## What is the differece between pervious flow, impervious flow and baseflow?

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rcoville
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### What is the differece between pervious flow, impervious flow and baseflow?

The model estimates total streamflow using three flow components: pervious area runoff, impervious area runoff, and baseflow.
• Pervious runoff is surface flow generated by infiltration or saturation excess flows. Infiltration excess flow is generated when the amount of water attempting to infiltrate is more than the infiltration rate. Saturation excess flow is generated when water attempting to infiltrate into soil storage cannot because the soil storage is already at maximum capacity.
• Impervious runoff is surface flow generated by precipitation on Directly Connected Impervious Area (DCIA), which is a fraction of total impervious area. For more information about DCIA, please see this FAQ: viewtopic.php?f=35&t=1233
• Baseflow is subsurface flow generated through infiltration and percolation of precipitation. Water that infiltrates first enters the root zone (aka 'the upper active soil zone' from which evapotranspiration occurs), then the unsaturated zone (above the water table), and finally the saturated zone (beneath the water table). Water in the saturated zone is presumed to contribute to streamflow, and this component of streamflow is known as baseflow. Baseflow can also be thought of as the streamflow in between precipitation events, when there are no other flow components contributing to streamflow.
The ratio of pervious area runoff to impervious area runoff will shift as the ratio of pervious area to impervious area changes; if pervious area (as a percentage of the watershed) increases, pervious area runoff may increase while impervious area runoff decreases, and vice versa. Also, as pervious area increases or decreases, baseflow should increase or decrease, since increasing or decreasing the pervious area (as a percentage of the watershed) will affect the amount of infiltrated water which in turn affects the amount of baseflow generated.

If different scenarios produce unexpected differences in flow components, give careful attention to the following parameters:
• DCIA %: This parameter has a significant impact on the amount of impervious runoff generated. It represents a percentage of total impervious area. When changing the amount of total impervious area, it is likely that DCIA % should also be changed.
• Land Cover beneath Tree Canopy: changing the amount of total tree canopy may also call for changing the amount of land cover beneath tree canopy. This is especially relevant in Hydro v5 where these parameters represent a proportion of total tree canopy. For example, if the total tree canopy area is increased, but the proportion of impervious to pervious land cover beneath tree canopy remains the same, then that increase in tree canopy is also increasing impervious cover in the project area.
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