The CWC lists 116 river-monitoring stations in Bihar in its database, of which 108 were operational last month when the floods occurred. Before analysing how many of these stations reported dangerous water levels in the past month, it is important to look at their location. As the accompanying map shows, some districts have more stations than others, although nearly all districts are covered. This is because the CWS stations are particularly interested in forecasting fluvial floods (floods that occur when rivers and streams overflow) and not all districts have the same density of rivers.

The uneven distribution of the CWC stations means that they can only point to broad trends for places close to a river. However, even these broad trends suggest the threat of floods increasing in Bihar in September. For example, of the 108 stations in the state, only eight reported the river crossing the danger mark in June. This number increased to 23 and 26 in July and August and then 40 in September. The danger mark is a level that the CWC uses to forecast the threat of a “moderate flood”. Therefore, the number of stations reaching/crossing that mark increasing in September suggests that flood threats were more widespread in September.

The trends discussed above suggest that the threat of flood covered a wider area in September. However, it is also useful to check the number of districts where rivers ran above the danger mark in each month. This is because station data alone can also suggest more parts of the same district getting flooded. However, the district-wise breakup also shows flood vulnerability increasing in September. The number of districts where rivers crossed the danger mark was six in June. This number was 16 and 13 in July and August, but increased to 20 in September. Bihar has 38 districts in total, of which 35 had operational CWC stations this year up to September.

Clearly, floods in the state were widespread in the past month. However, this does not mean that they were unusually prolonged everywhere. The duration for which the river flowed above the danger mark (up to noon on October 2) was among the top 10% longest for the station at only five stations: Dheng Bridge in Sitamarhi, Chargharia in Kishanganj, Jainagar in Madhubani, and Birpur in Supaul. To be sure, this metric must be read with caution. Rivers did not necessarily stop flowing above the danger mark on October 2. Similarly, not all stations have seen the same number of monsoon seasons, because they became operational at different points in time. However, the five stations listed here are indeed among the older stations, four having started their operation in either 1970 or 1971 and one in 1956. This means that the duration of the flood threat there was definitely unusual.

The five stations listed above also hint at the proximate cause of the floods in Bihar: water flowing in from Nepal, which witnessed intense rainfall in the past week. All the five stations are located in districts bordering Nepal. However, this does not mean that rain in Bihar had no role to play. The week ending September 30 was the rainiest in Bihar, according to the gridded data of the India Meteorological Department (IMD), although it was the last week of the monsoon season. It was also the only week when the state reported extreme rain or rain of at least 244.5 mm at a place in 24 hours. This means that the state dealt with excess water of its own, apart from what flowed in from Nepal. This explains the floods becoming widespread in the past week.

Abhishek Jha, HT’s assistant editor-data, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.