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2 2021

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Complexity analyses of Godavari and Krishna river streamflow using the concept of entropy

Singh Rahul Kumar, Jain Manoj Kumar

Acta Geophysica

2021

Vol. 69, no. 6

2325--2338

The hydrological regime in both the Godavari and Krishna River has been altered due to both human-induced and environmental changes. The present study utilizes the sample entropy and its more generalized approach known as multiscale entropy to investigate the temporal and spatial distribution of complexity and quantify them using SampEn values. Daily streamflow for five stations, three from Godavari River (Dhalegaon, Nowrangpur, and Polavaram), and two from Krishna River (Yadgir and K. Agraharam), was analysed for the complexity analyses. Trends in the streamflow for the selected gauging stations and their annual entropy values have also been evaluated using the Mann–Kendall test. The trend results revealed that three (Dhalegaon and Nowrangpur in Godavari basin and Yadgir in Krishna basin) out of five stations showed significant decreasing trends for both monthly and annual streamflow series. The declining trend in streamflow could be attributed to both anthropogenic (reservoir operation, increased water abstraction, etc.) and climatic (change in monsoon rainfall, temperature, etc.) factors. The most significant reduction in annual streamflow during the post-impact period was observed at Dhalegaon station in Godavari Basin (from 53,573 to 19,555 m3/s) signifying maximum alteration in annual flow regime. The entropy analysis results of streamflow showed that there was obvious spatial and temporal variation in the complexity, as indicated by the annual SampEn values. Although not profound, a negative correlation exists between the annual runoff and SampEn values (highest −0.42 at K. Agraharam) and hence a reverse correspondence exists between them. In MSE analysis, the original streamflow series increased with time scale (up to 30 days was chosen for this study), whereas entropy decreased with an increased time scale. Due to the fully operational state of the dams upstream of the gauging stations, the entropy values during the post-impact period were less the pre-impact period. The present study can be used as a scientific reference to use information science to detect hydrologic alterations in the river basins. Future studies should focus on considering both climatic and land-use changes in conjunction with the human-induced changes for more comprehensive river system disorder analysis.

A simplifed approach for the evaluation of groundwater fow in stream–aquifer interaction

Saxena Mamta, Singh Rahul Kumar

Acta Geophysica

2021

Vol. 69, no. 1

231--241

Stream–aquifer interaction process plays an important role in modulating food wave propagation in a channel. The most elementary understanding of stream–aquifer interaction can be interpreted by the fux direction between a surface water body and the underlying aquifer. At the time of foods, stream stage rises, and the water gets infltrated into the aquifer, and this process gets reversed at the time when the stream stage gets declined. Therefore, an integrated mechanism between the surface and subsurface fows is particularly important for models, where the response of the system is based on simultaneous interactions between these two major fow domains. In this study, numerical simulation of a food wave has been demon strated considering stream–aquifer interaction. The calibration has been executed on a hypothetical food event accessed by routing a known stage hydrograph for a channel reach having a rectangular cross section which fully penetrates the adjoining aquifer given by Zitta and Wiggert (Water Resour Res 7:1341–1345, 1971). A simplifed mathematical approach, based on Darcy’s law, has been presented here for the solution of groundwater fow equations. The results obtained from the adopted procedure are also compared with the solution proposed by Zitta and Wiggert in 1971. The NSE and RMSE (m3 /s) estimate assessed for the simulated hydrographs using the proposed methodology with respect to the procedure adopted by Zitta and Wiggert (Water Resour Res 7:1341–1345, 1971) is 0.9983 and 0.8544, respectively. Therefore, the use of Simpson’s (3/8)- rule is not suggestible due to its complicated calculation and its sensitivity, and it is better to use the proposed simplifed approach for the evaluation of lateral fow.