2018 SWCS International Annual Conference July 29-August 1, 2018 Albuquerque Convention Center

1. Upper Snake-Rock Watershed – Idaho
Conservation Effects Assessment Project

3. Upper Snake Rock Watershed – Idaho
1. 82,000 ha (85% cropland).
2. Irrigation water diverted from Snake River by Twin
Falls Canal Company since 1905. Water flows by
gravity through 180 km of main canal and +1500
km of laterals.
3. TFCC has a natural flow water right. Project
designed to evenly distribute river flow to all
shareholders. (full allocation is 5.6 gpm/acre or 7.5 mm/day)
4. 250 mm annual precipitation, 90 mm May to Sept.
All crop land is irrigated.
5. 5% of crop land sprinkler irrigated prior to 1990,
45% in 2006 and 60% in 2016.

4. Winter

5. Summer

6. • Irrigation return flow measured at 14 sites from 2006 to 2016 in cooperation with
Idaho Department of Water Resources.
• Irrigation canal (MLA) and an ephemeral stream (RCH) are the only inflow streams
into the watershed.

7. Summer
Winter

8. Deep Creek Winter - 2017
Deep Creek Winter - 2015
Deep Creek – July, 2018

9. Perrine Coulee and
Snake River Canyon
at Twin Falls, Idaho

10. Drainage tunnels
(man-made springs)
flow all year.

11. Water Balance
Inflow = Outflow
Irrigation Water + Rock Creek + Precipitation = ET + Return Flow + Remainder
Remainder includes all measurement errors, canal evaporation, canal seepage,
deep percolation, unmeasured return flow and changes in soil water.
Delivery Efficiency
Annual ET/Annual Total Inflow

12. Irrigation: 1120 to 1290 mm
(83 to 94% of Inflow)
Precipitation: 70 to 220 mm
Rock Creek: 7 to 30 mm
ET: 770 to 850 mm
(56 to 66% of Inflow)
Return Flow: 260 to 380 mm
(20 to 27% of Inflow)
Remainder: 110 to 280 mm
(8 to 19% of Inflow)
Irrigation Season Water Balance (mm)
April - October
Inflow Outflow

13. Irrigation = 81% of the total
(mean = 1200 mm)
Precipitation = 17% of total
(mean = 260 mm)
ET = 61% of total inflow
Return Flow = 31% of total
(450 mm or 500 billion L)
Remainder = 8%
(120 mm or 130 billion L)
Annual Water Balance (mm)
Inflow Outflow

18. Conclusions
• Increasing the amount of sprinkler irrigated has not
improved water delivery efficiency in the watershed.
• Large canal systems cannot match water delivery to
irrigation demand.
• Flow rate water allocation gives little incentive to
use less water.
• Sprinkler irrigation may be more efficient at the field
scale but water rights and allocation schemes have
a bigger impact at the watershed scale.

19. Questions?