InVEST Sediment Retention Hands-on Session InVEST Sediment Retention NASA photo, not sure where

Erosion Increase maintenance cost Soil erosion and sediment can cause: Decrease in agricultural productivity Degradation of fish habitat and aquatic life Water quality degradation Risk of structural failures Increased dam maintenance costs Increase maintenance cost Soil erosion can cause a lot of problems… A healthy ecosystem can do a lot to prevent these things from happening. Dense vegetation can slow down and stop soil particles from eroding away, which helps keep soil where we want it (like on farms) and keep it out of where don’t want it (like streams.), providing us with a valuable service.

Ecosystem Service questions Where are the sediment sources? Where is sediment retained? How much is retained? What is the value of this retention? In order to quantify how an ecosystem helps mitigate erosion, we consider the following questions…

InVEST Sediment Retention Model Based on the Revised Universal Soil Loss Equation (RUSLE) Includes geomorphology and climate Potential erosion on a parcel Enhanced by hydraulic connectivity What happens as the parcel’s sediment moves downslope? Influence of intervening landcover Sediment retention valued as ecosystem service For modeling sediment we provide a Sediment Retention model. First the model determines how much sediment will leave a parcel of land, using the Universal Soil Loss Equation (USLE), which is a commonly-used method for estimating potential erosion from a parcel. It includes both geomorphology and climate. We make some modifications to the USLE to make it more generally applicable, for example working with some Chinese colleagues (Wang Yu Kuan’s group) to adjust the equation in terraced areas. Once sediment leaves a parcel, it travels downslope until it reaches a stream, so we also model how much of that sediment is taken up by the vegetation that lies between the parcel and the stream. The total amount of sediment retained by a parcel is valued as the ecosystem service.

InVEST Sediment Retention Model Erosivity (R) Slope (LS) c Conservation Factor (P) To get more specific, the USLE includes these factors: How steep the parcel is (steep slopes will erode more easily); Rainfall erosivity (if rainfall is intense, it’s more likely to cause erosion); Conservation factor (whether erosion-reduction practices are done on the land, like contour farming); Crop factor (how the vegetation is managed, is tilling done); Soil erodibility (how susceptible the soil is to eroding, based primarily on soil texture). All of these are combined to give the potential erosion from each parcel on the landscape. Soil Erodibility (K) Soil loss= R x K x LS x C x P Crop factor (C)

Hydraulic Connectivity load (USLE) Flow direction Corn Forest export Wheat Forest retention Once we know how much sediment leaves a parcel, we then take into account what happens to it as it flows downslope until it reaches a stream. In this example, sediment leaves a parcel of corn, as it moves downslope the forest takes up some of it, and the rest keeps moving to a parcel of wheat, which also takes up some of it, as does another parcel of forest. Whatever does not get retained by the intervening landscape enters the stream and is considered export. The same thing happens with each parcel, and the final retention value for a parcel of land is the sum of how much that parcel retains from what comes to it from upstream. Stream to reservoir

Valuation Based on avoided treatment costs Can value for dredging and/or water quality Time Loading Service Critical Loading A reservoir can hold a certain amount of sediment before it needs to be dredged. And a water treatment facility might have a maximum amount of sediment that is allowed to remain in the water that is delivered to its customers. We call both of these ‘critical loading’. Since this amount does not need to be dredged or treated, we do not include it in our valuation. The ecosystem service value is based on the amount of sediment removed by the landscape that is above the critical loading threshold, since that’s how much the facility would have to pay to remove otherwise.

Biophysical Outputs Potential Soil loss Sediment Retained Calculated from USLE Per sub-watershed Sediment Retained Per watershed and sub-watershed Used in valuation Three main outputs are provided, all are per year. First is the potential soil loss, which is the result of the USLE equation. Next is the actual soil loss, how much of the potential erosion actually makes it to the stream. And finally, the amount of sediment retained by the landscape, which is the biophysical ecosystem service. Here’s an example of an output map, from a watershed in Colombia. The darker green areas are where larger amounts of sediment are being retained. Sediment retention Sediment Exported Per watershed and sub-watershed High Low + Total export to reservoir

Assumptions/Limitations Predicts erosion from sheet wash alone Sediment gets to outlet within a year No limit to retention Accuracy limited in mountainous areas Because the model is simple, it has some limitations… Only models sheet wash, no gully or bank erosion Future: Sediment delivery ratio: the ratio of actual sediment load to gross erosion estimated from USLE equation, is included in our model to account for sediment export and retention processes at catchment scale Takes into account for a catchment-specific parameter 𝛾 (proportional to a particle size) and the travel time in the flow path. The travel time through a single cell ( 𝑡 𝑖,𝑗 ) is estimated from the ith cell as a function of flow length ( 𝑙 𝑖,𝑗 ) (m), vegetation roughness ( 𝛼 𝑖,𝑗 ) and slope (%). The amount of sediment retained on a pixel is a function of the amount of sediment inflow and the inverse of the sediment delivery ratio on that pixel as well as the amount of sediment retained on pixel due to crop and land management practice Accuracy limited in mountainous areas – limitation of USLE, we have added functionality to do a better job, but it’s still not optimal in very karsty or mountainous areas.

Outlook Sediment delivery ratio Gully and bank erosion Dam retention Tier 2 sediment model West Coast East Coast Belize Colombia Mexico Sediment delivery ratio: ratio of sediment yield of a drainage basin to the total amount of sediment moved by sheet erosion and channel erosion. The ratio of actual sediment load to gross erosion estimated from USLE equation, is included in our model to account for sediment export and retention processes at catchment scale. Dam retention – effect of sediment trapping in dams Multiflow algorithm – replacement for Arc’s flow direction, instead of only one direction for flow, can go in several directions. Ecuador Amazon Basin Tanzania Indonesia

Sediment retention in Colombia Application Sediment retention in Colombia Dam 1 Dam 2 Supply Bogotá Service (dams) National analysis of ecosystem services in Colombia. The supply of sediment retention is how much happens whether we use it or not. Service is when people use it. Thus, when we mapped the supply in Colombia, we could look at the whole country. But when looking at service, we needed reservoirs or other points where sediment retention is useful to us. We only had the locations of a few major dams, so you see them here, along with the sediment retained in the watersheds that flow to them. What I wanted to point out here is what happens when several dams are placed along a river. Their contributing watersheds are overlapping. So any sediment retained at the top of the watershed not only provides a service to the first dam, but also the others downstream, since dams will only trap some sediment, and let some continue downstream to the next dam. If we know the trapping rate, then we can accurately calculate the service provided to subsequent streams downstream. In this case we don’t know, so we simply multiply by the number of dams downstream.

Scenarios for Mine Expansion in Columbia Current Mines Permits Granted Pending All possible permits

Mining in Columbia High Impact Zones should avoided Permits Granted Permits Pending All possible Permits

Hands-on Session Questions? NASA photo, not sure where