--- title: your city is upstream and downstream at once canonical_url: https://ensurance.app/guide/your-city-is-upstream-and-downstream-at-once markdown_url: https://ensurance.app/guide/your-city-is-upstream-and-downstream-at-once.md subtitle: "bidirectional spillover: trace what your city depends on, and what depends on your city, with the same map" category: natural-capital --- # your city is upstream and downstream at once *bidirectional spillover: trace what your city depends on, and what depends on your city, with the same map* **Bidirectional spillover** is the idea that every piece of ground is upstream of something and downstream of something else, so the services it gives and receives can be traced in both directions. Start from any node, whether a city, a hospital, a creek or a park, and you can ask two questions with the same map. What does this place depend on? And what depends on this place? For a city, both answers matter, and many valuations only ask one. A city depends on ground it does not own: the forest that feeds its reservoir, the wetlands upstream that slow the flood, the open ground that recharges the aquifer under it. And a city's own ground serves people who do not live there: its parks, canopy and river edges cool, drain and filter for its neighbors and for the towns downstream. This post is about running the valuation both ways, and why that is how the funding question gets answered. The forest, the wetland, the creek and the canopy do their work across the city line whether or not anyone traces it. Tracing it is how the people on each side of the line find each other. Ensurance is how they can fund the ground in between and the people who keep it. It is not the ground. :::johnson **every city is upstream of someone and downstream of someone else.** Run the valuation from your city outward and you find who depends on you. Run it inward and you find what you depend on. The same map gives both, and the overlap is where the payors may be. [see the places on the map →](/explore?from=guide) ::: ## spillover, in one paragraph Spillover is what happens when the flows from a natural asset cross the boundary of the asset. Water leaves the wetland, pollinators leave the meadow, shade falls on the next block. Ecology, economics and real estate have measured these crossings for decades under different names. [What is a spillover effect in nature](/guide/what-is-a-spillover-effect-in-nature?from=guide) covers the vocabulary, and [does protecting one parcel help the land around it](/guide/does-protecting-one-parcel-help-the-land-around-it?from=guide) covers the evidence. This post is about the direction of the arrow. ## the usual direction: outward from the asset Most spillover work starts at the natural asset and looks outward. Here is a forest. What does it do for the land around it? Who benefits from its water, its shade, its slope stability? This is how conservation planners pick which forest to protect. It is the right question for the forest's steward. It is not the question a city starts with. A city does not start from the forest. It starts from itself: its water supply, its flood risk, its hot wards. The question it asks is the other way round. ## the other direction: inward from the city Start from the city and trace inward. Which ground does our drinking water depend on? Which wetlands slow the river before it reaches us? Which forest keeps sediment out of our reservoir? New York City asked that question about its water. Its drinking water comes from a supply system of more than 1,900 square miles, mostly outside the city. In 1996 the city estimated that a filtration plant for the Catskill and Delaware supply would cost $4–6 billion to build and $300 million a year to run. Instead it signed a watershed agreement it expected would let it avoid the plant, at about $7 a year added to the average bill over five years. The plant estimate was the city's own. As the city set it out in 1996, ratepayers would fund the program, and it would pay upstream landowners and towns who agreed to it. [What is the beneficiary pays principle](/guide/what-is-the-beneficiary-pays-principle?from=guide) carries the case further. That is bidirectional spillover in practice. The watershed's water flows to the city. The ratepayers' money flows back to the people who keep the watershed in good condition. Same map, both directions. ## why both directions, not one Run only the inward question and you find what your city depends on. That is half of a funding plan. Run only the outward question from your own parks and canopy and you find who depends on you. That is the other half. Run both and something more useful appears: the **overlap**. | question | what you find | what it gives you | |---|---|---| | What does our city depend on? (inward) | The upstream forest, the regional wetlands, the aquifer recharge area | The assets outside your control that you cannot afford to lose | | What depends on our city's ground? (outward) | The downstream towns your floodplain protects, the neighbors your canopy cools, the utility your parks save money for | The parties who benefit from what you already maintain | | Who else depends on the same ground? (both) | The hospital, the utility, the rail yard and the town downstream, all relying on the same wetland | The co-payors: everyone who shares the dependency | The third row is the one that matters for funding. When a city, a utility, a hospital and a downstream town all depend on the same upstream wetland, none of them has to carry it alone. That is the hypothesis we work from: the dependency points to a likely payor, and the shared dependency points to likely co-payors. It has limits, and they are worth saying. Water runs one way, so upstream and downstream are not mirror images. Shade reaches only the blocks near the trees, so the neighbors a canopy cools are the ones at its edge. Two