Water use for agriculture represents, by far, the greatest consumption of water within the boundaries of the Powder/Tongue River Basin Plan area. An accurate estimate of existing irrigation water use is therefore central to a comprehensive water use inventory. Estimates of water use by irrigated agriculture can generally be divided into the following three components:

1. Quantity of irrigated lands
2. Types of crops grown and geographic distribution
3. Amount of water consumed by the crops

The irrigated lands mapping work indicates a total of 169,641 acres of irrigated agriculture in the planning area. Of this total, 8,280 acres are currently idle. In addition, 4,275 acres are currently undergoing development and gradually being removed, to varying degrees, from traditional agriculture. There are also 14,714 acres of irrigation served by spreader dikes or 'kick-out" ditches in the planning area (Irrigation Classification S and H). The remaining area (142,372 acres) is comprised of full-service or partial service irrigation or man-induced subirrigation (Irrigated Lands Mapping and Water Rights Data, HKM, 2002).

Also from the recent irrigated lands mapping work, together with agricultural statistics and interviews with SEO Hydrographer / Commissioners and area landowners, the primary irrigated crops grown in the basin are forage crops (alfalfa and grass hay or pasture grass) accounting for 88 percent of the actively irrigated acreage. Small grains and corn account for the remaining 12 percent of the irrigation (Irrigated Crops, HKM, 2002).

The methodology used to determine the amount of water consumed by the irrigated crops (depletions), and the diversion demands is the subject of this memorandum.

HISTORY OF AGRICULTURAL WATER USE IN THE BASINS

Irrigated agriculture in the planning area is primarily associated with forage production for the livestock industry. Ranchers depend on irrigated cropland to provide winter feed and summer grazing for a successful ranching enterprise. According to the 1972 Framework Water Plan, well over 80 percent of the water consumed by man's activities in the planning area is used for irrigation.

Early in Wyoming's history, ranchers had discovered that successful farming required irrigation. By the 1890's, many ranchers were taking advantage of cheap irrigation of bottomlands along the small tributary streams, but most of the water from the large rivers was being carried, unused, into other states (SEO, 1972). As the direct flow supply of the streams became fully appropriated, reservoirs were constructed, where suitable sites could be found, to store spring runoff for late-season irrigation uses. One of the earliest of these was Cloud Peak Reservoir with a water right of 1896. This reservoir began as a natural lake on the main stem of South Piney Creek. An earthen embankment and an outlet works were constructed to increase the storage regulation potential of this site. Today there are 14 significant storage facilities in the Powder/Tongue River Basin planning area, allowing for expanded irrigation use of the natural flows in the planning area (Storage Operation and Description, HKM, 2002).

Agricultural water use has changed little in recent decades with the exception that there has been a progressive shift, in many areas, from flood irrigation practices to sprinkler irrigation.

RELATIONSHIP BETWEEN ACTUAL HISTORIC DIVERSIONS AND THEORETICAL MAXIMUM DIVERSIONS

The amount of irrigation water use is primarily dependent on the number of acres irrigated, the crop water demands, and the amount of water available to meet these demands.

Total crop water demands are dependent on the consumptive use of the crop. The crop consumptive use requirement is the maximum water use of a well-watered crop under optimum growing conditions (Pochop; et.al, October 1992). A portion of the total annual rainfall is available to meet this consumptive use requirement. Effective precipitation is defined as that part of the total rainfall during the growing season which is available to meet the consumptive water requirements of the crops (Dastane, 1974). The remaining consumptive use requirement, unmet by natural rainfall, is referred to as the Crop Irrigation Requirement (CIR).

Knowing the number of acres of irrigation, the types of crops grown, and the climatic conditions, estimates of the theoretical maximum diversions can be made for all of the irrigated lands throughout the basin. In practice however, actual diversions commonly fall short of the amount necessary to meet the full CIR of the crops. This is especially true with direct-diversion, flood-irrigated, forage crops. Actual conditions seldom represent the optimum conditions necessary to achieve maximum water consumption. The objective of this exercise, then, is to compare actual historic diversions, to the extent available, to theoretical maximum diversions in order to develop a relationship that can be used to estimate actual diversions and depletions where records of actual use are unavailable.

