diff --git a/pywr-core/src/lib.rs b/pywr-core/src/lib.rs
index 2b3ed952..764a5988 100644
--- a/pywr-core/src/lib.rs
+++ b/pywr-core/src/lib.rs
@@ -5,8 +5,9 @@ extern crate core;
 use crate::derived_metric::DerivedMetricIndex;
 use crate::models::MultiNetworkTransferIndex;
 use crate::node::NodeIndex;
-use crate::parameters::{IndexParameterIndex, InterpolationError, MultiValueParameterIndex, ParameterIndex};
+use crate::parameters::{InterpolationError, ParameterIndex};
 use crate::recorders::{AggregationError, MetricSetIndex, RecorderIndex};
+use crate::state::MultiValue;
 use crate::virtual_storage::VirtualStorageIndex;
 use pyo3::exceptions::{PyException, PyRuntimeError};
 use pyo3::{create_exception, PyErr};
@@ -44,11 +45,11 @@ pub enum PywrError {
     #[error("virtual storage index {0} not found")]
     VirtualStorageIndexNotFound(VirtualStorageIndex),
     #[error("parameter index {0} not found")]
-    ParameterIndexNotFound(ParameterIndex),
+    ParameterIndexNotFound(ParameterIndex<f64>),
     #[error("index parameter index {0} not found")]
-    IndexParameterIndexNotFound(IndexParameterIndex),
+    IndexParameterIndexNotFound(ParameterIndex<usize>),
     #[error("multi1 value parameter index {0} not found")]
-    MultiValueParameterIndexNotFound(MultiValueParameterIndex),
+    MultiValueParameterIndexNotFound(ParameterIndex<MultiValue>),
     #[error("multi1 value parameter key {0} not found")]
     MultiValueParameterKeyNotFound(String),
     #[error("inter-network parameter state not initialised")]
@@ -72,9 +73,9 @@ pub enum PywrError {
     #[error("node name `{0}` already exists")]
     NodeNameAlreadyExists(String),
     #[error("parameter name `{0}` already exists at index {1}")]
-    ParameterNameAlreadyExists(String, ParameterIndex),
+    ParameterNameAlreadyExists(String, ParameterIndex<f64>),
     #[error("index parameter name `{0}` already exists at index {1}")]
-    IndexParameterNameAlreadyExists(String, IndexParameterIndex),
+    IndexParameterNameAlreadyExists(String, ParameterIndex<usize>),
     #[error("metric set name `{0}` already exists")]
     MetricSetNameAlreadyExists(String),
     #[error("recorder name `{0}` already exists at index {1}")]
@@ -158,7 +159,7 @@ pub enum PywrError {
     #[error("parameters do not provide an initial value")]
     ParameterNoInitialValue,
     #[error("parameter state not found for parameter index {0}")]
-    ParameterStateNotFound(ParameterIndex),
+    ParameterStateNotFound(ParameterIndex<f64>),
     #[error("Could not create timestep range due to following error: {0}")]
     TimestepRangeGenerationError(String),
     #[error("Could not create timesteps for frequency '{0}'")]
diff --git a/pywr-core/src/metric.rs b/pywr-core/src/metric.rs
index 82e88ea6..daa7ad0a 100644
--- a/pywr-core/src/metric.rs
+++ b/pywr-core/src/metric.rs
@@ -5,8 +5,8 @@ use crate::edge::EdgeIndex;
 use crate::models::MultiNetworkTransferIndex;
 use crate::network::Network;
 use crate::node::NodeIndex;
-use crate::parameters::{IndexParameterIndex, MultiValueParameterIndex, ParameterIndex};
-use crate::state::State;
+use crate::parameters::ParameterIndex;
+use crate::state::{MultiValue, State};
 use crate::virtual_storage::VirtualStorageIndex;
 use crate::PywrError;
 #[derive(Clone, Debug, PartialEq)]
@@ -18,8 +18,8 @@ pub enum Metric {
     AggregatedNodeOutFlow(AggregatedNodeIndex),
     AggregatedNodeVolume(AggregatedStorageNodeIndex),
     EdgeFlow(EdgeIndex),
-    ParameterValue(ParameterIndex),
-    MultiParameterValue((MultiValueParameterIndex, String)),
+    ParameterValue(ParameterIndex<f64>),
+    MultiParameterValue((ParameterIndex<MultiValue>, String)),
     VirtualStorageVolume(VirtualStorageIndex),
     MultiNodeInFlow { indices: Vec<NodeIndex>, name: String },
     MultiNodeOutFlow { indices: Vec<NodeIndex>, name: String },
@@ -87,7 +87,7 @@ impl Metric {
 
