Alamosa-La Jara Water Users Protection Ass'n v. Gould

This is an appeal from a judgment of the district court for Water Division 3 (water court) regarding rules promulgated by the Colorado State Engineer1 (proposed rules) *917limiting the use of surface and underground water in the San Luis Valley. Curtailment of water use in the valley is required by the terms of an interstate stipulation under which Colorado must meet its Rio Grande Compact (compact) obligation to deliver scheduled amounts of Rio Grande water at the Colorado-New Mexico border on an annual basis. The dispute over the proposed rules concerns the distribution of that curtailment: between Rio Grande and Conejos River water users, between Rio Grande mainstem and tributary users, and between well owners and surface diverters. The water court approved rules for administering separate obligations for deliveries from the Conejos River and the Rio Grande mainstem, and disapproved rules which phased out all wells in the San Luis Valley unless each well owner could demonstrate a lack of material injury to senior surface water users or provide a plan of augmentation to replace the water taken by a well. The water court also held that the compact applies to all tributaries of the Rio Grande. We reverse the water court’s tributary ruling and affirm its separate delivery and underground water rulings.

I.

The San Luis Valley in south-central Colorado extends approximately ninety miles from north to south and fifty miles from east to west at an elevation varying between 7,500 feet and 8,000 feet above sea level. The major mountain boundaries are the San Juan mountains to the west and the Sangre de Cristo mountains to the east.2 The Rio Grande mainstem rises in the San Juan mountains, flows south-easterly through the valley to Alamosa, and then runs south through a break in the San Luis hills, which border the valley on the south, into the state of New Mexico, then along the border between Texas and Mexico, emptying into the Gulf of Mexico. The Conejos River rises in the Conejos Mountains to the south-west and flows north-easterly along the southern edge of the valley, joining the Rio Grande mainstem at Los Sauces. Despite its high altitude, short growing season, and average annual precipitation of only about 7.5 inches, the valley sustains a productive agricultural economy dependent upon irrigation water.

The upper 6000 feet of fill below the valley surface consists of unconsolidated clay, silt, sand, and gravel, and interbedded lava flows, containing an estimated two billion acre-feet of underground water. Some of the underground water is in an unconfined aquifer system at shallow depths. Beneath the unconfined aquifer are relatively impermeable beds of clay and basalt and beneath these confining layers are substantial quantities of water which comprise the confined aquifer. The confining clay layer generally does not exist around the valley’s perimeter, and the confined aquifer system is recharged from surface flow to the underground water system at the edges of the valley. Because the recharge areas are higher in elevation than the floor of the valley, the confined aquifer is under artesian pressure, resulting in the free flow of water from some artesian wells and springs at natural breaks in the confining layer. In some places, where the confining layer is less thick and more transmissive, water from the confined aquifer will leak upward through the confining clay lay*918ers into the unconfined aquifer. The unconfined aquifer is directly connected with the surface streams in some places. To varying degrees, the surface streams, the unconfined aquifer, and the confined aquifer are hydraulically connected.3

The first appropriations from streams in the valley began in the 1850s on the Conejos River. The first appropriation on the Rio Grande mainstem was in 1866, and the most extensive development for irrigation purposes on both rivers was between 1880 and 1890. By 1900, the natural flow of all surface streams in the valley was over-appropriated. High spring runoff and low summer flows in valley streams, coupled with years of severe drought, resulted in undependable water supplies for irrigation; thus, farmers turned to wells and reservoirs to supplement and regulate their water supply.4

Well construction in the valley began as early as 1850. Between 1880 and 1891, about 2,000 artesian wells were drilled. Withdrawals from the confined aquifer by wells remained relatively constant until the early 1950s when a number of large capacity wells were constructed. In 1972, the state engineer ceased issuing permits for wells to be drilled into the confined aquifer after determining that both aquifers were tributary to the surface streams in the valley, based on studies by the United States Geologic Survey and state water agencies. There are more wells on land irrigated by surface water from the Rio Grande main-stem than on land in the Conejos River Basin because many of the Conejos farms are too small to use sprinklers connected to wells and wells in the Conejos River Basin west of Antonito must be drilled to such depths that pumping them is uneconomical.

