move check for snow depth

pvlib/snow.py

@@ -155,6 +155,14 @@ def coverage_nrel(snowfall, poa_irradiance, temp_air, surface_tilt,
     # don't slide in the interval preceding the snowfall data
     slide_amt.iloc[0] = 0
 
+    if snow_depth is not None:
+        # all slides off if there is no snow on the ground
+        # described in [2] to avoid non-sliding snow for low-tilt systems.
+        # default threshold_depth of 1cm is from SAM's implementation and
+        # is different than the value of 0cm implied in [2].
+        # https://github.com.mcas-gov.ms/NREL/ssc/issues/1265
+        slide_amt[snow_depth < threshold_depth] = 1.
+
     # build time series of cumulative slide amounts
     sliding_period_ID = new_snowfall.cumsum()
     cumulative_sliding = slide_amt.groupby(sliding_period_ID).cumsum()
@@ -166,13 +174,13 @@ def coverage_nrel(snowfall, poa_irradiance, temp_air, surface_tilt,
     snow_coverage.ffill(inplace=True)
     snow_coverage -= cumulative_sliding
 
-    if snow_depth is not None:
-        # no coverage when there's no snow on the ground
-        # described in [2] to avoid non-sliding snow for low-tilt systems.
-        # default threshold_depth of 1cm is from SAM's implementation and
-        # is different than the value of 0cm implied in [2].
-        # https://github.com.mcas-gov.ms/NREL/ssc/issues/1265
-        snow_coverage[snow_depth < threshold_depth] = 0.
+    # if snow_depth is not None:
+    #     # no coverage when there's no snow on the ground
+    #     # described in [2] to avoid non-sliding snow for low-tilt systems.
+    #     # default threshold_depth of 1cm is from SAM's implementation and
+    #     # is different than the value of 0cm implied in [2].
+    #     # https://github.com.mcas-gov.ms/NREL/ssc/issues/1265
+    #     snow_coverage[snow_depth < threshold_depth] = 0.
     # clean up periods where row is completely uncovered
     return snow_coverage.clip(lower=0)
 

pvlib/tests/test_snow.py

@@ -55,20 +55,21 @@ def test_coverage_nrel_hourly_with_snow_depth():
     surface_tilt = 45
     slide_amount_coefficient = 0.197
     threshold_depth = 0.5
-    dt = pd.date_range(start="2019-1-1 10:00:00", end="2019-1-1 17:00:00",
+    dt = pd.date_range(start="2019-1-1 10:00:00", end="2019-1-1 18:00:00",
                        freq='1h')
-    poa_irradiance = pd.Series([400, 200, 100, 1234, 134, 982, 100, 100],
+    poa_irradiance = pd.Series([400, 200, 100, 1234, 134, 982, 100, 100, 100],
                                index=dt)
-    temp_air = pd.Series([10, 2, 10, 1234, 34, 982, 10, 10], index=dt)
-    snowfall_data = pd.Series([1, .5, .6, .4, .23, -5, .1, .1], index=dt)
-    snow_depth = pd.Series([1, 1, 1, 1, 0, 1, 0, .1], index=dt)
+    temp_air = pd.Series([10, 2, 10, 1234, 34, 982, 10, 10, 10], index=dt)
+    # restarts with new snow on 5th time step
+    snowfall_data = pd.Series([1, .5, .6, .4, .23, 5., .1, .1, 0.], index=dt)
+    snow_depth = pd.Series([1, 1, 1, 1, 0, 1, 1, 0, .1], index=dt)
     snow_coverage = snow.coverage_nrel(
         snowfall_data, poa_irradiance, temp_air, surface_tilt,
         snow_depth=snow_depth, threshold_snowfall=0.6,
         threshold_depth=threshold_depth)
 
     slide_amt = slide_amount_coefficient * sind(surface_tilt)
-    covered = 1.0 - slide_amt * np.array([0, 1, 2, 3, 4, 5, 6, 7])
+    covered = 1.0 - slide_amt * np.array([0, 1, 2, 3, 0, 0, 1, 0, 0])
     expected = pd.Series(covered, index=dt)
     expected[snow_depth < threshold_depth] = 0
     assert_series_equal(expected, snow_coverage)