WSP11413

• File Number: 8443.600
• Description: Narrows Unit - Studies
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 2/1/1963
• Author: Bor,
• Title: Site Selecion Presentation to the South Platte Steering Committee
• Water Supply Pro - Doc Type: Report/Study

Preparatory Studies

~efore starting the historic studies, ~ determination had
to be made of the crop'irrigation requirement in the basin so that
we cou1d compare it with the historic water supply. Determining
the irrigation requirement involved the consideration of the
requirements at the crop level due to temperature, the amount of
lands irrigated in the basin, and the extent to which groundwater
is being used in the basin for meeting crop irrigation
req,uirements.

C~op Irrigation Requirement

Consumptive use requirement--In several western areas
studies have been made of the crop evaporation and transpiration
needs~ A correlation was obtained between the crop consumption
and the corresponding tenrperature. By using the recorded tenrper-
atures in this area and applying the correlation as found in these
other study areas, an approximate crop consumption requirement was
obtained.

The entire study area encompasses all lands irrigated from
the South Platte River between Denver and Julesburg--a distance of
about 220 valley miles. Because of differences in temperature and
rainfall, the basin was divided into four subbasins for ~etermining
the consUlllptive use requirements. Subbasin A extends from Denver
to Greeley; subbasin B from Greeley to Fort Morgan; subbasin C from
Fort Morgan to Julesburg, and the fourth area is Prospect Valley.
Table 1, General Hydrology for 1942-1960, line 1, shows the
consUlllptive use req,uirement in feet that we found for these subbasins.

Recorded precipitation--The crop consUlllptive use requirement
can be satisfied from two sources, precipitation or irrigation.
Because we are interested in finding the irrigation req,uirement, we
must subtract from the crop consumptive use req,uirement the amount
that is met from precipitation. Line 2 of Table 1 lists the
precipitation contribution to the subbasins in the main South Platte
Basin.

Effective precipitation--Not all the precipitation that falls
on an irrigated area, however, is effective towards meeting the crop
requirement, some runs off the surface because of the intensity of
the rain, and the soil conditions existing when the rain occurs, and
some percolates through the soil mantle and enters the ground water
table. Line 3 of Table I lists the effective precipitation in feet
,I
for each subbasin.

By subtracting the effective precipitation from the consumptive
use requirement, we determine the crop irrigation requirement. Line 4.

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