diff --git a/pywr-core/src/network.rs b/pywr-core/src/network.rs
index 13a44a2b..de960951 100644
--- a/pywr-core/src/network.rs
+++ b/pywr-core/src/network.rs
@@ -9,7 +9,7 @@ use crate::parameters::{MultiValueParameterIndex, ParameterType, VariableConfig}
 use crate::recorders::{MetricSet, MetricSetIndex, MetricSetState};
 use crate::scenario::ScenarioIndex;
 use crate::solvers::{MultiStateSolver, Solver, SolverFeatures, SolverTimings};
-use crate::state::{ParameterStates, State, StateBuilder};
+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};
@@ -201,9 +201,9 @@ pub struct Network {
     aggregated_nodes: AggregatedNodeVec,
     aggregated_storage_nodes: AggregatedStorageNodeVec,
     virtual_storage_nodes: VirtualStorageVec,
-    parameters: Vec<Box<dyn parameters::Parameter>>,
-    index_parameters: Vec<Box<dyn parameters::IndexParameter>>,
-    multi_parameters: Vec<Box<dyn parameters::MultiValueParameter>>,
+    parameters: Vec<Box<dyn parameters::Parameter<f64>>>,
+    index_parameters: Vec<Box<dyn parameters::Parameter<usize>>>,
+    multi_parameters: Vec<Box<dyn parameters::Parameter<MultiValue>>>,
     derived_metrics: Vec<DerivedMetric>,
     metric_sets: Vec<MetricSet>,
     resolve_order: Vec<ComponentType>,
@@ -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, PywrError> {
+    pub fn get_parameter(&self, index: &ParameterIndex) -> Result<&dyn parameters::Parameter<f64>, PywrError> {
         match self.parameters.get(*index.deref()) {
             Some(p) => Ok(p.as_ref()),
             None => Err(PywrError::ParameterIndexNotFound(*index)),
@@ -1097,7 +1097,10 @@ impl Network {
     }
 
     /// Get a `Parameter` from a parameter's name
-    pub fn get_mut_parameter(&mut self, index: &ParameterIndex) -> Result<&mut dyn parameters::Parameter, PywrError> {
+    pub fn get_mut_parameter(
+        &mut self,
+        index: &ParameterIndex,
+    ) -> Result<&mut dyn parameters::Parameter<f64>, PywrError> {
         match self.parameters.get_mut(*index.deref()) {
             Some(p) => Ok(p.as_mut()),
             None => Err(PywrError::ParameterIndexNotFound(*index)),
@@ -1105,7 +1108,7 @@ impl Network {
     }
 
     /// Get a `Parameter` from a parameter's name
-    pub fn get_parameter_by_name(&self, name: &str) -> Result<&dyn parameters::Parameter, PywrError> {
+    pub fn get_parameter_by_name(&self, name: &str) -> Result<&dyn parameters::Parameter<f64>, PywrError> {
         match self.parameters.iter().find(|p| p.name() == name) {
             Some(parameter) => Ok(parameter.as_ref()),
             None => Err(PywrError::ParameterNotFound(name.to_string())),
@@ -1121,7 +1124,7 @@ impl Network {
     }
 
     /// Get a `IndexParameter` from a parameter's name
-    pub fn get_index_parameter_by_name(&self, name: &str) -> Result<&dyn parameters::IndexParameter, PywrError> {
+    pub fn get_index_parameter_by_name(&self, name: &str) -> Result<&dyn parameters::Parameter<usize>, PywrError> {
         match self.index_parameters.iter().find(|p| p.name() == name) {
             Some(parameter) => Ok(parameter.as_ref()),
             None => Err(PywrError::ParameterNotFound(name.to_string())),
@@ -1311,7 +1314,10 @@ impl Network {
     }
 
