Precipitation Runoff Modeling System (PRMS) Soil Zone Modules

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Detailed Description

Presents descriptions of the USGS Precipitation Runoff Modeling System (PRMS) Soilzone module.
 

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Image Dimensions: 480 x 360

Date Taken:

Length: 00:13:17

Location Taken: US

Transcript

Hello. This is Steve Regan of the Modeling of Watershed Systems group. This presentation describes the PRMS soilzone module, which computes hydrologic processes related to the water content of the capillary, gravity, and preferential-flow reservoirs.

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This cartoon illustrates flow paths in the hydrologic cycle. The yellow lines represent flows computed by the soilzone module. Flows computed are: soil evapotranspiration, interflow, preferential flow, Dunnian runoff, and recharge.

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This cartoon illustrates the hydrologic cycle as simulated by PRMS. Precipitation, air temperature, and solar radiation drive the storage and exchange of water in and from each water-storage reservoir. The primary reservoirs are the plant canopy, snow pack, impervious surfaces, soil zone, and groundwater storage. The dashed lines represent internal flows and the solid lines flows from or to an HRU. The area outlined in red is the subject of this presentation.

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This cartoon illustrates the three soil zone water-holding reservoirs: the capillary, preferential-flow, and gravity reservoirs. The impervious and surface depression reservoirs are shown to illustrate that the capillary reservoir is the pervious fraction of the HRU land surface. Thus, the area of the capillary reservoir is the HRU area minus the sum of the impervious and surface-depression storage areas. Processes for impervious and surface depression reservoirs are computed by the surface runoff module; described in another presentation. The dashed lines represent internal flows and the solid lines flows from or to a reservoir.

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While computations for the soilzone processes are compartmentalized using the three reservoirs, the soil zone is considered one physical space from the land surface to the rooting depth. Each reservoir represents different pore spaces and water-holding capacities. <click> The capillary reservoir has water that does not flow laterally and is where soil-water evaporation and plant transpiration is computed. <click> The gravity reservoir holds soil water greater than the water-holding capacity of the capillary reservoir. This water can flow laterally as interflow and drain as recharge. <click> The preferential flow reservoir holds macropore soil water that can flow laterally, typically at a greater rate than from the gravity reservoir and does not drain.

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This figure illustrates available water within the soil profile for the continuum of soil characteristics (X‑axis) and percent water-holding capacity (Y‑axis). <click>

Unavailable water, that is water less than the wilting point threshold, is not included in a PRMS reservoir or water budget. The diagram illustrates that as clay content increases the unavailable water increases. This is primarily due to the pore size decreasing, thus more and more soil water is bound to the grains and not available for hydrologic flux computations. <click>

The capillary reservoir can be thought of as the water between field capacity and the wilting threshold. However, the capillary reservoir is defined as the soil water between the land surface and the rooting depth of the dominant plant species, for the pervious fraction of the HRU. One method to estimate the water-holding capacity of the capillary reservoir is as the rooting depth times the permeability of the soil. <click>

Gravity driven lateral flows and drainage are computed from the soil-water content above the water-holding capacity of the capillary reservoir to total soil saturation of the entire HRU. This water content is partitioned into the gravity and preferential-flow reservoirs.

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The parameters that define the water-holding capacity for each reservoir are highlighted in red.

The water-holding capacity of the capillary reservoir is specified by soil_moist_max, which occupies the pervious fraction, which is equal to: 1 – hru_percent_impervdprst_frac_hru. Each HRU must have at least 0.1% pervious area. The capillary reservoir is partitioned into two zones using parameter soil_rechr_max. Water in the recharge zone can both evaporate and is available for plant transpiration. Water in the lower zone is available for plant transpiration. Infiltration fills the recharge zone prior to being available for the lower zone.

The gravity reservoir receives infiltration in excess of the water-holding capacity of the capillary reservoir and is where gravity drainage (or recharge) to the groundwater reservoir and slow interflow are computed. The water-holding capacity is the portion of sat_threshold minus the water-holding capacity of the preferential-flow reservoir. Computations are based on the full HRU areal extent.

The preferential-flow reservoir receives a portion of infiltration and water in excess of the water-holding capacity of the gravity reservoir. This water is used to compute fast interflow. It has a water-holding capacity that is estimated to account for shallow subsurface storm flow. Computations are based on the entire HRU area extent.

