delivered the opinion of the court.
This controversy between the appellant corporation (plaintiff below) and appellees (defendants below) The Larimer and Weld Irrigation Company and The Windsor Reservoir and Canal Company, consists of two branches. The first relates to the extent of the priority of the canal owned by the appellee irrigation company over the ditch owned by the appellant; the second concerns the seniority of rival reservoir appropriations owned by the appellant and the appellee reservoir company.
The appellant is the owner of the Larimer county ditch and of reservoirs known as Nos. 1, 2, 3, 4 and 5 ; the appellee irrigation company of the Larimer and Weld canal and the appellee reservoir company of the Windsor reservoir. In proceedings duly instituted under our statutes, framed for the purpose of establishing the priority of rights to the *324use of water for irrigation belonging to the various irrigating ditches and canals throughout the different water districts of the state, to which the then owners of these ditches arid canals were parties, a decree was duly entered in the district court of Larimer county on the 11, of April, 1882, adjudicating such priorities in water district No. 3, in which all the ditches and reservoirs in controversy are situate.
It is conceded as a matter of fact, and it so appears from the decree, that the priority of the canal of the appellee irrigation company is superior to that of the ditch of the appellant company; and in so far as the controversy with respect to them is concerned, the contention of appellant is that such priority is only to the extent of 560 cubic feet per second of time, while the position assumed by appellee is that such priority is to the extent of 720 cubic feet per second of time.
All of the reservoirs were constructed subsequent to the decrees mentioned; hence the controversy as to them is not affected by any prior judicial decree, but must be determined entirely from the evidence introduced before the trial court. The avowed object of the action is to obtain a judicial determination of the respective rights of the corporation litigants to divert water from the Cache la Poudre river, both for irrigation and for storage in reservoirs.
The second branch of the case we proceed first to dispose of. The evidence was submitted to the trial court without a jury; and findings of fact made that reservoirs Nos. 2, 3 and 4 of the plaintiff, to a specified capacity, were prior in right to the reservoir of the defendant; and that, as between the plaintiff’s reservoirs known as Nos. 1 and 5, or respectively as “Rocky Ridge” and “Long Pond” reservoirs, and that of the defendant reservoir company, the priority was awarded to the latter.
It is practically conceded that the work of construction of the Windsor reservoir (that of the defendant) was begun in July, 1890, and substantially completed in May, 1893. Work upon the Long Pond and the Rocky Ridge reservoirs of the *325plaintiff was commenced about the first day of September, 1891 (about one year after the initiation of defendant’s enterprise), and was finished before that of the defendant.
The proper solution of the respective rights of the litigating parties with respect to these reservoirs depends very largely upon the fact whether reasonable diligence was used by the defendant in prosecuting work. It would serve no useful purpose to detail the evidence upon this disputed point, and we content ourselves with the statement that, after a careful examination of the entire evidence, we are satisfied that the finding of the trial court was correct, and that, in the circumstances disclosed by the record, considering the magnitude and cost of the work, and the obstacles put in its way, the defendant reservoir company was diligent in the prosecution of such construction, and that its priority should, by relation, date from the beginning, and not from the completion, of the work. From this it follows as a matter of law that the priority was properly awarded by the district court to the defendant company as between these three reservoirs in question.
By the decree of 1882 there was awarded to the Larimer and Weld irrigation canal five several priorities, the first by original construction, and the other four by successive enlargements. The decree, after specifying the several amounts to which the canal was entitled by construction and the first three enlargements, proceeds as follows with respect to the fourth:
“ And further, as to said appropriation by fourth enlargement—priority No. 79—in said findings and herein above mentioned, it having been found in manner and form aforesaid that said fourth enlargement of said ditch No. 9 has been, in fact, made, commencing on the month of September, A. D. 1878, by means of which said ditch is actually enlarged to a carrying capacity of 43,200 cubic feet of water per minute of time (i. e., 720 cubic feet per second), the same having been made at great expense and in good faith, for use for said purpose of irrigation, without the fact that actual *326appropriation by use of water bad. been made of increased quantity intended to be carried by means of said fourth enlargement, it is further adjudged and decreed that nothing in said findings or in this decree contained shall prejudice the right of said claimant or other party or parlies interested in said ditch or in appropriations of water from said river by means thereof, in or concerning said fourth enlargement, if the said appropriation of water intended to be made thereby has been, or shall be, in fact, made, or any part thereof, with due diligence according to the nature of the work of said enlargement, within a reasonable time from the commencement thereof, and said priority shall stand as Ho. 79, to cover any such actual appropriation so made or to be made until further order and judgment of this court in that behalf.”
