Siyambalanduwa Heda Oya water level

Steady: about 1.8 ft in an hour.

Published as 0.56 m (alert 4.5 m, minor flood 6.0 m, major flood 7.0 m)

Rain that fell

0.7 mm in the last 3 hours, 4.4 mm in the last 24 hours at this gauge (light rain)

Rain forecast

19 mm in the next 24 hours over the catchment (moderate rain). Forecast model estimate.

Satellite estimate of rain over the catchment: 0.0 mm in the last 6 hours, 4.3 mm in the last 24 hours (to 12:30 am). In this dry-zone area it often reads much higher than the gauges, so treat it as an upper figure. GSMaP data by Japan Aerospace Exploration Agency (JAXA)

• · Trend only: a rain model was fitted and tested on 8 days but was no more accurate than "level stays put" at any horizon (best: 24 h, 0.27 vs 0.26 error).
• · Forecast model estimate of catchment rain (Open-Meteo, area average over 5 grid cells above this gauge): 1.0 mm modelled for the last 6 hours, 14 mm modelled for the last 24 hours, 19 mm forecast for the next 24 hours. The model's past hours often read lower than the gauges, so they are not what fell; this note is for context.
• · Topmost gauge on its river: no gauge upstream, so only its own trend (plus catchment rain) is available.

• How this is worked out
• · This gauge's own recent changes and the other gauges on the river, fitted to about 11 months of hourly readings (one hour ahead only). In a backtest on the last 60 days it missed by less than 'no change'. This is the topmost gauge on its river, so there is no upstream gauge; the catchment rain above is the only upstream signal.