yukonhorizon2
Yukon Agriculture: in the context of global climate change


Abstract
Introduction and Background 
Data
Methods 
Results
Discussion
Conclusion
References
Contact
Methods  


GDD Calculations
 
Layers: Soil Texture, Proximity to Water, Proximity to Roads, Slope, EGDD
Analysis: Multicriteria Evaluation


GDD Calculations


The best way to calculate growing degree days is to use daily mean temperature values. Equipped with that data and a decided base temperature (5º C for this project), here is what you do: Tmean-Tbase = Daily GDD for example, a day with a mean temperature of 22 degrees Cº contributes 17 GDDs. Then, sum the Daily GDD to get the GDDs for the year. To get Effective GDDs, you calculate a DLF or “day length factor” that accounts for the different sunlight hours at different latitudes. I found that for the Yukon, the DLF ranges from 18% in Whitehorse (near the BC border) to 22% in Dawson City (farther north, near the arctic circle). I chose to use 18% across the Yukon, as this would underestimate the values. Further, I did not have mean daily temperatures, so I estimated with the mean monthly values I had. This inevitably leads to some error. However, I compared EGDD values calculated from monthly temperatures and the reference map of EGDD from 1990, and I found that the main effect of using monthly values was to decrease the overall variability between values and either underestimate or match the reference values. I talk more about this in the discussion section.

Layers:  (click on maps for large images)


Soil Texture:
This map was created by joining polygons from the Land Potential Database of Canada with corresponding data in an attribute table called Soil Inventory. The data joined included characteristics of soil texture, with the categories “Clayey,” “Sandy,” “Loamy,” and blank areas which appear to mostly correlate with densely glaciated, mountainous regions (such as the bottom left corner) or lakes (Kluane Lake is the little forked shape in bottom left). The layer was then converted to raster and reclassified into five classes of ascending desirability (shown by darker brown) to prepare it for the weighted overlay analysis.

isoiltexture!

Proximity to Water: This map was created from a line map of the drainage networks in the Yukon. A path distance analysis was run with a base raster of derived from a polygon of the Yukon Territory, and the distance map was reclassified into five classes with the darkest regions showing areas close to some kind of river. Unfortunately, time and metadata constraints meant I couldn't find a way to separate major rivers from creeks. This will be further addressed in the discussion section.

iriverdistancemage2

 

Proximity to Roads: This map was created similarly to the water map, with major roads as the network from which path distance was calculated. There was a limit to the extent of the calculated map, and I'm not sure where this arose from. I redid the analysis many times and made sure to use the Yukon territory as the base raster, and did not assign a maximum distance, but there continued to be a limit. This will also be discussed in the discussion section.

imagEGDDe3

Effective Growing Degree Days Reference Map: This map reflects effective growing degree days based on the EGDDs calculated by NRCAN. To build it, I joined the ecodistrict polygons to the EGDD attribute table, used a symbology of quantities, and assigned 5 discrete classes based on the classes described in the background section.

image!EGDD

 

Slope:This map was created from a 90m DEM of the Yukon Territory. Slopes were calculated with Spatial Analyst and reclassified into five classes according to flat slopes (0-5º), moderately flat slopes (5-10º), moderate slopes (10-15º), moderately steep slopes (15-25º), and very steep slopes (25º-89.9). The un-equal interval is a result of the functionality of steep slopes: past a certain point they will be unlikely to be stable and will be equally difficult to work with whether they are at 35º or 75º. Therefore I grouped all of the higher slopes into the single lowest class.

image!slope

Effective Growing Degree Days Present Day:  Each of the following maps were created in the following way:

 Using Excel, I started with mean monthly temperature values for each ecodistrict. I then built calculated tables using If/Then statements that summed all values above 5º for each month, multiplied those values by 30 to estimate the number of days in each month, and multiplied that value by 1.18 in order to obtain Effective GDD for the latitude around Whitehorse (info from Infarmation bulletin). For the 2050 and 2100 values, I first recalculated the mean monthly temperature tables to be 3º higher and 6º higher, respectively, and then carried out the same analysis. Temperature increases are based on this map for 2050 and this map for 2100 .

I also performed a minor error analysis to show that the majority of the calculated EGDD Present Day values underestimated rather than overestimated EGDD (which is good given that more EGDD is better, and an underestimate won't tend to raise false hopes). A graph of this can be found in the discussion section.

imageegdd1990

EGDD 2050

egdd2050

EGDD 2100

egdd2100
m
age!






Analysis: MCE

The MCE itself was fairly simple. Each of the above maps had 5 classes, and I reclassified them all to a simple 1 to 5 system in order to make them comparable. Then I performed a weighted overlay using these weights:

EGDD:      40%
Rivers:       25%
Slope:        15%
Roads:       10%
Soil:           10%

 
Justification for weights:

 EGDD is the most important factor, the factor which cannot be changes by human manipulations and has historically been one of the biggest limitations in the North.

 Water is a highly limiting resource in the dry Yukon, and is second most important.

 Slope is classed as third most important because of the rough terrain typical of the Yukon, and well as the uncertainty present in the road and texture layers that make them less valuable as criteria.

 Roads: From personal experience I know there are many more roads than shown on the main road map, including mining and logging roads. I believe remoteness is a general problem in the Yukon which will be delt with if the drive for access is strong enough.

 Texture: These classes were defined on a rather broad scale, and don't necessarily reflect the localized characteristics of the soil. Further, there is also the possibility of greenhouses which would decrease the necessity for good soils.