The CIR used in this planning effort was provided directly by Dr. Pochop for each month and year of his study period (1951 through 1990), for each crop evaluated, and for each climatic station included (Pochop; et.al., October 1992). Only data specific to the study period of the current investigation (1970 through 1999) was used. Average values were used for those years outside of the Pochop study (1991 through 1999). These CIR values were averaged for the specific wet years, normal years, and dry years selected through the surface water hydrology work to represent these three hydrologic conditions (Surface Water Hydrology, HKM, 2002).

Diversion Records

HKM compiled diversion records for approximately 70 key ditches in the planning area as described in the Irrigation Diversion Operation and Description memorandum (HKM, 2002). Monthly diversion volumes are summarized as well as the first and last days of irrigation for each year of record. Key ditches to water use in the basin were selected with assistance from the SEO Division 2 Superintendent. These diversions provide a representative coverage of diversion operations in the basin. These records took the form of continuous monthly records as well as periodic instantaneous spot measurements. Missing daily flow records were estimated using the methodology described in the Irrigation Diversion Operation and Description memo. Although these records have shortcomings, they represent the best available information regarding actual irrigation diversion amounts.

Service Area Delineation

Lands served by a common supply ditch are grouped together into Service Areas as described in the Irrigated Lands Mapping and Water Rights Data memorandum (HKM, 2002). In total, approximately 1200 service areas were delineated in the Powder/Tongue River Basin planning area including the service areas for the selected ditches with diversion records. A crop distribution was assigned to each of these service areas as described in the Irrigated Crops memorandum (HKM, 2002). Additionally, the representative climatic station(s) were also assigned to each service area. With this information, Crop Irrigation Requirements (CIRs) can be determined for each service area.

Theoretical Maximum Diversion Requirements during Period of Recorded Diversions

Based on conversations with SEO hydrographer/commissioners, the first and last entry in the annual record of diversions for the selected "spot measurement" ditches was sometimes after diversions began in the spring and before diversions ended in the fall. The records for the ditches with "continuous recorders" are less subject to this limitation. In order to provide a common basis for comparison of recorded diversions to theoretical maximum diversions, the CIR was first adjusted to reflect the crop water demand during the recorded period of irrigation for each respective ditch. To this end, the number of days of recorded irrigation was determined for each month and year of record and for each of the selected ditches. The average recorded number of days of irrigation for each ditch selected for this study and each hydrologic condition is provided at the back of this memorandum. The average number of days of recorded diversion for the selected ditches in comparison to the representative number of growing days per Dr. Pochop's study are summarized in Table 1 for wet, normal, and dry years.

Table 1 Average Number of Days of Irrigation
Hydrologic Condition	Apr	May	Jun	Jul	Aug	Sep	Oct	Annual
Wet	1	10	24	28	29	19	0	112
Normal	2	16	25	29	29	18	1	120
Dry	5	18	23	25	23	16	0	111
Average Growing Days (From Pochop Study)	25	31	30	31	31	30	14	192

Not all of the water diverted for irrigation goes to meeting the CIR, in fact, the CIR often times represents only a minority portion of the total diversion amount. A significant portion of the diverted flow is lost to seepage from the main conveyance ditch, lateral ditches, and 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 to the total volume of water diverted from the stream is referred to as the overall efficiency (Cuenca, 1989). The required amount of irrigation water diverted from the stream to fully meet the CIR during the period of irrigation is then defined as follows:

Theoretical Maximum Diversion Requirement = (Acres x adjusted CIR) / Overall Efficiency

Table 2 Overall Monthly Irrigation Efficiency
Apr	May	Jun	Jul	Aug	Sep	Oct	Annual
25%	30%	45%	65%	60%	30%	25%	40%

Proportion of Actual Historic Diversions to Theoretical Maximum Diversions

The proportion of actual historic diversion volumes to the theoretical maximum diversion volumes during the period of recorded irrigation was calculated for each month and year of record for each of the selected ditches. These proportions were then averaged for each ditch for the wet years, normal years, and dry years. The resulting proportions are provided at the back of this memorandum for each ditch and for each hydrologic condition (wet, normal, and dry years). These proportions vary considerably from ditch to ditch and likely represent differences in ditch priorities, relative position on the stream, availability of reservoir storage, physical condition of the ditch and diversion facilities, as well as inherent inaccuracies in the diversion records. The area-weighted average proportions for the ditches directly serving irrigated lands (excluding transbasin diversion ditches and reservoir supply ditches), are summarized in Table 3.

