From 78e980f755c63844bf8e08eb983feff0eb028b83 Mon Sep 17 00:00:00 2001
From: cristianchiriac <80774326+cristianchiriac@users.noreply.github.com>
Date: Sat, 17 Oct 2026 20:58:54 +0300
Subject: [PATCH] fix(model_splitter): keep maxats when copying the ATS package

_create_sln_tdis passed only the ATS perioddata to the new TDIS package,
so the split simulation's ATS file was written with the default MAXATS 1
while PERIODDATA still held every record, and MODFLOW 6 failed to read it.
Pass maxats together with perioddata.

Closes #2868
---
 autotest/test_model_splitter.py   | 37 +++++++++++++++++++++++++++++++
 flopy/mf6/utils/model_splitter.py |  9 ++++++--
 2 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/autotest/test_model_splitter.py b/autotest/test_model_splitter.py
index 041ecc430..9f5d196e8 100644
--- a/autotest/test_model_splitter.py
+++ b/autotest/test_model_splitter.py
@@ -1938,6 +1938,43 @@ def test_ats(function_tmpdir):
     assert success
 
 
+def test_ats_multiple_periods(function_tmpdir):
+    nper = 4
+    sim = flopy.mf6.MFSimulation(sim_ws=function_tmpdir)
+    tdis = flopy.mf6.ModflowTdis(
+        sim,
+        nper=nper,
+        perioddata=[(1.0, 1, 1.0)] + [(10.0, 10, 1.0)] * (nper - 1),
+    )
+    ats_perioddata = [(kper, 1.0, 0.1, 5.0, 2.0, 5.0) for kper in range(1, nper)]
+    tdis.ats.initialize(maxats=len(ats_perioddata), perioddata=ats_perioddata)
+
+    flopy.mf6.ModflowIms(sim)
+    gwf = flopy.mf6.ModflowGwf(sim, modelname="ats")
+    flopy.mf6.ModflowGwfdis(gwf, nlay=1, nrow=1, ncol=10, top=10.0, botm=0.0)
+    flopy.mf6.ModflowGwfic(gwf, strt=5.0)
+    flopy.mf6.ModflowGwfnpf(gwf, k=1.0)
+    flopy.mf6.ModflowGwfchd(
+        gwf, stress_period_data=[[(0, 0, 0), 6.0], [(0, 0, 9), 4.0]]
+    )
+
+    array = np.ones((1, 10), dtype=int)
+    array[0, 5:] = 2
+
+    mfs = flopy.mf6.utils.Mf6Splitter(sim)
+    new_sim = mfs.split_model(array)
+
+    ats = new_sim.tdis.ats
+    assert ats.maxats.get_data() == len(ats_perioddata)
+    assert len(ats.perioddata.get_data()) == len(ats_perioddata)
+
+    split_ws = function_tmpdir / "split"
+    new_sim.set_sim_path(split_ws)
+    new_sim.write_simulation(silent=True)
+    ats_text = (split_ws / ats.filename).read_text().upper().split()
+    assert ats_text[ats_text.index("MAXATS") + 1] == str(len(ats_perioddata))
+
+
 @requires_exe("mf6")
 def test_reconstruct_recarray(function_tmpdir):
     sim_path = get_example_data_path() / "mf6-freyberg"
diff --git a/flopy/mf6/utils/model_splitter.py b/flopy/mf6/utils/model_splitter.py
index 19df3656e..5790a8d8b 100644
--- a/flopy/mf6/utils/model_splitter.py
+++ b/flopy/mf6/utils/model_splitter.py
@@ -1362,8 +1362,13 @@ def _create_sln_tdis(self):
                 elif key == "ats_perioddata":
                     data = getattr(pak, "ats")
                     if len(data._packages) > 0:
-                        data = data._packages[0].perioddata.array
-                        d[key] = data
+                        # pass maxats along with perioddata, otherwise the new
+                        # ATS package keeps the default maxats of 1
+                        ats = data._packages[0]
+                        d[key] = {
+                            "maxats": ats.maxats.get_data(),
+                            "perioddata": ats.perioddata.array,
+                        }
 
                 else:
                     data = getattr(pak, key)