 #[derive(Clone, Debug, PartialEq)]
 pub enum IndexMetric {
-    IndexParameterValue(IndexParameterIndex),
+    IndexParameterValue(ParameterIndex<usize>),
     Constant(usize),
 }
 
diff --git a/pywr-core/src/network.rs b/pywr-core/src/network.rs
index de960951..0d6a3782 100644
--- a/pywr-core/src/network.rs
+++ b/pywr-core/src/network.rs
@@ -5,14 +5,14 @@ use crate::edge::{EdgeIndex, EdgeVec};
 use crate::metric::Metric;
 use crate::models::ModelDomain;
 use crate::node::{ConstraintValue, Node, NodeVec, StorageInitialVolume};
-use crate::parameters::{MultiValueParameterIndex, ParameterType, VariableConfig};
+use crate::parameters::{ParameterType, VariableConfig};
 use crate::recorders::{MetricSet, MetricSetIndex, MetricSetState};
 use crate::scenario::ScenarioIndex;
 use crate::solvers::{MultiStateSolver, Solver, SolverFeatures, SolverTimings};
 use crate::state::{MultiValue, ParameterStates, State, StateBuilder};
 use crate::timestep::Timestep;
 use crate::virtual_storage::{VirtualStorage, VirtualStorageIndex, VirtualStorageReset, VirtualStorageVec};
-use crate::{parameters, recorders, IndexParameterIndex, NodeIndex, ParameterIndex, PywrError, RecorderIndex};
+use crate::{parameters, recorders, NodeIndex, ParameterIndex, PywrError, RecorderIndex};
 use rayon::prelude::*;
 use std::any::Any;
 use std::collections::HashSet;
@@ -1089,7 +1089,7 @@ impl Network {
     }
 
     /// Get a `Parameter` from a parameter's name
-    pub fn get_parameter(&self, index: &ParameterIndex) -> Result<&dyn parameters::Parameter<f64>, PywrError> {
+    pub fn get_parameter(&self, index: &ParameterIndex<f64>) -> Result<&dyn parameters::Parameter<f64>, PywrError> {
         match self.parameters.get(*index.deref()) {
             Some(p) => Ok(p.as_ref()),
             None => Err(PywrError::ParameterIndexNotFound(*index)),
@@ -1099,7 +1099,7 @@ impl Network {
     /// Get a `Parameter` from a parameter's name
     pub fn get_mut_parameter(
         &mut self,
-        index: &ParameterIndex,
+        index: &ParameterIndex<f64>,
     ) -> Result<&mut dyn parameters::Parameter<f64>, PywrError> {
         match self.parameters.get_mut(*index.deref()) {
             Some(p) => Ok(p.as_mut()),
@@ -1116,7 +1116,7 @@ impl Network {
     }
 
     /// Get a `ParameterIndex` from a parameter's name
-    pub fn get_parameter_index_by_name(&self, name: &str) -> Result<ParameterIndex, PywrError> {
+    pub fn get_parameter_index_by_name(&self, name: &str) -> Result<ParameterIndex<f64>, PywrError> {
         match self.parameters.iter().position(|p| p.name() == name) {
             Some(idx) => Ok(ParameterIndex::new(idx)),
             None => Err(PywrError::ParameterNotFound(name.to_string())),
@@ -1132,17 +1132,20 @@ impl Network {
     }
 
     /// Get a `IndexParameterIndex` from a parameter's name
-    pub fn get_index_parameter_index_by_name(&self, name: &str) -> Result<IndexParameterIndex, PywrError> {
+    pub fn get_index_parameter_index_by_name(&self, name: &str) -> Result<ParameterIndex<usize>, PywrError> {
         match self.index_parameters.iter().position(|p| p.name() == name) {
-            Some(idx) => Ok(IndexParameterIndex::new(idx)),
+            Some(idx) => Ok(ParameterIndex::new(idx)),
             None => Err(PywrError::ParameterNotFound(name.to_string())),
         }
     }
 
     /// Get a `MultiValueParameterIndex` from a parameter's name
-    pub fn get_multi_valued_parameter_index_by_name(&self, name: &str) -> Result<MultiValueParameterIndex, PywrError> {
+    pub fn get_multi_valued_parameter_index_by_name(
+        &self,
+        name: &str,
+    ) -> Result<ParameterIndex<MultiValue>, PywrError> {
         match self.multi_parameters.iter().position(|p| p.name() == name) {
-            Some(idx) => Ok(MultiValueParameterIndex::new(idx)),
+            Some(idx) => Ok(ParameterIndex::new(idx)),
             None => Err(PywrError::ParameterNotFound(name.to_string())),
         }
     }
@@ -1317,7 +1320,7 @@ impl Network {
     pub fn add_parameter(
         &mut self,
         parameter: Box<dyn parameters::Parameter<f64>>,
-    ) -> Result<ParameterIndex, PywrError> {
+    ) -> Result<ParameterIndex<f64>, PywrError> {
         if let Ok(idx) = self.get_parameter_index_by_name(&parameter.meta().name) {
             return Err(PywrError::ParameterNameAlreadyExists(
                 parameter.meta().name.to_string(),
@@ -1339,7 +1342,7 @@ impl Network {
     pub fn add_index_parameter(
         &mut self,
         index_parameter: Box<dyn parameters::Parameter<usize>>,
-    ) -> Result<IndexParameterIndex, PywrError> {
+    ) -> Result<ParameterIndex<usize>, PywrError> {
         if let Ok(idx) = self.get_index_parameter_index_by_name(&index_parameter.meta().name) {
             return Err(PywrError::IndexParameterNameAlreadyExists(
                 index_parameter.meta().name.to_string(),
@@ -1347,7 +1350,7 @@ impl Network {
             ));
         }
 
