1 Introduction

Abundant surface water storage in the form of lakes, ponds, and wetlands is a defining characteristic of many Arctic landscapes [Smith et al., 2007]. Ice cover dominates the annual cycle of these lakes and essentially isolates water bodies from exchange of moisture and gas fluxes with the atmosphere for 60–80% of the year [Brown and Duguay, 2010; Prowse et al., 2011]. Thus, both lake extent and ice cover play a key role in the regional water and energy balance of Arctic landscapes [Rouse et al., 2005; Labrecque et al., 2009].

Lake area extent can exceed 25% of the land surface in the Arctic including the Canadian Shield region and ice‐rich continuous permafrost areas along coastal plains [Smith et al., 2007]. For example, on the Arctic Coastal Plain (ACP) of northern Alaska, lakes are of thermokarst origin [Jorgenson and Shur, 2007] with their depth being strongly controlled by the distribution of excess ice content in the permafrost [Sellmann et al., 1975]. Jeffries et al. [1996] found that 77% of the lakes located near Barrow, AK, froze completely to the lake bottom with bedfast ice during the 1991/1992 winter. In lakes deeper than the maximum ice thickness, ice remains floating above perennial liquid water that forms a thaw bulb or talik [Brewer, 1958; Sellmann et al., 1975; Jeffries et al., 1996]. A recent inventory of 85,000 lakes (>0.6 ha) across the entire North Slope of Alaska identified that only 28% had floating ice regimes (Figure 1) [Grunblatt and Atwood, 2014]. The landscape distribution of lake ice regimes is shown to be shifting, however, toward more lakes with floating ice regimes because of a trend toward thinner ice on the ACP [Arp et al., 2012; Surdu et al., 2014]. This form of lake change causes permafrost degradation, expands overwintering fish habitat, and increases the availability of winter water supply [Jones et al., 2009; Arp et al., 2011]—a particularly valuable resource to the petroleum industry for building winter ice roads. This form of regime shift may also affect hydrological processes, such as evaporative loss and corresponding water balance due to changes in ice‐out timing [Arp et al., 2011].

Figure 1 Open in figure viewer PowerPoint Lake density and size increase northward from the Brooks Range to the Coastal Plain in northern Alaska, as do the number of bedfast ice lakes relative to floating ice lakes as determined by Grunblatt and Atwood [2014] in April 2009. The major physiographic regions and study areas and lakes are indicated.

The timing associated with ice‐melt duration is affected by whether a lake has a bedfast or floating ice regime. Ice‐out in bedfast ice lakes may occur up to 1 month earlier than ice‐out in floating ice lakes [Sellmann et al., 1975; Arp et al., 2011] (Figure 2). Bedfast ice melts more rapidly due to both thinner ice and meltwater ponding atop ice that is anchored to the lake bed. Ponded water atop bedfast ice rapidly absorbs solar radiation and further accelerates the melt process [Brewer, 1958]. At the same time, in lakes deeper than the maximum ice thickness (typically greater than 1.5 m), ice remains floating and reflects much of the incoming solar radiation, causing floating ice to melt slower and resulting in later ice‐out. Meltwater forming on the surface of floating ice tends to percolate easily through the degrading ice column or drains laterally to lake margins, such that even when the ice pan is much degraded before complete ice‐out, the surface lacks ponded water and appears dry. This consistent difference in ice cover phenology between lakes with bedfast and floating ice has been used to classify lakes by depth across broad regions of the ACP using Landsat imagery [Sellmann et al., 1975]. In Arctic regions where lakes have open‐water for only 3–4 months a year, the difference in ice‐out timing between lakes with contrasting ice regimes may be energetically and hydrologically consequential [Arp et al., 2011], especially depending on the ice‐out timing relative to the annual peak in solar insolation (summer solstice).

Figure 2 Open in figure viewer PowerPoint An example of melt‐out conditions of a bedfast ice lake in the foreground showing ponded water atop grounded (bedfast) ice. A floating ice lake is in background indicated by a floating ice pan covering the majority of the lake surface with a narrow ice‐free margin where the ice is shallower and ice grounded. This photo was taken in the Fish Creek Watershed near Nuiqsut, Alaska, in early June 2008.

The dichotomy in ice‐out timing between bedfast and floating ice lakes likely results in differences in evaporative losses across the landscape. Bedfast ice lakes typically become ice free earlier in the Arctic summer when insolation is greatest [Arp et al., 2011], which is a time when potential evaporation is correspondingly greater [Marsh and Bigras, 1988]. This observation follows notably wide inter‐lake variability in evaporation rates in relation to climate, ice‐cover duration, and lake size gradients [Marsh and Bigras, 1988; Blanken et al., 2000; Rouse et al., 2005; Labrecque et al., 2009; Turner et al., 2010], although variation due to ice regimes (i.e., bedfast versus floating ice conditions) have not previously been considered. General assessments of Arctic water balance find that lake evaporation typically exceeds summer precipitation, suggesting the need to identify how and to what extent evaporative losses vary among the abundant lakes and ponds [Marsh and Bigras, 1988; Rovansek et al., 1996; Bowling and Lettenmaier, 2010]. All other hydrologic fluxes being equal (i.e., direct precipitation and stream inputs and other lateral inflows), understanding how evaporation varies among lakes will help decipher how not only lake, but also watershed, water balance dynamics are expected to respond to a changing climate. Such an understanding is particularly germane to uncertainty regarding hydrologic intensification in the Arctic, where both evapotranspiration and precipitation are expected to increase in a warmer climate [Rawlins et al., 2010]. In Arctic landscapes with abundant shallow thermokarst lakes such as northern Alaska, Siberian Russia, and northwestern Canada [Duguay et al., 2003], lake ice regimes may factor importantly into how hydrologic intensification is manifested at scales from individual lakes to large watersheds [Arp et al., 2012].

Accordingly, we hypothesized that lakes with bedfast ice regimes have higher evaporative loss than lakes with floating ice regimes due to earlier ice‐out timing. However, we expect that this difference in lake evaporation will lessen as the climate warms for two reasons: (1) observations show bedfast ice lakes transforming to floating ice regimes during recent decades because of thinner ice growth, which is thought to be a consequence of warmer winter temperatures and greater snowfall accumulation [Arp et al., 2012]; (2) a warmer climate may move ice‐out closer to the summer solstice, particularly for floating ice lakes. Accordingly, the effect of a difference in ice‐out timing between lake types may become less apparent, though overall evaporative losses are expected to increase. To examine this set of hypotheses, we estimated open‐water evaporation for paired sets of lakes (bedfast ice and floating ice) across a climatic gradient from the Beaufort Sea coast to the Brooks Range in northern Alaska over 3 years (2012–2014) with contrasting climate conditions (warm to cool summers). Differences in lake evaporation between ice regimes were additionally evaluated using synoptic measurement of water isotopes for a subset of lakes. Statistical relationships developed from all lakes studied over the 3 year period were used to assess patterns over a longer period during which the Arctic has been warming (1960–2009) and is projected to warm further (2010–2099). This temporal analysis used data sets from the outer ACP (Barrow, AK) where climate change has been most pronounced in Alaska [Wendler et al., 2014] and where shallow lakes are abundant and dynamic [Zhang and Jeffries, 2000; Duguay et al., 2003; Surdu et al., 2014].