An introduction to OCNet"

  collapse = TRUE,
  comment = "#>"

Graphical abstract

OCN <- create_OCN(30, 20, outletPos = 1)
OCN <- aggregate_OCN(landscape_OCN(OCN), thrA = 3)
par(mfrow = c(1, 3), mai = c(0, 0, 0.2, 0.2))
draw_simple_OCN(OCN, thrADraw = 3)
title("Optimal Channel Network")
draw_elev3D_OCN(OCN, drawRiver = FALSE, addColorbar = FALSE, expand = 0.2, theta = -30)
draw_thematic_OCN(OCN$AG$streamOrder, OCN, discreteLevels = TRUE, colPalette = rainbow(4))
title("Strahler stream order")


OCNet enables the creation and analysis of Optimal Channel Networks (OCNs). These are oriented spanning trees (built on rectangular lattices made up of square pixels) that reproduce all scaling features characteristic of real, natural river networks [@rodriguez1992; @rinaldo2014]. As such, they can be used in a variety of numerical and laboratory experiments in the fields of hydrology, ecology and epidemiology. Notable examples include studies on metapopulations and metacommunities [e.g. @carrara2012], scenarios of waterborne pathogen invasions [e.g. @gatto2013] and biogeochemichal processes in streams [e.g. @helton2018].

OCNs are obtained by minimization of a functional which represents total energy dissipated by water flowing through the network spanning the lattice. Such a formulation embeds the evidence that morphological and hydrological characteristics of rivers (in particular, water discharge and slope) follow a power-law scaling with drainage area. For an overview of the functionalities of the package, see @carraro2020. For details on the theoretical foundation of the OCN concept, see @rinaldo2014.

Some useful definitions

In graph theory, an oriented spanning tree is a subgraph of a graph $G$ such that:

At the simplest aggregation level (flow direction - FD; see Section \@ref(sec:agg) below), OCNs are oriented spanning trees whose nodes are the pixels consituting the lattice and whose edges represent flow directions.

Moreover, OCNs, just like real rivers, are constituted of nodes whose indegree (i.e. the number of edges pointing towards a node) can assume any value while the outdegree (number of edges exiting from the node) is equal to 1, except for the root (or outlet node), whose outdegree is equal to 0. Nodes with null indegree are termed sources. Nodes with indegree larger than 1 are confluences.

OCNet also allows building multiple networks within a single lattice. Each of these networks is defined by its respective outlet, which represents the root of a subgraph; the union of all subgraphs contains all elements of $G$. For simplicity, we will still refer to "OCNs" with regards to these multiple-outlet entities. In this case, strictly speaking, OCNs are not trees but rather forests.

An OCN is defined by an adjacency matrix $\mathbf{W}$ with entries $w_{ij}$ equal to 1 if node $i$ drains into $j$ and null otherwise. Owing to the previously described properties, all rows of $\mathbf{W}$ have a single non-zero entry, except those identifying the outlet nodes, whose entries are all null. Each adjacency matrix uniquely defines a vector of contributing areas (or drainage areas) $\mathbf{A}$, whose components $A_i$ are equal to the number of nodes upstream of node $i$ plus the node itself. Mathematically, this can be expressed as $(\mathbf{I}-\mathbf{W}^T)\mathbf{A}=\mathbf{1}$, where $\mathbf{I}$ is the identity matrix and $\mathbf{1}$ a vector of ones.

OCNet functions dependency tree

The generation of an OCN is performed by function create_OCN. Its only required inputs are the dimensions of the rectangular lattice, but several other features can be implemented via its optional inputs (see the function documentation for details). The output of create_OCN is a list, which can be used as input to the subsequent function landscape_OCN, as shown by the dependency tree below (indentation of an item implies dependency on the function at the above level).

OCNet functions are intended to be applied in sequential order: for each non-drawing function, the input list is copied into the output list, to which new sublists and objects are added.

