Tutorial: Solution of an NPZD model

This tutorial is about the efficient solution of production-destruction systems (PDS) with a small number of differential equations. We will compare the use of standard arrays and static arrays from StaticArrays.jl and assess their efficiency.

Definition of the production-destruction system

The NPZD model we want to solve was described by Burchard, Deleersnijder and Meister in Application of modified Patankar schemes to stiff biogeochemical models for the water column. The model reads

\[\begin{aligned} N' &= 0.01P + 0.01Z + 0.003D - \frac{NP}{0.01 + N},\\ P' &= \frac{NP}{0.01 + N}- 0.01P - 0.5( 1 - e^{-1.21P^2})Z - 0.05P,\\ Z' &= 0.5(1 - e^{-1.21P^2})Z - 0.01Z - 0.02Z,\\ D' &= 0.05P + 0.02Z - 0.003D, \end{aligned}\]

and we consider the initial conditions $N=8$, $P=2$, $Z=1$ and $D=4$. The time domain of interest is $t\in[0,10]$.

The model can be represented as a conservative PDS with production terms

\[\begin{aligned} p_{12} &= 0.01 P, & p_{13} &= 0.01 Z, & p_{14} &= 0.003 D,\\ p_{21} &= \frac{NP}{0.01 + N}, & p_{32} &= 0.5 (1.0 - e^{-1.21 P^2}) Z,& p_{42} &= 0.05 P,\\ p_{43} &= 0.02 Z, \end{aligned}\]

whereby production terms not listed have the value zero. Since the PDS is conservative, we have $d_{i,j}=p_{j,i}$ and the system is fully determined by the production matrix $(p_{ij})_{i,j=1}^4$.

Solution of the production-destruction system

Now we are ready to define a ConservativePDSProblem and to solve this problem with a method of PositiveIntegrators.jl or OrdinaryDiffEq.jl.

As mentioned above, we will try different approaches to solve this PDS and compare their efficiency. These are

an out-of-place implementation with standard (dynamic) matrices and vectors,
an in-place implementation with standard (dynamic) matrices and vectors,
an out-of-place implementation with static matrices and vectors from StaticArrays.jl.

Standard out-of-place implementation

Here we create a function to compute the production matrix with return type Matrix{Float64}.

using PositiveIntegrators # load ConservativePDSProblem

function prod(u, p, t)
    N, P, Z, D = u

    p12 = 0.01 * P
    p13 = 0.01 * Z
    p14 = 0.003 * D
    p21 = N / (0.01 + N) * P
    p32 = 0.5 * (1.0 - exp(-1.21 * P^2)) * Z
    p42 = 0.05 * P
    p43 = 0.02 * Z

    return [0.0 p12 p13 p14;
            p21 0.0 0.0 0.0;
            0.0 p32 0.0 0.0;
            0.0 p42 p43 0.0]
end

The solution of the NPZD model can now be computed as follows.

u0 = [8.0, 2.0, 1.0, 4.0] # initial values
tspan = (0.0, 10.0) # time domain
prob_oop = ConservativePDSProblem(prod, u0, tspan) # create the PDS

sol_oop = solve(prob_oop, MPRK43I(1.0, 0.5))

Plotting the solution shows that the components $N$ and $P$ are in danger of becoming negative.

using Plots

plot(sol_oop; label = ["N" "P" "Z" "D"], xguide = "t")

PositiveIntegrators.jl provides the function isnonnegative (and also isnegative) to check if the solution is actually nonnegative, as expected from an MPRK scheme.

isnonnegative(sol_oop)

true

Standard in-place implementation

Next we create an in-place function for the production matrix.

function prod!(PMat, u, p, t)
    N, P, Z, D = u

    p12 = 0.01 * P
    p13 = 0.01 * Z
    p14 = 0.003 * D
    p21 = N / (0.01 + N) * P
    p32 = 0.5 * (1.0 - exp(-1.21 * P^2)) * Z
    p42 = 0.05 * P
    p43 = 0.02 * Z

    fill!(PMat, zero(eltype(PMat)))

    PMat[1, 2] = p12
    PMat[1, 3] = p13
    PMat[1, 4] = p14
    PMat[2, 1] = p21
    PMat[3, 2] = p32
    PMat[4, 2] = p42
    PMat[4, 3] = p43

    return nothing
end

The solution of the in-place implementation of the NPZD model can now be computed as follows.

prob_ip = ConservativePDSProblem(prod!, u0, tspan)
sol_ip = solve(prob_ip, MPRK43I(1.0, 0.5))

plot(sol_ip; label = ["N" "P" "Z" "D"], xguide = "t")

We also check that the in-place and out-of-place solutions are equivalent.

