.l.), x- axis: Distance (m). Vegur - road, Farvegur - river channel.


Mynd 3.  Breytingar á grunnvatnsborði í Bessastaðanesi fyrir virkjun (2001 - 2007) og eftir virkjun (2010 - 2011)

Figure 3. Average water level observed in three holes before (2000-2007, blue) and after (2010-2011, red) construction of power plant compared to ground level (black line) in holes BES 1-3.
Labels: Y-axis: groundwater level (m a.s.l.), x- axis: Distance (m). Farvegur = river channel.

Við hól á Úthéraði

Mynd 4.  Breytingar á grunnvatnsborði við Lagarfljót hjá Hóli á Úthéraði samanborið við vatnsborð í fljótinu fyrir virkjun (2001 - 2007) og eftir virkjun (2010 - 2011).

Figure 4. Average water level observed in five holes before (2000-2007, blue) and after (2010-2011, red) construction of power plant compared to ground level (black line).
Labels: Y-axis: groundwater level (m a.s.l.), x- axis: Distance (m). Farvegur = river channel.


Niðurstöður grunnvatnsmælinga á sniði við Hólmatungu eru sýndar á tveimur myndum; einstakar vikulegar mælingar yfir tvö tímabil fyrir og eitt eftir virkjun samanborið við sískráningu vatnsborðs í viðkomandi vatnsfalli (Mynd 5). Á efri myndina er safnað mælingum yfir sumarið, þ.e. á þeim tíma sem vatnsborð í Jöklu er sem lægst eftir virkjun. Á þeim tíma lækkar grunnvatnsborð um 40-60 cm í 600-1300 m fjarlægð frá ánni, samanborið við 30-35 cm að meðaltali yfir allt tímabilið, eins og sést á neðri myndinni.

Mynd 5.  Vatnshæð við Hólmatungu

Figure 5. Water level at Hólmatunga

Mynd 6.  Grunnvatnsborð við Hólmatungu fyrir virkjun (2001 - 2007) og eftir virkjun (2010 - 2011)

Figure 6. Ground water level by Hólmatunga before (2000-2007, blue) and after (2010-2011, red) construction of power plant compared to ground level (black line).
Labels: Y-axis: groundwater level (m.a.s.l.), x- axis: Distance (m). Farvegur = river channel.


Measurements generally are based on both recording gauge in specific holes and weekly measurements during specific periods in other holes on same Gauging stations. This provides information about the relationship between ground water levels on flat land, discharge in the closest river and precipitation.

  1. On the flat vegetated land water level, adjacent river has a major impact on the groundwater table. This can refer to at least 700-800 m from the river.
  2. In a slope end close to the foot a-of a slope the groundwater table is more influenced by rainfall. This has not changed after construction of the power plant.
  3. As expected the ground water table in both Jökulá í Fljótsdal river and Lagarfljót river has risen but subsided in Jökulsá á Dal river.
  4. It was assumed that the plant could cause up to 25 cm rise in groundwater in Valþjófsstaðanes. Given the topography and the depth of groundwater in areas adjacent to Jökulsá í Fljótsdal river it was expected that the ground would get wetter if nothing was done. To counteract this the drainage canals were cleared and deepened, and water pumped from them to Jökulsá í Fljótsda river. If nothing had been done the ground water table would probably now be about 1 m higher in Valþjófsstaður peninsula and probably the lowest areas under water.

Updated: September 5, 2017 - Reviewed April 20, 2020
Source: Landsvirkjun

Metrics, Targets and Monitoring Protocol

What is measured?

Ground water levels in holes. (Project effect: indirect). Measurements repeated at regular intervals in 2009-2011.

The purpose of measurements is i.a. to analyze the effects of the operation of the power plant from natural fluctuations.

Monitoring protocol

Four stations to measure groundwater have been located in wells close to the rivers. Two are in Fljótsdalur and two in Úthérad. In each gauging station there is digital equipment, a groundwater station and a water level station, that measure and register the water level every hour. A report is piled every year for each station. Groundwater level is also measured manually in other groundwater holes with regular intervals.

Map below show the measuring stations.

Mynd 2. Vatnsborðsmælar á Úthéraði

This map shows water level gauges in Jökulsá river in Fljótsdalur, used for measuring groundwater in flatlands by the river. The blue dots mean constantly measuring gauges in groundwater holes, and the black dots indicate groundwater holes that are measured manually.

Changed monitoring: Following the Icelandic Meteorological Office's report on a comparison of water levels and groundwater in the lowlands of Hérað and an assessment made of vegetation changes, it was decided to add one groundwater profile in the lowest areas out by Héraðssandur in Húsey. Furthermore, measurement at the top of the groundwater profiles was stopped. At present, it is only recommended in the new format at Húsey.

Updated: April 17th, 2020

Changes of indicator

This indicator was originally number 25.1. It was then named Changes in Hydrology and can be found under that number in documents of the project from 2005 and 2006.

The indicator number has been changed twice.

Table 1. Changes to name and number of indicator
Year Nr. Indicator name
2020 2.1.3 Ground water levels in holes
2007 2.2b Changes in Hydrology


Measurements on groundwater levels in two measuring stations at Jökulsá in Fljótsdalur, one by Jökulsá river in Dal and one by Lagarfljót river. Measurements started in 2000 and were performed every week unntil end of 2001 by the local people. Repeat gauges were placed in a few groundwater holes in 2003-2004. Local people started measuring again in the all of 2005 and 2006 in holes which do not have repeat gauges.