Aggregation levels {#sec:agg}

Adjacency matrices and contributing area vectors of an OCN can be defined at different aggregation levels. In the output of OCNet functions, variables characterizing the OCN at the different aggregation levels are grouped within separate sublists, each of them identified by a two-letter acronym (marked in bold in the list below). Figure \@ref(fig:ex-net) provides a graphical visualization of the correspondence among the main aggregation levels.

  1. Nearest neighbours (N4, N8). Every pixel of the lattice constitutes a node of the network. Each node is connected to its four (pixels that share an edge) or eight (pixels that share an edge or a vertex) nearest neighbours. At this level, $\mathbf{W}$ is defined but $\mathbf{A}$ is not. Note that this level does not describe flow connectivity, but rather proximity among pixels. Hence, $\mathbf{W}$ does not describe an oriented spanning tree.
  2. Flow direction (FD). At this level, every pixel of the lattice is a node, but connectivity follows the flow directions that have been found by the OCN search algorithm (operated by function create_OCN). Edges' lengths are equal to either cellsize (the size of a pixel side, optional input in create_OCN) or cellsize*sqrt(2), depending on whether flow direction is horizontal/vertical or diagonal.
  3. River network (RN). The set of nodes at this level is a subset of the nodes at the FD level, such that their contributing area is larger than a certain threshold (optional input A_thr in aggregate_OCN). Such a procedure is customary in the hydrological problem of extracting a river network based on digital elevation models of the terrain [@ocallaghan1984], and corresponds to the geomorphological concept of erosion threshold [associated to a threshold in landscape-forming runoff, of which drainage area represents a proxy @rodriguez2001]. Edges' lengths are again equal to either cellsize or cellsize*sqrt(2).
  4. Aggregated (or reach - AG). The set of nodes at this level is a subset of the nodes at the RN level (see details in the next section). Accordingly, vector $\mathbf{A}$ is a subset of the vector of the same name defined at the RN level. Edges can span several pixels and therefore have various lengths.
  5. Subcatchment (SC). The number of nodes at this level is generally^[This might not be the case when there are many outlets. If the area drained by one of these outlets is lower than the threshold imposed, the cluster of pixels drained by that outlet constitutes a node at the SC level that does not have a correspondence at the AG level.] equal to that at the AG level. Each node is constituted by the cluster of pixels that directly drain into the edge departing from the corresponding node at the AG level. Here $\mathbf{W}$ does not represent flow connectivity but rather identifies terrestrial borders among subcatchments and is therefore symmetric.
  6. Catchment (CM). In this level, the number of nodes is equal to the number of outlets. Every node represents the portion of the lattice drained by its relative outlet. $\mathbf{A}$ stores drainage area values for each of these catchments, while $\mathbf{W}$ identifies terrestrial borders among catchments.

Relationship between nodes at the RN and AG levels

Nodes at the AG level correspond to a subset of nodes at the RN level. In particular, nodes at the AG level belong to at least one of these four categories:

Outlet nodes at the AG level might also be sources, confluences or breaking nodes. All AG nodes except outlet nodes have outdegree equal to 1. All RN nodes that do not correspond to AG nodes constitute the edges of the network at the AG level: more specifically, each edge is formed by an AG node and a sequence of RN nodes downstream of the AG node, until another AG node is found.

Figure \@ref(fig:ex3-net) shows an alternative aggregation scheme for the network showed in Figure \@ref(fig:ex-net) when the optional input maxReachLength is set to a finite value.


Correspondence between indices at different levels

The output of aggregate_OCN contains objects named OCN$XX$toYY, where XX and YY are two different aggregation levels. These objects define the correspondences between indices among aggregation levels. OCN$XX$toYY contains a number of elements equal to the number of nodes at XX level; each element OCN$XX$toYY[[i]] contains the index/indices at YY level corresponding to node i at XX level. For aggregation level AG, additional correspondence objects are marked by the string Reach: these consider the whole sequence of RN nodes constituting the edge departing from an AG node as belonging to the AG node.