Table 3 Area-Weighted Average Proportion of Historic to Theoretical Maximum Diversions
Hydrologic Condition	Apr	May	Jun	Jul	Aug	Sep	Oct	Annual
Wet	3%	40%	69%	78%	75%	79%	5%	73%
Normal	13%	64%	73%	68%	77%	68%	4%	71%
Dry	26%	91%	56%	56%	50%	44%	4%	56%

As shown in Table 3, the average proportion of actual historic diversions to theoretical maximum diversions during the period of irrigation, is at a maximum in wet years (73 %), is somewhat lower in normal years (71 %), and is considerably lower in dry years (56 %). The ditches included in these averages serve a total of 79,046 acres and constitute 49 percent of the total active irrigated acreage in the planning area (161,360 acres).

ESTIMATED ACTUAL HISTORIC DIVERSIONS

Records of irrigation diversions are not available for all of the irrigation in the basin. Further, even the records that are available are not complete for all years of the study period or every month within any particular year. The proportions presented in the preceding section provide a relationship between actual historic diversions and theoretical maximum diversions that make it possible to estimate actual historic diversions during wet, normal, and dry years where records are unavailable. The area-weighted averages developed from all of the ditches across the Powder/Tongue River Basin planning area (Table 3) are used for this planning level effort. The exceptions are the South Fork, Middle Fork, and mainstem Powder River, where the proportions specific to these areas are used to reflect the unique conditions, predominately lack of supplemental storage water, of these basins. The crop types, climatic conditions, period of irrigation, overall irrigation efficiency, and number of acres for each service area are used to determine the theoretical maximum diversions. The wet year, normal year, and dry year proportions are then applied to estimate actual historic diversions. The beginning and ending dates of irrigation for those ditches with continuous recorders are considered to be the best reflection of the actual period of diversions. The period of irrigation used to estimate actual historic diversions is therefore based on the number of days of irrigation derived from the records of these ditches. The average number of days of irrigation from the continuous recorder ditches is provided in Table 4.

Table 4 Average Number of Days of Irrigation . Continuous Recorder Ditches
Hydrologic Condition	Apr	May	Jun	Jul	Aug	Sep	Oct	Annual
Wet	7	16	26	31	31	29	2	142
Normal	7	18	28	30	31	29	3	147
Dry	11	26	30	31	31	30	2	160

The theoretical maximum diversion requirements during the irrigation season and the estimated actual diversions for wet, normal, and dry years are provided in Tables 5 and 6. This information is aggregated into areas represented by the same climate station(s). These climatic areas are shown on Figure 1.

click to enlarge

Table 5
Theoretical Maximum Surface Water Diversion Requirements (Acre-Feet)

