review improvements

pvlib/soiling.py

@@ -78,15 +78,15 @@ def hsu(rainfall, cleaning_threshold, tilt, pm2_5, pm10,
 
     # determine the time intervals in seconds (dt_sec)
     dt = rainfall.index
-    dt2 = np.roll(dt.copy(), 1)
     # subtract shifted values from original and convert to seconds
-    dt3 = np.delete((dt-dt2), 0).total_seconds()
-    # append last value to end so same number of elements in the array
-    dt_sec = np.append(dt3[0], dt3).astype('float64')
+    dt_diff = (dt[1:] - dt[:-1]).total_seconds()
+    # ensure same number of elements in the array, assuming that the interval
+    # prior to the first value is equal in length to the first interval
+    dt_sec = np.append(dt_diff[0], dt_diff).astype('float64')
 
     horiz_mass_rate = (
         pm2_5 * depo_veloc['2_5'] + np.maximum(pm10 - pm2_5, 0.)
-        * depo_veloc['10']) * dt_sec  # noqa: W503
+        * depo_veloc['10']) * dt_sec
     tilted_mass_rate = horiz_mass_rate * cosd(tilt)  # assuming no rain
 
     # tms -> tilt_mass_rate