New NADP Concentration and Deposition Maps
In 2011, the NADP Technical Committee instituted changes in the way NADP annual concentration and deposition maps are produced.Two significant changes were made:
- The discrete contour map style previously used was replaced with a continuous color gradient map style
- An external, highly resolved precipitation dataset (PRISM) was used to develop the deposition surfaces.
The interpolation procedures used to produce the concentration maps are the same as before. However, the methods used to develop the deposition maps are significantly different. These differences arise due to the fact that the annual precipitation surfaces, used in creating the new deposition maps, no longer rely only on NADP precipitation measurements. Instead, the annual composite precipitation surfaces are now derived from an adapted version of a high resolution precipitation model developed by the PRISM Climate Group, and supplemented with NADP precipitation observations. PRISM stands for "Parameter-elevation Regression on Independent Slopes Model". The PRISM modeled precipitation estimates incorporate point observation data, a reliable digital elevation model (DEM), and expert knowledge of complex climatic variables that result in high resolution, continuous, digital grid estimates of total annual precipitation.
NADP modified the original PRISM surfaces by adding in the additional NADP precipitation observations. The new annual precipitation surfaces were created by using an inverse distance weighting (IDW) method to calculate a weighted value for every grid cell within a 30 km radius of each NADP precipitation site. The weighted values are calculated using a combination of the PRISM modeled precipitation data, and the NADP observed precipitation values. The weighting function was established so that as you approached the edge of the 30 km radius the values of the weighted grid cells approached that of PRISM. Outside of the 30 km radius the annual precipitation grid cells were populated using only PRISM data. The new annual precipitation surfaces have a spatial resolution of 2338.383 meters, or half the cell size of the original projected PRISM data. An Albers Equal Area USGS map projection was used.
This process resulted in annual precipitation, and subsequently deposition maps that should show significant improvement in the estimation of interpolated values, especially in regions of highly complex terrain such as the Rocky Mountains.