Since before the turn of the century, valley water users have had to contend with out-of-state demands for Rio Grande water. In 1896, complaints and claims for damages from the Republic of Mexico led the United States Department of Interior to deny permission for the utilization of federal land in the construction of most reservoirs planned for the valley. The dispute with Mexico was resolved by treaty in 1906, 34 Stat. 2953 (1906), but the next year, the United States Supreme Court, in Kansas v. Colorado, 206 U.S. 46, 27 S.Ct. 655, 51 L.Ed. 956 (1907), articulated the doctrine of equitable apportionment, opening the door for the assertion of justiciable rights to Rio Grande water by the states of New Mexico and Texas.

To avoid litigation, Colorado, New Mexico, and Texas began in 1923 to make efforts towards a negotiated apportionment of Rio Grande water. Negotiators from the three states signed a permanent compact in 1938. The compact subsequently was ratified by the legislature of each state and approved by the United States Congress in 1939. 53 Stat. 785 (1939). Codified at section 37-66-101, C.R.S., the compact obligates Colorado to deliver water in the Rio Grande at the New Mexico border based upon two schedules tying delivery obligations to levels of inflow, as measured at upstream gauges on the Rio Grande mainstem and the Conejos River, to which is added the flow of the Los Pinos and San Antonio rivers (tributaries of the Conejos) measured near Ortiz, New *919Mexico. The amount of required discharge varies according to natural supply. In low water years, small deliveries are required; in high water years, large deliveries are required. The compact fixes Colorado’s overall obligation in the equitable interstate apportionment of the Rio Grande at a level intended to protect water use as it existed from 1928-1937 (the compact study period). In recognition that variations from predicted performance for each river would occur in the future because of the sequencing of wet and dry years, variable runoff patterns,5 and new depletions, the compact allows accumulated debits up to 100,000 acre-feet. See Article VI of the compact.6

Beginning in 1952, Colorado accumulated debits in excess of 100,000 acre-feet. Colorado water officials did not curtail surface appropriations to satisfy the compact, and by the end of 1965, Colorado’s accrued debit was 939,900 acre-feet. In 1966, Texas and New Mexico brought an original proceeding before the United States Supreme Court seeking repayment by Colorado of the accrued debit. The three states filed a motion for continuance, stipulating that the litigation would be stayed if Colorado met its delivery obligation on an annual basis, without an allowance for accumulated debits, and used all available administrative and legal powers, including curtailment of diversions, to assure annual compliance. This motion was granted by the United States Supreme Court. Texas v. Colorado, 391 U.S. 901, 88 S.Ct. 1649, 20 L.Ed.2d 416 (1968).

Governed by the stipulation, the state engineer is required to administer the Cone-jos River and Rio Grande mainstem on the basis of projected annual runoff. Since 1968, when the state engineer began enforcing the stipulation, water users on both the Conejos and Rio Grande have experienced substantial curtailments of their diversions. Between 1969 and 1975, the state engineer developed annual operating criteria to deliver water to the state line.

In 1975, the state engineer promulgated the proposed rules, publishing them in all counties of Water Division No. 3, which is generally coterminous with the San Luis Valley. The proposed rules fall into three categories. First, the separate delivery rules identify the respective obligations of the Conejos River and the Rio Grande mainstem to deliver water according to schedules, applicable to each stream, in Article III of the compact. The effect of these rules is to subject use on each river to separate administration. Second, the tributary rule provides that the state engineer has the authority to require curtailment of all tributaries of the Rio Grande to satisfy compact obligations. Third, the underground water rules provide for the phasing out of underground water diversions unless the underground water user submits proof to the division engineer that the user’s well is operating under a decreed plan of augmentation or has a decree as an alternate point of diversion, or that the underground water appropriation can occur without impairing the right of a senior appropriator.7 Numerous protests to the proposed rules were filed, requiring a hearing before a water judge under section 37-92-501(2)(h), C.R.S.

