WSP11081

• File Number: 8273.600.30
• Description: Colorado River Basin Salinity Control - Federal Agencies - USDA
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/1/1989
• Title: Intercepting Isolating and Reusing Drainage waters for Irrigation to Conserve Water and Protect Water Quality
• Water Supply Pro - Doc Type: Report/Study

DRAINAGE WATER REUSE STRATEGY

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This water is judged suitable for the irrigation of beans, since the product
(EC;w)' (F,) is less than EC~ at practical levels of leaching. For example, the
predicted level of average salinity within the rootzone resulting from long-term
irrigation with this water supply atL=0.15 isonlyO.75 dS/m (0.5 dS/mx 1.51;
the value 1.51 was obtained from Fig. 1). Beans can tolerate a soil salinity of
EC~ = 1.0 dS/m without any loss in yield using conventional irrigation man-
agement (Maas, 1986). The leaching requirement for this case, as obtained
from Fig. 1, is even lower, i.e., 0.09. If beans were irrigated at this latter'most-
efficient level of leaching, the EC of the drainage water (ECdw) resulting from
irrigation would be 5.55 dS/m (ECiw/ L; i.e., 0.5/0.09). Obviously one could not
use this latter drainage water again to grow beans, since the resulting average
rootzone salinity could not be kept within acceptable limits at any reasonable
level of L.

Case 2

The conditions: use of the saline drainage water of Ec=5.55 dS/m, as ob-
tained in Case 1, for the irrigation of cotton (EC~ = 7.7 dS/m),
Water of EC=5,55 dS/m, which was judged unsuitable for growing beans
(see Case 1), is quite acceptable for growing cotton, since the predicted level
of average rootzone salinity resulting from its use for irrigation is less than the
EC~ of cotton at practical levels of leaching. For example, the average EC, will
be less than EC~ for any value of L in excess of 0,17 (see Fig. 1 for the case of
F; =7.7/5.5). When irrigated at L=0.17, EC, will be 7.7 dS/m and ECdw will
be 32dS/m (5.5/0.17).
Thus it is apparent that the saline drainage water of Ec=5.55 dS/m (that
resulted from the irrigation of beans with the good quality water) could be used.
satisfactorily to grow salt-tolerant crops like cotton, barley, sugar beets, etc, It.
is also true thatthe drainage volume needing ultimate disposal out of the proj-
ect area would be greatly reduced through such reuse for irrigation within the
project. In this case the percent reduCtion in volume of drainage water ulti-
mately needing to be discharged is 83 [100 - 17; one can also calculate this
vahie using eq. (4) (see later );i.e., 1'- 5,55/32 J. The secondary saline drainage
water OfEC=32 dS/m that resulted in this case can not be used again to grow
cotton (or sugar beets, etc.), since excessive yield losses would result, But this
water is in a favorable condition for disposal or desalting, i,e., it is in a relatively'
small volume and at a relatively high salt concentration,

COlJe 3

The conditions: use ofa blend of the good quality water (Ec=O,5 dS/m) and
the very saline drainage water (EC= 32 dS/m) achieved in Case 2 for the irri-

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