The example shown in Figure \@ref(fig:ex-ind) corresponds to the dataset OCN_4 included in the package. Note that index numbering starts from the lower-left (southwestern) corner of the lattice.


The R code below displays the different OCN$XX$toYY objects corresponding to the example in Figure \@ref(fig:ex-ind):

ex <- aggregate_OCN(landscape_OCN(OCN_4), thrA = 2)





A working example

Let's build an OCN on a 20x20 lattice and assume that each cell represents a square of side 500 m. The total size of the catchment is therefore 100 km^2^. Let's locate the outlet close to the southwestern corner of the lattice. Function draw_simple_OCN can then be used to display the OCN.

OCNwe <- create_OCN(20, 20, outletPos = 3, cellsize = 500)

Now, let's construct the elevation field subsumed by the OCN. Let's suppose that the outlet has null elevation and slope equal to 0.01. Then, we use draw_elev3D_OCN to draw the three-dimensional elevation map (values are in m).

OCNwe <- landscape_OCN(OCNwe, slope0 = 0.01)
draw_elev3D_OCN(OCNwe, drawRiver = FALSE)

Next, the OCN can be aggregated. Let's suppose that the desired number of nodes at the AG level be as close as possible^[It is not guaranteed that an OCN can be aggregated to an arbitrary number of nodes. This is due to the fact that the indegree of confluence nodes is typically 2 or 3, depending on the flow direction patterns. For example, if the outlet is not a confluence node, all confluence nodes have indegree equal to 2, and maxReachLength = Inf, then the resulting number of aggregated nodes will be even.] to 20. With function find_area_threshold_OCN we can derive the corresponding value of drainage area threshold:

thr <- find_area_threshold_OCN(OCNwe)
# find index corresponding to thr$Nnodes ~= 20
indThr <- which(abs(thr$nNodesAG - 20) == min(abs(thr$nNodesAG - 20)))
indThr <- max(indThr) # pick the last ind_thr that satisfies the condition above
thrA20 <- thr$thrValues[indThr] # corresponding threshold area

The resulting number of nodes is^[The number of aggregated nodes is here uneven because one node has indegree equal to 3 (see figure below).] r thr$nNodesAG[indThr], corresponding to a threshold area thrA20 = r thrA20/1e6 km^2^. The latter value can now be used in function aggregate_OCN to obtain the aggregated network. Function draw_subcatchments_OCN shows how the lattice is partitioned into subcatchments. It is possible to add points at the locations of the nodes at the AG level.

OCNwe <- aggregate_OCN(OCNwe, thrA = thrA20)
points(OCNwe$AG$X,OCNwe$AG$Y, pch = 21, col = "blue", bg = "blue")

Finally, draw_thematic_OCN can be used to display the along-stream distances of RN-level nodes to the outlet (in m), as calculated by paths_OCN.

OCNwe <- paths_OCN(OCNwe, includePaths = TRUE)
draw_thematic_OCN(OCNwe$RN$downstreamPathLength[ , OCNwe$RN$outlet], OCNwe, 
                  backgroundColor = "#606060")

Application: metapopulation model


Let's build a simple discrete-time, deterministic metapopulation model on the previously built OCN. In particular, let's assume that:

Therefore, the model equation is: $$ \begin{split} P_i(t+1) &= \frac{r P_i(t)}{1+ (r-1)P_i(t)/K_i} - (p_d D_i + p_u U_i)G(K_i,K_i)\frac{P_i(t)}{K_i} \ &\quad + p_d \left( \sum_{j=1} w_{ji} G(K_j,K_j)\frac{P_j(t)}{K_j}\right) + p_u \left( \sum_{j=1} w_{ij} Y_i G(K_j,K_j)\frac{P_j(t)}{K_j}\right) \end{split} $$ where $D_i$ ($U_i$) is equal to one if there is a downstream (upstream) connection available from node $i$ and is null otherwise. Weights $Y_i$ are defined as: $$ Y_i = \frac{A_i}{\sum_{k=1} w_{kj}A_k}, $$ where $j$ identifies the node downstream of $i$. Moreover, it is $Y_o = 1$.