The first judge who heard the case remanded the proposed rules to the state engineer on the belief that the underground water rules were adopted under different statutory authority than the separate delivery and tributary rules and therefore should be promulgated separately. This court, in Kuiper v. Gould, 196 Colo. 197, 583 P.2d 910 (1978), reversed the water court’s ruling and remanded the proceeding to a different water judge for hearing and disposition of the protests to the proposed rules.

*920A lengthy trial ensued. The state engineer, who is also the ex-officio Rio Grande Compact Commissioner for Colorado, was the proponent of the rules. The Rio Grande Water Users Association, comprised of the mutual ditch companies and individual surface appropriators (the Rio Grande ditches), also advocated the separate delivery rules and the tributary rule. Surface water users on Alamosa, La Jara and Trinchera Creeks, all tributaries to the Rio Grande mainstem (surface water users on the tributaries), challenged the tributary rule.8 The Conejos Water Conservancy District, which encompasses irrigated lands supplied with water from the Conejos River and its tributaries (the Conejos District), protested the separate delivery rules, but supported the underground water rules.9 The San Luis Valley Irrigation Well Owners Association (the well owners), whose members own wells throughout the valley, and a number of municipal entities in the valley (the communities), which rely on confined aquifer wells to supply water to their residents, protested the underground water rules.

At trial, the proposed rules’ proponents and the protestants submitted documents detailing the legislative history of the compact and testimony from administrators about operative interpretations of the compact. All parties agreed that historical hy-drologic patterns in the valley have changed dramatically, particularly since 1950, because of a combination of natural and man-made causes, resulting in an overall reduction in surface water supplies. The state and the Conejos District asserted that the primary cause of the reduction is increased well pumping, but there was also evidence that decreased snowpack runoff, and more efficient irrigation are responsible causes. The theories were supported by studies, which contrasted stream system inflow and outflow, and computer modelling, which estimated overall stream depletions from well withdrawals. Farmers testified to the effect of loss of surface water on their ability to grow crops and the need for supplemental well water when surface supplies are undependable. Finally, evidence was presented that the natural consumptive use of water by evapotranspiration from native grasses and phreatophytes, such as cottonwood, greasewood and rabbitwood, accounts for a large portion of the annual loss of water in the valley. Expert testimony indicated, and the water court found, that when wells are pumped, lowering the water table below phreatophyte root zones, the result is a substantial salvage of water, perhaps as much as one million acre-feet a year, that would otherwise be lost through evapotranspiration.

The water court approved the separate delivery rules, holding that the compact was clear on its face in requiring separate delivery obligations. The court also upheld the tributary rule, holding that the compact applies to all tributaries of the Rio Grande, but if the state engineer determines that delivery of water from these tributaries to the mainstem would be futile or wasteful, he has the authority not to curtail diversions. The water court disapproved the underground water rules, suggesting that section 37-92-502, C.R.S. requires that the state engineer must determine that each well causes material injury to a senior appropriator before that well may be curtailed and holding the rules to be inconsistent with the statutory and judicial policy of maximum utilization of water. The court held, relying on section 37-92-102, C.R.S. and Fellhauer v. People, 167 Colo. 320, 447 P.2d 986 (1968), that in some instances senior appropriators may be required to drill new wells to augment or replace their surface water diversions before curtailment of junior rights can be required.

*921The Conejos District, and Mogote appeal the water court’s decision on the separate delivery rules. The surface water users on the tributaries appeal the tributary ruling. The state and the Conejos District appeal the water court’s disapproval of the underground water rules.

II.

A.

The proposed rules are based on the premise that the separate delivery schedules provided for the Conejos and Rio Grande in Article III of the compact mandate separate administration of the rivers.10 This interpretation of the compact reflects administration of the rivers under the compact since 1968 when, as a result of the stipulation with New Mexico and Texas, the Colorado state engineer began to require curtailment of diversions in order to meet compact obligations.

