Drought Risk for Any Location: Read Rain, Soil Moisture and NDVI Together

In October 2023 the Rio Negro at Manaus fell to its lowest level since measurements began more than a century earlier, in the middle of the largest rainforest on Earth. Seen from space, the forest around it was as green as ever. Rainfall and soil moisture told a very different story, and that gap is why one number is never enough to judge a drought.

This guide shows how to read the three signals that matter for land, rainfall, soil moisture and vegetation, for any location on Earth. It walks through five real droughts using the same monthly numbers you can pull yourself, explains what a ten-year record can and cannot tell you, and points to the official services for today's drought status.

What Counts as Drought

Drought has no fixed rainfall threshold, because it is measured against what is normal for a place. A month with 40 mm of rain is generous in a desert and a serious shortfall in the tropics. Three working definitions cover most decisions:

• Meteorological drought: rainfall stays well below normal for months.
• Agricultural drought: soil water falls too low for crops or pasture when they need it.
• Hydrological drought: rivers, reservoirs and groundwater drop. It arrives last and recovers last.

They usually arrive in that order, which makes the order useful. If the rain has failed but the soil is still holding, you have time. If the vegetation already shows it, you are late.

The Three Signals, in the Order They Move

1. Rainfall against normal

Add up the last 3, 6 or 12 months and divide by the normal total for the same months. Short windows catch a failed season. Long windows catch the slow drought that never looks dramatic in any single month. Always compare like with like: a dry August means nothing in a Mediterranean climate, where August is always dry.

2. Soil moisture

The ERA5-Land top layer covers the first 7 cm of soil, in cubic meters of water per cubic meter of soil. It responds within days, and one storm can lift it for a week, so read the run of months rather than a single value. Compare it with the same month in other years, not with a fixed number, because a clay soil holds more water than a sandy one at the same level of stress.

3. Vegetation

NDVI measures how green the landscape is. It usually moves last, weeks to months after the soil, and its seasonal cycle is much larger than most drought signals, so again compare a month with the same month in other years. Two things can hide a drought from NDVI: irrigation, which keeps crops green while the rain fails, and deep-rooted forest, which can reach water the topsoil no longer holds.

The European Drought Observatory builds its Combined Drought Indicator on the same chain: a rainfall shortage sets a Watch, a soil moisture deficit raises it to a Warning, and a vegetation response makes it an Alert.

Five Real Droughts, Read From the Data

Each row compares a period with the same calendar months in the other years of the record. A rainfall figure of 29% means the period had 29% of its normal rain. The soil moisture and NDVI columns show the difference from normal.

Walgett: every link fails. Australia's Bureau of Meteorology called January 2017 to August 2019 the driest such period on record for the Murray-Darling Basin. At Walgett rainfall ran at 71%, 47% and then 29% of normal across 2017, 2018 and 2019, while NDVI fell 27%, 40% and 43% below normal. That is drought with nothing to cushion it.

Mandera: drought across seasons. The Horn of Africa went through five failed rainy seasons in a row. Over 27 months Mandera had less than a third of its normal rain. This is where long windows matter: no single month tells the story, the sum does.

Manaus: green is not wet. Rain was about half of normal and topsoil moisture fell by nearly a third, yet NDVI barely moved. Anyone watching the satellite greenness alone would have seen nothing while the rivers hit record lows.

Cremona: the farmland buffer. The Po had its worst drought in 70 years in 2022. Rainfall at Cremona ran at 69% of normal, but soil moisture and NDVI were only 7% down. This is heavily irrigated land, which is one likely reason, and the value covers a whole landscape cell rather than one field.

Catalonia: the slow squeeze. Catalonia declared a drought emergency in February 2024, with reservoirs below 16% of capacity and more than six million people affected. Rainfall near Manresa for the 18 months before it was 73% of normal, which looks moderate in a table. It came after earlier dry years, on a water system with little margin. Rainfall tells you what the sky did. What it means depends on how much water a place has in reserve.

