All years since the modern Compact accounting began in 1949 were sorted into four categories. The bold colored lines show the average of each category’s years. Individual year traces illustrate the record of each year’s hydrologic accrual.
The four patterns, and where each ends the year on average:
The −100 KAF cut was tested against every threshold from −200 to −30 KAF and separates the two poor outcomes at a standardized difference of 1.8. The two poor categories are distinguishable. A year in the Collapse category dips below −100 KAF and ends the year at −77 KAF on average, against ending at an average of −30 KAF for a year in Behind and stayed behind.
This graphic can also aid projections of the year-end accrual before the year is finished. The running position through May accounts for 31 percent of the variation in where the year ends; through July 59 percent, through August 71 percent, and through September 79 percent, on the 1960-onward basis.
A projection uses the record to make a forecast. The matched years used to study outcomes closest to a calculated status point in any month are the nearest by position alone; they are not matched on supply, on storage, or on what the monsoon then did. Read the spread the figure reports, not the median on its own. In many cases the closest years’ outcomes span under- and over-deliveries.
What is plotted. Running delivery position by month, KAF: cumulative delivery minus the year’s hydrologic delivery obligation, prorated by the fraction of index supply received to date. Heavy lines are the four class means; fine lines are individual years. Every trace starts at zero on New Year’s Day, so the January point is that month’s movement rather than an offset.
Basis. 72 non-spill years, 1949–2025. The hydrologic delivery obligation is effective supply less the hydrologic accrual, so the position ends the year at what the river delivered rather than at what the Compact paper adjustments generate. That is the quantity a forecast needs: paper adjustments are a management decision and do not correspond to flows of water. Whether accrued credit is relinquished is an example. Four years sit in a different class on this basis: 1977, 2004 and 2008 recovered, and 2010 was never behind at all, but all ended the year with a Compact accounting annual debit.
Graphics features. Blue is never behind, green fell behind and recovered, amber behind and stayed behind, red collapse. The dashed horizontal is zero — on schedule. A dashed vertical and a ringed point mark the position a projection is running from.
Interactive features. Checkboxes overlay the member years of each class. The projection control takes a month and a running position, finds the years that stood nearest it, draws them, and reports what they did by December. Show these years takes a comma-separated list and draws exactly those years at full color and heavier than the other member years, reporting any entry that is not in the record; it adds to the class, era and projection filters rather than replacing them. Hovering a year names it and widens its trace. The figure opens with the projection running through August from −50 KAF over the nearest 10 years, and no class overlays ticked; the years drawn are the matched ones.
Data source. Rio Grande Compact Commission annual accounting records, monthly delivery and index supply determined after the end of the year.