As a result of separate administration of the rivers, senior water rights on the Cone-jos River have been curtailed at times when users with more recently acquired rights on the Rio Grande have continued to divert water. Moreover, although the Conejos system contributes only thirty percent of the inflow of water to the San Luis Valley, administration in accordance with the compact schedules requires the Conejos system to provide forty-five percent of Colorado’s deliveries at the New Mexico state line. The Conejos District maintains that the Conejos River is a losing stream,11 and that the state engineer has curtailed a large percentage of inflow at the headgates of the Conejos water users so that the stream will produce the amount required by the compact. In contrast the Rio Grande is a gaining stream, and its delivery obligation is satisfied to a large extent by return flows of water already diverted for irrigation. Since 1968, except for the severe drought years of 1972 and 1977, the state has curtailed diversions to some extent on the Conejos and its tributaries during the irrigation season. The effect of such curtailment in the Conejos area is to reduce the irrigation season by about one week at each end of the season, resulting in numerous hardships on farmers and ranchers in the area. Consequently, intense controversy has developed over whether administration according to the separate delivery schedules of Article III is required by the compact or permissible under state law.

The water court ruled that the compact is clear on its face: the purpose of the separate delivery schedules for the Conejos and Rio Grande in Article III is to establish separate obligations for administration of these two rivers in Colorado. The water court supported its reading of the compact by reference to the compact’s legislative history as well as the other factors set forth in section 2-4-203, C.R.S. (1980 Repl.Vol. IB).12 The water court found that water *922rights on the Conejos River historically have been administered independently of water rights on the Rio Grande mainstem and noted an absence of evidence that compact negotiators intended to break with that history. The court found that the compact negotiators and their advisors intended the compact to continue separate administration of the Conejos River and the Rio Grande mainstem, citing numerous statements by compact principals recognizing separate delivery obligations.13 The water court, considering the equities of separate delivery, concluded that although adherence to separate delivery schedules yields less water to Conejos appropriators than would a unitary obligation, the Cone-jos appropriators attain a larger volume of water per acre of irrigated land than do Rio Grande mainstem appropriators. Finally, the water court concluded that even if the compact were silent as to intrastate administration of the two streams, anything other than separate administration would violate section 37-80-104, C.R.S., which provides that when a compact is deficient in establishing standards for administration, its provisions shall be implemented “so as to restore lawful use conditions as they were before the effective date of the compact insofar as possible.”

Despite the water court’s detailed findings of fact and conclusions of law, the Conejos District (and Mogote, with slightly different emphases)14 asserts that the compact cannot allocate intrastate water distribution without violating the Colorado Constitution, that the compact is not clear on its face, that the water court misinterpreted the legislative history of the compact, and that the water court’s reading of the compact produces inequitable results in violation of section 37-80-104, C.R.S. The Conejos District contends that a unitary state obligation, which would combine the delivery obligations of both streams as a total obligation for Colorado, to be reallocated in conjunction with the prior appropriation doctrine, best meets the constitutional and statutory prior appropriation doctrine.

In rejecting the argument that an interstate compact cannot allocate intrastate water distribution, since Colorado’s Constitution and statutes direct priority administration of water rights, Colo. Const. Art. XVI, §§ 5, 6; sections 37-92-103(10), 37-92-301(3), 37-92-401, and 37-92-501, C.R.S.; Coffin v. Left Hand Ditch Co., 6 Colo. 443 (1882), the water court held,

“In an equitable apportionment of an interstate stream, the State of Colorado has legal power and authority to allocate by Compact different burdens and entitlements between various sections of the river. This is especially true where, as here, the burden represents only that quantity of water which was not consumed on each river at the time of the Compact.”

We agree. The equitable apportionment of the waters of interstate streams may be accomplished either by the United States Supreme Court, Kansas v. Colorado, 206 U.S. 46, 27 S.Ct. 655, 51 L.Ed. 956 (1907), or by interstate compact, Colorado v. Kansas, 320 U.S. 383, 64 S.Ct. 176, 88 L.Ed. 116 (1943). Equitable apportionment, a federal doctrine, can determine times of delivery and sources of supply to satisfy that delivery without conflicting with state law, for state law applies only to the water which has not been committed to other states by the equitable apportionment. Hinderlider v. LaPlata River and Cherry Creek Ditch Co., 304 U.S. 92, 58 S.Ct. 803, 82 L.Ed. 1202 *923(1938). In an equitable apportionment, strict adherence to prior appropriations may not always be possible. Colorado v. New Mexico, - U.S. -, 103 S.Ct. 539, 74 L.Ed.2d 348 (1982); Nebraska v. Wyoming, 325 U.S. 589, 65 S.Ct. 1332, 89 L.Ed. 1815 (1945).