At carrying capacity, the system is at equilibrium, which implies that the (expected) number of individuals moving from a node $i$ to its downstream connection $j$ is equal to the (expected) number of individuals moving from $j$ to $i$: $$ p_d G(K_i,K_i) = p_u Y_i G(K_j,K_j). $$ The iterative application of the above equation allows the calculation of $G(K_i,K_i)$ for all $i$ up to a constant. To this end, let's assume $G(K_o,K_o) = G_o$. We therefore obtain $$ G(K_i,K_i) = G_o \left(\frac{p_u}{p_d}\right)^{|P_{io}|} \prod_{k \in P_{io}} Y_k, $$ where $P_{io}$ is the set of nodes constituting the downstream path from $i$ to the outlet $o$, while $|P_{io}|$ is its cardinality.


For this example, let's use the previously derived OCN_we aggregated at the RN level. Let's assume that carrying capacity is proportional to the river width evaluated at the nodes, while the initial population distribution is randomly assigned.

## Input data
OCNwe <- rivergeometry_OCN(OCNwe, widthMax = 5)   # evaluate river width 
K <- 10*OCNwe$RN$width                             # calculate carrying capacity 
pop0 <- 2*mean(K)*runif(OCNwe$RN$nNodes)           # initial random population vector
nTimestep <- 100                                   # number of timesteps
r <- 1.05                                          # proliferation rate
pd <- 0.5                                          # probability to move downstream
pu <- 1 - pd                                       # probability to move upstream
Go <- 5                                            # parameter controlling mobility 
# (no. individuals exiting from outlet node at carrying capacity is pu*Go) 

We can now compute weights $Y$:

## Weights for upstream movement
Y <- rep(1,OCNwe$RN$nNodes)                    
for (i in 1:OCNwe$RN$nNodes){
  if (i != OCNwe$RN$outlet){
    Y[i] <- OCNwe$RN$A[i]/(OCNwe$RN$W[ , OCNwe$RN$downNode[i]] %*% OCNwe$RN$A)

and $G(K_i,K_i)$:

## Evaluate expected number of individuals moving at carrying capacity
GKK <- rep(0, OCNwe$RN$nNodes)
for (i in (1:OCNwe$RN$nNodes)){
  path <- OCNwe$RN$downstreamPath[[i]][[OCNwe$RN$outlet]] # select path
  prod <- Go                                                # initialize product of Y 
  for (j in path){
    prod <- prod*Y[j]
  GKK[i] <- (pu/pd)^(length(path))*prod  

We can now run the metapopulation model:

## Run metapopulation model
pop <- matrix(data=0,ncol=nTimestep,nrow=OCNwe$RN$nNodes)  # metapopulation matrix
pop[,1] <- pop0                                              # initialization
for (t in 2:nTimestep){
  for (i in 1:OCNwe$RN$nNodes){
    pop[i, t] <- 
      # Beverton-Holt growth model
      r*pop[i, t-1]/(1 + pop[i, t-1]*(r-1)/K[i]) +
      # individuals exiting from node i
                - (pu*(sum(OCNwe$RN$W[ , i])>0) + pd*(sum(OCNwe$RN$W[i, ])>0)) * 
      GKK[i] * (pop[i,t-1]/K[i]) +
      # individuals entering in i from the upstream nodes
                + pd * OCNwe$RN$W[ , i] %*% (GKK*pop[ , t-1]/K) +
      # individuals entering in i from the downstream node
                + pu * Y[i] * OCNwe$RN$W[i, ] %*% (GKK*pop[ , t-1]/K) 


The left panel of the graph below shows the time evolution of the local population at the outlet (red) and at the node at highest distance from the outlet (blue). In the right panel, the evolution total metapopulation size is shown. Dashed lines indicate population values at carrying capacity.