-        let parameter_index = IndexParameterIndex::new(self.index_parameters.len());
+        let parameter_index = ParameterIndex::new(self.index_parameters.len());
 
         self.index_parameters.push(index_parameter);
         // .. and add it to the resolve order
@@ -1360,7 +1363,7 @@ impl Network {
     pub fn add_multi_value_parameter(
         &mut self,
         parameter: Box<dyn parameters::Parameter<MultiValue>>,
-    ) -> Result<MultiValueParameterIndex, PywrError> {
+    ) -> Result<ParameterIndex<MultiValue>, PywrError> {
         if let Ok(idx) = self.get_parameter_index_by_name(&parameter.meta().name) {
             return Err(PywrError::ParameterNameAlreadyExists(
                 parameter.meta().name.to_string(),
@@ -1368,7 +1371,7 @@ impl Network {
             ));
         }
 
-        let parameter_index = MultiValueParameterIndex::new(self.multi_parameters.len());
+        let parameter_index = ParameterIndex::new(self.multi_parameters.len());
 
         // Add the parameter ...
         self.multi_parameters.push(parameter);
@@ -1457,7 +1460,7 @@ impl Network {
     /// This will update the internal state of the parameter with the new values for all scenarios.
     pub fn set_f64_parameter_variable_values(
         &self,
-        parameter_index: ParameterIndex,
+        parameter_index: ParameterIndex<f64>,
         values: &[f64],
         variable_config: &dyn VariableConfig,
         state: &mut NetworkState,
@@ -1487,7 +1490,7 @@ impl Network {
     /// Only the internal state of the parameter for the given scenario will be updated.
     pub fn set_f64_parameter_variable_values_for_scenario(
         &self,
-        parameter_index: ParameterIndex,
+        parameter_index: ParameterIndex<f64>,
         scenario_index: ScenarioIndex,
         values: &[f64],
         variable_config: &dyn VariableConfig,
@@ -1511,7 +1514,7 @@ impl Network {
     /// Return a vector of the current values of active variable parameters.
     pub fn get_f64_parameter_variable_values_for_scenario(
         &self,
-        parameter_index: ParameterIndex,
+        parameter_index: ParameterIndex<f64>,
         scenario_index: ScenarioIndex,
         state: &NetworkState,
     ) -> Result<Option<Vec<f64>>, PywrError> {
@@ -1533,7 +1536,7 @@ impl Network {
 
     pub fn get_f64_parameter_variable_values(
         &self,
-        parameter_index: ParameterIndex,
+        parameter_index: ParameterIndex<f64>,
         state: &NetworkState,
     ) -> Result<Vec<Option<Vec<f64>>>, PywrError> {
         match self.parameters.get(*parameter_index.deref()) {
@@ -1563,7 +1566,7 @@ impl Network {
     /// This will update the internal state of the parameter with the new values for scenarios.
     pub fn set_u32_parameter_variable_values(
         &self,
-        parameter_index: ParameterIndex,
+        parameter_index: ParameterIndex<f64>,
         values: &[u32],
         variable_config: &dyn VariableConfig,
         state: &mut NetworkState,
@@ -1593,7 +1596,7 @@ impl Network {
     /// Only the internal state of the parameter for the given scenario will be updated.
     pub fn set_u32_parameter_variable_values_for_scenario(
         &self,
-        parameter_index: ParameterIndex,
+        parameter_index: ParameterIndex<f64>,
         scenario_index: ScenarioIndex,
         values: &[u32],
         variable_config: &dyn VariableConfig,
@@ -1617,7 +1620,7 @@ impl Network {
     /// Return a vector of the current values of active variable parameters.
     pub fn get_u32_parameter_variable_values_for_scenario(
         &self,
-        parameter_index: ParameterIndex,
+        parameter_index: ParameterIndex<f64>,
         scenario_index: ScenarioIndex,
         state: &NetworkState,
     ) -> Result<Option<Vec<u32>>, PywrError> {
diff --git a/pywr-core/src/parameters/mod.rs b/pywr-core/src/parameters/mod.rs
index 289b8fc6..5243758b 100644
--- a/pywr-core/src/parameters/mod.rs
+++ b/pywr-core/src/parameters/mod.rs
@@ -28,7 +28,7 @@ pub use self::rhai::RhaiParameter;
 use super::PywrError;
 use crate::network::Network;
 use crate::scenario::ScenarioIndex;
-use crate::state::{ParameterState, State};
+use crate::state::{MultiValue, ParameterState, State};
 use crate::timestep::Timestep;
 pub use activation_function::ActivationFunction;
 pub use aggregated::{AggFunc, AggregatedParameter};
@@ -59,76 +59,59 @@ pub use profiles::{
 pub use py::PyParameter;
 use std::fmt;
 use std::fmt::{Display, Formatter};
+use std::marker::PhantomData;
 use std::ops::Deref;
 pub use threshold::{Predicate, ThresholdParameter};
 pub use vector::VectorParameter;
 
