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).
Potential evapotranspiration is calculated for each month. In the maps below yellow represents the highest PET and the months are April, June, and August.
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