Source of Supply	Climate Stations1	Active Irrigation (Acres)	Hydrologic Condition	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Annual
Little Bighorn Basin	Sheridan	1,781	Wet Normal Dry	0 0 0	0 0 0	0 0 0	13 184 449	459 660 1,250	1,183 1,279 1,588	1,443 1,489 1,474	1,372 1,378 1,232	868 1,058 1,217	3 13 8	0 0 0	0 0 0	5,341 6,061 7,219
Tongue River Basin	Sheridan	62,760	Wet Normal Dry	0 0 0	0 0 0	0 0 0	3,088 5,597 15,906	16,368 24,194 41,849	39,257 46,793 58,430	49,234 51,187 58,116	45,263 46,886 47,147	39,797 34,069 40,983	150 282 578	0 0 0	0 0 0	193,157 209,007 263,035
Upper Clear Creek	Buffalo	39,176	Wet Normal Dry	0 0 0	0 0 0	0 0 0	2,734 4,592 12,588	13,238 21,057 22,652	27,122 34,067 43,852	29,945 31,432 36,699	29,473 29,040 30,757	26,449 25,429 28,179	313 588 809	0 0 0	0 0 0	129,274 146,206 175,536
Lower Clear Creek	Buffalo & Weston	7,174	Wet Normal Dry	0 0 0	0 0 0	0 0 0	859 1,146 2,916	2,580 3,927 5,026	4,808 6,386 7,730	5,830 5,841 6,916	5,487 5,340 5,595	5,121 4,922 5,384	56 109 149	0 0 0	0 0 0	24,742 27,671 33,716
Upper Crazy Woman Creek	Buffalo	12,324	Wet Normal Dry	0 0 0	0 0 0	0 0 0	859 1,441 3,943	4,167 6,625 7,123	8,563 10,748 13,830	9,425 9,893 11,551	9,239 9,106 9,640	8,302 7,978 8,843	98 184 254	0 0 0	0 0 0	40,654 45,976 55,184
Lower Crazy Woman Creek	Buffalo & Weston	1,418	Wet Normal Dry	0 0 0	0 0 0	0 0 0	414 579 1,026	774 1,083 1,101	980 1,258 1,474	1,158 1,162 1,361	1,144 1,106 1,173	1,064 1,024 1,109	38 53 98	0 0 0	0 0 0	5,572 6,263 7,342
Upper Powder River	Kaycee	18,107	Wet Normal Dry	0 0 0	0 0 0	0 0 0	2,129 2,162 6,031	6,392 8,928 11,027	12,404 16,218 21,246	14,479 14,983 18,481	14,461 13,960 14,403	14,925 13,135 14,227	191 282 412	0 0 0	0 0 0	64,982 69,667 85,827
South Fork Powder River	Kaycee & Midwest	2,103	Wet Normal Dry	0 0 0	0 0 0	0 0 0	277 279 812	758 1,080 1,472	1,719 2,058 2,583	1,778 1,810 2,126	1,724 1,713 1,736	1,900 1,675 1,800	21 38 47	0 0 0	0 0 0	8,178 8,652 10,575
Lower Powder River	Buffalo & Weston	6,440	Wet Normal Dry	0 0 0	0 0 0	0 0 0	2,245 3,150 5,296	3,931 5,400 5,178	4,574 5,790 6,706	5,276 5,295 6,182	5,195 5,016 5,337	4,848 4,662 5,036	216 296 563	0 0 0	0 0 0	26,286 29,609 34,298
Little Powder River Basin	Weston	9,873	Wet Normal Dry	0 0 0	0 0 0	0 0 0	4,934 5,162 6,176	5,578 7,036 10,231	8,277 8,320 9,734	8,037 8,404 9,565	8,180 7,821 7,906	8,115 7,913 6,641	169 181 179	0 0 0	0 0 0	43,290 44,837 50,431

Notes: ¹Where more than one climate station is listed, the stations were weighted 50-50.

Table 6
Estimated Actual Surface Water Diversions (Acre-Feet)

Source of Supply	Climate Stations1	Active Irrigation (Acres)	Hydrologic Condition	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Annual
Little Bighorn Basin	Sheridan	1,781	Wet Normal Dry	0 0 0	0 0 0	0 0 0	0 24 118	186 420 1,141	822 936 893	1,124 1,007 829	1,030 1,067 615	688 722 541	0 0 0	0 0 0	0 0 0	3,851 4,176 4,137
Tongue River Basin	Sheridan	62,760	Wet Normal Dry	0 0 0	0 0 0	0 0 0	108 757 4,195	6,648 15,381 38,073	27,262 34,218 32,810	38,329 34,559 32,645	33,942 36,236 23,513	31,537 23,198 18,193	9 13 30	0 0 0	0 0 0	137,835 144,362 149,459
Upper Clear Creek	Buffalo	39,176	Wet Normal Dry	0 0 0	0 0 0	0 0 0	70 591 3,302	5,353 13,410 20,674	18,849 24,951 24,658	23,338 21,248 20,643	22,122 22,482 15,345	20,984 17,338 12,515	14 21 32	0 0 0	0 0 0	90,731 100,041 97,169
Lower Clear Creek	Buffalo & Weston	7,174	Wet Normal Dry	0 0 0	0 0 0	0 0 0	107 224 811	1,095 2,452 4,417	3,313 4,590 4,277	4,485 3,890 3,827	4,072 4,052 2,760	4,007 3,305 2,378	11 15 23	0 0 0	0 0 0	17,090 18,529 18,493
Upper Crazy Woman Creek	Buffalo	12,324	Wet Normal Dry	0 0 0	0 0 0	0 0 0	24 187 1,036	1,687 4,218 6,497	5,950 7,870 7,774	7,344 6,686 6,496	6,934 7,047 4,808	6,585 5,438 3,927	5 7 11	0 0 0	0 0 0	28,528 31, 453 30,549
Lower Crazy Woman Creek	Buffalo & Weston	1,418	Wet Normal Dry	0 0 0	0 0 0	0 0 0	218 263 383	432 576 612	617 726 672	766 650 621	745 657 509	708 575 460	21 24 37	0 0 0	0 0 0	3,507 3,471 3,294
Upper Powder River	Kaycee	18,107	Wet Normal Dry	0 0 0	0 0 0	0 0 0	841 827 2,203	7,627 3,618 4,046	11,793 8,619 10,130	14,363 8,792 12,363	14,279 9,849 9,103	14,437 5,757 7,976	79 109 153	0 0 0	0 0 0	63,419 37,571 45,974
South Fork Powder River	Kaycee & Midwest	2,103	Wet Normal Dry	0 0 0	0 0 0	0 0 0	18 16 23	1,013 84 24	1,611 559 485	1,780 653 1,052	1,714 944 761	1,828 215 585	2 3 4	0 0 0	0 0 0	7,966 2,474 2,934
Lower Powder River	Buffalo & Weston	6,440	Wet Normal Dry	0 0 0	0 0 0	0 0 0	1,288 1,510 1,975	2,596 2,491 1,826	2,943 2,715 2,478	3,465 2,543 2,557	3,408 2,545 2,164	3,156 2,086 1,961	126 143 221	0 0 0	0 0 0	16,983 14,034 13,181
Little Powder River Basin	Weston	9,873	Wet Normal Dry	0 0 0	0 0 0	0 0 0	2,621 2,363 2,319	3,130 3,730 5,592	5,199 4,761 4,405	5,284 4,675 4,329	5,305 4,604 3,413	5,371 4,417 2,751	95 83 68	0 0 0	0 0 0	27,004 24,633 22,876

