Introduction to the InVEST Water Yield Model
My name is Stacie Wolny, and I’m a GIS analyst. I’ll be introducing you to the InVEST Water Yield Model. The Water Yield Model follows the supply service value framework common across the InVEST tool set. Here, supply represents the biophysical processes of rainfall and runoff on a landscape, which produce rivers, and we call this water yield. Meanwhile, people in the landscape use some of that water for things like drinking and irrigation. What remains is the service called water scarcity. This remaining water can then be used for hydropower production, among other things. And we can put a monetary value on the amount of energy produced by water, running off of the landscape. So, this model allows us to start answering the question: What is the value of the landscape for water production? InVEST Water Yield is a basic rainfall runoff model. Think of it as a bucket, where precipitation fills the bucket with water. But some of that water is lost, due to evaporation and transpiration by plants.
And what remains is the water yield. Essentially, precipitation minus evapotranspiration equals water yield. To be more specific, the model starts with the total amount of precipitation falling on a landscape each year. From that, it subtracts the amount of water lost due to evaporation and transpiration, which will vary based on vegetation properties, like rooting death, and soil properties, such as soil death and available water content.
The result is the amount of water predicted to run off of the landscape each year. The primary limitation of this model is that it works on an annual average basis. So, there is no information given on seasonal variation in run off which needs to be considered when interpreting an increase or decrease in annual water yield. Depending on the climate, this change may occur homogeneously throughout the year or only during the dry season. And not knowing this timing may be important to the decision context. It also does not differentiate between the surface and sub-surface flow, or consider surface to ground water interactions. This is important when considering water extraction for uses like irrigation. For valuation, the model uses a very generalized and simplified relationship between water yield and reservoir hydropower production and assumes the pricing remains the same throughout the year, which is generally not the case. The inputs to the water yield model include the climate variables, precipitation and potential evapotranspiration, soil properties of soil depth, and plant available water content, land use and land cover with associated values for rooting depth and evapotranspiration.
It also requires the boundary for the watershed that drains into the point of interest, as well as the sub-watersheds making up the main basin. If you choose to do valuation, the model requires information on water demand, specified by land cover type, and economic information including a variety of hydropower plant data and the price of energy. The main model outputs include actual evapotranspiration, which is the amount of water that is lost each year expressed in millimeters, water yield also expressed in millimeters, which is the amount of water that runs off the landscape each year. If you input data on water demand, the model will also output amount of water scarcity, which is the water yield minus the amount of water you used in the basin. And if you choose to do valuation, net present value of hydropower production will be calculated based on water scarcity and economic data inputs. In summary, the InVEST Water Yield Model provides biophysical and economic information to help understand the spatial pattern of water supply and the value of the landscape for hydropower production..