From the findings of fact, as well as by the express language of the decree, it will be seen that the fourth enlargement had actually been made at the time the decree was rendered, though the increased supply of water thus secured had not been applied to a recognized use. An opportunity and a right, however, were given to the owner of the canal, or other interested party, within a reasonable time, by proper application, to present proof of, or disprove, the actual application of the enlarged supply of water to a beneficial use, when the original decree would be supplemented and perfected to comply with the facts. Based upon this express order of the court, and in virtue of the irrigation statutes so providing, such application for further proceedings was made, and thereupon an order of the court was entered in March, 1883, for the further hearing of evidence; and on the 11, of April, 1884,- a supplemental decree of the district court was entered, the portions of which that are material to the present discussion are as follows:
“ It is * * * decreed that everything in this decree shall be subject to the provisions of the original decree in said matter, as to everything that is and as to everything that is not decided in this decree.”
The taking of the additional evidence in this further pro*327ceeding necessitated a renumbering of the priorities, as well as certain changes in other respects. That portion of the supplemental decree affecting the Larimer and Weld Irrigation Company is as follows :
“No. Nine.
“ Larimer and Weld Irrigation Canal.
“ That the work referred to in the original decree, on the' completion of which said ditch should be entitled to priority No. 79 in said decree, has been completed, and that, by reason thereof, said ditch is now entitled to priority No. 88 of the new'numbering, dating in the month of September, 1878, and that the present dimensions of said ditch are 28 feet in width on the bottom with a berme 5 feet wide on each side so as to carry a flow of 5 feet in depth in’ all, and so built as to have a section area of 180 feet, with a grade of 2 and 4/10 feet per mile, and that there be allowed to flow in said ditch on all of its priorities so much water as it will carry with the dimensions above given, computed at 720 cubic feet of water per second of time.”
The contention of the appellant is that the capacity of the Larimer and Weld canal, as determined by the original decree, was not supported by the evidence, and so far as the decree referred to the last enlargement, when construed in connection with the irrigation statutes, was merely interlocutory and conditional, and left the whole subject of the amount of the appropriation and every matter pertaining thereto, open, unsettled, contingent and uncertain.
It is further claimed that section 2403 of Mills’ Annotated Statutes (which is the law governing this matter) does not empower the court to determine the carrying capacity of a ditch or an enlargement, unless an appropriation of the water had then been actually made, and that such ascertainment in the original decree, therefore, is of no effect. It is further contended that the decree of 1882, if the carrying capacity of the canal was properly determined, merged in, and was superseded by, the second decree of 1884. Therefore, it is said, *328that after the new decree was entered, the former was of no further force, and by the latter decree the capacity of the canal was erroneously computed; and, as attempted to be fixed, must yield to other data, contained in the decree, establishing the physical dimensions of the canal. It is also urged that the canal has been greatly enlarged since the decrees were rendered.
Upon the other hand, the contention of the appellees is that the two degrees supplement each other, and together constitute the measure of the rights of the parties ; and that thereby the defendant irrigation company has a priority over the plaintiff of 720 cubic feet of water per second of time, and that no subsequent enlargements have been made.
Under the theory of the plaintiff, a large volume of evidence was introduced, including all of the evidence that was submitted to the refereés prior to the adjudication of 1882 and which was before the district court as the basis for the first decree, and testimony of witnesses first taken upon the trial of the case, for the purpose of showing, among other things, that the original evidence did not warrant so large a quantity of water as the decrees specified; and that a ditch of the physical dimensions described in the decree could not carry 720 cubic feet of water per second of time. Much evidence was directed to the point that by reason of enlargements of the ditch made at various times since the entry of the second decree, the capacity of the canal had so been increased- as to permit its owner to obtain more water than originally the canal would carry.
Whatever be the declared object of the appellant in this suit, it is manifest that it can be attained only by treating this action as one for the review of the decrees, in evidence, for mere error of the trial court, long after appellant’s statutory time for a review in the district court, and an appeal to this court, has expired;—unless we place upon the decrees the construction which appellant asks. But its contention, in this respect, is altogether untenable-.