Notes: ¹ Where more than one climate station is listed, the stations were weighted 50-50.

ESTIMATED ACTUAL HISTORIC DEPLETIONS

The estimated actual depletions for wet, normal, and dry years are provided in Tables 7 and 9. The depletions summarized here represent the crop water consumption during the period of irrigation. It should be noted that depletions to the river system for any given month are calculated as diversions in that month less lagged return-flows which return back to the stream in that same month. These diversions minus return flows may, in fact, actually result in negative depletions (accretions) to the stream system in the post irrigation season months, when return flows arrive back into the system and diversions have ceased. The difference is typically small on an annual basis but is significant from month to month. The method which utilizes estimated actual diversions and subsequent return flows is used in the water availability models developed for this planning effort rather than the CIR based depletions reported here (Spreadsheet Model Development and Calibration, HKM, 2002).

Lacking diversion data for the lands served from ground water, the same methodology is applied to these lands. The estimated actual depletions for the lands supplied by ground water are provided on Tables 8 and 9.

A total of 14,714 acres of irrigation served by spreader dikes or "kick-out" ditches were mapped in the planning area (Irrigation Classification S and H). These systems are typically located in the bottomlands of ephemeral tributaries and use the water to the extent it is available, whenever it is available. Depletions for these lands do not depend on operation of stream diversion works as do conventional irrigation systems and therefore are treated differently. Depletions for these lands are not constrained by the number of days of irrigation diversion but, rather, are based on the growing season as presented in the Pochop study (Pochop; et.al, October 1992). It is assumed that the actual annual water use for these lands is 40% of the theoretical maximum water use in dry years, 50% of theoretical maximum in normal years, and 60% in wet years.

Table 7
Estimated Actual Surface Water Depletions (Acre-Feet)