     /// Add a `parameters::Parameter` to the network
-    pub fn add_parameter(&mut self, parameter: Box<dyn parameters::Parameter>) -> Result<ParameterIndex, PywrError> {
+    pub fn add_parameter(
+        &mut self,
+        parameter: Box<dyn parameters::Parameter<f64>>,
+    ) -> Result<ParameterIndex, PywrError> {
         if let Ok(idx) = self.get_parameter_index_by_name(&parameter.meta().name) {
             return Err(PywrError::ParameterNameAlreadyExists(
                 parameter.meta().name.to_string(),
@@ -1332,7 +1338,7 @@ impl Network {
     /// Add a `parameters::IndexParameter` to the network
     pub fn add_index_parameter(
         &mut self,
-        index_parameter: Box<dyn parameters::IndexParameter>,
+        index_parameter: Box<dyn parameters::Parameter<usize>>,
     ) -> Result<IndexParameterIndex, PywrError> {
         if let Ok(idx) = self.get_index_parameter_index_by_name(&index_parameter.meta().name) {
             return Err(PywrError::IndexParameterNameAlreadyExists(
@@ -1353,7 +1359,7 @@ impl Network {
     /// Add a `parameters::MultiValueParameter` to the network
     pub fn add_multi_value_parameter(
         &mut self,
-        parameter: Box<dyn parameters::MultiValueParameter>,
+        parameter: Box<dyn parameters::Parameter<MultiValue>>,
     ) -> Result<MultiValueParameterIndex, PywrError> {
         if let Ok(idx) = self.get_parameter_index_by_name(&parameter.meta().name) {
             return Err(PywrError::ParameterNameAlreadyExists(
diff --git a/pywr-core/src/parameters/aggregated.rs b/pywr-core/src/parameters/aggregated.rs
index 50e7349c..6b77864d 100644
--- a/pywr-core/src/parameters/aggregated.rs
+++ b/pywr-core/src/parameters/aggregated.rs
@@ -47,7 +47,7 @@ impl AggregatedParameter {
     }
 }
 
-impl Parameter for AggregatedParameter {
+impl Parameter<f64> for AggregatedParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/aggregated_index.rs b/pywr-core/src/parameters/aggregated_index.rs
index e4ec3bf7..45f6f8b9 100644
--- a/pywr-core/src/parameters/aggregated_index.rs
+++ b/pywr-core/src/parameters/aggregated_index.rs
@@ -2,10 +2,11 @@
 ///
 use super::PywrError;
 use crate::network::Network;
-use crate::parameters::{IndexParameter, IndexValue, ParameterMeta};
+use crate::parameters::{IndexValue, Parameter, ParameterMeta};
 use crate::scenario::ScenarioIndex;
 use crate::state::{ParameterState, State};
 use crate::timestep::Timestep;
+use std::any::Any;
 use std::str::FromStr;
 
 pub enum AggIndexFunc {
@@ -49,7 +50,11 @@ impl AggregatedIndexParameter {
     }
 }
 
-impl IndexParameter for AggregatedIndexParameter {
+impl Parameter<usize> for AggregatedIndexParameter {
+    fn as_any_mut(&mut self) -> &mut dyn Any {
+        self
+    }
+
     fn meta(&self) -> &ParameterMeta {
         &self.meta
     }
diff --git a/pywr-core/src/parameters/array.rs b/pywr-core/src/parameters/array.rs
index 0cb69f38..8c9d816c 100644
--- a/pywr-core/src/parameters/array.rs
+++ b/pywr-core/src/parameters/array.rs
@@ -23,7 +23,7 @@ impl Array1Parameter {
     }
 }
 
-impl Parameter for Array1Parameter {
+impl Parameter<f64> for Array1Parameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
@@ -66,7 +66,7 @@ impl Array2Parameter {
     }
 }
 
-impl Parameter for Array2Parameter {
+impl Parameter<f64> for Array2Parameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/asymmetric.rs b/pywr-core/src/parameters/asymmetric.rs
index 775fa535..489df8f1 100644
--- a/pywr-core/src/parameters/asymmetric.rs
+++ b/pywr-core/src/parameters/asymmetric.rs
@@ -1,9 +1,10 @@
 use crate::network::Network;
-use crate::parameters::{downcast_internal_state_mut, IndexParameter, IndexValue, ParameterMeta};
+use crate::parameters::{downcast_internal_state_mut, IndexValue, Parameter, ParameterMeta};
 use crate::scenario::ScenarioIndex;
 use crate::state::{ParameterState, State};
 use crate::timestep::Timestep;
 use crate::PywrError;
+use std::any::Any;
 
 pub struct AsymmetricSwitchIndexParameter {
     meta: ParameterMeta,
@@ -21,7 +22,11 @@ impl AsymmetricSwitchIndexParameter {
     }
 }
 
