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Fix delta_kt_prime halving single valid neighbor difference (gh-1847) #2821

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Fix delta_kt_prime halving single valid neighbor difference (gh-1847) #2821

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969d39b
Fix delta_kt_prime halving single valid neighbor difference (gh-1847)
adi-IL Jul 12, 2026
9045a48
Trim verbose comment in _delta_kt_prime_dirint (ponytail pass)
adi-IL Jul 13, 2026
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7 changes: 6 additions & 1 deletion docs/sphinx/source/whatsnew/v0.15.3.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,11 @@ Deprecations


Bug fixes
~~~~~~~~~
~~~~~~~~

* Fixed ``irradiance.dirint`` so that an interior missing value in the input
series no longer halves the single valid neighbor difference when computing
the stability index ``delta_kt_prime`` (Perez eqn 3). (:issue:`1847`)


Enhancements
Expand Down Expand Up @@ -53,4 +57,5 @@ Contributors
* Karl Hill (:ghuser:`karlhillx`)
* Mathias Aschwanden (:ghuser:`maschwanden`)
* Yonry Zhu (:ghuser:`yonryzhu`)
* Aditya (:ghuser:`adi-IL`)
* Darshan Gowda (:ghuser:`dgowdaan-cmyk`)
14 changes: 6 additions & 8 deletions pvlib/irradiance.py
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Expand Up @@ -2035,16 +2035,14 @@ def _delta_kt_prime_dirint(kt_prime, use_delta_kt_prime, times):
for use with :py:func:`_dirint_bins`.
"""
if use_delta_kt_prime:
# Perez eqn 2
# row-wise mean of neighbor abs-differences; pandas skips NaN so this
# covers 0/1/2 valid neighbors (Perez eqn 2 interior, eqn 3 boundary/gap)
kt_next = kt_prime.shift(-1)
kt_previous = kt_prime.shift(1)
# replace nan with values that implement Perez Eq 3 for first and last
# positions. Use kt_previous and kt_next to handle series of length 1
kt_next.iloc[-1] = kt_previous.iloc[-1]
kt_previous.iloc[0] = kt_next.iloc[0]
delta_kt_prime = 0.5 * ((kt_prime - kt_next).abs().add(
(kt_prime - kt_previous).abs(),
fill_value=0))
delta_kt_prime = pd.DataFrame({
'next': (kt_prime - kt_next).abs(),
'prev': (kt_prime - kt_previous).abs(),
}).mean(axis=1)
else:
# do not change unless also modifying _dirint_bins
delta_kt_prime = pd.Series(-1, index=times)
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11 changes: 11 additions & 0 deletions tests/test_irradiance.py
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Expand Up @@ -719,6 +719,17 @@ def test_dirint_nans():
np.array([np.nan, np.nan, np.nan, np.nan, 893.1]), 1)


def test_delta_kt_prime_interior_nan():
# regression test for gh-1847: an interior NaN in kt_prime must not
# halve the single valid neighbor difference. When only one neighbor
# is available (Perez eqn 3) delta_kt_prime equals that difference.
times = pd.date_range(start='2020-01-01', periods=5, freq='1h')
kt_prime = pd.Series([0.5, 0.6, np.nan, 0.7, 0.4], index=times)
result = irradiance._delta_kt_prime_dirint(kt_prime, True, times)
expected = pd.Series([0.1, 0.1, np.nan, 0.3, 0.3], index=times)
assert_series_equal(result, expected)


def test_dirint_tdew():
times = pd.DatetimeIndex(['2014-06-24T12-0700', '2014-06-24T18-0700'])
ghi = pd.Series([1038.62, 254.53], index=times)
Expand Down