Add positive chill units (#2003)

### What kind of change does this PR introduce?

* Adds a new argument to `chill_unit` `positive_only` to compute the
daily positive chill units

CHANGELOG.rst

@@ -4,7 +4,7 @@ Changelog
 
 v0.54.0 (unreleased)
 --------------------
-Contributors to this version: Trevor James Smith (:user:`Zeitsperre`), Pascal Bourgault (:user:`aulemahal`), Éric Dupuis (:user:`coxipi`).
+Contributors to this version: Trevor James Smith (:user:`Zeitsperre`), Pascal Bourgault (:user:`aulemahal`), Éric Dupuis (:user:`coxipi`), Sascha Hofmann (:user:`saschahofmann`).
 
 Breaking changes
 ^^^^^^^^^^^^^^^^
@@ -30,6 +30,10 @@ CI changes
 ^^^^^^^^^^
 * Added the `green-coding-solutions/eco-ci-energy-estimation` GitHub Action to the workflows to establish energy and carbon usage of CI activity. (:pull:`1863`).
 
+New features and enhancements
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+* ``chill_unit`` now accepts a new argument `positive_only` to compute the daily positive chill units. (:pull:`2003`).
+
 v0.53.2 (2024-10-31)
 --------------------
 Contributors to this version: Éric Dupuis (:user:`coxipi`), Pascal Bourgault (:user:`aulemahal`), Trevor James Smith (:user:`Zeitsperre`).

tests/test_indices.py

@@ -372,6 +372,10 @@ def test_chill_units(self, tas_series):
         out = xci.chill_units(tas)
         assert out[0] == 0.5 * num_cu_05 + num_cu_1 - 0.5 * num_cu_min_05 - num_cu_min_1
 
+        out = xci.chill_units(tas, positive_only=True)
+        # Only the last day contains negative chill units.
+        assert out[0] == 0.5 * num_cu_05 + num_cu_1 - 0.5 * 3
+
     def test_cool_night_index(self, open_dataset):
         ds = open_dataset("cmip5/tas_Amon_CanESM2_rcp85_r1i1p1_200701-200712.nc")
         ds = ds.rename(dict(tas="tasmin"))

xclim/indices/_agro.py

@@ -1635,17 +1635,21 @@ def chill_portions(
 
 
 @declare_units(tas="[temperature]")
-def chill_units(tas: xarray.DataArray, freq: str = "YS") -> xarray.DataArray:
+def chill_units(
+    tas: xarray.DataArray, positive_only: bool = False, freq: str = "YS"
+) -> xarray.DataArray:
     """Chill units using the Utah model
 
     Chill units are a measure to estimate the bud breaking potential of different crop based on Richardson et al. (1974).
     The Utah model assigns a weight to each hour depending on the temperature recognising that high temperatures can actual decrease,
-    the potential for bud breaking.
+    the potential for bud breaking. Providing `positive_only=True` will ignore days with negative chill units.
 
     Parameters
     ----------
     tas : xr.DataArray
         Hourly temperature.
+    positive_only : bool
+        If `True`, only positive daily chill units are aggregated.
 
     Returns
     -------
@@ -1680,4 +1684,8 @@ def chill_units(tas: xarray.DataArray, freq: str = "YS") -> xarray.DataArray:
         ),
     )
     cu = cu.where(tas.notnull())
+
+    if positive_only:
+        daily = cu.resample(time="1D").sum()
+        cu = daily.where(daily > 0)
     return cu.resample(time=freq).sum().assign_attrs(units="")