Hydrologic Model Setup (for upper-level courses only)

Overview

In this session, we will learn how a hydrologic numerical model can be used to model the rainfall-runoff processes in the watershed and predict its response to a real rainfall event (Storm Matthew in our case). The numerical model that we will use in this session is a physically-based, spatially-distributed hydrologic model known as the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) system, which is developed by the US Department of Defense. This model is non-commercial; it provides physically-based simulations of rainfall-runoff processes; and it allows for spatially distributed input and output information. The current setup of the model is based on the following features: two-dimensional diffusive wave for overland flow, one-dimensional explicit diffusive wave method for channel flow, and the Green&Ampt infiltration for flow simulation in the unsaturated zone (discuss these methods with your Hydrology Instructor).


Your Task

1. The first step in the model setup is to create a computational grid for the numerical calculations. For our case, the watershed topographic and hydrologic properties are represented using a square 25x25 m2 Cartesian grid. Turn on the Computational Grid layer and zoom-in for a closer look.
   
2. Next step in our model setup procedure is to populate the computational grid with information on land elevation. We used the 1/9 Arc-second elevation layer to assign an elevation value to every point in the computational grid. Turn on the Overland Elevation layer under Model Setup and zoom-in for a closer look and see how it matches with the original 1/9 Arc-second map.
   
3. Next, we translate the soil-type map into a soil-type index map where each point in the computational grid is assigned a unique soil-type index (turn on the soil index map layer and match with the original soil-type map).
   
4. Next, we translate the land-use map into a land-use index map where each point in the computational grid is assigned a unique land-use index (turn on the land-use index map layer and match with the original historical land-use map).
   
5. Next, the land-use and soil-type index maps are combined into a single index map (Turn on the Combined ST + LU index map layer under Model Setup).
   
6. The next step is to assign model parameters to each grid point based on soil type and land-use information. See this table for a list of the assigned parameters. Note that the over-land roughness Manning parameter is assigned based on Land-Use information only while the infiltration parameters (e.g., hydraulic conductivity and porosity) is assigned based on the land-use and soil-type combination.
   
7. The last step in the model setup is to assign cross sectional dimensions to the different channels in the watershed. These are based on field surveys of the channels. Turn on the Channels Cross Sections layer and examine some examples of the cross sections (indicated as white dashes).
   
   
   

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