WEPP climate files want daily averaged dew point. This value is provided by the IEM Daily ReAnalysis . It is derived by a gridded analysis of surface observation data.
Of course, precipitation is vital to erosion modeling. One of the unique aspects of our project is how we use high resolution depictions of precipitation to drive our estimates. One of the major updates included with IDEPv2 was the change from using NCEP stage IV precipitation estimates to NCEP MRMS estimates. Here is an example:
The MRMS product provides intensity estimates on a 2 minute timestep. The prospect of directly including these 2 minute interval data within the WEPP climate file would inflate the filesize and runtime. So an algorithm is used to collapse down the 2 minute interval data into periods that mostly retain the largest rates. This algorithm writes breakpoints whenever the instantaneous 2 minute accumulatio is over 1 mm and/or the accumulated precipitation since the last breakpoint is 2 mm. The result is a WEPP climate file with fewer break points, but still have the peak intensities captured. Any day without at least 0.254 mm accumulation is set to zero. If the day accumulation was less than 2 mm, the breakpoints used are simply the start and end timestamp of the precipitation event.
The amount of energy received by the Earth's surface from the sun is the important driver of plant growth. The WEPP model contains a crop growth model that is sensitive to daily solar radiation data. For IDEPv2, this information is provided on a 0.01 degree grid based on a nearest neighbor analysis of Iowa State Soil Moisture Network data. This routine does not produce fine scale detail as there are only 20 some observations in the state. In the future, the hope is to use high resolution numerical modelling analysis to provide fine scale detail.
The realtime runs are started at 12:30 AM each day. On some days, for various reasons, the solar radiation summary data is not yet available at this time. When this situation happens, the climate file generation script uses the previous day's data as today's data. A process later in the day attempts to correct this estimate as the actual data comes in. Having missing solar radiation data at 12:30 AM is not considered a show stopping issue and the previous day's estimates are "good enough".
WEPP requires the daily high and low temperature. In general and with Iowa's flat terrain, these values are very smooth over the state. An approximate 24 km analysis of Iowa Mesonet observations provides these variables to WEPP.
WEPP wants the daily average wind speed and direction. For our purposes, the wind direction is unused so is always set to north (0 degrees). The averaged wind speed is provided by the IEM Daily ReAnalysis.
The initial choice of a 10 mm precipitation accumulation prior to
writing a breakpoint file was a bit arbitrary. A sensitivity test was
done to investigate two options available for the breakpoint calculation
These two parameters were varied from 1 to 20 mm, with each combination producing a WEPP run for comparison. This comparison was made for 11 June 2015 at longitude 94.86W and 40.84N. Each plot shown below is normalized to the maximum value found in the plot. This allows for simple relative comparison to the amount of change that existed with the parameter change.
This figure displays the number of breakpoints computed.
This figure displays the WEPP computed amount of soil displacement.
This figure displays the resulting largest precipitation rate based on the break points generated.
This figure displays the WEPP computed delivery.
This figure displays the WEPP computed runoff. Interestingly, it appears that runoff is not sensitive to the intensities found in the climate file.