This rarely gets reported in media, but IMD (India Meteorological Department) also provides river basin wise rainfall figures. Here is an overview of the river basin wise rainfall during SW Monsoon 2020 (June-Sept 2020, though the monsoon withdrew much later), like the way we have been doing for the last three years[i]. Our earlier monsoon 2020 articles provided district wise figures for rainfall in June 2020[ii], June-July 2020[iii], June-Aug 2020[iv] and June Sept 2020[v].
Arguably, the most appropriate way to look at the rainfall figures is to report it river basin wise, as river basins are the hydrological units and the run off from the rainfall ends up in the rivers, and creates floods many times, as happened during 2020 monsoon at many places. There are of course issues of quality of the river basin wise rainfall figures, and also about rigour or seriousness with which even IMD reports these figures, some key issues we have been highlighting remains unresolved as we see below.
River Basin Wise Rainfall Figures These figures, in fact are more useful in understanding the hydrological implications of rainfall. For anyone working on rivers, water and floods related issues, these should be much more useful figures. The only source of the river basin wise rainfall figure is the map that IMD provides, there are several gaps here as we can see.
In Tables below we are reporting the sub basin wise rainfall figures for 2020 monsoon for each basin, along with normal rainfall and departure from normal. We are also providing some key interesting highlights.
Ganga Basin IMD map divides Ganga basin into three sections (Upper Ganga Valley, Yamuna Valley and Lower Ganga Valley) and 15 sub basins.
Six (Two in 2019) of the 15 sub basins here are categorised to have deficit rainfall: Upper, middle & lower Yamuna; Upper Ganga, Ganga and Ramganga. Two (one in 2019) sub basins are categorised as Surplus: Kosi and Bhagirathi. However, it was Gandak including Burhi Gandak basin that suffered more floods. Bagmati basin also suffered floods. The lowest rainfall was again, like in 2019, in Middle Yamuna sub basin (420.4 mm) and the highest (1541.1 mm) in Bhagirathi basin again like in 2019, in W Bengal. Kosi (1486.5) was the second highest rainfall basin. Sone, Gandak and Ghagra also had over 1000 mm rainfall.
Indus Basin IMD divides Indus basin into eight sub basins, but for three of the sub basins, namely Beas, Jhelum and Upper Indus, it reports no rainfall figures as is the usual practice. Same situation has prevailed for several years. Moreover, there is no mention of BEAS river basin in the map.
Among the five sub basins for which IMD has reported rainfall figure, three sub basins were in red (Ravi, with deficit of 43%, Chenab with deficit of 40% & Upper Sutlej with deficit of 24%) and none in surplus category. Lowest rainfall in absolute terms, at 387.6 mm was reported in Ghaghar basin, this basin had the lowest rainfall in last two years too. Upper Sutlej basin had the highest rainfall, like in 2019, at 437.4 mm, it had surplus rainfall in last three years.
North East India IMD divides North East India into six sub. In all the sub basins except in Barak and Naoch et al, the rainfall in SW Monsoon 2020 has been higher than that last year.
Kynichiang is the sub basin that has reported the HIGHEST rainfall of all sub basins in SW Monsoon 2020, at huge 11034.7 mm
Godavari Basin IMD map divides this basin into eight sub basins with normal rainfall varying from the lowest of 609.9 mm in Upper Godavari basin (Rain shadow area) to over double that amount in Indravati sub basin, east facing Eastern Ghats area. Indravati basin also had the highest rainfall in SW Monsoon 2020 among all the eight sub basins here, as was the case in 2019.
As an be seen above, in five of the sub basins, there was in Surplus or Large Surplus (Upper Godvari) category this year, highest surplus of 76% in Upper Godavari sub basin again like in 2019. Only two sub basins had lower rainfall this year compared to last year: Wainganga and Indravati sub basins.
Krishna Basin IMD divides the basin into six sub basins, for the Upper Tungabhadra sub basin, the IMD river basin wise rainfall map provides no information. Lower Tungabhadra sub basin, at 342 mm, has the lowest normal rainfall not only of the Krishna basin, but also one of the lowest among all the sub basins of entire India.
In 2020 monsoon, all sub basins here had higher rainfall than 2018 and 2019 monsoons, except Upper Bhima and Upper Krishna. The Lower Tungabhadra basin had the lowest rainfall of 595.3 mm. Upper Krishna basin had the highest rainfall (1070.2 mm). Lower Tungabhadra had the highest surplus (74%) among all the sub basins here. Lower Bhima basin had the highest increase in rainfall compared to last year, from deficit 16% last year to surplus 42% this year.
West Flowing Rivers There are six rivers here, as given below, four of them are individual rivers (Sabarmati, Mahi, Narmada and Tapi) and two are composite basins (Luni-Saraswati-Bhadar and west flowing rivers south of Tapi) comprising of large number of basins. In fact, to club all the west flowing rivers south of Tapi into one sub basin is clearly inappropriate and thoughtless. These rivers must be divided into at least ten sub basins, including one for rivers of Gujarat, Maharashtra, Goa, Karnataka and Kerala. Among rivers of Kerala, Periyar, Pamba and Chalakudy needs to be listed as separate sub basins. Similarly, the Kali and Nethravathi sub basin of Karnataka needs to be listed as separate sub basins.
It’s amazing to see the unique situation where ALL the six sub basins here are again having above normal rainfall, but unlike last year, only two are in Surplus/ Large Surplus category, where as last year all were in Surplus category. The highest surplus (83%), from Luni-Saraswati-Bhadar sub basin of Kutch-Saurashtra and the lowest surplus (13%) is in Narmada basin. The highest rainfall is again in: W Flowing rivers South of Tapi, while the lowest rainfall is in Sabarmati basin.
East Flowing Rivers The IMD map shows eleven sub basins under this category, as can be seen below. Six are individual basins, while five are composite basins. Here again IMD needs to report individual basin wise rainfall in a number of cases including Brahmani, Baitarni, Palar and Pamba.
One sub basin (Pennar) is in Large Surplus category and three sub basins in fact surplus category: E flowing rivers between Godavari & Krishna, E flowing rivers between Krishna and Pennar and Vaippar-Pamba sub basin, the last one has the lowest normal rainfall of all the sub basins. It also has the lowest actual rainfall among all sub basins of India.
Summary Following table provides summary situation of categories of sub basins for each of the basins as per IMD categorization of Large Deficit (over 59% deficit compared to Long Period Average (LPA)), Deficit (deficit between 19 and 59% of LPA), Normal (19% above LPA to 19% below LPA), Surplus (19 to 59% above LPA) and Large Surplus (over 59% above LPA) and ND (NO DATA).
In Conclusion The reporting of river basin wise rainfall figures is surely useful, but IMD possibly needs to be more serious rigorous about this. One hopes that IMD will break the larger river basins into smaller sub basins, so that the figures are more useful for all kind of hydrological assessments, impact based forecasts and even flood management and preparedness. We also hope media, research and advocacy groups will start using these figures, critique them rather than reporting only state and sub division figures.
Unfortunately, map is the only source of figures for River wise rainfall and some of the figures on the map are not even readable. IMD should possibly provide the figures also in tabular form. It should also provide the area for each of the sub basin that it has taken into calculation so that if someone wants to calculate the rainfall for say Ganga Basin as a whole, it can be done. Currently it is not possible and IMD also does not provide these figures for larger basins. The area of each district included in the respective sub basins should also be provided by IMD.
Current reporting by IMD of these figures are clearly rather callous with no improvement in last five years.