Changes from the original module are written in bold text
This module does soil moisture accounting, including addition of infiltration, computation of actual evapotranspiration, and seepage to subsurface and ground-water reservoirs.
The module has been modified to include water as a soil type for estimating evaporation from water bodies.
The amount of the soil water excess for an HRU that is routed directly to the associated ground-water reservoir each day, in inches.
Maximum available water holding capacity of soil profile, in inches. Soil profile is surface to bottom of rooting zone. Default for maximum set to 30 inches.
Initial value for available water in the soil recharge zone (upper part of soil_moist), in inches. Must be less than or equal to soil_moist_init. Default for initial set to 30 inches.
Maximum value for available water in the soil recharge zone (upper portion of soil_moist where losses occur as both evaporation and transpiration), in inches. Must be less than or equal to soil_moist. Default for maximum set to 30 inches.
Summer vegetation cover density for the major vegetation type on an HRU, in decimal percentage. [intcp]
Winter vegetation cover density for the major vegetation type on an HRU, in decimal percentage.[intcp]
Vegetation cover type designation for an HRU: 0 = bare soil, 1 = grasses, 2 = shrubs, 3 = trees.[intcp]
The amount of water transferred from the soil zone to a ground-water reservoir for each HRU, in inches.
The amount of water transferred from the soil zone to a subsurface reservoir for each HRU, in inches.
Indicator for whether transpiration is occurring: 0 = not occurring, 1 = occurring. [potet]
Soil moisture accounting is performed as the algebraic sum of all moisture accretions to, and depletions from, the active soil profile. Depletions include evapotranspiration and recharge to the subsurface and ground-water reservoirs. Accretions are rainfall and snowmelt infiltration. The depth of the active soil profile is considered to be the average rooting depth of the predominant vegetation on the HRU. The maximum available water-holding capacity of the soil zone, soil_moist_max, is the difference between field capacity and wilting point of the profile. The active soil profile is divided into two layers. The upper layer is termed the recharge zone, and the lower layer is termed the lower zone. The recharge zone is user-definable as to depth and maximum available water-holding capacity, soil_rechr_max. The maximum available water-holding capacity of the lower zone is the difference between soil_moist_max and soil_rechr_max. Losses from the recharge zone occur from evaporation and transpiration; they occur only as transpiration from the lower zone. For the case in which the soil type is set = 4 for water, the maximum available water-holding capacity is set to an amount that would provide enough water to meet evaporation from a free-water surface for the area being studied. This quantity may vary from as little as 10 inches to more than 60 inches, depending on latitude and the prevailing climatic regime. It is best to set soil_rechr_max = soil_moist_max for HRUs that are identified as water. An HRU identified as water still functions as other HRUs in that it can provide water to surface runoff, to subsurface runoff, or to a ground-water reservoir. Because the water storage of 10-60 inches will be depleted with evaporation and other losses and recharged with precipitation, the capacity may need to be set higher in order to keep it always full or it can be set so that it dries out each year.
Infiltration from rainfall or snowmelt, infil, is added to the soil zone. Water in excess of soil_moist_max is distributed to the subsurface and ground-water reservoirs. The excess is first used to satisfy the maximum ground-water recharge, soil_to_gw, and any remaining excess (soil_to_ssr) is added to the subsurface reservoir associated with the HRU.
Actual
evapotranspiration, hru_actet, is the computed rate of water loss, which reflects
the availability of water to satisfy potet. When available water is nonlimiting,
hru_actet equals potet. For HRUs identified as water, the hru_actet is
set equal to potet. Evaporation of intercepted water, intcp_evap, evaporation
from impervious area retention storage, imperv_evap, and evaporation/sublimation
from a snowpack, snow_evap, are first used to satisfy potet, in that order.
Remaining potet demand then is applied to the soil-zone storage. perv_actet
is computed separately for the recharge zone and the lower zone using the
unsatisfied demand and the ratio of currently available water in the soil
zone to its maximum available water-holding capacity. For the recharge zone,
this ratio is 
. For the lower zone, the ratio 
is used. The perv_actet for the recharge zone is first used to satisfy potet;
any remaining demand is attempted to be met from the lower zone. HRU soil
are designated as being predominantly sand, loam or clay, using parameter
soil_type. The potential-actual relations for these soil types as a function
of the soil-water ratio are shown in figure 1 (Zahner, 1967). hru_actet is
computed as the weighted average of perv_actet and imperv_evap.
This module also computes basin weighted averages, basin_actet, basin_soil_moist, and basin_soil_rechr.
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.
Zahner, R., 1967, Refinement in empirical functions for realistic soil-moisture regimes under forest cover, in Sopper, W. E., and Lull, H. W., eds., International Symposium of Forest Hydrology: New York, Pergamon Press, p. 261-274.
URL for this page is http://pubsdata.usgs.gov/pubs/of/2002/ofr02362/htdocs/smbal/smbal_prms_wtr.htm
Page contact: Mark Mastin (mcmastin@usgs.gov),
253-428-3600, ext. 2609
Last modified: Friday, 11-Jan-2013 03:19:51 EST