-impl IndexParameter for AsymmetricSwitchIndexParameter {
+impl Parameter<usize> for AsymmetricSwitchIndexParameter {
+    fn as_any_mut(&mut self) -> &mut dyn Any {
+        self
+    }
+
     fn meta(&self) -> &ParameterMeta {
         &self.meta
     }
diff --git a/pywr-core/src/parameters/constant.rs b/pywr-core/src/parameters/constant.rs
index 658ee86f..d59f1a30 100644
--- a/pywr-core/src/parameters/constant.rs
+++ b/pywr-core/src/parameters/constant.rs
@@ -37,7 +37,7 @@ impl ConstantParameter {
     }
 }
 
-impl Parameter for ConstantParameter {
+impl Parameter<f64> for ConstantParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/control_curves/apportion.rs b/pywr-core/src/parameters/control_curves/apportion.rs
index 5aaaffe1..578f1e03 100644
--- a/pywr-core/src/parameters/control_curves/apportion.rs
+++ b/pywr-core/src/parameters/control_curves/apportion.rs
@@ -1,10 +1,11 @@
 use crate::metric::Metric;
 use crate::network::Network;
-use crate::parameters::{MultiValueParameter, ParameterMeta};
+use crate::parameters::{Parameter, ParameterMeta};
 use crate::scenario::ScenarioIndex;
 use crate::state::{MultiValue, ParameterState, State};
 use crate::timestep::Timestep;
 use crate::PywrError;
+use std::any::Any;
 use std::collections::HashMap;
 
 /// A parameter which divides a apportions a metric to an upper and lower amount based
@@ -31,7 +32,11 @@ impl ApportionParameter {
     }
 }
 
-impl MultiValueParameter for ApportionParameter {
+impl Parameter<MultiValue> for ApportionParameter {
+    fn as_any_mut(&mut self) -> &mut dyn Any {
+        self
+    }
+
     fn meta(&self) -> &ParameterMeta {
         &self.meta
     }
diff --git a/pywr-core/src/parameters/control_curves/index.rs b/pywr-core/src/parameters/control_curves/index.rs
index 34d157a3..07b80dc4 100644
--- a/pywr-core/src/parameters/control_curves/index.rs
+++ b/pywr-core/src/parameters/control_curves/index.rs
@@ -1,10 +1,11 @@
 use crate::metric::Metric;
 use crate::network::Network;
-use crate::parameters::{IndexParameter, ParameterMeta};
+use crate::parameters::{Parameter, ParameterMeta};
 use crate::scenario::ScenarioIndex;
 use crate::state::{ParameterState, State};
 use crate::timestep::Timestep;
 use crate::PywrError;
+use std::any::Any;
 
 pub struct ControlCurveIndexParameter {
     meta: ParameterMeta,
@@ -22,7 +23,11 @@ impl ControlCurveIndexParameter {
     }
 }
 
-impl IndexParameter for ControlCurveIndexParameter {
+impl Parameter<usize> for ControlCurveIndexParameter {
+    fn as_any_mut(&mut self) -> &mut dyn Any {
+        self
+    }
+
     fn meta(&self) -> &ParameterMeta {
         &self.meta
     }
diff --git a/pywr-core/src/parameters/control_curves/interpolated.rs b/pywr-core/src/parameters/control_curves/interpolated.rs
index 15f48e31..784fad81 100644
--- a/pywr-core/src/parameters/control_curves/interpolated.rs
+++ b/pywr-core/src/parameters/control_curves/interpolated.rs
@@ -26,7 +26,7 @@ impl ControlCurveInterpolatedParameter {
     }
 }
 
-impl Parameter for ControlCurveInterpolatedParameter {
+impl Parameter<f64> for ControlCurveInterpolatedParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/control_curves/piecewise.rs b/pywr-core/src/parameters/control_curves/piecewise.rs
index a2e3522e..e3ca6a13 100644
--- a/pywr-core/src/parameters/control_curves/piecewise.rs
+++ b/pywr-core/src/parameters/control_curves/piecewise.rs
@@ -37,7 +37,7 @@ impl PiecewiseInterpolatedParameter {
     }
 }
 