What a Ten-Year Record Can and Cannot Tell You

• It can show how a place behaves. How often a year fell short, how long soil deficits lasted, how quickly vegetation came back. For a land purchase or a crop decision, that is often the real question.
• It cannot give formal drought statistics. The Standardized Precipitation Index needs a long record: the WMO guide asks for at least 20 to 30 years of monthly values and prefers 50 to 60. Our monthly series covers December 2015 to December 2025, so we show plain comparisons with normal and do not compute SPI.
• The normal contains droughts too. In a ten-year record, one long drought pulls the normal down, so a shortfall can look smaller than it would against a 30-year baseline.
• It is a grid, not a rain gauge. ERA5-Land is a model constrained by observations, on a 0.1 degree grid of about 11 km. Local rainfall can differ, above all in mountains and on coasts.
• It is not today's drought status. The monthly history currently ends in December 2025. For the current situation use your national meteorological service, the U.S. Drought Monitor or the European Drought Observatory.

What to Do With the Answer

• Buying land: count the years that fell well short, and how long the soil took to recover. Then ask the questions the data cannot answer: water rights, wells, irrigation supply and what the neighbors did in the last dry year.
• Choosing a crop or a planting date: look at which months fail most often, not just the annual total. A site with a reliable spring and a dry summer is a different bet from one with erratic rain all year.
• Reporting and insurance: a dated record of monthly rainfall gives a claim or a due diligence file something to point to, next to the official declarations.

How to Check a Location on PixelGust

1. Open the dashboard at pixelgust.com/app and click your location, or enter its coordinates.
2. Read today's picture: soil moisture in the Weather panel and the latest NDVI in the Environment panel.
3. Open the record (paid plans): switch the weather mode to Historical and open the charts for Precipitation and Soil Moisture, then the NDVI chart in the Environment panel. Each one runs month by month.
4. Compare like with like: read a month against the same month in other years, and add up the months you care about.
5. Keep it: export the numbers to Excel (paid plans), or save a Property Climate Report as a dated PDF.

AI assistants connected to the PixelGust MCP server can pull the same monthly series with the get_climate_timeseries tool, with no key inside the daily allowance.

Data Sources and Method

Monthly rainfall and top-layer soil moisture come from ERA5-Land (ECMWF, Copernicus Climate Change Service), December 2015 to December 2025, on a 0.1 degree grid of about 11 km. ECMWF's monthly averaged product is wrong for accumulated variables, rainfall included, from September 2022 to February 2024 and will not be replaced, so for those 18 months we use the by-hour-of-day product at 00:00 that ECMWF recommends. Today's soil moisture comes from NOAA GFS. The monthly NDVI series comes from NASA MODIS (MOD13A3) on cells of about 10 km, and the latest NDVI from Copernicus 300 m composites. The table compares each period with the same calendar months in the rest of the record. All values are model or satellite estimates for a grid cell, so check them on the ground before acting on them.

Frequently Asked Questions

What is the difference between meteorological, agricultural and hydrological drought?

Meteorological drought is a long shortfall in rainfall. Agricultural drought is soil water too low for crops or pasture when they need it. Hydrological drought is low rivers, reservoirs and groundwater. They usually arrive in that order, and hydrological drought recovers last.

How can I check the drought history of a location?

Open pixelgust.com/app, click the location and switch to Historical mode (paid plans) to see monthly rainfall, soil moisture and NDVI since 2015. A one-off Property Climate Report (EUR 19) has the monthly rainfall and temperature record in a dated document. Compare each period with the same months in other years, because a dry month only matters against what is normal for that place.

Can NDVI detect drought?

Often, but late and not always. NDVI usually falls weeks to months after rainfall and soil moisture. Irrigation can keep crops green while the rain fails, and deep-rooted forest can stay green through a severe drought, so read NDVI together with rainfall and soil moisture.

Does PixelGust calculate the Standardized Precipitation Index?

No. The Standardized Precipitation Index needs a long record: the WMO guide asks for at least 20 to 30 years of monthly values and prefers 50 to 60. The PixelGust monthly series covers ten years, so it shows rainfall, soil moisture and NDVI against the same months in other years instead.

Is this an official drought status?

No. PixelGust shows modelled and satellite estimates for a grid cell, and the monthly history currently ends in December 2025. For the current official status, use your national meteorological service, the U.S. Drought Monitor or the European Drought Observatory.