Dunnian runoff is the result of soil-water in excess of the water-holding capacity of the preferential-flow reservoir. If a preferential-flow reservoir is not present, Dunnian runoff occurs when water content in the gravity reservoir exceeds sat_threshold.

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This information rich slide is used to illustrate the fourteen computation steps in the soilzone module. Parameters are shown in red text, storage components in purple, and processes in light blue.

The antecedent water content plus inputs is the starting point for the sequence. Inputs are infiltration, unsatisfied potential ET, transpiration state, and snow cover. Infiltration is computed in the surface-runoff module based on throughfall, snowmelt, and cascading Hortonian runoff. Unsatisfied PET is the potential ET as computed by the selected potential ET module minus the sum of canopy evaporation as computed by the interception module; sublimation as computed by the snow computation module, and evaporation from impervious and surface-depression storage as computed by the surface runoff module. Snow cover is computed by the snow computation module and transpiration state is computed by the transpiration module.  <click>

The first step is to add a portion, based on the value of parameter pref_flow_den, of infiltration to the preferential-flow reservoir. If the addition of this water plus the antecedent water exceeds the storage capacity, the excess water is the first component of Dunnian surface runoff. <click>

Step 2 adds the remainder of the infiltration to the storage capacity of the recharge zone.

Step 3 adds any cascading Dunnian surface runoff and interflow from upslope HRUs to the storage capacity of the recharge zone.

Step 4 partitions excess water from steps 2 and 3 to the lower zone, up to its storage capacity. <click>

Step 5 partitions excess water from step 4 up to the value of parameter soil2gw_max as the first component of recharge. Typically, the value of soil2gw_max is set to zero unless direct recharge to groundwater is likely, such as when karst features exist.

Step 6 adds excess water from step 5 to the antecedent storage of the gravity reservoir, up to its storage capacity. <click>

Step 7 adds excess water from step 6 to the antecedent storage of the preferential-flow reservoir, up to its storage capacity. <click>

Step 8 sets excess water from step 7 as the second component of Dunnian surface runoff. <click>

Step 9 computes slow interflow from the water content of the gravity reservoir and subtracts this water from the storage. <click>

Step 10 computes gravity drainage from the adjusted water content of the gravity reservoir and subtracts this water from the storage. <click>

Step 11 computes fast interflow from the water content of the preferential-flow reservoir and subtracts this water from the storage. <click>

Step 12 sums the first and second components of Dunnian surface runoff.

Step 13 computes cascading fast and slow interflow and Dunnian surface runoff when the cascading flow option is active.

Step 14 computes soil-water evaporation and the first component of plant transpiration from the current water content of the recharge zone of the capillary reservoir and the second component of plant transpiration from the lower zone. The water is then subtracted from the capillary reservoir storage. <click>

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See the PRMS manual for a more complete description of the computation order and equations used in soilzone computations.

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Soil ET is determined based on unsatisfied potential ET, water content of the recharge zone and capillary reservoir, transpiration state, snow cover, cover type, and soil type. ET is computed for the recharge zone first. The unsatisfied potential ET available for the lower zone is reduced by recharge zone ET.

Whether or not plant transpiration is computed is determined by transpiration module. There are two modules, transp_tindex and transp_frost. Transp_tindex determines the transpiration period, or growing season, based on an index temperature (parameter transp_tmax) and beginning and ending months (parameters transp_beg and transp_end, respectively). Transp_frost is used to specify the growing season as the days from the last spring frost to the first fall frost (parameters spring_frost and fall_frost, respectively). These modules are described in another training video.

If plant transpiration is inactive and snow covered area is greater than 1 percent, soil ET is set to 0. Additionally, if transpiration is active, snow covered area is greater than 1 percent, and bare ground (cov_type=0), soil ET is set to 0.

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These graphs are the basis for the algorithms used to compute ET for the three soil types. The x-axis is the fraction of water content of the maximum water content of the recharge zone or capillary reservoir storages. The y-axis is the fraction of unsatisfied ET that is set to actual ET.

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See the PRMS-IV manual for a more detailed explanation of the soilzone module. 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 the parameters that can be specified in the Control File. Table 1-3 contains descriptions of the parameters that can be specified in the Parameter File. Table 1-5 contains descriptions of all variables that can be written to the various output files. The FAQ 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 soilzone module or need help for other issues related to PRMS you can click on the Help tab and fill out the contact form.