-impl Parameter for PiecewiseInterpolatedParameter {
+impl Parameter<f64> for PiecewiseInterpolatedParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/control_curves/simple.rs b/pywr-core/src/parameters/control_curves/simple.rs
index a59553d2..f13013c6 100644
--- a/pywr-core/src/parameters/control_curves/simple.rs
+++ b/pywr-core/src/parameters/control_curves/simple.rs
@@ -25,7 +25,7 @@ impl ControlCurveParameter {
     }
 }
 
-impl Parameter for ControlCurveParameter {
+impl Parameter<f64> for ControlCurveParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/control_curves/volume_between.rs b/pywr-core/src/parameters/control_curves/volume_between.rs
index f1f11880..e4abcff8 100644
--- a/pywr-core/src/parameters/control_curves/volume_between.rs
+++ b/pywr-core/src/parameters/control_curves/volume_between.rs
@@ -26,7 +26,7 @@ impl VolumeBetweenControlCurvesParameter {
     }
 }
 
-impl Parameter for VolumeBetweenControlCurvesParameter {
+impl Parameter<f64> for VolumeBetweenControlCurvesParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/delay.rs b/pywr-core/src/parameters/delay.rs
index 6ca3dc85..d734b530 100644
--- a/pywr-core/src/parameters/delay.rs
+++ b/pywr-core/src/parameters/delay.rs
@@ -26,7 +26,7 @@ impl DelayParameter {
     }
 }
 
-impl Parameter for DelayParameter {
+impl Parameter<f64> for DelayParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/discount_factor.rs b/pywr-core/src/parameters/discount_factor.rs
index 4994ea91..1ecd5b95 100644
--- a/pywr-core/src/parameters/discount_factor.rs
+++ b/pywr-core/src/parameters/discount_factor.rs
@@ -24,7 +24,7 @@ impl DiscountFactorParameter {
     }
 }
 
-impl Parameter for DiscountFactorParameter {
+impl Parameter<f64> for DiscountFactorParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/division.rs b/pywr-core/src/parameters/division.rs
index 4a3a749b..efec1576 100644
--- a/pywr-core/src/parameters/division.rs
+++ b/pywr-core/src/parameters/division.rs
@@ -24,7 +24,7 @@ impl DivisionParameter {
     }
 }
 
-impl Parameter for DivisionParameter {
+impl Parameter<f64> for DivisionParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/indexed_array.rs b/pywr-core/src/parameters/indexed_array.rs
index d83a0361..fdc15a8a 100644
--- a/pywr-core/src/parameters/indexed_array.rs
+++ b/pywr-core/src/parameters/indexed_array.rs
@@ -23,7 +23,7 @@ impl IndexedArrayParameter {
     }
 }
 
-impl Parameter for IndexedArrayParameter {
+impl Parameter<f64> for IndexedArrayParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/interpolated.rs b/pywr-core/src/parameters/interpolated.rs
index 25111aeb..a717bf63 100644
--- a/pywr-core/src/parameters/interpolated.rs
+++ b/pywr-core/src/parameters/interpolated.rs
@@ -27,7 +27,7 @@ impl InterpolatedParameter {
     }
 }
 
-impl Parameter for InterpolatedParameter {
+impl Parameter<f64> for InterpolatedParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/max.rs b/pywr-core/src/parameters/max.rs
index 2074d4b9..062c3ada 100644
--- a/pywr-core/src/parameters/max.rs
+++ b/pywr-core/src/parameters/max.rs
@@ -24,7 +24,7 @@ impl MaxParameter {
     }
 }
 
-impl Parameter for MaxParameter {
+impl Parameter<f64> for MaxParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/min.rs b/pywr-core/src/parameters/min.rs
index 62c3e1ca..a407220c 100644
--- a/pywr-core/src/parameters/min.rs
+++ b/pywr-core/src/parameters/min.rs
@@ -24,7 +24,7 @@ impl MinParameter {
     }
 }
 