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
-pub struct ParameterIndex(usize);
-
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
-pub struct IndexParameterIndex(usize);
-
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
-pub struct MultiValueParameterIndex(usize);
-
-impl ParameterIndex {
-    pub fn new(idx: usize) -> Self {
-        Self(idx)
-    }
-}
-
-impl IndexParameterIndex {
-    pub fn new(idx: usize) -> Self {
-        Self(idx)
-    }
+/// Generic parameter index.
+///
+/// This is a wrapper around usize that is used to index parameters in the state. It is
+/// generic over the type of the value that the parameter returns.
+#[derive(Debug)]
+pub struct ParameterIndex<T> {
+    idx: usize,
+    phantom: PhantomData<T>,
 }
 
-impl MultiValueParameterIndex {
-    pub fn new(idx: usize) -> Self {
-        Self(idx)
+// These implementations are required because the derive macro does not work well with PhantomData.
+// See issue: https://github.com/rust-lang/rust/issues/26925
+impl<T> Clone for ParameterIndex<T> {
+    fn clone(&self) -> Self {
+        *self
     }
 }
 
-impl Deref for ParameterIndex {
-    type Target = usize;
+impl<T> Copy for ParameterIndex<T> {}
 
-    fn deref(&self) -> &Self::Target {
-        &self.0
+impl<T> PartialEq<Self> for ParameterIndex<T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.idx == other.idx
     }
 }
 
-impl Deref for IndexParameterIndex {
-    type Target = usize;
+impl<T> Eq for ParameterIndex<T> {}
 
-    fn deref(&self) -> &Self::Target {
-        &self.0
+impl<T> ParameterIndex<T> {
+    pub fn new(idx: usize) -> Self {
+        Self {
+            idx,
+            phantom: PhantomData,
+        }
     }
 }
 
-impl Deref for MultiValueParameterIndex {
+impl<T> Deref for ParameterIndex<T> {
     type Target = usize;
 
     fn deref(&self) -> &Self::Target {
-        &self.0
-    }
-}
-
-impl Display for ParameterIndex {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", self.0)
-    }
-}
-
-impl Display for IndexParameterIndex {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", self.0)
+        &self.idx
     }
 }
 
-impl Display for MultiValueParameterIndex {
+impl<T> Display for ParameterIndex<T> {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", self.0)
+        write!(f, "{}", self.idx)
     }
 }
 
@@ -271,7 +254,7 @@ pub trait Parameter<T>: Send + Sync {
 #[derive(Copy, Clone)]
 pub enum IndexValue {
     Constant(usize),
-    Dynamic(IndexParameterIndex),
+    Dynamic(ParameterIndex<usize>),
 }
 
 impl IndexValue {
@@ -284,9 +267,9 @@ impl IndexValue {
 }
 
 pub enum ParameterType {
-    Parameter(ParameterIndex),
-    Index(IndexParameterIndex),
-    Multi(MultiValueParameterIndex),
+    Parameter(ParameterIndex<f64>),
+    Index(ParameterIndex<usize>),
+    Multi(ParameterIndex<MultiValue>),
 }
 
 /// A parameter that can be optimised.
diff --git a/pywr-core/src/state.rs b/pywr-core/src/state.rs
index 6f896b6e..194beeae 100644
--- a/pywr-core/src/state.rs
+++ b/pywr-core/src/state.rs
@@ -3,7 +3,7 @@ use crate::edge::{Edge, EdgeIndex};
 use crate::models::MultiNetworkTransferIndex;
 use crate::network::Network;
 use crate::node::{Node, NodeIndex};
-use crate::parameters::{IndexParameterIndex, MultiValueParameterIndex, ParameterIndex};
+use crate::parameters::ParameterIndex;
 use crate::timestep::Timestep;
 use crate::virtual_storage::VirtualStorageIndex;
 use crate::PywrError;
@@ -271,27 +271,30 @@ impl ParameterStates {
         }
     }
 
-    pub fn get_value_state(&self, index: ParameterIndex) -> Option<&Option<Box<dyn ParameterState>>> {
+    pub fn get_value_state(&self, index: ParameterIndex<f64>) -> Option<&Option<Box<dyn ParameterState>>> {
         self.values.get(*index.deref())
     }
 
-    pub fn get_mut_value_state(&mut self, index: ParameterIndex) -> Option<&mut Option<Box<dyn ParameterState>>> {
+    pub fn get_mut_value_state(&mut self, index: ParameterIndex<f64>) -> Option<&mut Option<Box<dyn ParameterState>>> {
         self.values.get_mut(*index.deref())
     }
 