sol_oop.t ≈ sol_ip.t && sol_oop.u ≈ sol_ip.u

true

Using static arrays

For PDS with a small number of differential equations like the NPZD model the use of static arrays will be more efficient. To create a function which computes the production matrix and returns a static matrix, we only need to add the @SMatrix macro.

using StaticArrays

function prod_static(u, p, t)
    N, P, Z, D = u

    p12 = 0.01 * P
    p13 = 0.01 * Z
    p14 = 0.003 * D
    p21 = N / (0.01 + N) * P
    p32 = 0.5 * (1.0 - exp(-1.21 * P^2)) * Z
    p42 = 0.05 * P
    p43 = 0.02 * Z

    return @SMatrix [0.0 p12 p13 p14;
                     p21 0.0 0.0 0.0;
                     0.0 p32 0.0 0.0;
                     0.0 p42 p43 0.0]
end

In addition we also want to use a static vector to hold the initial conditions.

u0_static = @SVector [8.0, 2.0, 1.0, 4.0] # initial values
prob_static = ConservativePDSProblem(prod_static, u0_static, tspan) # create the PDS

sol_static = solve(prob_static, MPRK43I(1.0, 0.5))

using Plots

plot(sol_static; label = ["N" "P" "Z" "D"], xguide = "t")

This solution is also nonnegative.

isnonnegative(sol_static)

true

The above implementation of the NPZD model using StaticArrays can also be found in the Example Problems as prob_pds_npzd.

Performance comparison

Finally, we use BenchmarkTools.jl to show the benefit of using static arrays.

using BenchmarkTools
@benchmark solve(prob_oop, MPRK43I(1.0, 0.5))

BenchmarkTools.Trial: 2422 samples with 1 evaluation per sample.
 Range (min … max):  1.508 ms … 73.403 ms  ┊ GC (min … max):  0.00% … 96.45%
 Time  (median):     1.593 ms              ┊ GC (median):     0.00%
 Time  (mean ± σ):   2.061 ms ±  2.803 ms  ┊ GC (mean ± σ):  14.54% ± 12.82%

  █▂  ▁▅▃                                                     
  ██▅▅███▆▁▁▁▁▁▁▃▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▅▁▃▃▅▅▅▆▅▅ █
  1.51 ms      Histogram: log(frequency) by time       11 ms <

 Memory estimate: 2.89 MiB, allocs estimate: 41328.

using BenchmarkTools
@benchmark solve(prob_ip, MPRK43I(1.0, 0.5))

BenchmarkTools.Trial: 5590 samples with 1 evaluation per sample.
 Range (min … max):  766.435 μs … 70.795 ms  ┊ GC (min … max): 0.00% … 98.36%
 Time  (median):     799.518 μs              ┊ GC (median):    0.00%
 Time  (mean ± σ):   891.630 μs ±  1.433 ms  ┊ GC (mean ± σ):  5.48% ±  4.01%

     ▂█▅                         ▁▁                             
  ▂▅▅███▅▂▂▁▂▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▂▃▄██▆▃▂▁▁▁▁▁▁▁▂▂▂▂▁▁▂▃▄▃▂▁▁▁▁▁▁ ▂
  766 μs          Histogram: frequency by time          985 μs <

 Memory estimate: 477.21 KiB, allocs estimate: 6570.

@benchmark solve(prob_static, MPRK43I(1.0, 0.5))

BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):  268.169 μs …  52.421 ms  ┊ GC (min … max): 0.00% … 98.70%
 Time  (median):     282.576 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   338.024 μs ± 818.675 μs  ┊ GC (mean ± σ):  8.00% ±  5.24%

  ▁█▂                                                            
  ███▇▃▅▆▅▄▃▃▂▂▂▂▂▂▂▂▂▂▂▁▂▂▂▂▂▂▁▂▂▁▁▂▂▁▁▂▁▂▃▄▆▇▇▆▅▅▅▅▄▃▃▂▂▂▂▂▂▂ ▃
  268 μs           Histogram: frequency by time          383 μs <

 Memory estimate: 251.09 KiB, allocs estimate: 1743.

Package versions

These results were obtained using the following versions.

using InteractiveUtils
versioninfo()
println()

using Pkg
Pkg.status(["PositiveIntegrators", "StaticArrays", "LinearSolve"],
           mode=PKGMODE_MANIFEST)

Julia Version 1.11.7
Commit f2b3dbda30a (2025-09-08 12:10 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 4 × AMD EPYC 7763 64-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver3)
Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)
Environment:
  JULIA_PKG_SERVER_REGISTRY_PREFERENCE = eager

Status `~/work/PositiveIntegrators.jl/PositiveIntegrators.jl/docs/Manifest.toml`
  [7ed4a6bd] LinearSolve v3.40.1
  [d1b20bf0] PositiveIntegrators v0.2.13 `~/work/PositiveIntegrators.jl/PositiveIntegrators.jl`
  [90137ffa] StaticArrays v1.9.15