FLOOD08228

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Basin: South Platte
• Title: Urban Drainage and Flood Control District
• Date: 3/1/1990
• Prepared For: Boulder Creek
• Prepared By: Goerge V. Sabol
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

;
;
,
,
,
f
~
~
~
,
,.
,
i
,
i
,
;
;
;

Hydrologic Design Manual
for Maricopa County

Rainfall Losses

However, Maricopa County also has a large segment of its land area under irrigated
agriculture, and it is reasonable to assume that the design frequency storm could
occur during or shortly after certain lands have been irrigated. Therefore, it would
be reasonable to assume that soil moisture for irrigated lands could be at or near
effective saturation during the start of the design rainfall.

Three conditions for DTIIET A have been defined for use in Maricopa County based
on the antecedent soil moisture condition that could be expected to exist at the start
of the design rainfall. These three conditions are:

>> "Dry" for antecedent soil moisture near the vegetation wilting point;

>> "Normal" for antecedent soil moisture condition near field capacity due to
previous rainfall or irrigation applications grfnonagrlculturallands; and
0/
>> . "Saturated" for antecedent soil moisture near effective saturation due to
recent irrigation of agricultural lands.

Values of DTHETA have been estimated by subtracting the initial volumetric soil
moisture for each of the three conditions from the soil porosity. '
.

The value of DTHETA "Dry" as a function of soil texture is shown in Figure 4.8.
This figure was prepared by subtracting the wilting point soil moisture in Figure
4.6 from the soil porosity in Figure 4.5. The value of DTHETA "Nonnal" as a
function of soil texture is shown in Figure 4.9. This figure was prepared by
subtracting the field capacity soil moisture in Figure 4.7 from the soil porosity in
Figure 4.5. The value of DTHET A "Saturated" is always equal to 0.0 because for
this condition there is no available pore space in the soil matrix at the start of rainfall.
Values of DTHETA for the three antecedent soil moisture conditions are shown in
Table 4.2 DTHET A "Dry" should be used for soil that is usually in a state of low
soil moisture such as would occur in the desert and rangelands of Maricopa County.
DTHETA "Nonnal" should be used for soil that is usually in a state of moderate
soil moisture such as would occur in Irrigated lawns, golf courses, parks, and
irrigated pastures. DTHETA "Saturated" should be used for soil that can be
expected to be in a state of high soil moisture such as irrigated agricultural land.

The hydraulic conductivity <XI<SA T) can be affected by several factors besides soil
texture. For example, hydraulic conductivity is reduced by soil crusting. increased
by tillage, and increased by the influence of ground cover and canopy cover. The
values of XI<SAT that have been presented for bare ground as a function of soil
texture alone should be adjusted under certain soil cover conditions.

Ground cover, such as grass, litter, and rock will generally increase the infiltration
rate over that of bare ground conditions. Similarly, canopy cover-such as from
trees, brosh, and tall grasses--can also increase the bare ground infiltration rate.
The procedures and data that have been presented are for estimating the Green and
Ampt parameters based solely on soil texture and would be applicable for bare
ground conditions. Past research has shown that the wetting front capillary suction

~~::m:~:'Ii C :::';:;;:*:: ~<<<<<<<<-::<<,:<<,;,;{*x>;~<<<w,,:~:*^<<-:':~~';-:~~{~';0:-:':'''''-~'>>~<<<<-:{-;':'*..wit-:<<:~~-x=:-:';W;,*''<<';-:<<':<<{-:-:';v>>:<<V:-:N'~*'';':**
53