-    pub fn get_mut_index_state(&mut self, index: IndexParameterIndex) -> Option<&mut Option<Box<dyn ParameterState>>> {
+    pub fn get_mut_index_state(
+        &mut self,
+        index: ParameterIndex<usize>,
+    ) -> Option<&mut Option<Box<dyn ParameterState>>> {
         self.indices.get_mut(*index.deref())
     }
 
     pub fn get_mut_multi_state(
         &mut self,
-        index: MultiValueParameterIndex,
+        index: ParameterIndex<MultiValue>,
     ) -> Option<&mut Option<Box<dyn ParameterState>>> {
         self.multi.get_mut(*index.deref())
     }
 }
 
-#[derive(Debug, Default, Clone)]
+#[derive(Debug, Default, Clone, PartialEq)]
 pub struct MultiValue {
     values: HashMap<String, f64>,
     indices: HashMap<String, usize>,
@@ -328,14 +331,14 @@ impl ParameterValues {
         }
     }
 
-    fn get_value(&self, idx: ParameterIndex) -> Result<f64, PywrError> {
+    fn get_value(&self, idx: ParameterIndex<f64>) -> Result<f64, PywrError> {
         match self.values.get(*idx.deref()) {
             Some(s) => Ok(*s),
             None => Err(PywrError::ParameterIndexNotFound(idx)),
         }
     }
 
-    fn set_value(&mut self, idx: ParameterIndex, value: f64) -> Result<(), PywrError> {
+    fn set_value(&mut self, idx: ParameterIndex<f64>, value: f64) -> Result<(), PywrError> {
         match self.values.get_mut(*idx.deref()) {
             Some(s) => {
                 *s = value;
@@ -345,14 +348,14 @@ impl ParameterValues {
         }
     }
 
-    fn get_index(&self, idx: IndexParameterIndex) -> Result<usize, PywrError> {
+    fn get_index(&self, idx: ParameterIndex<usize>) -> Result<usize, PywrError> {
         match self.indices.get(*idx.deref()) {
             Some(s) => Ok(*s),
             None => Err(PywrError::IndexParameterIndexNotFound(idx)),
         }
     }
 
-    fn set_index(&mut self, idx: IndexParameterIndex, value: usize) -> Result<(), PywrError> {
+    fn set_index(&mut self, idx: ParameterIndex<usize>, value: usize) -> Result<(), PywrError> {
         match self.indices.get_mut(*idx.deref()) {
             Some(s) => {
                 *s = value;
@@ -362,7 +365,7 @@ impl ParameterValues {
         }
     }
 
-    fn get_multi_value(&self, idx: MultiValueParameterIndex, key: &str) -> Result<f64, PywrError> {
+    fn get_multi_value(&self, idx: ParameterIndex<MultiValue>, key: &str) -> Result<f64, PywrError> {
         match self.multi_values.get(*idx.deref()) {
             Some(s) => match s.get_value(key) {
                 Some(v) => Ok(*v),
@@ -372,7 +375,7 @@ impl ParameterValues {
         }
     }
 
-    fn set_multi_value(&mut self, idx: MultiValueParameterIndex, value: MultiValue) -> Result<(), PywrError> {
+    fn set_multi_value(&mut self, idx: ParameterIndex<MultiValue>, value: MultiValue) -> Result<(), PywrError> {
         match self.multi_values.get_mut(*idx.deref()) {
             Some(s) => {
                 *s = value;
@@ -382,7 +385,7 @@ impl ParameterValues {
         }
     }
 
-    fn get_multi_index(&self, idx: MultiValueParameterIndex, key: &str) -> Result<usize, PywrError> {
+    fn get_multi_index(&self, idx: ParameterIndex<MultiValue>, key: &str) -> Result<usize, PywrError> {
         match self.multi_values.get(*idx.deref()) {
             Some(s) => match s.get_index(key) {
                 Some(v) => Ok(*v),
@@ -637,35 +640,35 @@ impl State {
         &mut self.network
     }
 
-    pub fn get_parameter_value(&self, idx: ParameterIndex) -> Result<f64, PywrError> {
+    pub fn get_parameter_value(&self, idx: ParameterIndex<f64>) -> Result<f64, PywrError> {
         self.parameters.get_value(idx)
     }
 
-    pub fn set_parameter_value(&mut self, idx: ParameterIndex, value: f64) -> Result<(), PywrError> {
+    pub fn set_parameter_value(&mut self, idx: ParameterIndex<f64>, value: f64) -> Result<(), PywrError> {
         self.parameters.set_value(idx, value)
     }
 
-    pub fn get_parameter_index(&self, idx: IndexParameterIndex) -> Result<usize, PywrError> {
+    pub fn get_parameter_index(&self, idx: ParameterIndex<usize>) -> Result<usize, PywrError> {
         self.parameters.get_index(idx)
     }
 
-    pub fn set_parameter_index(&mut self, idx: IndexParameterIndex, value: usize) -> Result<(), PywrError> {
+    pub fn set_parameter_index(&mut self, idx: ParameterIndex<usize>, value: usize) -> Result<(), PywrError> {
         self.parameters.set_index(idx, value)
     }
 
