Bugfix when using pyet method to calculate PET (Deltares#918)

DavidChoi76 · Aug 1, 2024 · a707d44 · a707d44
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diff --git a/docs/changelog.rst b/docs/changelog.rst
@@ -13,11 +13,13 @@ New
 ---
 - Further automate release mechanism. (#1019)
 
+
 Changed
 -------
 
 Fixed
 -----
+- Bugfix related to use of pyet in Penman-Monteith PET calculation. (#918)
 
 Deprecated
 ----------

diff --git a/hydromt/workflows/forcing.py b/hydromt/workflows/forcing.py
@@ -666,7 +666,7 @@ def pm_fao56(
     if var == "temp_dew":
         avp = pyet.calc_e0(tmean=temp_dew)
     elif var == "rh":
-        avp = pyet.calc_e0(tmax=temp_max, tmin=temp_min, rh=temp_dew)
+        avp = pyet.calc_ea(tmax=temp_max, tmin=temp_min, rh=temp_dew)
 
     # Net radiation
     dates = pyet.utils.get_index(kin)

diff --git a/tests/test_forcing.py b/tests/test_forcing.py
@@ -43,7 +43,14 @@ def test_pet(data_catalog):
     et_data = data_catalog.get_rasterdataset("era5_daily_zarr")
     dem = data_catalog.get_rasterdataset("era5_orography").squeeze("time", drop=True)
 
-    peto = pet(et_data, et_data, dem, method="penman-monteith_tdew")
-
-    assert peto.raster.shape == dem.raster.shape
-    np.testing.assert_almost_equal(peto.mean(), 0.57746, decimal=4)
+    peto_tdew = pet(et_data, et_data, dem, method="penman-monteith_tdew")
+    assert peto_tdew.raster.shape == dem.raster.shape
+    np.testing.assert_almost_equal(peto_tdew.mean(), 0.57746, decimal=4)
+
+    # Convert to relative humidity using simple approach
+    # as relative humidity is not provided in ERA5 data
+    # Lawrence (2005) dx.doi.org/10.1175/BAMS-86-2-225
+    et_data["rh"] = 5 * et_data["temp_dew"] + 100 - 5 * et_data["temp"]
+    peto_rh = pet(et_data, et_data, dem, method="penman-monteith_rh_simple")
+    assert peto_rh.raster.shape == dem.raster.shape
+    np.testing.assert_almost_equal(peto_rh.mean(), 0.51279, decimal=4)