par(mfrow = c(1, 2))
plot(pop[OCNwe$RN$outlet, ], type = "l", ylim = c(0, 1.05*K[OCNwe$RN$outlet]), col = "red", 
     xlab = "Time", ylab = "Population", lwd = 2)
title("Evolution of local pop. size")
lines(c(1, nTimestep),c(K[OCNwe$RN$outlet], K[OCNwe$RN$outlet]), col = "red", lty = 2)
farthestNode <- which(OCNwe$RN$downstreamPathLength[ , OCNwe$RN$outlet]
                      == max(OCNwe$RN$downstreamPathLength[ , OCNwe$RN$outlet]))[1]
lines(pop[farthestNode, ], type="l", col="blue",lwd=2)
lines(c(1, nTimestep), c(K[farthestNode], K[farthestNode]), col = "blue", lty = 2)

plot(colSums(pop), type = "l", xlab = "Time", ylab = "Population", lwd = 2, ylim = c(0, 1.05*sum(K)))
lines(c(1, nTimestep), c(sum(K),sum(K)), lty = 2)
title("Evolution of metapop. size")

Function draw_thematic_OCN can be used to visualize the spatial distribution of the metapopulation at given time points.

par(mfrow = c(2, 2), mai = c(0.1, 0, 0.2, 0))
draw_thematic_OCN(pop[,1], OCNwe, colLevels = c(0, max(K), 1000),
                  drawNodes = TRUE)
title("Time = 1")
draw_thematic_OCN(pop[,5], OCNwe, colLevels = c(0, max(K), 1000),
                  drawNodes = TRUE)
title("Time = 5")
draw_thematic_OCN(pop[,20], OCNwe, colLevels = c(0, max(K), 1000),
                  drawNodes = TRUE)
title("Time = 20")
draw_thematic_OCN(pop[,100], OCNwe, colLevels = c(0, max(K), 1000),
                  drawNodes = TRUE)
title("Time = 100")

Peano networks

Function create_peano can be used in lieu of create_OCN to generate Peano networks on square lattices. Peano networks are deterministic, plane-filling fractals whose topological measures (Horton's bifurcation and length ratios) are akin to those of real river networks [@marani1991] and can then be used in a variety of synthetic experiments, as it is the case for OCNs [e.g. @campos2006]. Peano networks are generated by means of an iterative algorithm: at each iteration, the size of the lattice side is doubled (see code below). As a result, Peano networks span squares of side equal to a power of 2. The outlet must be located at a corner of the square.

par(mfrow = c(2, 3), mai = c(0, 0, 0.2, 0))
peano0 <- create_peano(0)
title("Iteration: 0 - Lattice size: 2x2")

peano1 <- create_peano(1)
title("Iteration: 1 - Lattice size: 4x4")

peano2 <- create_peano(2)
title("Iteration: 2 - Lattice size: 8x8")

peano3 <- create_peano(3)
title("Iteration: 3 - Lattice size: 16x16")

peano4 <- create_peano(4)
title("Iteration: 4 - Lattice size: 32x32")

peano5 <- create_peano(5)
title("Iteration: 5 - Lattice size: 64x64")

The output of create_peano is a list containing the same objects as those produced by create_OCN. As such, it can be used as input for all other complementary functions of the package.

par(mai = c(0, 0, 0, 0))
peano5 <- landscape_OCN(peano5)

List of ready-made OCNs

OCNet contains some ready-made large OCNs built via function create_OCN. Their features are summarized in the Table below. Refer to the documentation of create_OCN for the definition of column names. Note that:

