Simulation of Net Infiltration for Present-Day and Potential Future Climates Page: 58 of 456

                                
Simulation of Net Infiltration for Present-Day and Potential Future Climates

and is limited by the field-scale-saturated hydraulic conductivity of the soil or bedrock
underlying the root zone. A unit gradient is assumed because INFIL is a simple bucket model,
and it cannot accommodate Richards' Equation flow calculations in which a unit gradient is not
required. A detailed description of the method used for calculating net infiltration is provided in
Sections 6.4.5 and 6.4.6.
The daily water-balance calculation performed for a root zone is illustrated in Figure 6-2, which
was discussed in Section 6.3.1. In this figure, water balance of the root zone is schematically
represented for a single soil layer. In modeling the daily water-balance, parameters affecting the
daily water balance, such as soil thickness, soil and bedrock properties, and various surface and
vegetation characteristics, are uniquely defined for each grid cell. The difference between field
capacity and residual water content is commonly referred to as available water capacity in soil
science terms and that is the water available for plants. Therefore, the transpiration part of
evapotranspiration process takes place this is the zone. The infiltration rate of precipitation,
snowmelt, or surface-water run-on into the root zone from the land surface is limited by the
saturated hydraulic conductivity of the grid cell soil type (or the bulk saturated hydraulic
conductivity of the grid cell bedrock type in cases of no soil cover). Precipitation and
surface-water flow rates are defined using an estimated 2-hour storm duration for summer
storm events and an estimated 12-hour storm duration for winter storm events. This is because
summer storms are usually high-intensity short-duration convective storms, while winter
storms are usually low-intensity, long-duration  low-pressure  storms (Winograd  and
Thordarson 1975 [DIRS 101167]). If the precipitation or snowmelt rate exceeds the saturated
hydraulic conductivity of the top root-zone layer, the excess precipitation or snowmelt is added
to the runoff term for that grid cell. During the simulation of surface-water flow, the infiltration
of surface-water run on is also limited by the saturated hydraulic conductivity of the top root-
zone layer. Surface-water run on exceeding the saturated hydraulic conductivity of the top root-
zone layer is added to the runoff term routed to the downstream grid cell.
6.4.2  Daily Climate Input
Infiltration occurs in response to daily precipitation that occurs in particular temporal and spatial
patterns. Stochastic representations of infiltration would be required to predict infiltration for
long time periods without daily input; however, no infiltration data are available for the
development of long-term patterns. Therefore, using stochastic representations of precipitation
and daily input to simulate infiltration is appropriate.
The daily climate input file is the primary control for the timing and duration of the simulation.
The daily climate input file defines the time domain through which the simulation occurs by
providing a real-time sequential input of daily climate parameters. The file is ASCII column
formatted and at minimum consists of the year number, the day of year number, and the total
daily precipitation amount but can also consist of maximum, minimum, and average daily air
temperature, along with total daily snowfall accumulation.

MDL-NBS-HS-000023 REV 00

November 2004

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