payors can claim the same benefit, so their shares must not be added up as if the benefit were counted twice. And a city that wants to pay for ground in another jurisdiction needs a vehicle to do it: an intergovernmental agreement, or a utility program, as New York used. [TNFD](https://tnfd.global/), the disclosure framework many large organizations now report against, asks the same two questions in its LEAP approach: what you depend on, and what you affect. ## what this looks like for a C40 city Many metros import services. Their water, and much of their flood protection, often come from beyond their limits. They also export: large parks and river corridors can serve suburbs and downstream communities. Run inward, the valuation finds the source watershed, the regional forests and the floodplains upstream. Those are assets the city may need to help fund, often across a jurisdictional line, as New York did. [The dependency your map can't see](/guide/the-dependency-your-map-cant-see?from=guide) and [your place depends on a place](/guide/your-place-depends-on-a-place?from=guide) walk the inward trace. Run outward, it finds which neighbors and institutions benefit from the city's own green infrastructure. Those are the parties who could share its cost. A C40 climate office that has both lists can bring them to its water utility, its regional government and its neighbors with a concrete proposal instead of a general appeal. ## what this looks like for a GCoM town A small town is usually smaller than its watershed. The creek that floods Main Street may start several towns upstream. A floodplain meadow on the edge of town may protect the towns below it. Run inward, the town learns which ground upstream holds back its flood. That ground is often in another municipality. Run outward, it learns which neighbors its own floodplain protects. Put the two lists together and the town sees that it is one link in a chain of towns that all depend on the same creek. The [water cycle](/water-cycle.syndicate?from=guide) and [flood resilience](/flood-resilience.syndicate?from=guide) lines are live for that kind of shared work. That is why [the regional line is named for the buyer, not the town](/guide/name-the-line-for-the-buyer-not-the-town?from=guide): the book is the watershed. ## what we can and cannot show today When we work a market, we read the dependency map both ways: from a place inward to what it depends on, and from a place outward to who depends on it. That is how we work. It is not a tool you can run yourself, and the inward trace from a city is not a finished product. The step from a payor to the specific parcels it depends on, and code that names co-payors, are planned, not built. Until then, a valuation does it by hand: an analyst traces the flows, names the assets and lists who depends on each. We have not yet published a completed payment from one parcel's beneficiaries to another parcel. When one happens, it will be here. ## frequently asked questions ### what is bidirectional spillover? It is the idea that every piece of ground both receives services from other ground and provides services to other ground. The same dependency map can be read from any starting point: what this place depends on, and what depends on this place. For a city, that means tracing both the natural assets it relies on and the neighbors who rely on its natural assets. ### how is it different from a serviceshed? A [serviceshed](/guide/what-is-a-serviceshed?from=guide) is the area where one natural asset delivers one service to its beneficiaries. Bidirectional spillover reads many servicesheds at once, from either end, and finds where they overlap. ### why does the direction matter for funding? Because the party that depends on an asset is a likely funder of it. Tracing inward from a city finds the assets it may need to help fund. Tracing outward from the city's own ground finds who might help fund what the city maintains. The overlap finds groups of funders who share one asset. ### is the New York watershed deal an example? Yes. New York traced its water dependency to the watershed beyond its limits and chose to fund protection there, through ratepayers, rather than build a filtration plant. The watershed's water flows to the city, and the ratepayers' money flows back to the people who keep the watershed. ## next steps Take your [natural asset inventory](/guide/how-to-build-a-natural-asset-inventory?from=guide) and add two columns: what each asset depends on, and who depends on it. Then [browse the map](/explore?from=guide) for named places like yours. If you would like the whole picture in both directions, the regional natural asset analysis on [our services page](/services?from=guide#finance) names every asset your town or region depends on and who else depends on it. Or [talk to us](/contact?from=guide&topic=city-nature-valuation). ## sources - [New York City, press release 431-96, 1996](https://www.nyc.gov/html/om/html/96/sp431-96.html): filtration plant estimate, watershed agreement, bill impact - [TNFD, LEAP approach](https://tnfd.global/): dependencies and impacts on nature ## the series 1. [how a city values the nature it already owns](/guide/how-a-city-values-the-nature-it-already-owns?from=guide) 2. [why a city should run an ecosystem service valuation](/guide/why-a-city-should-run-an-ecosystem-service-valuation?from=guide) 3. [how to build a natural asset inventory with the staff you have](/guide/how-to-build-a-natural-asset-inventory?from=guide) 4. [what a city's green space is worth, and what the number is for](/guide/what-a-citys-green-space-is-worth?from=guide) 5. [price what the creek already does](/guide/price-what-the-creek-already-does?from=guide) 6. **your city is upstream and downstream at once** (you are here) 7. [hire a natural asset valuation for your city or town](/guide/hire-a-natural-asset-valuation-for-your-city?from=guide)