By its express terms the decree of April, 1884, was in* *329tended as merely supplemental to that of April, 1882, and the two, taken together, constitute the decree in the case. By section 2403 of Mills’ Statutes, one of the things which the court was required to do was to determine the carrying capacity of the ditch, if the evidence contained sufficient data, not only by original construction, but the increased capacity occasioned by each enlargement. The decree of 1882 expressly. declares that the fourth enlargement had been completed at the date of the decree, and that, by virtue of this enlargement, the entire carrying capacity of the ditch was 720 cubic feet per second of time. But if this fact had not been determined and established by the former decree, the second decree of 1884 confirms the prior findings as well as the former decree, and expressly declares that the capacity of the ditch was computed at 720 cubic feet. So, whether the rights of the parties here depend upon the former, or the latter, decree, or upon both taken as one completed decree, the priority of the Larimer and Weld canal is fixed at 720 cubic feet.
By a somewhat ingenious argument counsel for appellant insist that the ascertainment of the carrying capacity of this ditch, as expressed in the decree, is not res judicata, and is of less dignity and importance than is the determined physical dimensions; and that it must yield to such other ascertainment of superior rank concerning physical dimensions. It is said to be a mere matter of computation from these other data, and that, under the latest approved formula for ascertaining the carrying capacity of water in ditches and canals, the computation, as originally made by the court, was erroneous, and that the specified quantity should be much less.
It must be remembered that the present action is not for the purpose of reforming the decree upon the ground of mistake or fraud, or any other recognized ground, but that it is in the nature of a collateral attack upon the decrees after the statutory time for their reformation or review in the court of original jurisdiction, upon the same or additional *330testimony, lias long gone by, and when the right of appeal is also lost by lapse of time. The present action cannot be allowed to usurp the function of an appeal or writ of error, and thus secure a correction or reformation of the decree, because of some erroneous calculation of the district court. If a mistake was made by the court in computing the capacity of the ditch, such a mistake cannot be corrected in this proceeding. The capacity is res judicata, as much as is any other fact which the special statute requires the court to determine. New Mercer D. Co. v. Armstrong, 21 Colo. 357.
But if we should adopt appellant’s views and hold that, under these decrees, the carrying capacity of the canal was still open to adjudication, and that the court, in the present action, might determine the same, the evidence before us would not, at this late period, justify the reduction asked. From the testimony of Prof. Carpenter it would appear that the latest approved and most trustworthy test for ascertaining the carrying capacity of streams and canals is that known as Ganguillet and Kutter’s. By the application of their formula, a ditch of the theoretical dimensions specified in the decree will not, so Prof. Carpenter testifies, carry more than about 560 cubic feet of water per second of time; at least, his computation so resulted. It is in evidence, however, that in accordance with a formula that was in general use in this state at the time the decrees were rendered, the computation, as determined by the decree, was substantially correct, and it further appears that no two computations are ever the same, even with the same formula. Hydraulics is said to be one of the most complex of sciences. Under the title hydromechanics, at p. 494, vol. 12, of the Encyclopedia Britannica, as well as from Prof. Carpenter’s testimony, it seems that in the formula used by the latter in estimating the capacity of the canal, n represents the coefficient of roughness. Without going into this technical and scientific subject at any length, it is sufficient merely to say that the mathematical value of this element varies with different conditions from .010 to .035. The formula is confessedly era*331pirical and not strictly scientific; it lias not been adopted by our statutes or by the courts as a rule of evidence, or as the exclusive test for ascertaining the carrying capacity of ditches.
The testimony upon this point is in substantial conflict; so, also, is there a substantial conflict in the testimony as to the enlargement of this canal after the two decrees were rendered, and as to the amount of water which the canal at various times since its construction has actually carried. In accordance with some of the testimony, it appears that, as a matter of fact, the ditch has at different times carried even more than the amount of water for which the decree was rendered. There is no serious contention by appellant that there has been any abandonment by appellee company of any of the waters which the decrees have awarded it; but if the point was urged, we would unhesitatingly affirm the finding of the trial court that proof of abandonment was not made out.
Such being our ruling upon the priorities of the reservoirs, and such being our construction of the decrees in this case, and holding, as did the trial court, that they determine the priorities attaching to the canal and the ditch, and that they award to the canal of the defendant irrigation company a priority of 720 cubic feet of water per second of time ahead of any priority belonging to the ditch of the plaintiff company, and it also appearing that if the entire question were reopened at the instance of the plaintiff, there is not in the record sufficient evidence to justify a reduction of this quantity, it follows that the decree should be affirmed in all respects as entered by the district court.
Affirmed.