Moving waste to a city's existing gas capture changes its emissions

SWEET_python/city_params.py

@@ -450,6 +450,54 @@ def __init__(self, code: str, message: str):
         super().__init__(self.message)
 
 
+def _move_to_existing_gas_capture(
+    split_fractions: SplitFractions, existing_gas_pct: float
+) -> SplitFractions:
+    """The city's disposal split with ``existing_gas_pct`` of it going to the
+    landfills it already has with gas capture: the City DST's "move waste to
+    existing gas capture" lever.
+
+    The landfill and dumpsite shares without capture take the rest,
+    ``1 - existing_gas_pct``, in the proportions they had to each other. A city
+    sending 34% of its waste to capture and 66% to a landfill without it, moved to
+    90%, sends the other 10% to that landfill.
+
+    Raises CustomError("INVALID_PARAMETERS") when the share is outside [0, 1], when
+    the city has no landfill with gas capture to move waste to, and when it has no
+    site without capture to move waste away to.
+    """
+    if not 0.0 <= existing_gas_pct <= 1.0:
+        raise CustomError(
+            "INVALID_PARAMETERS",
+            "The share of waste to existing gas capture must be between 0 and 1, "
+            f"not {existing_gas_pct}",
+        )
+    with_capture = split_fractions.landfill_w_capture
+    without_capture = split_fractions.landfill_wo_capture + split_fractions.dumpsite
+    if with_capture <= 0:
+        if existing_gas_pct > 0:
+            raise CustomError(
+                "INVALID_PARAMETERS",
+                "This city has no landfill with gas capture to move waste to",
+            )
+        return split_fractions.model_copy()
+    if without_capture <= 0:
+        if existing_gas_pct < 1:
+            raise CustomError(
+                "INVALID_PARAMETERS",
+                "This city sends all of its waste to landfills with gas capture, "
+                "so there is no site without capture to move waste to",
+            )
+        return split_fractions.model_copy()
+    scale = (1 - existing_gas_pct) / without_capture
+    return SplitFractions(
+        landfill_w_capture=existing_gas_pct,
+        landfill_wo_capture=split_fractions.landfill_wo_capture * scale,
+        dumpsite=split_fractions.dumpsite * scale,
+        new_w_capture=split_fractions.new_w_capture,
+    )
+
+
 class City:
     def __init__(self, city_name: str):
         """
@@ -7204,18 +7252,19 @@ def implement_dst_changes_simple_v1_5(
                 skip_ox = True
 
             if move_gas:
-                original_gas_pct = (
-                    scenario_parameters.split_fractions.landfill_w_capture
+                scenario_parameters.split_fractions = _move_to_existing_gas_capture(
+                    scenario_parameters.split_fractions, existing_gas_pct
+                )
+                # Each landfill's waste comes from its own fraction_of_waste
+                # (LandfillWasteMassDF.create below), not from split_fractions.
+                split = scenario_parameters.split_fractions
+                scenario_parameters.landfills[0].fraction_of_waste = (
+                    split.landfill_w_capture
                 )
-                scenario_parameters.split_fractions.landfill_w_capture = (
-                    existing_gas_pct
+                scenario_parameters.landfills[1].fraction_of_waste = (
+                    split.landfill_wo_capture
                 )
-                if original_gas_pct > 0:
-                    ratio = existing_gas_pct / original_gas_pct
-                else:
-                    ratio = 0
-                scenario_parameters.split_fractions.landfill_wo_capture *= ratio
-                scenario_parameters.split_fractions.dumpsite *= ratio
+                scenario_parameters.landfills[2].fraction_of_waste = split.dumpsite
 
             if new_gas_pct > 0:
                 scenario_parameters.landfills.append(
@@ -7232,7 +7281,8 @@ def implement_dst_changes_simple_v1_5(
                 total = sum(scenario_parameters.split_fractions.model_dump().values())
                 if abs(total - 1.0) > 1e-3:
                     raise CustomError(
-                        f"Invalid split fractions: {scenario_parameters.split_fractions}"
+                        "INVALID_PARAMETERS",
+                        f"Invalid split fractions: {scenario_parameters.split_fractions}",
                     )
                 scenario_parameters.landfills[0].fraction_of_waste = (
                     scenario_parameters.split_fractions.landfill_w_capture

