Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Application
2. Map unit values of XKSAT (bare ground) are calculated based on individual soil textures
in a map unit, percentages of soil textures in a map unit, XKSAT values from
Table 4.1, and a logarithmic area-weighting procedure. Since many of the soil groups
contain horizons of different textures, the top texture may or may not control the total
volume and rate of infiltration. The decision of which soil layer controls the infiltration
rate is based on soil texture, horizon thickness, and the accumulated depth of water
during the initial low intensity period of a design storm.
3. Impervious cover percentages, applied in an HEC-1 model using the RTIMP variable,
directly converts the assigned percentage of areal rainfall to runoff. This assumes
that the impervious area is hydraulically connected to the outlet. Impervious cover
percentages (i.e. rock outcrop) listed in the soil surveys may or may not be hydraulically
connected to the outlet. Judgement should be exercised in the assignment of
the effectiveness of impervious cover percentages based on the soil surveys.
4. The PSIF and DTHETA values are taken from Figure 4.3 as a function of the basin or
subbasin average value of XKSAT (bareground) not for each map unit value of
XKSAT.
5. XKSAT (bareground) is adjusted for the effects of vegetation cover by use of Figure
4.4. The PSIF and DTHETA values are not a function of the adjusted XKSAT value
and are not adjusted for vegetation cover.
6. For a partially developed basin or subbasin, DTHETA dry and DTHETA normal can be
readily averaged based on the percentages of the natural and developed areas.
7. The DTHETA “Saturated” condition should be used only if the entire area is under irrigation
simultaneously.
9-28 August 15, 2013