The question before us is how the prior appropriation doctrine may be reconciled with the equitable apportionment of water under federal law, which in this case subjugates the heretofore independent water rights on the Rio Grande mainstem and the Conejos River to a relatively recently created obligation to deliver an equitable share of the rivers’ water at the New Mexico state line. As a result of the doctrine of prior appropriation, local economies develop based on vested rights in appropriations, subject to the vagaries of nature, but with settled expectations — arising out of the pattern of development of a water source — as to how water is to be allocated. Under prior appropriation doctrine water is allocated according to chronology because such allocation has the effect of protecting historic patterns of use. This process of development based on vested rights occurred independently on the Rio Grande mainstem and the Conejos River. As the water court found, prior to and at the time of the signing of the compact, there were no decreed diversions from the Rio Grande in Colorado below the confluence of the mainstem and the Conejos. At no time have diversions on one stream been subject to curtailment by senior appropriators on the other.

To hold, as suggested by the Cone-jos District, that the compact obligation has the effect of re-sorting settled water rights on both streams into a single system of priorities based solely on dates of appropriation would reshuffle the economies of the valley according to a chronology of events unrelated to settled expectations derived from historical patterns of use and reflected in the independent priority systems. That this result is not compelled by the doctrine of prior appropriation was recognized by the General Assembly in section 37-80-104, C.R.S. which mandates that compacts which are deficient in provision for intrastate administration be implemented so as to “restore lawful use conditions as they were before the effective date of the compact insofar as possible.” We agree with the statutory implication that a compact obligation should not be viewed as a senior water right which upsets historical development and reshuffles rights according to a chronological formula. Under the doctrine of pri- or appropriation, streams which have been independently appropriated remain independent. If the water of those streams becomes subject to equitable apportionment by compact, the streams must be administered as mandated by the compact, or if the compact is deficient in providing for administration, according to section 37-80-104. The separate delivery rules, therefore, are not inconsistent with constitutional and statutory provisions for priority administration of water rights.15

The Conejos District argues that the drafters of the compact did not intend for the schedules of Article III to be used other than to define Colorado’s state line delivery obligations and to account for the effects of post-compact developments, particularly storage in reservoirs. The Conejos District asserts that historically independent administration in the absence of a defined interstate obligation is irrelevant to the question of which water rights must be curtailed to meet the state line obligation. The district contends that while the streams are separate in that an appropriator on one cannot make a call on decrees on the other, the superseding obligation to deliver water at the state line should be seen as a “senior” downstream obligation which, when necessary, “calls” upstream diversions on a uni*924tary basis in accord with the doctrine of prior appropriation. This approach would view the generally older Conejos diversions as senior to most of the Rio Grande diversions and thus result in significantly more Rio Grande than Conejos diversions being curtailed for compact purposes.

Accepting, arguendo, the Conejos District’s interpretation, we are left with the question of what the purpose of the separate delivery schedules in Article III might be. Since Article III identifies circumstances in which “application of these schedules” is to be adjusted, we must conclude that the schedules were intended to be applied in some fashion. The Conejos District’s argument is that these schedules were intended to allow Colorado to keep track of the responsibilities of various storage facilities for any debits and credits which accrue in Colorado’s compact obligations.16 While there is legislative history indicating that the compact negotiators expected the schedules to be used to allocate credits and debits to post-compact reservoirs,17 this intended use does not adequately explain the presence of separate schedules in Article III. If a unitary obligation had been intended, there would have been no need for separate accounting of storage since under a unitary obligation any post-compact reservoir would contribute to the overall Colorado delivery based on its relative priority, irrespective of the stream upon which it was located. Therefore, we conclude that the schedules must have been intended to govern separate administration of the Rio Grande and Conejos River.

