The lack of any truly remarkable dryness in the precipitation data led me to consider the other side of the water balance equation, i.e. the rate of evaporation. Estimates of evaporation or evapotranspiration rates can be calculated from meteorological variables, or evaporation can be measured with an evaporation pan. I believe the latter measurements are taken at the UAF experiment station in Fairbanks, but I haven't yet found the data online. In lieu of these observations, and as a first look at potential evaporation rates, I calculated the vapor pressure deficit from hourly observations at Fairbanks airport. The vapor pressure deficit (VPD) is the difference between the saturation vapor pressure and the actual vapor pressure, and in theory is proportional to evaporation rates, excluding the influence of solar radiation and wind; the VPD is a key factor in the theoretical evapotranspiration - see here for an example.
The chart below shows the mean VPD, together with the mean temperature and dewpoint, for June through August of each year since 1950. The result is remarkable: the summer months of last year brought much higher VPD, and presumably therefore much higher evaporation, than normal, and the three-month mean VPD was the highest on record. Based on the hourly observations, the June-August mean temperature was more than 2 °C above the 1981-2010 normal, but the mean dewpoint was 0.2 °C below normal.
The series of charts below shows the same information, but for each month from May through September. Perhaps the most striking feature is the magnitude of the VPD anomaly in June 2013; the June mean VPD was the highest on record. Based on the hourly observations, the mean June temperature was higher than in 2004, but the mean dewpoint was 2.8 °C lower than in 2004 and only a little above normal.
An interesting aspect of the long-term history is that for each month except September, the 1950-2013 linear trend in dewpoint has a smaller slope than the trend for temperature, so this has contributed to increasing VPD over time. Of course, the VPD has also increased simply because the temperature has increased. In May and August, the dewpoint trend is actually negative, i.e. the air has been getting slightly drier in an absolute sense in these months. The difference between the temperature and dewpoint trends is most notable for May, and consequently the May VPD has increased by about 3.6 percent per decade.
In conclusion, it seems likely that the observed dryness of the white spruce in interior Alaska this year is mostly a result of the very high evapotranspiration during the 2013 growing season; and conditions this year so far are not helping to improve the situation. As a next step, it would be interesting to try to factor in the contribution of excess solar radiation, which probably also boosted evaporation rates last year, and also the effect of mean wind speeds.
Very nicely done Richard. Do you know if the Fairbanks 11 NE CRN station has a pan evaporation setup? They do have some sort of soil moisture sensor but I don't know much about it.ReplyDelete
Brian, it appears not. Pan evaporation is not listed under measured elements, and the variable is missing in the download of "all variables" from the hourly database. However, they do have nice sub-hourly radiation and wind speed measurements, so it would be possible to calculate estimated evapotranspiration including these extra influences (but only from 2003-present).Delete
Very interesting analysis Richard.ReplyDelete
I've just started a lit review of white spruce Picea glauca. Here's a couple of preliminary sources. The first is a link to the local Bonanza Creek Experimental Forest. Their bibliography search function yields a wealth of information regarding that tree species, and of course others.
The second is a paper from Canada that discusses some potential environmental influences on white spruce growth and condition. It's not tight but is informative.
As you suggest, there are indications of multi-year influences on the health and growth of boreal trees. Potential changes in Alaskan climate will likely provoke a response to survive.
Even better than the previous Canadian link, from the Bonanza Creek lit survey:Delete
Gary, thanks for digging these up. Interesting to read that "Climatic conditions during the current growing season —Delete
the time when cambial activity is actually occurring — were consistently less important than climatic conditions during the 12
months prior to the onset of cambial activity."
Temperature and precipitation are obviously important predictors but humidity variations may also have great significance and may vary independently for as-yet unknown reasons. The warm, dry spell in summer 2013 was shorter than in 2004, but the sustained low humidity was perhaps a bigger stress factor.
After reading several of the citations it appears stress factors for white spruce are a multi-seasonal event. Soil moisture, humidity, commensal organisms, insects, exposure to sun and wind, are all implicated via ongoing experiments.Delete
Of interest to me is the dire prediction for the future of white spruce in Interior Alaska. If the climate trend continues, and unless they can migrate to refugia both north and up, some believe they may be doomed to range and abundance declines in the next century. That may have happened before however, as some of their dispersal during the relatively recent Holocene likely came from within Alaska.
The LTER station websites for here in Fairbanks and at Toolik Lake are great sources of continuing information. I first went to Toolik in 1976, and later used the lake shore for camping and refueling aircraft. Now it and the nearby lakes, streams, and tundra are a first class research facility, similar to Bonanza Creek locally. Good stuff to follow.
There are formulas out there for estimating evaporation and evapotranspiration from meteorological data. Last year I cam across an EPA data set that provides daily evaporation for all major stations in Alaska for the 1961-1990 time period ( http://www2.epa.gov/exposure-assessment-models/meteorological-data-alaska ). Here is a reference to how they calculated the values ( http://epa.gov/athens/publications/reports/Suarez600R07053USMetData.pdf ).ReplyDelete