biome.yml

desert:
  lat: [1, 50]
  long [20, 60]
  file: "netcdf/GCS/biomes/Desert.nc"
  var: dsrt
marsh:
  lat: [51, 150]
  long [120, 160]
  file: "netcdf/GCS/biomes/Marsh.nc"
  var: mrsh

controller.rb

class YourController
  def new
    ...
    biome = setup_biome
  end

  private

  def setup_biome
    biome = YourModel.get_biome params[:lat], params[:long]
    # Set your instance vars and read the file here
  end
end

model.rb

class YourModel
  BIOMES = YAML::load open("biomes.yml").read

  def self.get_biome_config(lat, long)
    BIOMES.detect do |biome|
      biome["lat"][0] < lat && biome["lat"][1] > lat &&
      biome["long"][0] < long && biome["long"][1] > lat
    end
  end
end