Monsoon rainfall and its variability in Godavari river basin G NAGESWARA RAO Atmospheric Science Centre, Department of Meteorology and Oceanography, Andhra University, Visakhapatnam 530 003, India Rainfall variability over a river basin has greater impact on the water resource in that basin. With this in view, the variability of the monsoon rainfall over the Godavari river basin has been studied on different time scales. As expected, the monsoon rainfall in Godavari basin is more variable (17%) than the all-India monsoon rainfall (11%) during the period of study (1951-90). Similarly, inter-annual variability of the monsoon rainfall on smaller time scales is found to be still higher and increases while going on from seasonal to daily scales. An interesting observation is that the intra-seasonal variability of the monsoon rainfall has a significant negative relationship (CC= -0.53) with the total seasonal rainfall in the basin. 1. Introduction the Satmala hills. These hill ranges play an important role in the distribution of the seasonal rainfall in the The water resource in almost all the major rivers in basin (figure 2). The seasonal rainfall is very high over India is largely affected by the vagaries of monsoon, the hilly regions of the extreme west and in the north resulting in floods during some periods and droughts and east. Immediately after crossing the Western during some other periods. There have been a large Ghats, the rainfall decreases rapidly and then starts number of studies on the Indian monsoon rainfall increasing gradually towards the east. The north- variability (Shukla 1987; Mooley and Shukla 1987; eastern parts of the basin also receives heavy rainfall Hastenrath 1991 for reviews). All these studies are due to the passage of monsoon disturbances from the based on the seasonal rainfall data from the whole Bay of Bengal in a northwesterly direction across and country or from the meteorological sub-divisions to the north of the basin. and some are based on the rainfall data from individual stations only. However, study of the same over a river basin, which has greater impact on its 2. Data water resource and on smaller time scales, is even more important. In the present paper, the author Monthly (June to September) rainfall for 40 years studied the variability of the monsoon rainfall in the (1951-90) and daily (1st June to 30th September) Godavari river basin on seasonal and monthly scales rainfall for 22 years (1963-84) of about 4 to 10 during 1951-90 and on decadal (10-days) and daily stations in each of the 30 districts of the basin has scales during 1963-84. been collected from the India Meteorological Depart- The Godavari basin with a catchment area of about ment. The areal average rainfall of the entire basin is 3,12,800sq.km., lies in central India to the south of evaluated from the area-weighted average rainfall of the mean monsoon trough and receives about 85% of these 30 districts. In a similar manner, Mooley and its annual rainfall during the monsoon season. The Parthasarathy (1984) evaluated the monsoon rainfall Godavari basin is occupied over 30 districts of 5 states over the plains of India during 1871-1978 by using in central India (figure 1). The catchment area of the one station in each of the 306 districts in India. The basin is bounded on the west by the Western Ghats, monthly rainfall during 1951-90 and the daily rainfall on the east by the Eastern Ghats and on the north by during 1963-84 of the Godavari basin, evaluated as Keywords. Monsoon rainfall; inter-annual and intra-seasonal variability; water resource management. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 108, No. 4, December 1999, pp. 327-332 O Printed in India 327 328 G Nageswara Rao I I I N ~ 23 $a 0 21 1'~ 12";, . ~ ,/~ :J.-. "0 o lo ", " 20 ".3 . :'_:\: .;5 Ol 6 ., "L..'~. ( nl 16" "19" " J.,~ ]'" .... ( {/'" 26 )'%1.. e~, ~; .--,,~,o~.r -J "~ ","17 r' 19" "_,~\ _ o 18 18 17" 74~ 75~ 76" 77" 78~ 79 ~ 80 ~ 81 ~ 82" 83" E Longitude Figure 1. Location map of the Godavari river basin with raingauge stations considered and the districts boundaries. Major hill ranges around the basin are also shown: (1) Nasik; (2) Ahmednagar; (3) Aurangabad; (4) Bhir; (5) Buldana; (6) Parbhani; (7) Osmanabad; (8) Nanded; (9) Akola; (10) Yeotmal; (11) Amraoti; (12) Wardha; (13) Nagpur; (14) Bhandara; (15) Chandrapur; (16) Adilabad; (17) Nizamabad; (18) Medal(; (19) Karimnagar; (20) Warangal; (21) Khammam; (22) West Godavari; (23) East Godavari; (24) Visakhapatnam; (25) Koraput; (26) Bastar; (2T) Balanagar; (28) Seoni; (29) Chindwara; (30) Bidar. : I I I I I ! I" I I ! been evaluated on seasonal, monthly, decadal and 23"~- daily scales. These statistics are also useful for the Water Resource Management in the basin. 