EI30.awk
 
 ================================================
 José Eugenio López Periago
 Universidade de Vigo
 Faculty of Sciences 32004 Spain
 vie oct 10 19:51:55 CEST 2014
 ================================================
 
 This is a awk(*) script that calculates and displays the EI30 erosivity
 index from rainfall gauge records.
 The sampling interval must be specified at the source code 

 [usage]: ' awk -v interval=5 -f EI30.awk < test_filename.dat'
  
 In this example use a raingauge with a sampling interval equals to 5 minutes
 -v interval =5

 For the EI calculations, a break between storms is
 defined as 6 h or more with less than 12.7 mm of
 precipitation. Rains less than 12.7 mm, and separated
 from other storms by 6 or more hours, are moitted
 as insignificant unless the maximum 15 min intensity
 exceeds 24 mm h^-1 (Wischmeier and Smith, 1978)

 This scrip separate storms with this criterion. Then,
 the rainfall energy  is calculated for each time interval as:


function energy(rain_intensity)
        {
        res = 0.29 * (1 - 0.72 * exp(-0.05 * rain_intensity))
        return(res)
        }

 where rain_intensity is the average rainfall intensity during 
 the time interval espressed as (mm hr -1) (Brown and Foster, 1987).
 The functions returns the energy  in  E(MJ/ha)



Note:

 (*)AWK is an interpreted programming language designed for text processing
 and typically used as a data extraction and reporting tool. It is a
 standard feature of most Unix-like operating systems.
 More info at:
 http://en.wikipedia.org/wiki/AWK 
 http://es.wikipedia.org/wiki/AWK  (spanish)

 A version for windows of awk can be found at:
 http://gnuwin32.sourceforge.net/packages/gawk.htm

References:


 Brown, L.C. and Foster, G.R. 1987. Storm erosivity using idealized
 intensity distributions. Transaction of the ASAE 30:379-386.

 Wischmeier, W.H. and D.D. Smith. 1978. Predicting rainfall-
 erosion losses - A guide to conservation farming. U.S. Dept.
 of Agric., Agr. Handbook No. 537.

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