fix(prt): fix default release timing

Author: wpbonelli

autotest/test_prt_release_timing.py

@@ -56,12 +56,20 @@
     f"{simname}multi",  # FIRST in period 1, ALL in period 2, FIRST in period 3
     # test explicit release time and period-block release on a time step boundary
     f"{simname}bndy",  # RELEASETIMES: 1.0; FIRST in period 2 (also t=1.0)
+    # test default release (no period block, no release times) with multiple periods
+    f"{simname}dflt",  # no config: should release only at t=0 despite 3 periods
 ]
 
 
 def get_perioddata(name, periods=1) -> Optional[dict]:
     opt = []
-    if "sgl" in name or "dbl" in name or "open" in name or "tol" in name:
+    if (
+        "sgl" in name
+        or "dbl" in name
+        or "open" in name
+        or "tol" in name
+        or "dflt" in name
+    ):
         return None
     if "bndy" in name:
         return {
@@ -111,10 +119,10 @@ def build_prt_sim(name, gwf_ws, prt_ws, mf6):
     )
 
     # create tdis package
-    # 3 periods for fill-forward, multi-period, and boundary, 1 period for others
+    # 3 periods for fill-forward, multi-period, boundary, and default, 1 for others
     nper = (
         3
-        if ("fill" in name or "multi" in name or "bndy" in name)
+        if ("fill" in name or "multi" in name or "bndy" in name or "dflt" in name)
         else FlopyReadmeCase.nper
     )
     if "multi" in name:
@@ -136,7 +144,7 @@ def build_prt_sim(name, gwf_ws, prt_ws, mf6):
                 FlopyReadmeCase.tsmult,
             ),  # Period 2: 1 time step
         ]
-    elif "fill" in name or "bndy" in name:
+    elif "fill" in name or "bndy" in name or "dflt" in name:
         perioddata = [
             (
                 FlopyReadmeCase.perlen,
@@ -335,8 +343,13 @@ def build_models(test):
         test.name, test.workspace, test.targets["mf6"]
     )
 
-    # For fill-forward, multi-period, and boundary, update GWF to use 3 periods
-    if "fill" in test.name or "multi" in test.name or "bndy" in test.name:
+    # For fill-forward, multi-period, boundary, and default, update GWF to use 3 periods
+    if (
+        "fill" in test.name
+        or "multi" in test.name
+        or "bndy" in test.name
+        or "dflt" in test.name
+    ):
         tdis = gwf_sim.get_package("tdis")
         tdis.nper = 3
         if "multi" in test.name:
@@ -457,7 +470,7 @@ def check_output(test, snapshot):
     # Multi-period tests use 3 periods, others use 1
     nper = (
         3
-        if ("fill" in name or "multi" in name or "bndy" in name)
+        if ("fill" in name or "multi" in name or "bndy" in name or "dflt" in name)
         else FlopyReadmeCase.nper
     )
     check_budget_data(
@@ -539,6 +552,15 @@ def check_output(test, snapshot):
         expected_kpers = [1, 2]
         assert unique_kpers == expected_kpers
 
+    # check default release case: no config at all, 3 stress periods.
+    # particles should be released exactly once at t=0 (start of simulation),
+    # not at the start of every stress period.
+    if "dflt" in name:
+        release_times = sorted(mf6_pls["trelease"].unique())
+        assert release_times == [0.0], (
+            f"Expected releases only at t=0.0, got: {release_times}"
+        )
+
     # convert mf6 pathlines to mp7 format
     mf6_pls = to_mp7_pathlines(mf6_pls)
 

doc/ReleaseNotes/develop.toml

@@ -77,4 +77,9 @@ description = "Fix the PRT PRP package's DRAPE option's behavior. A draped parti
 [[items]]
 section = "fixes"
 subsection = "basic"
-description = "Previously, GWF model output files could only be referenced by a single coupled model's FMI package. This might be necessary for certain configurations where multiple transport models or particle tracking models in the same simulation need flow information from a previous GWF simulation. Referencing the same GWF output files from multiple coupled models would cause a crash. Fix file-opening logic to allow multiple coupled models to refer to the same GWF model's output files."
+description = "Previously, GWF model output files could only be referenced by a single coupled model's FMI package. This might be necessary for certain configurations where multiple transport models or particle tracking models in the same simulation need flow information from a previous GWF simulation. Referencing the same GWF output files from multiple coupled models would cause a crash. Fix file-opening logic to allow multiple coupled models to refer to the same GWF model's output files."
+
+[[items]]
+section = "fixes"
+subsection = "model"
+description = "PRT was described as configuring a single release at the beginning of the simulation by default, but a release was in fact scheduled for the first time step of every stress period. Fix the default release behavior so a single release occurs at the beginning of the simulation."

src/Model/ParticleTracking/prt-prp.f90

@@ -752,11 +752,10 @@ subroutine prp_rp(this)
       ! If the user hasn't provided any release settings (neither
       ! explicit release times, release time frequency, or period
       ! block release settings), default to a single release at the
-      ! start of the first period's first time step.
-      ! Store default configuration; advance() will be called in prp_ad().
-      if (allocated(this%period_block_lines)) deallocate (this%period_block_lines)
-      allocate (this%period_block_lines(1))
-      this%period_block_lines(1) = "FIRST"
+      ! start of the simulation (t=0). Add t=0 directly to the time
+      ! selection rather than time step selection because the latter
+      ! would fill forward, releasing at the start of every period.
+      call this%schedule%time_select%extend([DZERO])
       return
     else if (iper /= kper) then
       return