Wyoming Framework Water Plan - Living Rivers Home Page

5.0 USE
River Basin
Table 5-3 Estimated Average Annual Irrigation Surface Water Diversions
Wet Normal Dry
Historical Diversions
Overall
Average 1
Surface
Irrigated
Overall
Unit
Average
Theoretical Diversions
Unit
Maximum Maximum
Diversions Diversions
ac-ft acres ac-ft/ac ac-ft ac-ft/ac
Bear 2 295,000 64,000 4.61 303,000 4.74
Green 2,3 1,011,000 330,000 3.06 1,394,000 4.22
Northeast Wyoming 5 153,000 152,000 152,000 152,000 64,000 2.38 258,000 4.03
Platte 4 1,553,000 612,000 2.54 1,860,000 3.04
Powder/Tongue 5 397,000 349,000 388,000 349,000 161,000 2.17 594,000 3.69
Snake/Salt 2 363,000 99,000 3.67 448,000 4.52
Wind/Bighorn 5 3,028,000 3,148,000 2,324,000 3,166,000 600,000 5.28 3,278,000 5.46
Total 6,889,000 1,930,000 3.57 8,135,000 4.21
1
Overall average in the long-term average for the period of record, and is not the average of the Wet, Normal, and Dry columns.
2
The Bear, Green, Platte, and Snake/Salt River Basin Plans did not provide normal, dry, and wet year scenarios for diversions.
3
Green River Basin diversion data were not developed due to lack of data; data here are estimated by dividing the annual
consumptive use (depletion) of irrigation water by the estimated overall irrigation efficiency. Green River Basin irrigation efficiency
(39.7%) was not reported but calculated as the average of the other six basins, which were calculated by dividing the annual
consumptive use by the estimated historical diversions.
4
Platte River Basin diversions and irrigated acres represent total values, as basin plan did not distinguish between surface water
and groundwater.
5
Only Northeast, Powder/Tongue and Wind/Bighorn River Basin Plans calculated theoretical maximum diversions. Other basin
theoretical maximum diversions are estimated by dividing the total irrigation water requirement by the estimated overall irrigation
efficiency.
Groundwater
Only the Bear River Basin Plan attempted to estimate groundwater diversions (pumping). Thus,
groundwater diversions by basin are not reported here. However, Figure 5-2 is provided to show the
nearly 3,000 active irrigation wells in the state.
Theoretical Maximum Diversion Requirement
Theoretical irrigation water requirement (IWR) of crops advises irrigators on the approximate
amount of irrigation water to fully meet crop needs and is, therefore, useful in determining the diversion
demand necessary to meet that requirement. Not all of the water diverted for irrigation goes to meeting
the IWR; in fact, the IWR oftentimes represents only a minority of the total diversion amount. A
significant portion of the diverted flow is lost to seepage from the main conveyance ditch, lateral ditches,
field ditches, headgate leakage, evaporative losses from sprinklers, ditch tailwater waste, field
wastewater, and deep percolation past the crop root zone. The proportion of water ultimately consumed
by the crop compared to the total volume of water diverted from the stream is referred to as the overall
efficiency (some refer to this as the “consumptive use fraction”). Using estimated overall irrigation
efficiency, one can calculate the theoretical maximum diversion requirement from the theoretical IWR.
Irrigation efficiency typically varies throughout the irrigation season but decreases as the amount
of water applied relative to the crop water requirement increases (Cuenca, 1989). For example, overall
efficiency would typically be at a minimum during the early irrigation season when water supply from the
snowmelt runoff is abundant but crop water requirements are minimal. Conversely, overall efficiency
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