The following figures show an example of the basic condition of one measuring station (borehole) in each profile in the years 2004-2007. In three cases it is the top station, i.e. the one furthest from the relevant watercourse. The groundwater level in these holes has proven to be independent of the water level in the streams, but gives a good picture of natural changes in groundwater due to the weather (thawing and precipitation). Only in Bessastaðanes did it appear that the measuring meter had turned down in a hole under the influence of the water level of a nearby waterfall.

In other respects, reference must be made to the figures published in the performance section, where the groundwater level after the power plant is compared with the groundwater level before the power plant and both in the relevant water level measurements in the next watercourse.

Grunnvatnshæð við Valþjófsstaðanes 2004-2007 (FLJ3)

Figure 8. Groundwater level at Valþjófsstaðanes 2004-2007 (FLJ3)

Grunnvatnshæð við Bessastaðagerði 2004-2007 (BES2)

Figure 9. Groundwater level at Bessastaðagerði 2004-2007 (BES2)

Grunnvatnshæð við Lagarfljót á Úthéraði, Hóll 2004-2007 (LAG4)

Figure 10. Groundwater level at Lagarfljót in Úthérað, Hóll 2004-2007 (LAG4)

Grunnvatnshæð við Jökulsá á Dal á Úthéraði, Hólmatunga 2004-2007 (DAL5)
Figure 11. Groundwater level at Jökulsá á Dal in Úthérað, Hólmatunga 2004-2007 (DAL5)

Rationale for Indicator Selection

With the construction of Karahnjukar power station, the river Jokulsa in Dal will be diverted from Halslon Reservoir into Jokulsá in Fljótsdal and Lagarfljot. Water from Jokulsá in Fljótsdal and from rivers in the Hraun area will also be diverted to the power plant. This has caused substantial changes in the hydrology of the area. Water discharge would increase in some areas but decrease in others. Water level changes, which in turn can affect ground water level, sediment transport, changes in erosion and so on, can also occur.

The discharge of the rivers Jokulsá in Fljótsdal, downstream of the tailrace canal, and Lagarfljot will increase considerably, on average just less than 90 m3/s. The mean annual discharge at Egilsstadir will increase by about half with the construction of the power plant.

The increase in discharge differs within the year. It is highest in winter (around 100 m3/s increase), but much less during the period of maximum discharge in summer when the power plant is mainly utilizing water from Jokulsá in Fljótsdal and rivers in the Hraun area. Increased discharge in floods will be proportionally much less.

At worst case scenarios in floods (all reservoirs full), the discharge of Jokulsá in Fljótsdal and Lagarfljot will increase by about 60 m3/s. In the largest recorded floods in Lagarfljót (October and November 2002), the maximum discharge was estimated about 1,650 – 1,700 m3/s. Maximum discharge at the Lagarfoss waterfall in the same floods was measured at 950 m3/s. The increase in maximum discharge into Lagarfljot and the flow by Lagarfoss waterfall is caused by dissemination effects of the lake by Egilsstadir. The Fljótsdalur Power Station therefore increases the flow to Lagarfljót river in floods by around 3-4%, and after the water has levelled in Lagarfljót river, the level by Lagarfoss increases by 30 m3/s (goes from around 950 to around 980 m3/s) a.

Just as discharge increases in the rivers Jokulsá in Fljótsdal and Lagarfljót, it is considerably reduced in the river Jokulsá in Dal. The mean discharge is reduced for most of the year, although least during summer and into the autumn when it is likely that water will be discharged over the spillway of Kárahnjukar dam. Floods due to glacial melt during summer are significantly reduced until August when the Halslón Reservoir fills up. Typical autumn-, winter- and spring-floods in the lower part of the river are only minimally reduced, as these originate mainly from the river catchment's area below Kárahnjúkar dam. The same applies to the Jokulsa in Fljótsdal upstream of the tailrace canal.

a: Updated from original terms

From phase I/II report on indicators and baseline from April 2005

Further reading

LV-2017-024 Kárahnjúkavirkjun - Framkvæmd skilyrða fyrir virkjunarleyfi

LV-2017-024 Kárahnjúkavirkjun - Framkvæmd skilyrða fyrir virkjunarleyfi


Eftir 10 ára rekstur Kárahnjúkavirkjunar þykir tímabært að fara yfir það hvernig Landsvirkjun hefur höndlað ýmis skilyrði sem sett voru fyrir leyfi til virkjunar. Þessi skilyrði eru aðallega þrennskonar; skilyrði sem umhverfisráðherra setti í tengslum við úrskurð sinn um mat á umhverfisáhrifum, fyrirheit Landsvirkjunar í matsskýrslu og viðbótarskilyrði iðnaðarráherra í virkjunarleyfi. Skilyrðunum má skipta í rannsóknir og vöktun og aðgerðir til að vinna gegn ætluðum umhverfisáhrifum. Í meginatriðum er markmið rannsókna og vöktunar að ganga úr skugga um hver umhverfisáhrifin eru í raun og hvort þau eru í samræmi við það sem álitið var eða meiri eða minni? Í þeim tilfellum þar sem mögulegt er að draga úr áhrifum er því lýst hvernig það hefur verið gert, í öðrum tilvikum hvernig aðferðir hafa verið þróaðar til að fást við áhrif svo sem af áfoki, sem frá upphafi var helsta áhyggjuefni um illviðráðanleg umhverfisáhrif. Mótvægisaðgerðir hafa gefið góða raun svo langt sem þær ná, en ekki hefur í öllum tilvikum reynt á þær, svo sem við firnamikið áfok.

The groundwater measurements described are related to vegetation monitoring, which assumed a decrease in the groundwater level at Jökla and an increase at Lagarfljót. In September 2013, regular groundwater measurements began in four wells in the land of Húsey, as it was clear that there had been changes in vegetation ( see memorandum from January 28th, 2015 (Icelandic Only)).