-    pub fn get_multi_parameter_value(&self, idx: MultiValueParameterIndex, key: &str) -> Result<f64, PywrError> {
+    pub fn get_multi_parameter_value(&self, idx: ParameterIndex<MultiValue>, key: &str) -> Result<f64, PywrError> {
         self.parameters.get_multi_value(idx, key)
     }
 
     pub fn set_multi_parameter_value(
         &mut self,
-        idx: MultiValueParameterIndex,
+        idx: ParameterIndex<MultiValue>,
         value: MultiValue,
     ) -> Result<(), PywrError> {
         self.parameters.set_multi_value(idx, value)
     }
 
-    pub fn get_multi_parameter_index(&self, idx: MultiValueParameterIndex, key: &str) -> Result<usize, PywrError> {
+    pub fn get_multi_parameter_index(&self, idx: ParameterIndex<MultiValue>, key: &str) -> Result<usize, PywrError> {
         self.parameters.get_multi_index(idx, key)
     }
 
diff --git a/pywr-schema/src/parameters/aggregated.rs b/pywr-schema/src/parameters/aggregated.rs
index 945a1543..022449c6 100644
--- a/pywr-schema/src/parameters/aggregated.rs
+++ b/pywr-schema/src/parameters/aggregated.rs
@@ -6,7 +6,7 @@ use crate::parameters::{
     TryIntoV2Parameter,
 };
 use pywr_core::models::ModelDomain;
-use pywr_core::parameters::{IndexParameterIndex, ParameterIndex};
+use pywr_core::parameters::ParameterIndex;
 use pywr_v1_schema::parameters::{
     AggFunc as AggFuncV1, AggregatedIndexParameter as AggregatedIndexParameterV1,
     AggregatedParameter as AggregatedParameterV1, IndexAggFunc as IndexAggFuncV1,
@@ -99,7 +99,7 @@ impl AggregatedParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let metrics = self
             .metrics
             .iter()
@@ -206,7 +206,7 @@ impl AggregatedIndexParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<IndexParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<usize>, SchemaError> {
         let parameters = self
             .parameters
             .iter()
diff --git a/pywr-schema/src/parameters/asymmetric_switch.rs b/pywr-schema/src/parameters/asymmetric_switch.rs
index 554b4506..3943e2c3 100644
--- a/pywr-schema/src/parameters/asymmetric_switch.rs
+++ b/pywr-schema/src/parameters/asymmetric_switch.rs
@@ -5,7 +5,7 @@ use crate::parameters::{
     DynamicFloatValueType, DynamicIndexValue, IntoV2Parameter, ParameterMeta, TryFromV1Parameter, TryIntoV2Parameter,
 };
 use pywr_core::models::ModelDomain;
-use pywr_core::parameters::IndexParameterIndex;
+use pywr_core::parameters::ParameterIndex;
 use pywr_v1_schema::parameters::AsymmetricSwitchIndexParameter as AsymmetricSwitchIndexParameterV1;
 use std::collections::HashMap;
 use std::path::Path;
@@ -34,7 +34,7 @@ impl AsymmetricSwitchIndexParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<IndexParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<usize>, SchemaError> {
         let on_index_parameter =
             self.on_index_parameter
                 .load(network, schema, domain, tables, data_path, inter_network_transfers)?;
diff --git a/pywr-schema/src/parameters/control_curves.rs b/pywr-schema/src/parameters/control_curves.rs
index 9e3477b2..a7f722d0 100644
--- a/pywr-schema/src/parameters/control_curves.rs
+++ b/pywr-schema/src/parameters/control_curves.rs
@@ -6,7 +6,7 @@ use crate::parameters::{
     DynamicFloatValue, IntoV2Parameter, NodeReference, ParameterMeta, TryFromV1Parameter, TryIntoV2Parameter,
 };
 use pywr_core::models::ModelDomain;
-use pywr_core::parameters::{IndexParameterIndex, ParameterIndex};
+use pywr_core::parameters::ParameterIndex;
 use pywr_v1_schema::parameters::{
     ControlCurveIndexParameter as ControlCurveIndexParameterV1,
     ControlCurveInterpolatedParameter as ControlCurveInterpolatedParameterV1,
@@ -33,7 +33,7 @@ impl ControlCurveInterpolatedParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let metric = self.storage_node.load(network, schema)?;
 
         let control_curves = self
@@ -136,7 +136,7 @@ impl ControlCurveIndexParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<IndexParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<usize>, SchemaError> {
         let metric = self.storage_node.load(network, schema)?;
 
         let control_curves = self
@@ -247,7 +247,7 @@ impl ControlCurveParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let metric = self.storage_node.load(network, schema)?;
 
         let control_curves = self
@@ -341,7 +341,7 @@ impl ControlCurvePiecewiseInterpolatedParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let metric = self.storage_node.load(network, schema)?;
 
