WSP05265

• File Number: 8170
• Description: Arkansas Basin Water Quality Issues
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 2/1/1998
• Author: Goff Lewis Person Ko
• Title: Simulated Effects of Irrigation on Salinity in the Arkansas River Valley in Colorado
• Water Supply Pro - Doc Type: Report/Study

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Year

_ . _ " - 25% Decrease in pump!ng
- - 50% Decrease in pumpi'1g
100% Decrease In pumping

Figure 11. Simulated change in ground water salinity for scenarios of
decreased pumpage, relath'e to baseline conditions.

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- Cessamn of Irrigation-Area 1
- . - . - Cessation of Irrigation-Area 2
_ Ceo..."", ot lniga';on-""'a 3
Cessation of Irrigation-All Areas

Figure 12. Simulated change in ground water salinit)' for scenarios of
decreased irrigated acreage, relative to baseline conditions.

through time (Figure 11). The variabillty is a function of the rela-
tive proportion of ground water to surface water that was histori-
cally applied for inigation. In the relatively dry years of 1971-82,
surface water diversions were relatively small (0.6 m1yr), and irr;.
gation from ground water (0.6 m1yr) represented 50% of the total
applied water. In the relatively wet years of 1983-95 when surface
water was more plentiful (0,8 m1yr), ground water represented
about 33% (0.4 m1yr) of the total applied water. Therefore, a fixed
percentage decrease in withdrawals had a slightly more pronounced
effect on salinity in the drier period, 1971-82 (Figure II). Decreased
pumping had a minimal effect on river salinity; in the three decreased
pumping scenarios, salinity in the river essentially remained
unchanged (Table 2).
As expected. streamflow gains generally increased in response
to decreased ground water withdrawals. The complete cessation of
pumping increased the gains from 0.18 mJls to 0.20 mJls (Table 2).
Simulated ground water levels relative to the baseline simulations,
however, were relatively insensitive to changes in pumping.

Decreased Irrigated Acreage
Water transfers between agricultural and urban users in arid to
semiarid regions of the United States are being used increasingly
because of competition for scarce water resources. During the
1987-93 drought, for example, I billion m' of water w" tran,ferred
between different entities with the California water banking system
(Bouwer 1994). However, the impacts of these transfers on the qual-
ity of surface and ground water remains largely unknown.
In response to increased demands for municipal water supplies
in Colorado, agricultural water rights have been sold and transferred
for municipal use along the Colorado Front Range. As of 1990, about
19,440 hectares of historically irrigated land in the lower Arkansas
River vaUey has ceased to be irrigated following the transfer of water
rights (Howe et a1. 1990). In accordance with the legal stipulations
of these transfers, it was mandated that these historically inigated
lands could no longer be inigated. Therefore, the calibrated model
W'a5 u'!.ed to estimate \he effects of decreased irrigaled acreage in me.
study area.
Four scenarios in the management category of inigation ces-
sation were simulated: (I to 3) individually ceasing irrigation on sub-
areas \,2, and 3; and (4) ceasing irrigation on all three sub-areas.

Table 3
Model Results for Scenarios of Decreased Irrigate-d Acreage
AUuvial Aquifer Arkansas River
Average Monthly Average Monthly Water Average .l\-lonthJy Average Monthly
Model Run Sa6nity (mg/L) Level (m above mean sea level) Salinity (mg/L) Srnaml10w Gains (m'/s)
Base Condition 2180 t224.42 1810 0.176
Study Area-Wide Decreases in Irrigated Acreage
Area 1 (20% of
irrigated study area) 2080 (-4.6%)' 1224.42 1810 (0.0%) 0.162(-79%)
Ma 2 (33% of
irrigated study area) 201O( -7.8%) 1224.42 1780(-1.7%) 0.151 (- 14%)
Area 3 (47% of
irrigated srudy area) 1960(-10%) 1224.]] 1780 (-1.7%) 0.087(-51%)
Areas I. 2, and 3 (100% of
irrigated study area) 1630(-25%) 1224.33 1730 (-4.4%) 0.064 (- 64%)
lNumbers in parentheses indicate the percent difference be[ween the modded base condillon and the indicated scenario.

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