changelog/2026-10.md

@@ -0,0 +1,52 @@
+# SWEET_python Changelog — October 2026
+
+**Highlights:** The City DST's "move waste to existing gas capture" lever now
+changes a city's emissions. For a city on the generic landfill split, it worked
+out a new split and never gave it to the landfills, so every setting returned the
+no-change scenario. Moved to 90% gas capture, Lahore now emits 45% less in 2040
+(model-output change).
+
+## Fixed
+
+- **Moving waste to a city's existing gas capture changes its emissions**
+  (model-output change). `City.implement_dst_changes_simple_v1_5` with
+  `move_gas=1` and `existing_gas_pct` is the City DST's "% of waste diverted to
+  existing landfill with gas capture" slider. Its generic branch rewrote
+  `split_fractions`, but each landfill's waste comes from its own
+  `fraction_of_waste`, and only the "new landfill with gas capture" block copied
+  the split there. WasteMAP offers this slider only to a city that already has gas
+  capture, and switches the new-landfill lever off for exactly those cities, so the
+  two never ran together. On the June 2026 cities table, all 7 cities with a live
+  slider (Mexico City, Lahore, Santiago, Amman, Tegucigalpa, Cebu City, Tunis)
+  returned the no-change scenario exactly, at every setting.
+
+  The arithmetic was wrong as well. It set the capture share to `existing_gas_pct`
+  and multiplied the shares without capture by `existing / original`, scaling them
+  up rather than down: Lahore's landfill without capture came out at 171% of the
+  city's waste, and Mexico City's at 3,848%. A city with no capture had both shares
+  zeroed. With a new landfill with gas capture as well, the inflated split failed
+  its sum check, and that check built `CustomError` with one argument, so the API
+  returned a 500.
+
+  The new `_move_to_existing_gas_capture` gives the capture share
+  `existing_gas_pct`. The landfill and dumpsite without capture share the rest in
+  the proportions they had, and the scenario applies the split to the landfills
+  from the implementation year. In 2040:
+  - Moved to 90% capture: Lahore −45%, Mexico City −43%, Cebu City −30%.
+  - Moved to 100%: Lahore −53%, Mexico City −48%.
+  - Moving waste off capture raises emissions. Santiago (98.8% to capture) moved
+    to 0% emits 96% more.
+
+  Three requests the lever can't carry out now return `INVALID_PARAMETERS`
+  instead of the no-change scenario:
+  - a share outside [0, 1];
+  - a share above 0 for a city with no landfill with gas capture (WasteMAP doesn't
+    offer the lever there);
+  - less than 100% for a city that sends all its waste to capture. WasteMAP
+    disables the slider for the 41 such cities in the June table and sends 100%,
+    which leaves the split as it is.
+
+  Nothing else moves. The no-change scenario, the other levers and the 41 cities at
+  100% give the same numbers as before. The sum check now raises a proper
+  `INVALID_PARAMETERS` too.
+  ([#75](https://github.com/RMI/SWEET_python/pull/75), [RMI/WasteMAP#854](https://github.com/RMI/WasteMAP/pull/854))

changelog/README.md

@@ -15,6 +15,7 @@ The project does not publish semantic version tags, so releases are tracked by
 
 Newest first:
 
+- [2026-10](2026-10.md) — the City DST's "move waste to existing gas capture" slider changes a city's emissions; it computed a new split and never applied it to the landfills, so every setting returned the no-change scenario for the 7 cities with a live slider (its arithmetic also scaled the shares without capture up, not down) (model-output change)
 - [2026-09](2026-09.md) — WasteMAP's site tool, city tool and Custom Location grow waste with the country's UN population year by year, as Climate TRACE does; big cities keep their own UN city rates (model-output change); model output frames get one fixed waste-type column order instead of a per-process one (a `set` fed the ordering, and Python randomizes set iteration per process), moving values in the last bit only, where a sum runs across the reordered columns; City DST's diversion sum stops passing pandas a keyword pandas is removing, which would have broken the model path on pandas 4 behind a misleading `AttributeError` (no output change); annual model no longer charges a flare's inefficiency twice (a hardcoded 2% slip stacked on the `flaring` destruction efficiency), so it agrees with the monthly model again and a site with gas capture emits ~3.5% less (model-output change); `gas_capture_efficiency` bounded to [0, 1], where an out-of-range value used to produce negative emissions silently; one methane density constant instead of two 9.1% apart
 - [2026-08](2026-08.md) — Single-site adst gains optional `depth` (deep-dump MCF bump) and `k_override` (caller-supplied decomposition rate) inputs, restoring the last two site-DST levers; `/sdst` flaring efficiency reaches the model again after a variable-name bug silently forced flare destruction to 0.98; annual model applies cover oxidation by emission year not deposit year, fixing biocover having no effect on closed landfills (WasteMAP #719); `City.sdst_v1_5` custom-site path holds the scenario equal to the baseline before the implementation year even when composition changes (was back-dating the new composition onto pre-implementation deposits) (model-output change); MCF consolidated into a new `SWEET_python.mcf` module and both dump types moved to the IPCC uncategorised-SWDS 0.6 (open dumps up from 0.4, controlled dumps down from 0.7), with a supplied depth now selecting the deep/shallow category (model-output change)
 - [2026-07](2026-07.md) — All ten waste types eligible for combustion (metal/glass/other added); methane-only model treats combustion as landfill diversion (model-output change)