Source of Supply	Climate Stations1	Active Irrigation (Acres)	Hydrologic Condition	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Annual
Little Bighorn Basin	Sheridan	1,781	Wet Normal Dry	0 0 0	0 0 0	0 0 0	0 6 29	56 126 342	370 421 402	731 654 539	618 640 369	206 216 162	0 0 0	0 0 0	0 0 0	1,981 2,064 1,844
Tongue River Basin	Sheridan	62,760	Wet Normal Dry	0 0 0	0 0 0	0 0 0	27 189 1,049	1,994 4,614 11,422	12,268 15,398 14,764	24,914 22,463 21,219	20,365 21,741 14,108	9,461 6,960 5,458	2 3 8	0 0 0	0 0 0	69,031 71,369 68,028
Upper Clear Creek	Buffalo	39,176	Wet Normal Dry	0 0 0	0 0 0	0 0 0	18 148 825	1,606 4,023 6,202	8,482 11,228 11,096	15,170 13,811 13,418	13,273 13,489 9,207	6,295 5,201 3,755	4 5 8	0 0 0	0 0 0	44,847 47,906 44,511
Lower Clear Creek	Buffalo & Weston	7,174	Wet Normal Dry	0 0 0	0 0 0	0 0 0	27 56 203	329 735 1,325	1,491 2,065 1,924	2,915 2,528 2,488	2,443 2,431 1,656	1,202 991 713	3 4 6	0 0 0	0 0 0	8,409 8,812 8,315
Upper Crazy Woman Creek	Buffalo	12,324	Wet Normal Dry	0 0 0	0 0 0	0 0 0	6 47 259	506 1,265 1,949	2,678 3,541 3,498	4,774 4,346 4,222	4,160 4,228 2,885	1,975 1,631 1,178	1 2 3	0 0 0	0 0 0	14,100 15,061 13,994
Lower Crazy Woman Creek	Buffalo & Weston	1,418	Wet Normal Dry	0 0 0	0 0 0	0 0 0	54 66 96	130 173 184	278 326 302	498 423 403	447 394 305	213 173 138	5 6 9	0 0 0	0 0 0	1,624 1,561 1,438
Upper Powder River	Kaycee	18,107	Wet Normal Dry	0 0 0	0 0 0	0 0 0	210 207 551	2,288 1,085 1,214	5,307 3,879 4,558	9,336 5,715 8,036	8,568 5,910 5,462	4,331 1,727 2,393	20 27 38	0 0 0	0 0 0	30,059 18,549 22,252
South Fork Powder River	Kaycee & Midwest	2,103	Wet Normal Dry	0 0 0	0 0 0	0 0 0	5 4 6	304 25 7	725 251 218	1,157 425 684	1,028 567 457	548 64 176	1 1 1	0 0 0	0 0 0	3,768 1,337 1,548
Lower Powder River	Buffalo & Weston	6,440	Wet Normal Dry	0 0 0	0 0 0	0 0 0	322 378 494	779 747 548	1,324 1,222 1,115	2,253 1,653 1,662	2,045 1,527 1,298	947 626 588	32 36 55	0 0 0	0 0 0	7,701 6,188 5,760
Little Powder River Basin	Weston	9,873	Wet Normal Dry	0 0 0	0 0 0	0 0 0	655 591 580	939 1,119 1,678	2,339 2,142 1,982	3,435 3,039 2,814	3,183 2,763 2,048	1,611 1,325 825	24 21 17	0 0 0	0 0 0	12,186 10,999 9,943

Notes: ¹ Where more than one climate station is listed, the stations were weighted 50-50.

Table 8
Estimated Actual Ground Water Depletions (Acre-Feet)

Source of Supply	Climate Stations1	Active Irrigation (Acres)	Hydrologic Condition	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Annual
Upper Clear Creek	Buffalo	20	Wet Normal Dry	0 0 0	0 0 0	0 0 0	0 0 0	1 2 3	4 6 6	8 7 7	7 7 5	3 3 2	0 0 0	0 0 0	0 0 0	23 25 23
Upper Crazy Woman Creek	Buffalo	97	Wet Normal Dry	0 0 0	0 0 0	0 0 0	0 0 2	4 10 15	20 27 27	37 34 33	34 35 24	16 13 10	0 0 0	0 0 0	0 0 0	112 119 110
Upper Powder River	Kaycee	58	Wet Normal Dry	0 0 0	0 0 0	0 0 0	0 0 0	8 0 0	17 6 6	31 11 19	28 15 12	14 1 4	0 0 0	0 0 0	0 0 0	98 34 41
Lower Powder River	Buffalo & Weston	28	Wet Normal Dry	0 0 0	0 0 0	0 0 0	0 0 0	4 0 0	8 3 2	15 5 9	14 7 6	6 1 2	0 0 0	0 0 0	0 0 0	46 16 19

Notes: ¹ Where more than one climate station is listed, the stations were weighted 50-50.

Table 9
Estimated Actual Depletions (Acre-Feet)

