Wednesday, March 25, 2015

Updates to the Climatic Water Deficit Toolbox part II

Here I walk through an example using the West Walker River watershed in Nevada/California USA. The watershed is 2575 square kmand took 5 minutes and 9 seconds to run for the 30 year normals. Lower elevations are extremely arid (7 inches or 170 mm) while upper elevations in the Sierra Nevada can receive significant snowfall (> 9000 mm at high elevations). Below is the DEM used as well as a SSURGO soils map (light colors represent higher soil water holding capacity).

The first steps involve partitioning precipitation into snow vs. rain fractions using a melt factor and then determining the amount of snowmelt + rain to get the water supply. The maps below show water supply for March, April, and May moving upslope due to snowmelt. The purple areas are the areas of greatest water availability.

We use the heat load index of McCune and Keon (2002) to apply a fine-scale topographic influence on potential evapotranspiration. The heat load index is fixed in space and gets applied as a multiplier in the potential evapotranspiration calculations. In the map below reds show areas of high heat load (e.g. south-facing slopes).