Changes from the original module are written in bold text
Sums inflow to ground-water reservoirs and computes outflow to streamflow and to a ground-water sink if specified. Includes a new parameter that specifies a minimum ground-water storage for each reservoir that is maintained at all times. If this parameter is set equal to zero then the module is the same as the original PRMS module.
Ground-water routing coefficient to obtain ground-water flow contribution to streamflow.
Ground-water sink coefficient to compute the seepage from each reservoir to a ground-water sink.
Storage in each ground-water reservoir at beginning of run, in inches.
Minimum storage in each ground-water reservoir, in inches.
Index of ground-water reservoir receiving excess water from the HRU soil zone.
Index of the ground-water reservoir that will receive flow from each subsurface reservoir.
Weighted average ground-water contribution to streamflow for the basin, in inches.
Weighted average of inflow to ground water for the basin, in inches.
Weighted average water being routed to a ground-water sink for the basin, in inches.
Weighted average for ground-water storage for the basin, in inches.
Sum of the ground-water inflow from soil water excess, in acre-inches.
Sum of the ground-water inflow from subsurface reservoirs, in acre-inches.
Ground-water reservoir area, in acres.
Ground-water contribution to streamflow from each ground-water reservoir, in inches.
Total inflow to each ground-water reservoir, in acre-inches.
Amount of water being routed to a ground-water sink, in inches.
Total storage in each ground-water reservoir, in inches
HRU pervious area, in acres. [basin]
The amount of water transferred from the soil zone to a ground-water reservoir for each HRU, in inches. [smbal]
Subsurface reservoir area, in acres. [ssflow]
Flow from each subsurface reservoir to its associated ground-water reservoir, in inches. [ssflow]
The ground-water system is conceptualized as a linear reservoir and is assumed to be the source of all baseflow. Inflow to the ground-water reservoir is from excess soil moisture, soil_to_gw , and from seepage from a subsurface reservoir, ssr_to_gw . The shape of the baseflow recession of the simulated hydrograph will be influenced by the relative proportion of ground-water recharge from the two sources. Recharge from soil_to_gw occurs only on days when soil_moist_max is exceeded by infiltration, while ssr_to_gw occurs at any time there is water available in the subsurface reservoir. Therefore, the use of ssr_to_gw to recharge ground-water preferentially over soil_to_gw could decrease subsurface flow and increase ground-water contributions to the simulated hydrograph.
One or more ground-water reservoirs can be delineated in a watershed. Using more than one reservoir requires sufficient data to estimate initial storage volumes and routing coefficients. On small watersheds, only one ground-water reservoir is normally specified.
The flow from each ground-water reservoir ( gwres_flow ) , expressed in acre-inches is computed by
,gwflow_coef is the ground-water routing coefficient to obtain ground-water flow contribution to streamflow, and
gwres_stor is the total storage in each ground-water reservoir.
gwflow_coef and the initial value of gwres_stor , gwstor_init , can be estimated from available streamflow records using the hydrograph separation technique described by Linsley, Kohler and Paulhus (1958). Integrating the characteristic depletion equation,
,q t , q 0 are streamflow at times t and 0, and
shows a relationship between gwres_flow and gwres_stor that is expressed as
,K r is the slope of the ground-water flow recession obtained from the semilog plot for discharge versus time.
shows that -log e K r is equivalent to gwflow_coef in equation 1.
For each ground-water reservoir, after gwres_flow is calculated and gwres_stor is reduced by that amount, gwres_stor is checked to determine if it is less than gwstor_min. If it is less than that quantity, it is made equal to gwstor_min. This minimum quantity adds additional water to the water budget that is not accounted for by precipitation during the simulation period. In high-relief, mountainous terrain, much of the water stored in the rock materials is older water. Even if precipitation did not occur over several years, certain geologic materials have residual storage that can still provide water to a lower basin outflow point. For example, a larger basin with elevation differences of 1,000 to 7,000 feet may have an effective (low porosity) storage reservoir depth of 3,000 feet that was filled over millenia. This reservoir can drain to the basin outflow without any additional input from precipitation. This concept would also hold in wetter mountainous terrain over timescales of hundreds of years.
The movement of water through the ground-water reservoir to points beyond the area of interest or measurement is treated using a ground-water sink. The accretion to gwres_sink is computed by
.For each ground-water reservoir, after gwres_sink is calculated and gwres_stor is reduced by that amount, gwres_stor is checked to determine if it is less than gwstor_min. If it is less than that quantity, it is made equal to gwstor_min.
One or more ground-water reservoirs can be delineated in a watershed. Using more than one reservoir requires sufficient data to estimate initial storage volumes and routing coefficients. On small watersheds, only one ground-water reservoir is normally specified.
This module also computes weighted averages for gwres_stor , gwres_flow and gwres_sink for the basin.
Leavesley, G. H., Lichty, R. W., Troutman, B. M., and Saindon, L. G., 1983, Precipitation-runoff modeling system--User's manual: U. S. Geological Survey Water-Resources Investigations Report 83-4238, 207 p.
Linsley, R. K., JR., Kohler, M. A., and Paulhus, J. L., 1958, Hydrology for engineers: New York, McGraw-Hill, p.151-155.
URL for this page is http://pubsdata.usgs.gov/pubs/of/2002/ofr02362/htdocs/gwflow/gwflow_prms_min.htm
Page contact: Mark Mastin (mcmastin@usgs.gov),
253-428-3600, ext. 2609
Last modified: Friday, 11-Jan-2013 03:19:44 EST