Source of Supply	Active Irrigation (Acres)	Hydrologic Condition	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Annual
Little Bighorn Basin	1,781	Wet Normal Dry	0 0 0	0 0 0	0 0 0	0 6 29	56 126 342	370 421 402	731 654 539	618 640 369	206 216 162	0 0 0	0 0 0	0 0 0	1,981 2,064 1,844
Tongue River Basin	62,760	Wet Normal Dry	0 0 0	0 0 0	0 0 0	27 189 1,049	1,994 4,614 11,422	12,268 15,398 14,764	24,914 22,463 21,219	20,365 21,741 14,108	9,461 6,960 5,458	2 3 8	0 0 0	0 0 0	69,031 71,369 68,028
Powder River Basin - Surface Supply	86,742	Wet Normal Dry	0 0 0	0 0 0	0 0 0	642 905 2,433	5,941 8,054 11,429	20,284 22,513 22,713	36,101 28,901 30,912	31,965 28,546 21,270	15,512 10,414 8,941	65 81 120	0 0 0	0 0 0	110,509 99,414 97,819
Powder River Basin - Ground Water Supply	203	Wet Normal Dry	0 0 0	0 0 0	0 0 0	0 0 3	17 13 18	49 42 40	91 58 68	83 64 47	39 18 18	0 0 0	0 0 0	0 0 0	279 194 194
Little Powder River Basin	9,873	Wet Normal Dry	0 0 0	0 0 0	0 0 0	655 591 580	939 1,119 1,678	2,339 2,142 1,982	3,435 3,039 2,814	3,183 2,763 2,048	1,611 1,325 825	24 21 17	0 0 0	0 0 0	12,186 10,999 9,943

FULL SUPPLY DIVERSION REQUIREMENTS

The results of the agricultural water use estimates are used, in part, as inputs to the water availability models prepared for this planning effort. The modeling is used to determine the availability of excess flows for future development and also as a means of determining where there are needs for supplemental water supplies under existing conditions. The need for these supplemental supplies should be consistent with irrigators' perceptions of those needs. HKM utilized the diversion records during non supply-limited conditions (wet years) as a measure of the level of supply that irrigators have settled on, under existing conditions, as a balance between maximizing crop yields while conserving their limited resources. This is defined here as the "full supply diversion requirement". This level of supply may be defined by a number of conditions including how much money and effort the irrigator is willing to expend on repairing diversion works, cleaning and repairing ditches, and managing on-farm irrigation. This level of supply is typically less than the theoretical maximum diversion requirement and this is especially true for relatively low-value forage crops.

The average proportion of historic diversions to theoretical maximum diversions during the period of irrigation in wet years is 73 percent as shown in Table 3. The full supply diversion requirement is, therefore selected as 75 percent of the theoretical maximum (slightly higher than the wet year proportion) for each service area with it's unique combination of acreage, crops, and climate and for each hydrologic condition (wet, normal, and dry years).

A graphical comparison of the estimated actual diversions, full supply diversion requirements, and theoretical maximum diversion requirements during the irrigation season are provided on Figures 2, 3, and 4 for wet, normal, and dry years respectively.

The full supply diversion requirements are provided in Table 10 for wet, normal, and dry years.

Figure 2
Comparison of Estimated Actual Diversions to Full Supply Diversion
Requirements and Theoretical Maximum Diversion Requirements
ALL BASINS
Wet Year Hydrologic Conditions

Figure 3
Comparison of Estimated Actual Diversions to Full Supply Diversion
Requirements and Theoretical Maximum Diversion Requirements
ALL BASINS
Normal Year Hydrologic Conditions

Figure 4
Comparison of Estimated Actual Diversions to Full Supply Diversion
Requirements and Theoretical Maximum Diversion Requirements
ALL BASINS
Dry Year Hydrologic Conditions

Table 10
Full Supply Surface Water Diversion Requirements (Acre-Feet)

