The effect of climate change on runoff from two watersheds in Iceland

Abstract

In order to investigate the effect of climate change on the hydrological regime in Iceland, future projections of river discharge were made for two watersheds with the WaSiM hydrological model. The projections were made for the period 2021–2050 and compared with the reference period 1961–1990. The runoff projections are based on thirteen different climate scenarios. Monthly d-changes, based on the climate scenarios, applied repeatedly to selected base years are used to construct the future climate input for the hydrological model. This methodology preserves the internal climate variability of the climate model runs.

The selected watersheds have different hydrological properties and climate characteristics. Sandá í Þistilfirði, vhm 26, is located close to the coast in the north-eastern part of Iceland and Austari-Jökulsá, vhm 144, is located in the northern part of the central highland with a 10% glacier coverage. Average warming for both watersheds between the reference period and the scenario period is on the order of 2°C. A precipitation increase of 16% is projected for Austari-Jökulsá and an increase of 3% for Sandá í Þistilfirði. During the reference period 1961–1990, snow storage has a dominating effect on the discharge seasonality and snowmelt originated spring floods are the largest floods of the year for both watersheds.

Compared with the reference period, the magnitude of spring floods is predicted to decrease in 2021–2050 and they will appear earlier in the year. The timing of maximum snow melting is predicted to be about a month earlier for both watersheds and the magnitude of the mean yearly maximum snowmelt is predicted to decrease by 5–70%. The time with considerable snow cover is predicted to diminish from 7 months to 3–5 months per year depending on watershed. Mean yearly maximum snow thickness decreases by 0–80%.

Winter flow is predicted to increase on average due to a higher number of melt events at relatively high and flat heath areas of the watersheds. For Sandá í Þistilfirði, vhm 26, the snowmelt generated spring/summer discharge peak largely disappears and the seasonal discharge becomes more evenly distributed with higher winter discharge.

For Austari-Jökulsá, vhm 144, runoff from the glacier will increase substantially due to increased snow and ice melting. The share of glacier originated runoff in the total annual volume is predicted to increase from 20% to 25–30% and the duration of glacier runoff is predicted to increase by nearly two months, reaching further into the spring and autumn. The increase of annual glacier melt, assuming unchanged glacier geometry, is predicted to be in the range from 75–150% depending on scenario. This leads to a late summer discharge maximum caused by increased glacier runoff. The discharge peaks caused by snowmelt and glacier melt will become more distinct and appear as two separate summer maxima with the one caused by glacier melt the largest runoff peak of the year.

Compared to the period 1961–1990, a warming of about 1°C has already been observed for both watersheds during the period 2000–2009, causing considerable discharge changes in the same direction as the predicted future changes.