# FloPy Tutorial 2 Benchmark Model - Daily Time-Stepping Architecture

## Overview
The GoldSim-MODFLOW interface implements a **daily time-stepping architecture** where GoldSim calls the interface 201 times (once per day), with each call running a single 1-day MODFLOW simulation. State persistence between calls enables long-term transient behavior while maintaining GoldSim's control over time-step progression and feedback loops.

## Key Changes Made

### 1. Interface Simplification (gspy_interface.py)
- **Old Interface**: 7 parameters (recharge, storage_rate, max_additional, base_flow, hk, sy, river_conductance)
- **New Interface**: 4 parameters only:
  1. `tutorial_period` (int): 1, 2, or 3
  2. `well_rate_m3_per_day` (float): -500.0 for Period 3, 0.0 otherwise  
  3. `hk` (float): Hydraulic conductivity (10.0 m/day)
  4. `sy` (float): Specific yield (0.3)
- **Return Value**: `(Center_Cell_Head,)` - Head at model center cell (0,4,4)

### 2. Core Logic Refactoring (groundwater_models.py)
- **Removed**: `_calculate_controlled_response()` fallback model
- **Removed**: `_controlled_storage_state` global variable
- **Renamed**: `calculate_return_flow()` → `calculate_tutorial_head()`
- **Updated**: Parameter validation for 4-input Tutorial 2 model
- **Updated**: Error handling returns safe default head (10.0 m) instead of flow

### 3. Static Model Setup (static_model_setup.py)
- **Grid**: Changed from 20×20 to 1×10×10 (Tutorial 2 specification)
- **Spacing**: Changed from 50m to 1000m cell spacing
- **Initial Heads**: Set to 10.0m everywhere (overridden by persistent state)
- **Removed**: All RIV (River) and RCH (Recharge) package logic
- **Retained**: Only core packages (DIS, BAS, LPF, OC, PCG)

### 4. MODFLOW Runner Implementation (modflow_runner.py)
- **CRITICAL - Daily Time Discretization**: **MANDATED** `nper=1, perlen=[1.0], nstp=[1]` for single 1-day step per call
- **State Persistence Integration**:
  - `run_simulation()` calls `self.state_manager.load_heads()` for initial conditions
  - `run_simulation()` calls `self.state_manager.save_heads()` after successful run
- **Dynamic Boundary Conditions**: GHB package configured based on `tutorial_period`:
  - Period 1: Left=10.0m, Right=10.0m
  - Periods 2&3: Left=10.0m, Right=0.0m
- **Dynamic Well Package**: WEL added for Period 3 pumping at center cell (0,4,4)
- **Output Control**: Configured to save heads at end of 1-day period for persistence
- **Removed**: `_calculate_transient_river_flow()` method
- **Added**: `_get_center_head()` method to extract head at (0,4,4)
- **Foundation**: Uses `create_static_model()` then dynamically adds boundary packages

### 5. Persistent State Management (state_manager.py)
- **No Changes**: Preserved as required - handles head persistence between daily calls

## FloPy Tutorial 2 Compliance

### Model Configuration
- **Grid**: 1 layer × 10 rows × 10 columns
- **Cell Size**: 1000m × 1000m
- **Top Elevation**: 25.0m
- **Bottom Elevation**: 0.0m
- **Hydraulic Conductivity**: 10.0 m/day (configurable input)
- **Specific Yield**: 0.3 (configurable input)

### Boundary Conditions by Period
1. **Period 1 (Day 1)**:
   - GHB: Left boundary = 10.0m, Right boundary = 10.0m
   - WEL: No pumping (0.0 m³/day)

2. **Period 2 (Days 2-101)**:
   - GHB: Left boundary = 10.0m, Right boundary = 0.0m  
   - WEL: No pumping (0.0 m³/day)

3. **Period 3 (Days 102-201)**:
   - GHB: Left boundary = 10.0m, Right boundary = 0.0m
   - WEL: Center well pumping = -500.0 m³/day

### Critical Daily Time-Stepping Architecture
- **GoldSim Calls Interface**: 201 times (once per day for 201-day simulation)
- **Each Call Runs**: Exactly 1 day (`nper=1, perlen=[1.0], nstp=[1]`)
- **State Persistence**: `state_manager.py` saves final heads → loads as initial heads next day
- **Chained Simulation**: 201 short runs create long-term transient behavior
- **GoldSim Control**: Maintains driver control for feedback loops and time progression

## Verification Features

### Input Validation
- Validates exactly 4 input parameters
- Checks `tutorial_period` is 1, 2, or 3
- Handles invalid inputs gracefully with default heads

### Error Handling  
- MODFLOW execution errors return safe default (10.0m head)
- Comprehensive logging for debugging
- Graceful fallbacks preserve system stability

### Testing
- `test_tutorial2_benchmark.py` validates all three periods
- Tests invalid inputs and error conditions
- Verifies tuple return format compliance

## Expected Daily Time-Stepping Behavior
- **Day 1 (Period 1)**: Establishes initial equilibrium with equal GHB boundaries (10.0m both sides)
- **Days 2-101 (Period 2)**: Hydraulic gradient (10.0m → 0.0m) drives progressive head decline  
- **Days 102-201 (Period 3)**: Center well pumping (-500 m³/day) accelerates head decline at center
- **State Chaining**: Each day's final head distribution becomes next day's initial conditions
- **Feedback Loop**: GoldSim can modify `well_rate_m3_per_day` based on previous day's head response
- **Long-term Response**: 201 chained 1-day runs achieve full transient behavior of long simulation

## Files Modified
1. `gspy_interface.py` - Simplified 4-parameter interface
2. `groundwater_models.py` - Removed fallback, renamed methods  
3. `static_model_setup.py` - 1×10×10 grid, removed RIV/RCH packages
4. `modflow_runner.py` - Tutorial 2 logic, GHB/WEL packages, center head extraction
5. `state_manager.py` - No changes (preserved)

## Files Added
1. `test_tutorial2_benchmark.py` - Validation test script

The refactored system now provides a clean, verifiable implementation of the FloPy Tutorial 2 benchmark model with persistent daily time stepping for GoldSim integration.