What the La Niña years left in the ground.
ENSO is a trajectory, not an event. Five La Niña winters in six drew down water that the landscape remembers: the soil (months), the snowpack (a season), groundwater and total storage (years) and the reservoirs we manage. This ledger keeps the running account of each since the La Niña run began in September 2020, for every U.S. county and every country, and asks what a record El Niño can do about it: of the very strong El Niño seasons on record, how many would have repaid today's deficit?
Where the debt is deepest, and least likely to be repaid
The 15 brief regions, ranked
Each region's reservoirs standardised against their own variability, and the share of very strong El Niño seasons that would have closed today's deficit. Select a row for the region's full account.
Every county, every country
How the ledger is kept
Flows and stocks. Precipitation and degree days are flows: the account is the running total of monthly departures from the 1991–2020 normal for that calendar month, from the start month to the latest month. Soil water, snow, total water storage, reservoirs and drought area are stocks: the balance is how far the reservoir sits from its normal for the time of year now, and the account since a start month is its average departure over those months.
The reservoirs. Root-zone soil water is the water in the top metre of soil (ERA5-Land layers 0–7, 7–28 and 28–100 cm, weighted by depth); topsoil is the top 7 cm. Percentiles rank this month against the same calendar month in 1991–2020. Total water storage comes from the GRACE (2002–2017) and GRACE-FO (2018 on) satellites, which weigh the water column from the change in Earth's gravity: soil, snow, surface water and groundwater together, at roughly 300 km resolution. The two missions leave an 11-month gap (July 2017 to May 2018) and a few missing months; the ledger never fills them. Its anomalies are against the 2004–2009 mean, as the data are distributed. Snow is ERA5's snow water equivalent at each place's peak month.
The ENSO lens. An El Niño season is named by its start year (1997 = 1997–98). The very strong seasons, with a peak Oceanic Niño Index (ONI) of 2.0 °C or more as NOAA publishes it, are only four: 1982–83, 1997–98, 2015–16 and 2023–24, so every reservoir from 1981 on has the same four, and GRACE only two (2015 and 2023). Strong seasons (1.5 °C or more) add 1957, 1965, 1972, 1987, 1991 and 2009 where a record reaches back that far. For stocks, a season's repayment is the change in the trend-removed anomaly from September to April; for precipitation it is the Oct–Apr departure, placed on today's trend level. Repayment odds count the very strong seasons whose change would have closed today's deficit, beside the same count over every season in the record (the base rate). With four or two cases these are hit counts, not probabilities; the Wilson interval says how wide the uncertainty is. A deficit repaid in one season can be taken back by a La Niña the next: the region panels show what the La Niña winters after El Niño did.