Python scripts for calculating Weather Severity Index using NetCDF NARR data
Each year millions of waterfowl migrate from their breeding grounds in arctic, northern and mid-latitudes to more southern locales to exploit abundant food and wetland resources as freezing wetlands and snowfall progress from north to south in North America. These migrating ducks, geese, and swans consume an abundance of wetland plants and seeds, as well as agricultural waste grain; they distribute seeds and invertebrates in their guts and on their feathers and provide a substantial cultural resource for wildlife watchers and hunters alike. The age old migration of waterfowl has been awaited by many generations of North Americans but, in an age of emerging and evolving science regarding climate change, discussions among scientists, waterfowl hunters and bird watchers about the stability of these traditional migrations are increasingly common. Anecdotal, as well as empirical evidence suggest that waterfowl are now migrating later than traditionally observed and remaining at northern latitudes later into autumn-winter.
Autumn migration is influenced by three main factors that result in yearly differences in the timing of duck movements in North America. Individual species of waterfowl migrate based on cues that helped their species survive for millennia including, 1) changes in day length (photoperiod), 2) habitat suitability, and 3) weather severity. The species of waterfowl we see migrating each autumn, the timing of their arrival to southern latitudes, and their abundance are a combination of these three main factors.
A Weather Severity Index (or WSI) to explain changes in abundance of Mallards and Other Dabbling Ducks at staging areas during autumn migration was developed because an experimental method was needed to estimate the potential influence of a warming climate on autumn-winter distributions of ducks in North America. Thus, we answered the question (Schummer et al. 2010), “What weather thresholds make dabbling ducks migrate?” Using different combinations of temperature and snow depth data available on the Internet and waterfowl counts from Conservation Areas in Missouri, it was determined that the cumulative effects of temperature and snow over several days significantly influenced southward migrations by Mallards.
This web application allows you to view a animated map of waterfowl migration and download a table which has summed the square kilometers of area with WSI values ≥ 7.2 (for mallards). You can select your Area of Interest category (Entire dataset, Flyway, LCC, Joint Venture, or state) and a sub-category (Example: If you select LCC as the category you will select one of the available LCCs as the sub-category). Once these have been selected you must input dates of interest. The dataset starts September 01, 1979 and ends December 31st, 2012.
WSI Description
Using different combinations of temperature and snow depth data available on the Internet and waterfowl counts from Wildlife Areas and Refuges, Schummer et al. (2010) determined that the cumulative affects of temperature and snow over several days influenced mallard migration from northern and mid-latitudes towards their southern wintering grounds. One cold evening may not force large numbers of mallards to migrate, but consecutive days below freezing makes it difficult for even a large duck like a mallard to maintain body fat. Fat is a key component for fall and winter survival , but when ducks are subjected to below freezing temperatures they burn fat to maintain core body temperature (which is ~104◦F) and often opt to head south warmer regions. Other factors also influence migration, such as snow and ice cover that reduces the functional availability of foods that mallards eat in wetlands and crop fields. Overall, the combined effects of several consecutive days of below freezing temperaturs and snow cover best explain mallard migration out of northern and mid-latitude staging areas. To calculate the mallard WSI, someone only needs the current daily mean temperature (in degrees Celsius), and snow cover (in inches). When the WSI increases, so does the likelihood that mallards will leave northern and mid-latidues for areas to the south.
To calculate the WSI 1/ use the daily mean temperature and switch the sign. For example, -5◦C would be given a value of 5 and +5◦C would be given a value of -5 2/ count up the consecutive days when the mean daily temperature was below zero, if two days in a row were below freezing your value would be 2 3/ determine the snow depth in inches, if there is 5 inches of snow on the ground your value would be 5 4/ determine the number of consecutive day when there was measurable snow on the ground, if the 5 inches of snow had been on the ground for 3 days, your value would be 3 5/ add the numbers from 1 through 4 above, this is your WSI value. When the WSI value is 7.2 or greater there is increased likelihood of decreasing mallard abundance between two days at your location.
Please visit http://gisweb.ducks.org/wsi/ for more information and to access the application