The Conejos District argues that the separate delivery rules are illegal and inequitable in violation of section 37-80-104, C.R.S. which provides:

“The state engineer shall make and enforce such regulations with respect to deliveries of water as will enable the state of Colorado to meet its compact commitments. In those cases where the compact is deficient in establishing standards for administration within Colorado to provide for meeting its terms, the state engineer shall make such regulations as will be legal and equitable to regulate distribution among the appropriators within Colorado obligated to curtail diversions to meet compact commitments, so as to restore lawful use conditions as they were before the effective date of the compact insofar as possible.”

Because we do not believe that the compact is deficient in establishing standards for administration within Colorado, we hold that, other than as the source of the corn-*925pact rule power, section 37-80-104 has no application to the instant case.

We affirm the water court’s holding that the compact is clear on its face and thus affirm the court’s approval of the separate delivery rules. Despite the introduction of massive amounts of legislative history, the Conejos District has been unable to explain away the presence of the separate delivery schedules of Article III. These schedules could only have been included for the separate administration of each stream’s compact obligations. We note that the legislative history18 and administrative history19 support the water court’s holding, but we do not consider it necessary to employ these or the other statutory construction aids set out in section 2-4-203, C.R.S. (for interpretation of ambiguous statutes) to interpret a clear statute. See Aranda v. Patterson, 146 Colo. 424, 361 P.2d 782 (1961).

Because we are convinced that the separate delivery obligation is clear on the face of the compact, we hold that the separate delivery rules have a reasonable basis in law. Ricci v. Davis, 627 P.2d 1111 (Colo.1981). Although the impact on the Conejos River of separate delivery administration has been severe, particularly on the small farmers and ranchers who depend exclusively on a surface supply, the state engineer did not have any means of addressing that problem in the exercise of his compact rule power; the state engineer did use his water rule power to address the supply problems in the Conejos Basin by means of well regulation. See Part III, infra.

B.

The proposed tributary rule includes Ala-mosa Creek, La Jara Creek and Trinchera Creek within the state engineer’s power of administration to secure delivery of water to meet the compact obligation.20 The water court concluded that the compact applies to all tributaries of the Rio Grande,21 but held that the state engineer may choose not to curtail diversions on the three creeks, if he determines that delivery of water from these tributaries to the mainstem of the Rio Grande would be futile or wasteful. We reverse, concluding that the compact negotiators did not intend to include Ala-mosa, La Jara, and Trinchera Creeks under compact administration.

Trial testimony established that Alamosa Creek and La Jara Creek flow through flat land, the stream channels are not clearly defined, and practically no water from either creek reaches the Rio Grande except *926during periods of flooding. The Alamosa Creek channel ends about eight or nine miles from the Rio Grande and the water spreads out and is absorbed by meadowland. The La Jara Creek channel is small, crooked, and obstructed with beaver dams. Most of the water soaks away or spreads over meadows before it reaches the Rio Grande. Both Alamosa Creek and La Jara Creek have their own reservoirs and the water supplies of the creeks were fully utilized by the time of the compact study period. The Trinchera Creek streambed, below two reservoirs, is obstructed with willows, beaver dams, and man-made dams. Water reaches the Rio Grande only under flood conditions, or as underground flow. Priorities on all three creeks always have been administered separately from those on the Rio Grande mainstem. The creeks have not been successfully administered for compact purposes.22

The water court recognized that, at the time of the compact study period, irrigation development and water use on the tributaries were such that the streams contributed little water to the mainstem except occasional flood flows. Because of this, the compact negotiators made no provision for gauging stations for the streams and made no specific allowance for their flow in the delivery schedules of Article III. The water court concluded, however, that the omission of gauging stations did not exclude the tributaries from the compact obligation since Article 111(4) refers to the total flow of the Rio Grande at the gauging station near Lobatos, less the discharge of the Conejos River at its mouth. The court noted that although the compact makes no mention of Alamosa Creek, La Jara Creek, or Trinchera Creek, it does not specifically exclude them.