Source of Supply	Climate Stations1	Active Irrigation (Acres)	Hydrologic Condition	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Annual
Little Bighorn Basin	Sheridan	1,781	Wet Normal Dry	0 0 0	0 0 0	0 0 0	10 138 337	344 495 937	887 959 1,191	1,082 1,117 1,106	1,029 1,033 924	651 794 913	3 10 6	0 0 0	0 0 0	4,006 4,546 5,414
Tongue River Basin	Sheridan	62,760	Wet Normal Dry	0 0 0	0 0 0	0 0 0	2,316 4,198 11,929	12,276 18,145 31,387	29,443 35,094 43,822	36,926 38,391 43,587	33,947 35,164 35,380	29,847 25,552 30,737	112 211 433	0 0 0	0 0 0	144,868 156,755 197,277
Upper Clear Creek	Buffalo	39,176	Wet Normal Dry	0 0 0	0 0 0	0 0 0	2,051 3,444 9,441	9,928 15,793 16,989	20,341 25,550 32,889	22,459 23,574 27,525	22,105 21,780 23,068	19,837 19,072 21,134	234 441 607	0 0 0	0 0 0	96,955 109,654 131,652
Lower Clear Creek	Buffalo & Weston	7,174	Wet Normal Dry	0 0 0	0 0 0	0 0 0	644 860 2,187	1,935 2,945 3,770	3,606 4,790 5,798	4,373 4,381 5,187	4,116 4,005 4,197	3,841 3,691 4,038	42 82 111	0 0 0	0 0 0	18,557 20,753 25,287
Upper Crazy Woman Creek	Buffalo	12,324	Wet Normal Dry	0 0 0	0 0 0	0 0 0	645 1,081 2,958	3,125 4,969 5,342	6,422 8,061 10,372	7,069 7,420 8,663	6,929 6,829 7,230	6,226 5,984 6,633	74 138 191	0 0 0	0 0 0	30,491 34,482 41,388
Lower Crazy Woman Creek	Buffalo & Weston	1,418	Wet Normal Dry	0 0 0	0 0 0	0 0 0	311 434 770	580 812 826	735 943 1,106	869 871 1,021	858 829 880	798 768 832	29 40 74	0 0 0	0 0 0	4,179 4,697 5,507
Upper Powder River	Kaycee	18,107	Wet Normal Dry	0 0 0	0 0 0	0 0 0	2,056 2,084 5,894	6,265 8,770 10,886	12,253 16,033 21,017	14,321 14,819 18,282	14,304 13,809 14,245	14,761 12,991 14,072	180 269 385	0 0 0	0 0 0	64,140 68,775 84,783
South Fork Powder River	Kaycee & Midwest	2,103	Wet Normal Dry	0 0 0	0 0 0	0 0 0	270 271 797	746 1,064 1,458	1,702 2,039 2,561	1,763 1,794 2,108	1,710 1,698 1,721	1,884 1,661 1,784	20 36 44	0 0 0	0 0 0	8,094 8,564 10,472
Lower Powder River	Buffalo & Weston	6,440	Wet Normal Dry	0 0 0	0 0 0	0 0 0	1,708 2,395 4,062	3,024 4,167 4,037	3,572 4,533 5,261	4,138 4,152 4,850	4,075 3,936 4,185	3,801 3,655 3,951	163 224 425	0 0 0	0 0 0	20,481 23,062 26,771
Little Powder River Basin	Weston	9,873	Wet Normal Dry	0 0 0	0 0 0	0 0 0	3,701 3,871 4,632	4,183 5,277 7,674	6,208 6,240 7,300	6,027 6,303 7,173	6,135 5,866 5,929	6,086 5,935 4,981	127 136 134	0 0 0	0 0 0	32,467 33,628 37,823

Notes: ¹ Where more than one climate station is listed, the stations were weighted 50-50.

REFERENCES

Cuenca, Richard H., 1989. Irrigation System Design . An Engineering Approach, Prentice-Hall, Inc., Englewood Cliffs, New Jersey

Dastane, N., 1974. Effective Rainfall in Irrigated Agriculture, Irrigation and Drainage Paper No. 25, FAO, United Nations

HKM Engineering Inc., 2002. Irrigated Crops, Technical Memorandum, Powder/Tongue River Basin Plan. Billings, Montana

HKM Engineering Inc., 2002. Irrigation Diversion Operation and Description, Technical Memorandum, Powder/Tongue River Basin Plan. Billings, Montana

HKM Engineering Inc., 2002. Irrigated Lands Mapping and Water Rights Data, Technical Memorandum, Powder/Tongue River Basin Plan. Billings, Montana

HKM Engineering Inc., 2002. Spreadsheet Model Development and Calibration, Technical Memorandum, Powder/Tongue River Basin Plan. Billings, Montana

HKM Engineering Inc., 2002. Storage Operation and Description, Technical Memorandum, Powder/ Tongue River Basin Plan. Billings, Montana

HKM Engineering Inc., 2002. Surface Water Hydrology, Technical Memorandum, Powder/Tongue River Basin Plan. Billings, Montana

Pochop, Larry, Travis Teegarden, Greg Kerr, Ronald Delaney and Victor Hasfurther, October 1992. Consumptive Use and Consumptive Irrigation Requirements . Wyoming, WWRC Publication #92-06. Laramie, Wyoming

Wyoming State Engineer's Office, April 1972. Wyoming Water Planning Program, Report 10, Water & Related Land Resources of Northeastern Wyoming. Cheyenne, Wyoming

Appendix A

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