import numpy as np 
def altblocks(idf,dur,dt,RP): aDur = np.arange(dt,dur+dt,dt) 
# in minutes 
aInt = (idf[0]*RP**idf[1])/((aDur+idf[2])**idf[3]) 
# idf equation - in mm/h 
aDeltaPmm = np.diff(np.append(0,np.multiply(aInt,aDur/60.0))) 
aOrd=np.append(np.arange(1,len(aDur)+1,2)[::-1],np.arange(2,len(aDur)+1,2)) 
prec = np.asarray([aDeltaPmm[x-1]) 
for x in aOrd] 
     aAltBl = np.vstack((a,b)) 
     return aAltBl