The tributary users urge the significance of the absence from the compact of delivery schedules or provision for gauging stations for the creeks, contrasting the fact that the compact specifically includes in a delivery schedule the natural flow of both the Los Pinos and San Antonio Rivers (tributaries of the Conejos), and. establishes gauging stations for them. Since the delivery schedules of Article III were intended to protect pre-existing uses, identifying only the remaining amount of water as subject to delivery at the state line, the tributary surface users argue that had the compact negotiators prepared a schedule for the creeks, the delivery obligation would have been zero.

The state, the Rio Grande ditches, and the Conejos District, seeking compact contributions from the tributary surface water users, rely on specific provisions of the compact. Article 1(c) of the compact defines “Rio Grande Basin” as “all of the territory drained by the Rio Grande and its tributaries in Colorado... . ” Article 1(e) of the compact defines “tributary” as “any stream which naturally contributes to flow of the Rio Grande.” The parties seeking contributions note that there are many tributaries to the Rio Grande and the Conejos River which were not named in the compact and for which delivery schedules were not supplied. In addition, these parties argue that the compact specifically mentioned gauging stations on the Los Pinos and San Antonio only because such gauging stations were necessary to prevent distortion of inflow-outflow correlations which would have skewed the delivery obligation schedules, while, because the Alamosa, La Jara, and Trinchera Creeks all had pre-compact reservoirs, their contribution to the Rio Grande mainstem was too small in comparison with the total inflow to significantly distort the compact study period analysis.23

We conclude that the compact is ambiguous: it could be read to include all tributaries of the Rio Grande and the Conejos River, or it could be read to authorize administration only of those streams which significantly contributed to the outflow of the Rio *927Grande at Lobatos during the compact study period. Interpretation of ambiguous legislation requires resort to its history, here represented by evidence of the intent of the compact negotiators. See section 2-4-203, C.R.S.

R.J. Tipton, Colorado’s engineering-advis- or to the compact commissioner, observed in 1935 that “[t]he interstate problem in the San Luis Valley concerns in general only the Rio Grande above Alamosa, and the Conejos River and its tributaries....” R.J. Tipton, Analysis of Report of Committee of Engineers to Rio Grande Compact Commissioners Dated December 27, 1937, app. “Extracts from Resume of the Problem Concerning the Rio Grande above Fort Quitman, Texas, by R.J. Tipton, July, 1935” 7 (1938).24 The Alamosa, La Jara, and Trinchera Creeks enter the Rio Grande below Alamosa.

The Rio Grande Joint Investigation found that the demands upon streams in the valley other than the Rio Grande and the Conejos “are not of concern [in developing an equitable division of water] ... since other than by occasional flood flows these streams contribute practically no water to the flow of Rio Grande leaving the valley.” National Resources Committee, Regional Planning, Part VI — The Rio Grande Joint Investigation In the Upper Rio Grande Basin in Colorado, New Mexico, and Texas, 1936-1937 93 (1938). The Rio Grande Joint Investigation also found that “high transportation and other losses in the use of the available supplies combine to leave practically no residual flow to the Rio Grande from Trinchera drainage.” Id. at 77.

We held, in Part IIA, supra, that the compact requires administration of the Rio Grande mainstem and Conejos River according to the delivery schedules of Article III, which were based on the contributions of those streams during the compact study period. There is no evidence that the contributions of the Alamosa, La Jara, and Trinchera Creeks were significant to the calculation of the Rio Grande mainstem’s delivery obligation. The compact negotiators knew that even if diversions on these tributaries were curtailed to provide compact water, that water would be lost in transit and not reach the Rio Grande.