         let control_curves = self
diff --git a/pywr-schema/src/parameters/core.rs b/pywr-schema/src/parameters/core.rs
index a2f0c456..7eac46e0 100644
--- a/pywr-schema/src/parameters/core.rs
+++ b/pywr-schema/src/parameters/core.rs
@@ -170,7 +170,7 @@ impl ConstantParameter {
         &self,
         network: &mut pywr_core::network::Network,
         tables: &LoadedTableCollection,
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let p = pywr_core::parameters::ConstantParameter::new(&self.meta.name, self.value.load(tables)?);
         Ok(network.add_parameter(Box::new(p))?)
     }
@@ -226,7 +226,7 @@ impl MaxParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let idx = self
             .parameter
             .load(network, schema, domain, tables, data_path, inter_network_transfers)?;
@@ -301,7 +301,7 @@ impl DivisionParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let n = self
             .numerator
             .load(network, schema, domain, tables, data_path, inter_network_transfers)?;
@@ -376,7 +376,7 @@ impl MinParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let idx = self
             .parameter
             .load(network, schema, domain, tables, data_path, inter_network_transfers)?;
@@ -433,7 +433,7 @@ impl NegativeParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let idx = self
             .parameter
             .load(network, schema, domain, tables, data_path, inter_network_transfers)?;
diff --git a/pywr-schema/src/parameters/data_frame.rs b/pywr-schema/src/parameters/data_frame.rs
index 57be6a7d..d0ce6a9e 100644
--- a/pywr-schema/src/parameters/data_frame.rs
+++ b/pywr-schema/src/parameters/data_frame.rs
@@ -75,7 +75,7 @@ impl DataFrameParameter {
         network: &mut pywr_core::network::Network,
         domain: &ModelDomain,
         data_path: Option<&Path>,
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         // Handle the case of an optional data path with a relative url.
         let pth = if let Some(dp) = data_path {
             if self.url.is_relative() {
diff --git a/pywr-schema/src/parameters/delay.rs b/pywr-schema/src/parameters/delay.rs
index 79021b85..de891c36 100644
--- a/pywr-schema/src/parameters/delay.rs
+++ b/pywr-schema/src/parameters/delay.rs
@@ -39,7 +39,7 @@ impl DelayParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let metric = self
             .metric
             .load(network, schema, domain, tables, data_path, inter_network_transfers)?;
diff --git a/pywr-schema/src/parameters/discount_factor.rs b/pywr-schema/src/parameters/discount_factor.rs
index bef78112..0ab9a321 100644
--- a/pywr-schema/src/parameters/discount_factor.rs
+++ b/pywr-schema/src/parameters/discount_factor.rs
@@ -40,7 +40,7 @@ impl DiscountFactorParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let discount_rate =
             self.discount_rate
                 .load(network, schema, domain, tables, data_path, inter_network_transfers)?;
diff --git a/pywr-schema/src/parameters/indexed_array.rs b/pywr-schema/src/parameters/indexed_array.rs
index 3534db62..af1fb583 100644
--- a/pywr-schema/src/parameters/indexed_array.rs
+++ b/pywr-schema/src/parameters/indexed_array.rs
@@ -42,7 +42,7 @@ impl IndexedArrayParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let index_parameter =
             self.index_parameter
                 .load(network, schema, domain, tables, data_path, inter_network_transfers)?;
diff --git a/pywr-schema/src/parameters/interpolated.rs b/pywr-schema/src/parameters/interpolated.rs
index 68bf67d2..b8de7960 100644
--- a/pywr-schema/src/parameters/interpolated.rs
+++ b/pywr-schema/src/parameters/interpolated.rs
@@ -58,7 +58,7 @@ impl InterpolatedParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let x = self
             .x
             .load(network, schema, domain, tables, data_path, inter_network_transfers)?;
diff --git a/pywr-schema/src/parameters/mod.rs b/pywr-schema/src/parameters/mod.rs
index af362eed..83e313d0 100644
--- a/pywr-schema/src/parameters/mod.rs
+++ b/pywr-schema/src/parameters/mod.rs
@@ -53,7 +53,7 @@ use crate::parameters::interpolated::InterpolatedParameter;
 pub use offset::OffsetParameter;
 use pywr_core::metric::Metric;
 use pywr_core::models::{ModelDomain, MultiNetworkTransferIndex};
-use pywr_core::parameters::{IndexParameterIndex, IndexValue, ParameterType};
+use pywr_core::parameters::{IndexValue, ParameterIndex, ParameterType};
 use pywr_v1_schema::parameters::{
     CoreParameter, ExternalDataRef as ExternalDataRefV1, Parameter as ParameterV1, ParameterMeta as ParameterMetaV1,
     ParameterValue as ParameterValueV1, TableIndex as TableIndexV1, TableIndexEntry as TableIndexEntryV1,
@@ -718,7 +718,7 @@ impl ParameterIndexValue {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<IndexParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<usize>, SchemaError> {
         match self {
             Self::Reference(name) => {
                 // This should be an existing parameter
diff --git a/pywr-schema/src/parameters/offset.rs b/pywr-schema/src/parameters/offset.rs
index 43d58440..3e35df93 100644
--- a/pywr-schema/src/parameters/offset.rs
+++ b/pywr-schema/src/parameters/offset.rs
@@ -55,7 +55,7 @@ impl OffsetParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let idx = self
             .metric
             .load(network, schema, domain, tables, data_path, inter_network_transfers)?;
diff --git a/pywr-schema/src/parameters/polynomial.rs b/pywr-schema/src/parameters/polynomial.rs
index 377058b3..674c35aa 100644
--- a/pywr-schema/src/parameters/polynomial.rs
+++ b/pywr-schema/src/parameters/polynomial.rs
@@ -23,7 +23,7 @@ impl Polynomial1DParameter {
         HashMap::new()
     }
 
