9626

• UCREFRP Catalog Number: 9626
• Author: Butler, D. L.
• Title: Effects of Piping Irrigation Laterals on Selenium and Salt Loads, Montrose Arroyo Basin, Western Colorado.
• USFW Year: 2001.
• USFW - Doc Type: Denver, CO.
• Copyright Material: NO

would be sufficient to determine effects of the project;
however, placing open laterals in pipe eliminated
lateral waste flows that previously discharged into
Montrose Arroyo or tributary ditches. Lateral waste
flows are the excess, or unused part of the water in a
lateral that is not diverted to on-farm use. Often, a
greater quantity of flow than what is required for on-
farm use is diverted into a lateral to maintain sufficient
head and depth at all the headgates on that lateral. The
excess water from the lateral is usually discharged into
the nearest drainage ditch or natural drainage. There-
fore, streamflow during the irrigation season in
Montrose Arroyo and tributary drainage ditches is a
mixture of a surface-water component composed of
irrigation tailwater from fields and suburban areas and
waste flows from canals and laterals and a ground-
water component. Elimination of waste flows from the
five laterals caused lower streamflow during the irriga-
tion season in the arroyo and the two drainage ditches
(table 1). For example, at site MA2, the mean irriga-
tion-season streamflow decreased from 19.4 ft3/s for
the pre-project samples to 12.4 ft3/S for the post-
project samples. Because there were no waste flows
from the piped laterals in the post-project period, what
previously was wastewater from the laterals remained
in the AM lateral or Loutzenhizer Canal, and much of
that water flowed out of the basin. The lateral waste
flows had a selenium and salt load associated with it;
therefore, the question could be asked, "How much of
the selenium and salt reductions in Montrose Arroyo
were caused simply by a reduction in lateral waste
water?" To address that question, the irrigation-season
loads were separated into ground-water and surface-
water components.
The analysis of irrigation-season loads was done
using concentration (C) data and discharge (Q) data.
The method involves solving two equations:
Qi = QSW +Qgw (1)
(0)(0) = (Qsw)(Csw) + (Qgw)(Cgw) (2)
The i terms are the irrigation-season streamflow
and sample concentrations; the sw terms are the
surface-water component, and the gw terms are the
ground-water component. The Qi and Ci terms are the
means of the measured streamflow and sample
concentrations for the irrigation season at each site.
The mean irrigation-season selenium and salinity
concentrations are listed in table 1. The surface-water
concentrations (Csw) were estimated as the mean of
Table 1. Mean streamflow, selenium concentrations, and salinity concentrations for
irrigation-season (April through October) and nonirrigation-season (November through March)
samples at the five monitoring sites
[Pre, pre-project period; Post, post-project period; the U.S. Geological Survey site identification numbers for the
monitoring sites: site MA4 is 382702107493701; site D2MA is 382704107493801; site MA3 is 382711107500501;
site D1MA is 3 8 27 26 1 07 5 05 802; site MA2 is 382802107513301]
Site Streamflow Selenium Salinity
(fig. 2) (cubic feet per second) (micrograms per liter) (milligrams per liter)
Pre Post Pre Post Pre Post
irrigation season
MA4' 0.91 1.01 18 18 2,996 3,732
D2MA 2.93 1.96 32 23 1,510 1,797
MA3 4.76 3.89 27 20 1,924 1,963
D1MA 4.54 3.58 26 27 1,398 1,778
MA2 19.4 12.4 25 27 1,277 1,703
Nonirrigation season
MA4 0.45 0.43 77 85 6,621 6,851
D2MA .49 .24 149 89 7,107 6,702
MA3 .96 .70 106 86 6,455 6,573
D1MA .63 .60 65 69 4,579 4,592
MA2 2.55 2.22 98 82 4,830 4,871
,control site
6 Effects of Piping Irrigation Laterals on Selenium and Salt Loads, Montrose Arroyo Basin, Western Colorado