Although findings of fact derived from evidence before the trial court normally will not be disturbed on appeal, where, as here, the evidence consists entirely of written documents, an appellate court may review independently the sufficiency of the evidence and determine how that evidence will assist in construction of a compact. See Colorado River Water Conservation Dist. v. Municipal Subdistrict, Northern Colorado Water Conservancy District, 198 Colo. 352, 610 P.2d 81 (1979); Sentinel Acceptance Corp. v. Colgate, 162 Colo. 64, 424 P.2d 380 (1967). Our independent evaluation of the legislative history, coupled with the water court’s finding that at the time of the compact the streams contributed little water to the mainstem, leads us to conclude that the drafters did not intend to include the normal surface flows of Ala-mosa Creek, La Jara Creek and Trinch-era Creek under Article III compact administration, and therefore, that the state engineer does not have the authority to apply the tributary rule to these creeks.

*928III.

The proposed underground water rules tie tributary underground water administration in the valley to regulation for compact requirements by integrating tributary underground water diversions into the priority system for surface streams. The rules are intended over a five-year period to curtail well diversions25 unless individual well owners prove that their wells do not cause injury to senior water rights or remedy such injury through plans for augmentation.26

In support of the underground water rules, the state engineer introduced voluminous exhibits which showed that well pumping decreases artesian pressure, resulting in increased recharge to the confined aquifer from the streams in the recharge areas and decreased flow into surface streams from springs fed from the confined aquifer, and, ultimately, in streamflow depletion. The state engineer estimated that well diversions from both aquifers annually deplete the Rio Grande mainstem by 17,700 acre-feet and the Conejos River by 16,400 acre-feet. He concluded that junior well, diversions were causing material injury to senior water rights throughout the valley.27

The water court found that the underground water was tributary to the surface streams; that surface decrees were experiencing increasing curtailment; and that underground water withdrawals had accelerated in recent years, affecting surface flows. The court also found that the effect of underground water withdrawals had not been specifically quantified and had not been attributed to individual wells. None of the parties take issue with the water court’s general factual findings. Instead the rules’ proponents challenge the legal bases of the water court’s disapproval of the underground water rules.

A.

At the outset, we address the state engineer’s claims, based on Citizens for Free Enterprise v. Department of Revenue, 649 P.2d 1054 (Colo.1982), that the water court failed to apply a properly deferential standard of review to factual and policy determinations embodied in the under*929ground water regulations.28 Because none of the parties challenge the factual findings of the water court, the question of court deference to the state engineer’s factual presentation is not before us. The state engineer’s argument that the water court did not defer to the policy determinations embodied in the rules ignores the basis of the water court’s decision: that the rules were not within the authority of the state engineer under sections 37-80-104 and 37-92-501 and 37-92-502, C.R.S. Court deference to policy determinations in rule-making proceedings does not extend to questions of law such as the extent to which rules and regulations are supported by statutory authority. Colorado Auto and Truck Wreckers Association v. Department of Revenue, 618 P.2d 646 (Colo.1980). We turn, therefore, to the propriety of the water court’s legal conclusions.

B.

In disapproving the underground water rules, the water court looked to language in section 37-92-502(2), C.R.S. which prohibits the curtailment of a diversion unless the diversion is causing material injury to senior water rights.29 The water court stressed the statutory reference to “each case” and “each diversion” and concluded that the materiality of injury must be determined individually for each well after consideration of the factors set out in section 37-92-502(2), stating: “[ujntil the Division Engineer determines the materiality of injury to senior priorities caused by a specific well . .. that well may not be curtailed.” To the extent that the water court ruling disapproves the rules because they presume material injury to senior rights from all junior groundwater diversions in the valley, and to the extent that the ruling requires the division engineer to re-prove in each individual well determination the existence of material injury which has already been proven on a valley-wide basis, the ruling is in error.

In Fellhauer v. People, 167 Colo. 320, 447 P.2d 986 (1968), this court responded to an *930argument that wells could only be regulated on a case-by-case basis and that injury to a particular senior appropriator must be shown before wells could be regulated as follows:

“[Wjhenever a court or water administration official can make a finding that the pumping of a junior well materially injures senior appropriators who are calling generally for more water, there exists a legitimate and constitutional ground and reason for the regulation of the well, and a showing of a call against that well by a particular senior user is not necessary. In other words, we hold that, subject to the conditions hereinafter mentioned, the State Assembly may