Shalini Balaram

Field Notes

Satellite Precipitation for Indian Hydrology: A Practical Introduction

Students and practitioners working on Indian catchments, monsoon hydrology, and drought/flood monitoring

A monsoon season advancing across India on the 0.1° IMERG grid, half-hour by half-hour.

India is a hard place to measure rain. The gauge network is uneven, many basins are transboundary, the monsoon delivers most of the year’s water in a few intense weeks, and a great many catchments are simply ungauged. Satellite precipitation doesn’t replace gauges — but it covers everywhere, in near real time, and for a lot of Indian hydrology that changes what’s possible. Here’s a practical introduction, built around a small IMERG toolkit I maintain.

The products worth knowing

  • GPM IMERG — NASA’s Integrated Multi-satellitE Retrievals for GPM. Half- hourly, 0.1° resolution, global. This is the workhorse, and it comes in three latencies: Early (~4 h, for nowcasting), Late (~14 h), and Final (gauge-corrected, the one for research and climatology). My tooling uses the half-hourly V07B product.
  • CHIRPS — daily, rainfall-focused, with a long record. Strong for drought indices and trends in data-sparse regions.
  • ERA5 — reanalysis precipitation; coarser and model-based, but complete and paired with every other forcing variable.
  • IMD gridded — the India Meteorological Department’s gauge-based gridded rainfall (0.25°). This is not a competitor to the satellite products; it’s your ground truth for validating and bias-correcting them.

A three-step IMERG workflow

The toolkit mirrors how I actually work with this data — download, clip, analyse:

  1. Download. Authenticate with NASA Earthdata credentials (store them in environment variables, not in the notebook), then pull HDF5 granules from GES DISC for your date range. IMERG filenames carry the timestamp and version, e.g. 3B-HHR.MS.MRG.3IMERG.YYYYMMDD-SHHMMSS-EHHMMSS.XXXX.V07B.HDF5. Monthly files are available too when you only need aggregates.
  2. Clip. Subset the global grid to your study boundary — a basin or an India shapefile in EPSG:4326 — and convert to NetCDF (*_subset.nc). Do this immediately; a continental half-hourly archive is unwieldy until you cut it down to the catchment you care about.
  3. Analyse. Generate precipitation maps, rank the top events, and compute summary statistics. Mind the units: IMERG is a rate in mm/hr, so to get a daily or event total you integrate over time — multiply the half-hourly rate by the interval and accumulate, don’t sum rates as if they were depths.

The biases you must respect

This is the part the tutorials skip, and it’s the part that decides whether your study is any good. Validate against IMD gauges before you trust IMERG over India, because it has well-documented, regionally structured errors:

  • Orographic terrain — the Western Ghats and the Himalayan foothills, where satellite retrievals systematically struggle.
  • Convective monsoon extremes — the intense, short-duration cells that drive Indian floods are exactly where instantaneous retrievals are least reliable.
  • Light rain and drizzle — often missed or misclassified.

The standard fix is bias correction — quantile mapping IMERG onto IMD gridded rainfall over a baseline period — after which the product is far more dependable. Treat the satellite estimate as a spatially complete first guess that you calibrate to the gauge network, not as truth.

What you can do with it

Once it’s clipped and bias-corrected, satellite precipitation is a genuine force multiplier for Indian hydrology:

  • Monsoon onset and progression mapping at sub-daily resolution.
  • Drought monitoring — compute SPI or SPEI directly from the satellite record where gauges are absent.
  • Flood nowcasting with the Early product.
  • Forcing ungauged basins and filling gaps in patchy gauge series.

The honest summary: satellite precipitation gives India spatial completeness the gauge network can’t, and the cost of admission is that you must understand and correct its biases. Do that, and a great many “ungauged” problems stop being ungauged.