I am currently working on an urban flood risk mitigation model. My runoff retention results range from 16 to 135 m³/m², while the flood volume ranges from 0 to 132.16 m³/m².
My question is regarding the minimum value of runoff retention. I expected it to be close to 0 m³/m² in urban areas due to the prevalence of impervious surfaces. However, my model shows a minimum value of 16 m³/m². Could you please help me understand why this value is not lower, or whether such a result is reasonable?
Like with many InVEST models, the InVEST Urban Flood Risk Mitigation model is highly dependent on the parameters you provide to the model, which in this model are mostly just the curve numbers for each soil group per LULC class. I would suggest reviewing these curve numbers to ensure that they reflect your study area and make any adjustments needed, such as by referring to published literature.
Additionally, the equations of the model are documented in our user’s guide chapter, so I would definitely suggest tracing through the equations with some of the values you’re providing in order to see how the model is doing its calculations to produce the values you’re seeing. That is often the best way to help understand what is happening in the model.
Of course, please feel free to reply back here with any questions you may have!
Thank you for taking the time to reply to my questions. As you mentioned, the user guide is very helpful for understanding how to use the model. I am already familiar with many InVEST models, and before running any model, I always read the user guide carefully. After fully understanding how the model works, I prepare the required data accordingly.
I raised these questions only after I was unable to find answers in the existing literature or the user guide. I prepared the model parameters carefully, based on both the user guide and recent literature. As far as I know, there is no option in the InVEST software or in the biophysical table to adjust the curve number. If there is still something I have overlooked in the user guide, I will review it again.
Since you are working with the InVEST Urban Flood Risk Mitigation model, you’ll appreciate that the curve numbers for each soil group are assigned for each landcover type in the biophysical table. So, if you have an LULC class that represents a more impervious surface, then you can configure the cn_a, cn_b, cn_c and cn_d fields in your biophysical table accordingly to reflect the impervious nature of the soils for that LULC class. In this model, this is how you configure the curve numbers.