| name | dimX | dimY | No. of outlets | Periodic Boundaries | Initial State | Cooling Schedule | Cellsize | seed | On CRAN? | | ----------:|:------:|:------:|:----------:|:-------------------:|:-------------:|:----------------:|:--------:|:----:|:-----:| |OCN_4^[This is actually not an OCN, but was rather generated manually for illustration purposes.] | 4 | 4 | 1 | FALSE | | | 1 | | | Yes | |OCN_20 | 20 | 20 | 1 | FALSE | I | default | 1 | 1 | Yes | |OCN_250 | 250 | 250 | 1 | FALSE | I | default | 1 | 2 | No | |OCN_250_T | 250 | 250 | 1 | FALSE | T | default | 1 | 2 | Yes | |OCN_250_V | 250 | 250 | 1 | FALSE | V | default | 1 | 2 | No | |OCN_250_cold | 250 | 250 | 1 | FALSE | I | cold | 1 | 2 | No | |OCN_250_hot | 250 | 250 | 1 | FALSE | I | hot | 1 | 2 | No | |OCN_250_V_cold | 250 | 250 | 1 | FALSE | V | cold | 1 | 2 | No | |OCN_250_V_hot | 250 | 250 | 1 | FALSE | V | hot | 1 | 2 | No | |OCN_250_PB | 250 | 250 | 1 | TRUE | I | default | 1 | 2 | Yes | |OCN_rect1 | 450 | 150 | 1 | FALSE | I | default | 1 | 3 | No | |OCN_rect2 | 150 | 450 | 1 | FALSE | I | default | 1 | 3 | No | |OCN_300_diag | 300 | 300 | 1^[outletPos = 1.] | FALSE | V | default | 50 | 4 | No | |OCN_300_4out | 300 | 300 | 4^[outletSide = c("S","N","W","E"), outletPos = c(1,300,149,150).] | FALSE | V | default | 50 | 5 | Yes | |OCN_300_4out_PB_hot| 300 | 300 | 4^[outletSide = c("S","N","W","E"), outletPos = c(1,300,149,150).] | TRUE | V | hot | 50 | 5 | Yes | |OCN_300_7out | 300 | 300 | 7 | FALSE | V | default | 50 | 5 | No | |OCN_400_T_out | 400 | 400 | 1 | FALSE | T | hot | 50 | 7 | No | |OCN_400_Allout | 400 | 400 | All | FALSE | H | hot | 50 | 8 | Yes | |OCN_500_hot^[This OCN was obtained with nIter = 50*dimX*dimY.] | 500 | 500 | 1^[outletSide = "E", outletPos = 100] | FALSE | I | hot | 50 | 9 | No | |OCN_500_PB_hot | 500 | 500 | 1 | TRUE | V | hot | 50 | 10 | No |

Compatibility with other packages


Adjacency matrices at all aggregation levels are produced as spam [@furrer2010] objects. In order to transform the OCN into an igraph [@csardi2006] graph object, the adjacency matrix must be converted into a Matrix object (via function as.dgCMatrix.spam of spam). Function graph_from_adjacency_matrix of igraph can then be used to obtain a graph object.

For example, let's transform the previously obtained OCN_we at the AG level into a graph:

g <- OCN_to_igraph(OCNwe, level = "AG")
plot.igraph(g, vertex.color = rainbow(OCNwe$AG$nNodes), 
     layout = matrix(c(OCNwe$AG$X,OCNwe$AG$Y),ncol = 2, nrow = OCNwe$AG$nNodes))

The same network can be displayed as an OCN:

draw_thematic_OCN(c(1:OCNwe$AG$nNodes), OCNwe, discreteLevels = TRUE, drawNodes = TRUE,
                  colPalette = rainbow,  cex = 3, riverColor = "#999999",
                  backgroundColor = "#00000000", addLegend = FALSE)
text(OCNwe$AG$X, OCNwe$AG$Y)


Function OCN_to_SSN transforms an OCN at a given aggregation level into an SSN [@verhoef2014] object. See the following example:

ssnOCN <- OCN_to_SSN(OCNwe, level = "RN", obsDesign = SSN::binomialDesign(50),
                     path = paste(tempdir(), "/OCN.ssn", sep = ""), importToR = TRUE)
plot.SpatialStreamNetwork(ssnOCN, "upDist", breaktype = "user", brks = seq(0,14000,2000), 
                          xlab = "x [m]", ylab = "y [m]", asp = 1)
title("Distance from outlet of observation points [m]")



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OCNet documentation built on May 17, 2021, 5:07 p.m.