-impl Parameter for MinParameter {
+impl Parameter<f64> for MinParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/mod.rs b/pywr-core/src/parameters/mod.rs
index 10f4af27..9d1c6c39 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::{MultiValue, ParameterState, State};
+use crate::state::{ParameterState, State};
 use crate::timestep::Timestep;
 pub use activation_function::ActivationFunction;
 pub use aggregated::{AggFunc, AggregatedParameter};
@@ -200,8 +200,10 @@ pub fn downcast_variable_config_ref<T: 'static>(variable_config: &dyn VariableCo
     }
 }
 
-// TODO It might be possible to make these three traits into a single generic trait
-pub trait Parameter: Send + Sync {
+/// A trait that defines a component that produces a value each time-step.
+///
+/// The trait is generic over the type of the value produced.
+pub trait Parameter<T>: Send + Sync {
     fn as_any_mut(&mut self) -> &mut dyn Any;
     fn meta(&self) -> &ParameterMeta;
     fn name(&self) -> &str {
@@ -223,7 +225,7 @@ pub trait Parameter: Send + Sync {
         model: &Network,
         state: &State,
         internal_state: &mut Option<Box<dyn ParameterState>>,
-    ) -> Result<f64, PywrError>;
+    ) -> Result<T, PywrError>;
 
     fn after(
         &self,
@@ -267,75 +269,6 @@ pub trait Parameter: Send + Sync {
     }
 }
 
-pub trait IndexParameter: Send + Sync {
-    fn meta(&self) -> &ParameterMeta;
-    fn name(&self) -> &str {
-        self.meta().name.as_str()
-    }
-
-    fn setup(
-        &self,
-        _timesteps: &[Timestep],
-        _scenario_index: &ScenarioIndex,
-    ) -> Result<Option<Box<dyn ParameterState>>, PywrError> {
-        Ok(None)
-    }
-
-    fn compute(
-        &self,
-        timestep: &Timestep,
-        scenario_index: &ScenarioIndex,
-        model: &Network,
-        state: &State,
-        internal_state: &mut Option<Box<dyn ParameterState>>,
-    ) -> Result<usize, PywrError>;
-
-    fn after(
-        &self,
-        #[allow(unused_variables)] timestep: &Timestep,
-        #[allow(unused_variables)] scenario_index: &ScenarioIndex,
-        #[allow(unused_variables)] model: &Network,
-        #[allow(unused_variables)] state: &State,
-        #[allow(unused_variables)] internal_state: &mut Option<Box<dyn ParameterState>>,
-    ) -> Result<(), PywrError> {
-        Ok(())
-    }
-}
-
-pub trait MultiValueParameter: Send + Sync {
-    fn meta(&self) -> &ParameterMeta;
-    fn name(&self) -> &str {
-        self.meta().name.as_str()
-    }
-    fn setup(
-        &self,
-        #[allow(unused_variables)] timesteps: &[Timestep],
-        #[allow(unused_variables)] scenario_index: &ScenarioIndex,
-    ) -> Result<Option<Box<dyn ParameterState>>, PywrError> {
-        Ok(None)
-    }
-
-    fn compute(
-        &self,
-        timestep: &Timestep,
-        scenario_index: &ScenarioIndex,
-        model: &Network,
-        state: &State,
-        internal_state: &mut Option<Box<dyn ParameterState>>,
-    ) -> Result<MultiValue, PywrError>;
-
-    fn after(
-        &self,
-        #[allow(unused_variables)] timestep: &Timestep,
-        #[allow(unused_variables)] scenario_index: &ScenarioIndex,
-        #[allow(unused_variables)] model: &Network,
-        #[allow(unused_variables)] state: &State,
-        #[allow(unused_variables)] internal_state: &mut Option<Box<dyn ParameterState>>,
-    ) -> Result<(), PywrError> {
-        Ok(())
-    }
-}
-
 #[derive(Copy, Clone)]
 pub enum IndexValue {
     Constant(usize),
diff --git a/pywr-core/src/parameters/negative.rs b/pywr-core/src/parameters/negative.rs
index f44621a7..b1a7eab1 100644
--- a/pywr-core/src/parameters/negative.rs
+++ b/pywr-core/src/parameters/negative.rs
@@ -21,7 +21,7 @@ impl NegativeParameter {
     }
 }
 
-impl Parameter for NegativeParameter {
+impl Parameter<f64> for NegativeParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/offset.rs b/pywr-core/src/parameters/offset.rs
index 65c8c1af..1b7be433 100644
--- a/pywr-core/src/parameters/offset.rs
+++ b/pywr-core/src/parameters/offset.rs
@@ -41,7 +41,7 @@ impl OffsetParameter {
     }
 }
 
