David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Strategies for Analyzing and Using Environmental Systems Models• So far, we have– Identified the component elements we can use in the models we build to represent systems– Looked at the kind of thinking required to assemble those elements into useful representations of systems– Examined a set of standard configurations of model elements that produce certain common behaviors• We’ve now got the tools we need to build models• Next, we need to learn how to determine if the models we build are in useful and worth the effortDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Strategies for Analyzing and Using Environmental Systems Models• When a system model is built, we want to know the following:– Does the model appropriately represent the system such that we can use it to answer the questions we want to ask?– How well does the model tell us about• the system’s behavior?•The factors that control the system?• We can formulate a general strategy that we can apply to any given systems model to evaluate these questionsDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Strategies for Analyzing and Using Environmental Systems Models1. Problem Definition – From the outset we need to know what questions we want to answer2. Model Validation and Calibration – We need to make sure that the model is structurally and predictively valued3. Exploratory Analysis – Through disturbance experiments and sensitivity analysis, we can determine how the system will perform under various conditions4. Case Studies – We apply the model to specific scenarios of interest so we can use it to help understand a system and/or make decisionsDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Strategies for Analyzing and Using Environmental Systems Models1. Problem Definition• Real environmental systems are incredibly complex• We can choose to try and represent as much of that complexity as we’d like to in models• Ultimately, however, we cannot represent everything … we have to make choices of what to include and what not to include based on the questions we want to answer• E.g. Our People on the Island model doesn’t include the coral reef near the island …• Models are always a simplified representation of reality, that can answer some questions, but not allDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Strategies for Analyzing and Using Environmental Systems Models1. Problem Definition Cont.•What kinds of questions can our People on the Island model answer?• Issues pertaining to the size of the population through time, and its survival•What sorts of question can the People on the Islandmodel not answer?• Anything else really … i.e. how will the presence of the people impact biodiversity on the island?•So, before we create a systems model, we really must have our ‘problem’ defined so we can decide how to build the model to help us with that problemDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Strategies for Analyzing and Using Environmental Systems Models2. Model Validation and Calibration• Does the model structure capture how we believe the system functions?• Does the model produce realistic values for the stocks over time?• We determine this by comparing simulated output with observations of the system• We need to do this to make sure that the processes and relationships defined in the model properly represent the system for the questions posed: Doing a good job here is what gives us confidencethat we can use the model to understand the systemDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Strategies for Analyzing and Using Environmental Systems Models3. Exploratory Analysis• Here, we use the model to explore the system and gain insights into how the system works, via two techniques:• Disturbance experiments – By using PULSE, STEP, and RAMP functions to disturb the system, we can see how the system reacts under these stimuli• Sensitivity Analysis – We can identify the model elements that have a high impact on the behavior4. Case Studies• We often have specific scenarios we want to simulate in order to provide information for managementdecisions, and use the model to examine those specificsDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Analysis and Use Strategy – An Illustrated Example• We begin with a population of fish, that contract a contagious disease which threatens to wipe out a significant number of fish and do considerable damage to the aquatic ecosystem• First, we need a conceptual model of how the disease effects any given fish:Infected FishdieEx-FishrecoverResistant Fish•May still passthe disease onto other fish1. Not all Infected Fish die, so we need a Death Rate2. Not all contacts lead to infection; Infection Rate3. We need to estimate how often fish have contactsDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Analysis and Use Strategy – An Illustrated Example - Diagram• The overall model looks like this:David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Analysis and Use Strategy – An Illustrated Example - Reservoirs• Because we are representing this disease as having a temporal aspect associated with how individual fish progress, two of the reservoirs are defined a little differently:– Susceptible Fish is a standard stock, consisting of individuals that are not infected, but could become infected with exposure– Sick Fish and Resistant Fish are different: Fish do not stay in these forever. They either die or become resistant, or lose resistance and become susceptible to infection once again• We need a new type of reservoir to model this, since we need to differentiate one fish from the nextDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Analysis and Use Strategy – An Illustrated Example - Conveyors• This new type of stock is called a conveyor• Unlike a reservoir, which does not distinguish between individuals, a conveyor does• Conveyors have a transit time, beyond which an individual will exit the conveyor– E.g. At the end of the allotted time, Sick Fish will either have died or become resistant• Conveyors have a flow through process, through which individuals will exit at the end of the transit time– E.g. At the end of the transit time, Sick Fish will flow through into Recovering, and become Resistant FishDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Analysis and Use Strategy – An Illustrated Example -