Precipitation Runoff Modeling System (PRMS) Cascading Flow Option

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Detailed Description

Presents descriptions of the USGS Precipitation Runoff Modeling System (PRMS) cascading-flow computation option, which allows for reinfiltration across the land surface, shallow subsurface, and saturated zone
 

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Image Dimensions: 480 x 360

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Length: 00:19:37

Location Taken: US

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Hello, this is Steve Regan of the Modeling of Watershed Systems project. This presentation describes the cascading flow option in PRMS-IV.

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The cascade module provides a method to route lateral flows from HRU to HRU towards the stream network and swales allowing for re-infiltration, cascading Hortonian and Dunnian surface runoff, interflow and groundwater flow. This allows surface and subsurface flow rates to be slowed or sped up across the model domain.

Use of the cascading flow option may allow for more realistic representation of the effects of topography, topology, geology, land use, land cover, and any other characteristic that is important in a modeling study, including requiring HRUs to be represented by a grid. HRUs can be discretized based any combination or combinations of surface and subsurface characteristics. For example, if agriculture is an important control on hydrologic response, each field could be its own HRU with each parameterized differently. The traditional PRMS discretization is based on the contributing area to stream segments with or without elevation bands to define HRUs for which all flows generated reach the stream network each timestep. This type of HRU could have a large variation in physical and hydrologic characteristics that are lumped together using a mean value of the characteristics for parameterization. Through use of cascading flow the model domain could be discretized in ways that can provide increased spatial distribution of parameter values compared to the traditional way of defining HRUs.

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All lateral flows can cascade with varied flow rates and with water-holding reservoirs, such as the capillary, gravity, preferential flow, groundwater, and surface-depression reservoirs, having varying storage capacities along flow paths. For example, the capillary reservoir storage may increase downslope allowing for increasing capacity for re-infiltration and soil saturation. If the slope varies along a flow path, perhaps decreasing downslope, parameter values could be specified to decrease flow rates as the slope decreases. Accounting for hydrogeologic properties could lead to parameter values that increase or decrease flow rates downslope.

Groundwater flow paths can be the same or different from the land surface/shallow subsurface flow paths. All groundwater cascades must be specified so that groundwater flow can be routed to one or more HRUs or terminate in one or more stream segments. Whereas, land surface/shallow subsurface cascading flows can terminate in an HRU as well as stream segments. Terminus HRUs are called swales. They can receive cascading lateral flows, but cannot generate lateral flows.

The cascading flow procedure is used in all GSFLOW applications. GSFLOW is an integration of PRMS with the USGS Modular Groundwater Flow Model-MODFLOW. Even though simulation of cascading flow is optional in GSFLOW, it is highly recommended that the cascade module be used.

The cascade module parameters provide a means to specify simple and complex flow paths throughout the model domain. There can be linear one-to-one flow paths and divergent and convergent flow paths as long as spatial continuity is maintained, that is, there are no circular flow paths.

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This cartoon shows most of the PRMS fluxes, such as surface runoff, interflow, groundwater flow, and evapotranspiration. <click> This animation illustrates the lateral fluxes and flow paths where each arrow represents a flow type leaving an HRU. Upslope flows are computed prior to downslope HRUs. The relative speed of the arrows represents the flow rates that could be generated based on parameters that differ between HRUs. In this case, higher flow rates correspond to steeper slopes. The groundwater flow varies in flow rates based on changing geology instead of land-surface slope. <pause>

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This cartoon is another way of representing fluxes within HRU water-holding reservoirs, the preferential flow, gravity, capillary, and groundwater reservoirs using black arrows. The figure has 6 HRUs that are delineated with vertical brown lines. The center dark blue line represents the stream segment. This animation illustrates the cascading lateral flows and flow paths where each arrow represents a flow type leaving an HRU. Upslope flows are computed prior to downslope HRUs. <click> These arrows represent the direction of surface runoff. Note the arrows go into the soil zone of the downslope HRU as re-infiltration, which cancontribute to surface runoff, evapotranspiration, interflow, gravity drainage, and groundwater flow in the receiving HRU. <click> These arrows represent the direction of cascading fast and slow interflow. These flows go into the capillary reservoir, which can lead this water contributing to surface runoff, evapotranspiration, interflow, gravity drainage, and groundwater flow in the receiving HRU. <click> These arrows represent the direction of groundwater flow. These flows cascade from the groundwater reservoir of one HRU to the groundwater reservoir of the next downslope HRU with the flow paths terminating in the stream segment.