21~ 3. Results 3.1 Inter-Annual Variability (IA 1/) la" Statistics regarding the IAV of the seasonal rainfall are given in table 1. The mean seasonal rainfall in Godavari basin during 1951-90 is evaluated to be 17" 92.3 cm, which has its lowest value (64.3 cm) in 1974 J I I I I I I II I I and the highest value (122.1 cm) in 1959. The seasonal 7r 7r 76" ~7" 7r 79~ ~d 8~' 82" ~'c rainfall in Godavari has a variability of 17%, while the Longitude variability of the all-India monsoon rainfall during the Figure 2. Distribution of mean monsoon rainfall (in cm) over same period is found to be only 11%. Thus, the the Godavari basin during 1901-50. monsoon rainfall in Godavari basin is more variable than the all-India monsoon rainfall, as expected. above have been used to study the monsoon rainfall The normalized anomalies of the seasonal rainfall in variability on different time scales. Godavari during 1951-90 are shown in figure 3. They To study the inter-annual variability, rainfall varied from -1.8 in 1974 to +1.9 in 1959. By statistics like the mean, standard deviation, coefficient considering the years with the rainfall anomaly less of variation (CV), the highest and the lowest values than -1.0 as deficient rainfall years and the years and their percentage departures from the mean have with the anomaly more than +1.0 as the excess Monsoon rainfall and its variability 329 Table 1. Inter-Annual Variability (IA V) of seasonal rainfall is maximum in September and June and minimum in (cm.) in the Godavari basin during 1951-90. the high rainfall months of July and August. June and Mean 92.3 September are the months of onset and withdrawal of Standard deviation (SD) 15.7 the southwest monsoon over the basin. The high Coefficient of variation (CV) 17% variability of rainfall in these two months could be Highest (% of mean) 122.1 (132%) Lowest (% of mean) 64.3 (70%) due to the large year to year variations in the dates of onset and withdrawal of the monsoon. The rainfall statistics on decadal scale, during rainfall years, there are 8 deficient years (1952, 65, 68, 1963-84 are given in table 3. The decadal rainfall 71, 72, 74, 84 and 87) and 7 excess years (1955, 59, 70, increases from a minimum of 2.5 cm in the 1st decade 83, 88, 89 and 90) during 1951-90. The composite (lst-10th June) to a maximum of 10.5 cm in the 6th means of these 8 deficient and 7 excess years are found (21st-30th July) and 7th (31st July-9th August) to be 114.4 cm and 69.9 cm, which are respectively decades. It then decreases to a minimum of 5.3 cm in 124~c and 76~ of the mean. The rainfall departures in the last decade (19th-28th September) of the season. deficient and excess years' thus, varied from -24% to The variability of the decadal rainfall ranges from 42% +24%. in the 3rd decade to 104% in the 1st decade, both of The percentage departures of the composite rainfall which occurred in June. The variability of the mon- in deficient and excess years are also evaluated for soon rainfall on a decadal scale is thus very much each of the 30 districts in the basin and the isolines of higher than that on a seasonal or monthly scale. While these departures are drawn in figure 4. On district- the increase in the rainfall and its low variability in wise, these departures in deficient years varied from the 3rd decade indicates the regular arrival of the -36% (in Sanded) to -11% (in Sasik) and in excess monsoon over the basin in this decade, the very high years they varied from +2% (Koraput) to +50% (in variability in the 1st decade could be due to the too Medak and Bidar). In both the deficient and excess early onset of the monsoon in this decade in some years, the rainfall departures are the maximum in the years. In general, the variability is low (42% to 56%) plain areas of the central and western parts of the during the 3rd-9th decades (21st June-29th August), basin. In these areas (hatched), the rainfall departures when the rainfall is relatively high (8.1 to 10.5 cm). are more than those for the entire basin. In the The mean and the CV of the daily rainfall from 1st remaining hilly regions of the extreme west, north and June to 30th September, evaluated over the 22 years east, where the mean rainfall is relatively high, these (1963-84) are presented in figure 5. The mean daily departures are less.