Articles (31)
Case Study · 24 min read
India's rainfall datasets agree on seasons, not storms
We put IMD's gauge-based rainfall grid side by side with CHIRPS, ERA5 and IMDAA for 2004–2020. They tell much the same story about a wet or dry monsoon for India as a whole. On a heavy-rain day in one place, they mostly disagree.
Case Study · 23 min read
Almost 1,000 mm in a day: India's wettest days on record
We searched 45 years of IMD gridded rainfall for India's wettest single days: where they fall, when, whether they are becoming more common, and how far a 25 km grid sits from what rain gauges record.
Case Study · 21 min read
Is heavy rain rising in India's cities? 463 cities tested
We compared heavy-rain days in 1979–93 and 2009–23 around every Indian city of 1 lakh people or more. Most show no clear change. Twenty-nine have clearly more heavy days and 37 clearly fewer, with the biggest gains around Mumbai, in Saurashtra and in the Uttarakhand foothills.
Case Study · 20 min read
India's dams are getting old. Who is watching downstream?
A building boom from the 1970s and 80s is turning 50, so we mapped India's large dams by age, earthquake zone and the change in heavy rain since they were built, and asked what the people downstream can rely on.
Case Study · 9 min read
Non-contact river discharge vs ADCP: DoorDrishti in a Himalayan field test
DoorDrishti measured river discharge from a bridge without touching the water. We checked it against boat-mounted ADCP gaugings on two days, a month apart, at very different flows.
Case Study · 22 min read
Is the monsoon arriving later where you live?
We dated the start and end of the rainy season in every district from 45 years of IMD rainfall. Kerala's official onset has not moved, and in most districts the rains start when they used to. What has changed is how long they last.
Case Study · 20 min read
Are monsoon dry spells getting longer? 45 years of data
We measured the dry gaps inside every June–September from 1979 to 2023, district by district. Across most of India they have become shorter, not longer. The exception is a belt across the east and the Northeast, where rainy days have been lost and the gaps have grown.
Case Study · 21 min read
Is India's monsoon getting burstier? 45 years of data
We measured how much of each year's rain falls on a handful of days, district by district, from 1979 to 2023. India as a whole has not shifted, but rain is concentrating in the Northeast and parts of the east, and spreading out across Rajasthan and the southern peninsula.
Case Study · 21 min read
India's heavy rain isn't rising everywhere. It's moving.
Forty-five years of IMD rainfall data show a flat national average and a very uneven map. Heavy rain days have climbed sharply across one belt of India and fallen across another, and the places gaining them are often the least ready.
Technology · 9 min read
Sentinel-1 flood mapping: floods from space with radar
How radar satellites see through monsoon cloud to show where floodwater has spread, how the maps are made, and where they can mislead.
Technology · 10 min read
SPI drought index and SPEI explained
How the Standardized Precipitation Index turns rainfall into a drought number, what its classes mean, and what the SPEI adds when temperature is included.
Technology · 6 min read
Soil moisture and runoff: the antecedent moisture condition
The same storm can give a trickle or a flood, depending on how wet the ground already was; here is how hydrologists account for it and how it can be measured.
Technology · 8 min read
Open water data in India: India-WRIS and IMD
Where India-WRIS river and groundwater data and IMD rainfall data come from, what you can download freely, and how to use them without getting caught out.
Technology · 9 min read
Bridge scour monitoring: why river level is not enough
Scour hides under the water and often refills before anyone looks, so a river level reading tells you when a bridge is at risk but not what the bed is doing.
Technology · 13 min read
Climate change and Indian floods: what the science says
What the IPCC and Indian assessments say about heavy rain, river floods and flood risk in India, and where the evidence is firm or still uncertain.
Technology · 13 min read
Flood-prone areas of India: where and why rivers flood
Which states and river regions flood most, what the official 40 million hectare figure really means, and the physical reasons behind each region's floods.
Technology · 8 min read
Surface velocity coefficient: where 0.85 comes from
Why a radar that sees only the water surface needs a number to get the mean velocity, how 0.85 became the default, and why it can be wrong.
Technology · 6 min read
Cloudburst vs flash flood: what's the difference?
A cloudburst is a burst of rain; a flash flood is what the water does next. How the two terms relate, and why one can happen without the other.
Technology · 6 min read
India's colour-coded weather warnings explained
What green, yellow, orange and red mean in IMD's own procedure, how the colour is chosen, and what each one asks of you.
Technology · 8 min read
Nowcasting: forecasting the next three hours
What nowcasting means, how it differs from ordinary forecasting, how radar extrapolation and models are combined, and why its skill fades within a few hours.
Technology · 6 min read
Monsoon onset: how IMD declares it and what it means
The three tests the India Meteorological Department applies before announcing the monsoon over Kerala, and why that announcement is not a forecast for your district.
Technology · 9 min read
Open channel flow meter types: contact vs non-contact
How the main open channel flow meters work, what each needs to touch, and when staying out of the water is worth the trade-offs.
Technology · 15 min read
Weather radar vs rain gauge vs satellite: who sees what
How each of the three measures rain, what a dBZ number means, where each one goes wrong in the mountains, and which to rely on when a flood warning is at stake.
Technology · 8 min read
Weather station siting: heights, obstacles and exposure
Where to put a weather station, how high each sensor should be, and how far it must stand from buildings and trees, following WMO and Indian guidance.
Technology · 12 min read
Rain gauge density: how many gauges does a district need?
How to size a rain gauge network: WMO and BIS minimum densities, the optimum number formula N = (Cv/ε)², why hills need more gauges, and how to check the network you already have.
Technology · 12 min read
Intensity-duration-frequency (IDF) curves in hydrology
What an IDF curve shows, how it is built from annual maximum rainfall and Gumbel frequency analysis, what the IDF formula means, and how to read a design intensity for drains and culverts.
Technology · 14 min read
Western disturbances: winter precipitation in the Himalaya
What the weather systems behind Himalayan winter snow are, where they come from, how much water they bring, how they turn dangerous, and what is known about how they are changing.
Technology · 12 min read
Measurement uncertainty for water data, in plain language
What the ± after a water level or a discharge really means, how to work it out without heavy mathematics, and how to report it so that others can use your data with confidence.
Technology · 13 min read
Snow and glacier melt: how winter shapes Himalayan rivers
Where the water in the Indus, Ganga and Brahmaputra comes from, why snowmelt and glacier melt arrive at different times, what 'peak water' means, and what all of it asks of a river gauge.
Technology · 11 min read
Normal, excess, deficient: how IMD classifies monsoon rainfall
What the India Meteorological Department means by a normal monsoon, how percentage departures are turned into categories for districts and the country, and why a normal year nationally can still bring floods and drought side by side.
Technology · 12 min read
Gauge zero, datum and benchmarks: why two water-level records disagree
What a river level is measured from, how staff gauges and automatic sensors are tied to benchmarks, and how to reconcile records that read differently for the same water.