Father appeals a trial court order modifying a dissolution judgment to increase his child support obligation. He argues that mother failed to establish a substantial change in circumstances and that the court improperly applied the Uniform Child Support Guidelines (guidelines). ORS 25.270 - ORS 25.285.1 On de novo review, we affirm.
The parties, parents of a four-year-old child, were divorced on August 28, 1987. The original dissolution judgment provided that mother would have custody and father would pay $260 per month in child support. On May 10,1989, mother moved to modify the judgment to increase child support to $500 per month on the basis of a substantial change in father’s economic circumstances. ORS 107.135(2)(a).2 She also requested that father be required to make future child support payments through the Support Enforcement Division (SED) of the Department of Human Resources. The court modified the decree, applied the guidelines to increase father’s child support obligation to $477.95 per month and required payment through SED.3
At the outset, it is important to point out that we have only the pro se brief filed by father, which raises only statutory, nonconstitutional issues, and the letter opinion of *124the trial court to guide our review of the record. In its letter opinion, the court simply stated that “[t]he Uniform Child Support Guidelines apply to the Court’s decision in this case.” The court then went on to explain that, because of mother’s decision to reduce her work schedule to three-quarters time to spend more time with the child, her monthly gross income would decrease from $2,710 to $2,030. On the basis of evidence in the record that the child suffers emotional and behavioral problems resulting from the divorce and the parties’ continuing hostile relations, the court concluded that mother’s voluntary reduction in her work schedule was in the child’s best interests. However, the trial court made no specific finding as to any substantial change in circumstances. It appears to have been struggling to determine whether the enactment of the guideline legislation created a “trigger” by which all modifications of support orders must be adjusted to comply with the guideline formulas.
The guidelines do not apply to all outstanding orders. There are only two situations, other than original determinations, in which they apply. First, if, in the traditional proceeding, the court finds that the movant has established a substantial change in circumstances so that an increase or decrease in child support is warranted, the guidelines will apply to determine the amount of the modified award. Such proceedings to modify based on a substantial change in circumstance may be initiated at any time. ORS 25.285; ORS 416.425. Second, effective July 1,1990, support obligations collected through an “enforcing agency” (i.e., SED or a district attorney) must be periodically reviewed and adjusted to comply with the guidelines if the obligee, obligor or Adult and Family Services Child Support Unit requests. However, that review may not be initiated sooner than two years after the support order was originally entered or was most recently modified or reviewed. ORS 25.285; OAR 461-195-072; see also 42 USC § 666(a)(10). The latter periodic review process is available only to an enforcing agency. See ORS 25.285; OAR 461-195-072(l)(c)(A).
It is difficult to discern from the trial court’s order which procedure it used to adjust the child support award. What is clear, however, is that, although the obligation would be collected through SED in the future, it was not being *125enforced through that agency when mother moved to modify it. Accordingly, the periodic review process was not available. However, because the trial court did focus on the change in mother’s financial circumstances resulting from the decrease in her work schedule, we conclude that it modified the award on that basis and then applied the guidelines to determine the appropriate amount.4 Thüs, the dispositive question is whether the change in circumstances was substantial enough to warrant the increase in support.
In general, a substantial change in either parent’s financial situation or in the child’s needs may justify a modification of child support. An involuntary decrease in the obligee’s income can constitute a change justifying a support increase. See Bell and Bell, 81 Or App 413, 725 P2d 938 (1986). We must decide whether that rule applies when, as in the present case, the decrease in income is the result of the obligee’s voluntary action.
A voluntary decrease in an obligor’s income may justify a reduction in child support, if the change is made in good faith, ORS 107.135(3)(b); Jones and Jones, 106 Or App 264, 267, 806 P2d 1170 (1991), and if the hardship to the obligor if the decrease is not allowed outweighs the hardship to the child that the reduction would cause. Nelson v. Nelson, 225 Or 257, 264, 357 P2d 536 (1960). Because the welfare of the child is the paramount consideration, it is appropriate to apply that standard here. To justify a modification based on a voluntary decrease in the obligee’s income, the obligee must show that her action was taken in good faith and not merely to compel an increase in support.
The court found that
“the Petitioner’s reduction in income is necessary for [the child’s] well being and that it is appropriate to use the reduced income in determining the amount of child support. *126Her reduced work schedule will have the effect of reducing the child care expenses and this factor does benefit the Respondent by reducing the total child support obligation under the Uniform Child Support Guidelines.”
It is therefore apparent that the court was satisfied that mother’s reduction in her work schedule was taken in good faith and for the child’s welfare. Our de novo review of the record supports that finding. A $700 per month decrease constitutes a substantial change in mother’s economic circumstances and her ability to provide for the child’s needs. Accordingly, we hold that the trial court did not err in ordering an increase in child support.
Father also argues that, even if mother established a substantial change in circumstances, the trial court improperly applied the guidelines to determine the amount of support. Under the guidelines, parents must share in the costs of support on the basis of their proportional incomes, and a “child is entitled to benefit from the income of both parents to the same extent that the child would have benefited had the family unit remained intact.” ORS 25.275(2). Father argues that the guidelines establish “unimplementable criteria,” because “[t]he spending on the child of two separated parents cannot be the same as it would have been if the marriage had not ended, because the total resources of both parents are now being spent on two households rather than one.” According to father, that objection is sufficient to rebut the presumption that the amount of support determined by the guidelines is correct. ORS 25.280.
Whatever merit that argument may have, it is not sufficient to rebut the presumption. That must be accomplished by “a written finding or a specific finding on the record that the application of the formula would be unjust or inappropriate in a particular case,” according to the criteria stated in ORS 25.280.5 See also Watson and Watson, 107 Or *127App 416, 418 n 1, 812 P2d 429 (1991). OAR 137-50-490 includes in Table 2 a worksheet to be completed by the trial court in making that finding from information provided by the parties. Father did not object to how the trial court applied the guidelines, and no findings were made that the formula was unjust or inappropriate in this case. Because mother established a substantial change in circumstances to justify a modification of child support, the guidelines control the amount of support to be awarded. Father has failed to show that the amount is incorrect.
Affirmed. No costs to either party.