-impl Parameter for OffsetParameter {
+impl Parameter<f64> for OffsetParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/polynomial.rs b/pywr-core/src/parameters/polynomial.rs
index 16d82126..5b125c07 100644
--- a/pywr-core/src/parameters/polynomial.rs
+++ b/pywr-core/src/parameters/polynomial.rs
@@ -27,7 +27,7 @@ impl Polynomial1DParameter {
     }
 }
 
-impl Parameter for Polynomial1DParameter {
+impl Parameter<f64> for Polynomial1DParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/profiles/daily.rs b/pywr-core/src/parameters/profiles/daily.rs
index 6f2411d2..386b853f 100644
--- a/pywr-core/src/parameters/profiles/daily.rs
+++ b/pywr-core/src/parameters/profiles/daily.rs
@@ -21,7 +21,7 @@ impl DailyProfileParameter {
     }
 }
 
-impl Parameter for DailyProfileParameter {
+impl Parameter<f64> for DailyProfileParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/profiles/monthly.rs b/pywr-core/src/parameters/profiles/monthly.rs
index 60b49679..9bf4244e 100644
--- a/pywr-core/src/parameters/profiles/monthly.rs
+++ b/pywr-core/src/parameters/profiles/monthly.rs
@@ -67,7 +67,7 @@ fn interpolate_last(date: &NaiveDateTime, first_value: f64, last_value: f64) ->
     }
 }
 
-impl Parameter for MonthlyProfileParameter {
+impl Parameter<f64> for MonthlyProfileParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/profiles/rbf.rs b/pywr-core/src/parameters/profiles/rbf.rs
index e92d935f..bcc71e55 100644
--- a/pywr-core/src/parameters/profiles/rbf.rs
+++ b/pywr-core/src/parameters/profiles/rbf.rs
@@ -101,7 +101,7 @@ impl RbfProfileParameter {
     }
 }
 
-impl Parameter for RbfProfileParameter {
+impl Parameter<f64> for RbfProfileParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/profiles/uniform_drawdown.rs b/pywr-core/src/parameters/profiles/uniform_drawdown.rs
index a34d4439..fe13f088 100644
--- a/pywr-core/src/parameters/profiles/uniform_drawdown.rs
+++ b/pywr-core/src/parameters/profiles/uniform_drawdown.rs
@@ -32,7 +32,7 @@ impl UniformDrawdownProfileParameter {
     }
 }
 
-impl Parameter for UniformDrawdownProfileParameter {
+impl Parameter<f64> for UniformDrawdownProfileParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/profiles/weekly.rs b/pywr-core/src/parameters/profiles/weekly.rs
index 4eadb5b4..b43e74a3 100644
--- a/pywr-core/src/parameters/profiles/weekly.rs
+++ b/pywr-core/src/parameters/profiles/weekly.rs
@@ -180,7 +180,7 @@ impl WeeklyProfileParameter {
     }
 }
 
-impl Parameter for WeeklyProfileParameter {
+impl Parameter<f64> for WeeklyProfileParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/py.rs b/pywr-core/src/parameters/py.rs
index 28714beb..ba88286f 100644
--- a/pywr-core/src/parameters/py.rs
+++ b/pywr-core/src/parameters/py.rs
@@ -1,7 +1,7 @@
 use super::{IndexValue, Parameter, ParameterMeta, PywrError, Timestep};
 use crate::metric::Metric;
 use crate::network::Network;
-use crate::parameters::{downcast_internal_state_mut, MultiValueParameter};
+use crate::parameters::downcast_internal_state_mut;
 use crate::scenario::ScenarioIndex;
 use crate::state::{MultiValue, ParameterState, State};
 use pyo3::prelude::*;
@@ -69,7 +69,7 @@ impl PyParameter {
     }
 }
 
-impl Parameter for PyParameter {
+impl Parameter<f64> for PyParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
@@ -164,7 +164,11 @@ impl Parameter for PyParameter {
     }
 }
 