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Two parameters are specified in the Control File; cascade_flag and cascadegw_flag.

cascade_flag specifies whether land surface/shallow subsurface cascades are to be computed. If specified equal to zero these cascades are not computed and any land surface/shallow subsurface cascade parameters specified in the Parameter File are ignored. If dimension ncascade is specified equal to 0, cascade_flag is ignored.

cascadegw_flag specifies whether groundwater cascades are to be computed. If specified equal to zero these cascades are not computed and any groundwater cascade parameters specified in the Parameter File are ignored. If dimension ncascdgw is specified equal to 0, cascadegw_flag is ignored.

Two additional dimensions are required: ncascade—the number of cascade links of the land surface/shallow subsurface cascade network and ncascdgw—the number of cascade links of the groundwater cascade network.

Parameter cascade_tol specifies the minimum contributing area that is considered an insignificant cascade link. If a cascade link has less than this value, the contributing area is equally distributed to the other cascade links from that HRU. The other stipulation on the tolerance is that the value of cascade_tol has to be less than 7.5% of the HRU area. This hard-coded requirement was added for the case that the value of cascade_tol is specified to a value that is likely significant and for models where there may be very large and very small cascades for which a single tolerance limit does not apply. Thus, PRMS considers any cascade link with at least 7.5% of the HRU area to be significant. For example, if cascade_tol is set to the default value of 5 acres and an HRU is 10 acres, then the largest insignificant cascade link is 0.75 acres.

Parameter cascade_flag is used to force the cascade network to have one-to-one or linear flow paths. Thus, it eliminates divergent flow, but, can still allow for convergent flow. If cascade_flag is specified equal to 1; any multiple cascade links from an HRU are reduced to a single link with all flow cascading to the downslope HRU with the largest contributing area. For example, if HRU 1 cascades to HRU 2 using 10% of the HRU area, cascades to HRU 3 using 40% of the area, and cascades to HRU 4 using 50%, then 100% would cascade to HRU 4.

The specified cascading flow network must be specified such that it is a directed, acyclic network. This means the flow network cannot have circles. The cascade module can check for circular paths. If found they are identified and the simulation stops. The circle_switch parameter is used to turn on or off checking for circles. It should be specified equal to 1 until the cascading flow network is free of circles, then circle_switch can be set to 0. Turning off checking for circles is especially important for large models as checking for circles is compute intensive for very complex cascading flow networks.

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Each flow path is comprised of a set of links that begin in an HRU that does not receive flow and can continues as HRU-to-HRU or HRU to stream segment links. An HRU can cascade to multiple HRUs and stream segments. Each cascade link is specified using 4 topological parameters for the land surface/shallow subsurface cascade network and 4 for the groundwater cascade network. Parameters hru_up_id, hru_down_id, hru_pct_up, and hru_strmseg_down_id specify the land surface/shallow subsurface cascading flow network. Each group of 4 specifies one cascade link. There must be ncascade number of groups. Parameters gw_up_id, gw_down_id, gw_pct_up, and gw_strmseg_down_id specify the groundwater cascading flow network. If hru_strmseg_down_id is specified greater than 0 for a cascade link the value of hru_down_id is ignored. Similarly, if gw_strmseg_down_id is specified greater than 0 the value of gw_down_id is ignored.

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In this example there are 14 cascade links, thus, ncascade is specified equal to 14. The cascade network routes water from 10 HRUs to 4 stream segments with divergent flow from HRUs 3, 5 and 6. For example HRU 5 receives 30% and HRU 6 receives 70% of lateral flows computed for HRU 3. The animation shows the relationship of the topological parameters to the flow network for 10 of the cascade links. <silence during animation>

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This cartoon is the same schematic of the previous slide. The network is made up of flow paths, such as HRU 4 to HRU 9 to stream segment 3, and links, such as HRU 1 to HRU 2. There are 14 cascade links. The network must be specified such that it does not have a circle, for example a circle would be HRU 3 to HRU 6 to HRU 5 to HRU 3.