-    pub fn add_to_model(&self, network: &mut pywr_core::network::Network) -> Result<ParameterIndex, SchemaError> {
+    pub fn add_to_model(&self, network: &mut pywr_core::network::Network) -> Result<ParameterIndex<f64>, SchemaError> {
         let metric =
             network.get_storage_node_metric(&self.storage_node, None, self.use_proportional_volume.unwrap_or(true))?;
 
diff --git a/pywr-schema/src/parameters/profiles.rs b/pywr-schema/src/parameters/profiles.rs
index 0a24b895..78bc1c89 100644
--- a/pywr-schema/src/parameters/profiles.rs
+++ b/pywr-schema/src/parameters/profiles.rs
@@ -31,7 +31,7 @@ impl DailyProfileParameter {
         &self,
         network: &mut pywr_core::network::Network,
         tables: &LoadedTableCollection,
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let values = &self.values.load(tables)?[..366];
         let p = pywr_core::parameters::DailyProfileParameter::new(&self.meta.name, values.try_into().expect(""));
         Ok(network.add_parameter(Box::new(p))?)
@@ -101,7 +101,7 @@ impl MonthlyProfileParameter {
         &self,
         network: &mut pywr_core::network::Network,
         tables: &LoadedTableCollection,
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let values = &self.values.load(tables)?[..12];
         let p = pywr_core::parameters::MonthlyProfileParameter::new(
             &self.meta.name,
@@ -175,7 +175,7 @@ impl UniformDrawdownProfileParameter {
         &self,
         network: &mut pywr_core::network::Network,
         tables: &LoadedTableCollection,
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let reset_day = match &self.reset_day {
             Some(v) => v.load(tables)? as u32,
             None => 1,
@@ -363,7 +363,7 @@ impl RbfProfileParameter {
         HashMap::new()
     }
 
-    pub fn add_to_model(&self, network: &mut pywr_core::network::Network) -> Result<ParameterIndex, SchemaError> {
+    pub fn add_to_model(&self, network: &mut pywr_core::network::Network) -> Result<ParameterIndex<f64>, SchemaError> {
         let function = self.function.into_core_rbf(&self.points)?;
 
         let p = pywr_core::parameters::RbfProfileParameter::new(&self.meta.name, self.points.clone(), function);
@@ -545,7 +545,7 @@ impl WeeklyProfileParameter {
         &self,
         network: &mut pywr_core::network::Network,
         tables: &LoadedTableCollection,
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         let p = pywr_core::parameters::WeeklyProfileParameter::new(
             &self.meta.name,
             WeeklyProfileValues::try_from(self.values.load(tables)?.as_slice()).map_err(
diff --git a/pywr-schema/src/parameters/tables.rs b/pywr-schema/src/parameters/tables.rs
index 6fe16f8b..5e9da5d8 100644
--- a/pywr-schema/src/parameters/tables.rs
+++ b/pywr-schema/src/parameters/tables.rs
@@ -33,7 +33,7 @@ impl TablesArrayParameter {
         network: &mut pywr_core::network::Network,
         domain: &ModelDomain,
         data_path: Option<&Path>,
-    ) -> Result<ParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<f64>, SchemaError> {
         // 1. Load the file from the HDF5 file (NB this is not Pandas format).
 
         // Handle the case of an optional data path with a relative url.
diff --git a/pywr-schema/src/parameters/thresholds.rs b/pywr-schema/src/parameters/thresholds.rs
index e24ba041..3770c6d0 100644
--- a/pywr-schema/src/parameters/thresholds.rs
+++ b/pywr-schema/src/parameters/thresholds.rs
@@ -5,7 +5,7 @@ use crate::parameters::{
     DynamicFloatValue, DynamicFloatValueType, IntoV2Parameter, ParameterMeta, TryFromV1Parameter, TryIntoV2Parameter,
 };
 use pywr_core::models::ModelDomain;
-use pywr_core::parameters::IndexParameterIndex;
+use pywr_core::parameters::ParameterIndex;
 use pywr_v1_schema::parameters::{
     ParameterThresholdParameter as ParameterThresholdParameterV1, Predicate as PredicateV1,
 };
@@ -77,7 +77,7 @@ impl ParameterThresholdParameter {
         tables: &LoadedTableCollection,
         data_path: Option<&Path>,
         inter_network_transfers: &[PywrMultiNetworkTransfer],
-    ) -> Result<IndexParameterIndex, SchemaError> {
+    ) -> Result<ParameterIndex<usize>, SchemaError> {
         let metric = self
             .parameter
             .load(network, schema, domain, tables, data_path, inter_network_transfers)?;