Guanajuato month by month: when the land has water
Hover over each month for the summary. The rain does not last long; what matters is when you can see what it leaves behind.
Guanajuato is usually told through its cities: the capital tucked into a ravine, San Miguel de Allende, Dolores Hidalgo, León. But the state is better understood through something you can barely see from a plaza: water, and how briefly it stays. Most of the territory is a semi-arid plateau where rain arrives packed into a few months and the air takes away, year after year, far more than falls.
That balance explains 18th-century dams, streets underground, fields that are green in the middle of the dry season and a mountain range that turns green later than everywhere else. And it points to a specific time of year, the end of the rains, between September and the first half of October, as the one when the territory shows most clearly what it is built for.
01 A balance that almost never comes out ahead
The evaporation recorded by the National Meteorological Service is measured in an open pan: it tells you how much water an exposed surface could lose to sun and wind, not exactly how much a field loses. Even so, it is a good measure of how thirsty the air is. And in Guanajuato that thirst is huge.
In Celaya, in the eastern Bajío, April gets 8 millimetres of rain while the pan loses 233.5: almost thirty times as much. In Dolores Hidalgo, further north, 468.9 millimetres fall each year and 1,357.7 evaporate. Neither in Celaya nor in Dolores is there a single month, in the 1991–2020 normal, when rain overtakes evaporation. The balance is not the same across the state: in León, to the west, July and August do come out ahead, with 187.9 millimetres of rain against 155.7 of evaporation and 170.4 against 152.7. But even there the full year loses far more than it receives: 1,834.9 millimetres evaporated against 730.3 of rain.
That changes the way you look at the landscape. In Guanajuato rain does not pile up on its own: it has to be caught, stored or pumped from underground. Almost everything that catches a traveller's eye, from a reservoir beside a tree-lined promenade to a field of vegetables in March, is a response to that balance.
Key moments at a glance
Jacarandas in bloom and irrigated fields beside dry hills: the other half of the story.
The wettest month and the one when the Presa de la Olla opens; stormy afternoons.
The last month in surplus in the capital and the wettest in the Sierra Gorda.
The rain retreats and the land stays green; the capital fills up with the Cervantino.
02 Three months of surplus in the hills
The clearest exception is in the mountains. In the city of Guanajuato, at around 2,000 metres and wedged among the ranges that surround the valley, the observatory station records 763.4 millimetres a year, the highest of the stations cited here. And here the balance does come out ahead, if only just: July adds 187.8 millimetres of rain against 141.8 of evaporation; August, 147 against 136.9; September, 131.3 against 120.7.
That makes three months of spare water in the entire year, one more than in León. For the rest, from October to June, dry air takes over again. July averages 15.6 days of rain and November, 2. That is why the hills around the capital change colour so much from one season to the next: the oak woods of the Sierra de Santa Rosa, north of the city, look deep green in midsummer, while the lower slopes spend winter in shades of brown.
September is the last of those months and, at the same time, the one that arrives with everything stored up. The reservoirs have taken in July and August's water, the soil is damp and the rains start to thin out. If you travel at the end of September, you see the territory with its tank full, just before it starts emptying again.
In Guanajuato rain does not pile up on its own: it has to be caught, stored or pumped from underground.
03 A city built for the flood
The capital has been organised around those few months for almost three centuries. The Presa de la Olla was built between 1741 and 1749 to supply water to the mining town, and from then on the custom took hold of opening its gates once the rains had settled in, so that the water would flush out the riverbed. The opening is still celebrated, in principle on the first Monday of July, though the date depends on how the rains arrive: in 2026 it was scheduled for 13 July.
The problem for a city in a ravine is not just scarcity but also sudden excess. Between 30 June and 1 July 1905, a great flood swept through part of Guanajuato. That flood drove the works to divert and channel the river beneath the city, a tunnel system that began with a first drainage channel in 1823 and ended, once the river was piped, as the underground street that cars and buses use today.
Walking through those tunnels in February, when it hardly ever rains, is seeing the infrastructure without its reason. In July and August, with afternoon storms, you understand what it is for. And at the end of September, with the dam full and the street dry, you see the result of the whole season.
04 The green that does not come from rain
If water is so scarce, you might expect a brown Bajío for most of the year. It is not, and the explanation lies in the balance too. Guanajuato is Mexico's leading broccoli producer: in 2021 it harvested 387,796 tonnes on 22,913 hectares, according to the state government, and the municipality that contributed the most was Dolores Hidalgo, with 44,681 tonnes. It is the same Dolores that gets 468.9 millimetres of rain a year.
That broccoli does not depend on rain. A study of the crop in the state notes that in 2010 the entire area planted with broccoli in Guanajuato was irrigated. Hence the most puzzling image of the dry season: green plots and freshly worked furrows beside parched hills, in March, when Celaya gets 10.7 millimetres in the whole month.
March also has a colour of its own. In San Miguel de Allende, the jacarandas bloom between late February and April, at their fullest in March, tinting squares and streets in the centre lilac. It is a spectacle of the driest time, not the wettest. That is why March is the other window for understanding Guanajuato: it shows how the territory lives without rain, just as September shows what it does with it.
05 The Sierra Gorda arrives late
To the northeast, the Sierra Gorda of Guanajuato keeps a different clock. It has been a biosphere reserve since 2 February 2007, covers 236,882 hectares spread across Atarjea, San Luis de la Paz, Santa Catarina, Victoria and Xichú, and drops into canyons far lower than the valley. In Xichú, at 1,318 metres, the wettest month is not July but September: 114.3 millimetres and 13.5 days of rain, the highest of the year.
The rain also lasts longer than in the rest of the state. October leaves Xichú with 54.5 millimetres and 8.5 days of rain, while Celaya gets 34.4. In January, by contrast, the range looks like the photo below: leafless scrub, dust on the tracks and grey slopes. Someone arriving in the dry season and someone arriving in early October do not see the same place.
It is a useful lag for the traveller. If you want to see the Bajío and the sierra green at the same time, the window from late September to early October is the one that brings them together best: the valley still holds summer's moisture and the sierra is at its wettest.
06 When to go and what you leave out
If you want to understand Guanajuato, go between mid-September and mid-October. That is when the capital closes out its only months with water to spare, when the reservoirs are full, like the Ignacio Allende dam near San Miguel, and when the Sierra Gorda reaches its peak. The state's grape-harvest festivals, with venues in San Miguel de Allende and Dolores Hidalgo, run in 2026 from 1 August to 18 October, so they overlap with this window.
What you lose is clear too. September still has rainy afternoons, 12.1 days on average in the capital, and some mountain roads can get tricky. The first half of October coincides with the Festival Internacional Cervantino, which in 2026 runs from 3 to 18 October: the city of Guanajuato gains cultural life, but it fills up and accommodation gets pricier. If what you are after is the territory, plan the capital before or after the festival, or stay outside and come in by day.
March is the alternative for anyone who wants the other face: jacarandas in San Miguel, clear skies and irrigated fields beside dry hills around Dolores Hidalgo. April and May take that drought to the extreme, with the Bajío at its brownest and dustiest. From December to February nights are cold, with average January lows of 3.9 degrees in Dolores Hidalgo, and the landscape is at rest. July and August bring storms and the first green, but not yet the full tank. In Celaya no month brings more rain than evaporates; late September is when you best see what the territory has managed to keep.