-impl MultiValueParameter for PyParameter {
+impl Parameter<MultiValue> for PyParameter {
+    fn as_any_mut(&mut self) -> &mut dyn Any {
+        self
+    }
+
     fn meta(&self) -> &ParameterMeta {
         &self.meta
     }
@@ -343,7 +347,7 @@ class MyParameter:
 
         let mut internal_p_states: Vec<_> = scenario_indices
             .iter()
-            .map(|si| Parameter::setup(&param, &timesteps, si).expect("Could not setup the PyParameter"))
+            .map(|si| Parameter::<f64>::setup(&param, &timesteps, si).expect("Could not setup the PyParameter"))
             .collect();
 
         let model = Network::default();
@@ -412,14 +416,14 @@ class MyParameter:
 
         let mut internal_p_states: Vec<_> = scenario_indices
             .iter()
-            .map(|si| MultiValueParameter::setup(&param, &timesteps, si).expect("Could not setup the PyParameter"))
+            .map(|si| Parameter::<MultiValue>::setup(&param, &timesteps, si).expect("Could not setup the PyParameter"))
             .collect();
 
         let model = Network::default();
 
         for ts in timesteps {
             for (si, internal) in scenario_indices.iter().zip(internal_p_states.iter_mut()) {
-                let value = MultiValueParameter::compute(&param, ts, si, &model, &state, internal).unwrap();
+                let value = Parameter::<MultiValue>::compute(&param, ts, si, &model, &state, internal).unwrap();
 
                 assert_approx_eq!(f64, *value.get_value("a-float").unwrap(), std::f64::consts::PI);
 
diff --git a/pywr-core/src/parameters/rhai.rs b/pywr-core/src/parameters/rhai.rs
index 8fcfbb5f..57c14ddc 100644
--- a/pywr-core/src/parameters/rhai.rs
+++ b/pywr-core/src/parameters/rhai.rs
@@ -55,7 +55,7 @@ impl RhaiParameter {
     }
 }
 
-impl Parameter for RhaiParameter {
+impl Parameter<f64> for RhaiParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/parameters/threshold.rs b/pywr-core/src/parameters/threshold.rs
index 6d02fc05..148a0cd9 100644
--- a/pywr-core/src/parameters/threshold.rs
+++ b/pywr-core/src/parameters/threshold.rs
@@ -1,10 +1,11 @@
 use crate::metric::Metric;
 use crate::network::Network;
-use crate::parameters::{downcast_internal_state_mut, IndexParameter, ParameterMeta};
+use crate::parameters::{downcast_internal_state_mut, Parameter, ParameterMeta};
 use crate::scenario::ScenarioIndex;
 use crate::state::{ParameterState, State};
 use crate::timestep::Timestep;
 use crate::PywrError;
+use std::any::Any;
 use std::str::FromStr;
 
 pub enum Predicate {
@@ -50,7 +51,11 @@ impl ThresholdParameter {
     }
 }
 
-impl IndexParameter for ThresholdParameter {
+impl Parameter<usize> for ThresholdParameter {
+    fn as_any_mut(&mut self) -> &mut dyn Any {
+        self
+    }
+
     fn meta(&self) -> &ParameterMeta {
         &self.meta
     }
diff --git a/pywr-core/src/parameters/vector.rs b/pywr-core/src/parameters/vector.rs
index 7a9263a0..87c25d0f 100644
--- a/pywr-core/src/parameters/vector.rs
+++ b/pywr-core/src/parameters/vector.rs
@@ -20,7 +20,7 @@ impl VectorParameter {
     }
 }
 
-impl Parameter for VectorParameter {
+impl Parameter<f64> for VectorParameter {
     fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }
diff --git a/pywr-core/src/test_utils.rs b/pywr-core/src/test_utils.rs
index 284825e4..941e4d3d 100644
--- a/pywr-core/src/test_utils.rs
+++ b/pywr-core/src/test_utils.rs
@@ -158,7 +158,7 @@ pub fn simple_storage_model() -> Model {
 /// See [`AssertionRecorder`] for more information.
 pub fn run_and_assert_parameter(
     model: &mut Model,
-    parameter: Box<dyn Parameter>,
+    parameter: Box<dyn Parameter<f64>>,
     expected_values: Array2<f64>,
     ulps: Option<i64>,
     epsilon: Option<f64>,