The cascade module determines the computation order of the specified network to ensure upslope HRUs are computed before any downslope HRUs. HRUs that do not receive flow are computed first, such as HRUs 1, 3 and 4. All cascading flows from an HRU are computed before computing cascading flows for the next HRU in the computation order, for example flows from HRU 3 to HRU 6 and to HRU 5 are computed before cascading flows from HRU 4This animation illustrates the computation order of the example network. <silence during animation>

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A method to compute cascade links for polygon HRUs is to compute the contributing area of an HRU to all adjacent stream segments and HRUs. For example a GIS analysis could be used to determine the stream segments and HRUs that each cell on the edge of an HRU is upslope, parameter hru_up_id. The contributing area for each edge cell is computed and then summed and divided by the area of the HRU, parameter hru_pct_up, for each adjacent stream segment, parameter hru_strmseg_down_id, and HRU, parameter hru_down_id. Some edge cells flow into the HRU and not to a stream segment or another HRU, thus they will be included in the contributing area of another edge cell.

If an edge cell flows to a stream segment and HRU, the contributing area is assigned to the stream segment. For example, in the figure, HRU's one and three are adjacent to stream segment eight is adjacent to both HRUs for all edge cells. Thus, stream segment eight receives all cascading lateral flows from both HRUs.

These topographic-based cascade links could be used for the initial groundwater cascades. Then they could be modified to account for geologic features that are not related to the land-surface topography, including adding cascade links for any swale HRUs.

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This cartoon illustrates a cascading network for polygon HRUs that were developed from the contributing area to each stream segment and subdividing them with 3 elevation bands. The network consists of one-to-one cascade links. The animation illustrates cascading flow for the flow path that begins in HRU 4 and terminates in stream segment 3. Note that HRU 2 and HRU 19 cascade to HRU 1 with HRU 1 cascading to stream segment 3. A more typical cascading network would split HRU 1 into left- and right-bank HRUs.

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A software tool has been developed to compute cascade links for grid-based HRUs. The links are computed based on elevation differences between adjacent HRUs and stream segments. Computing cascades for grids is easier than for polygon HRUs as the adjacency is built into the grid structure. The contributing area for each link is based on change in elevation of an HRU with each adjacent HRU; the greater the change in elevation, the greater the contributing area. If two HRUs have lower elevations than the source HRU, with one having a difference of 4 feet and the other a difference of 2 feet, the HRU with the 4 foot difference will receive 2/3rds of the flow and the other 1/3rd. See the CRT manual for more information. Note, that a GIS analysis could be used to determine contributing areas in the same manner described on the previous slide. This might be necessary when grid cells are large, such as a 1 kilometer grid-cell size.

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This cartoon illustrates a cascading network for grid-based HRUs that include swale and lake HRUs without stream segments. The shaded relief map is in the background, which gives an indication of change in elevation between HRUs. The thicker the arrow indicates a higher percentage, parameter hru_pct_up, of cascading flow from an upslope HRU, parameter hru_up_id, to a downslope HRU, parameter hru_down_id. The thickest arrows designate that 100 percent of the lateral flows from an upslope HRU is going to one downslope HRU. For this example all values of hru_strmseg_down_id are specified equal to 0.

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See the PRMS-IV manual for a more detailed explanation of the cascade module, which can downloaded from this webpage. Be sure to download the Changes in the specification of user inputs document, which provides updated tables. Table 2 contains descriptions of available modules. Table 1-2 contains descriptions to all parameters that can be specified in the Control File. Table 1-3 contains descriptions of all parameters that can be specified in the Parameter File. Table 1-5 contains descriptions of all variables that can be output to the various output files. The F-A-Q tab can be very helpful as it provides common questions with answers that users have submitted to the MoWS group over the years. If you have questions about the cascade module or need help for other issues related to PRMS you can click on the Help tab and fill out the contact form.