ENSO modulated groundwater variations in a river basin of Central India

Several previous studies have examined the traceable regional impacts of El Niño Southern Oscillation (ENSO) on groundwater level (GWL) but it remains a question whether the ENSO impacts on groundwater can be established in smaller basins using statistical techniques. The present study attempts to record the ENSO impacts on the groundwater availability in Venna basin, Maharashtra by proposing a combination of statistical and spatial analysis. Utilizing the GWL, the study estimates the spatial variability of GWL, groundwater anomalies, groundwater recharge and discharge using geographic information system (GIS) and quantitative variations in groundwater using statistical techniques. The study also highlights the applicability of the Kolmogorov–Smirnov test in hydrometeorological studies. Analyses reveal the association of deeper GWLs and higher discharge with the El Niño, as opposed to shallower GWLs and higher recharge with La Niña. The two-sample Kolmogorov–Smirnov test confirms the discrepancy in the cumulative distribution of GWL between different ENSO phases. Mann–Kendall, Sen slope and Mann–Whitney tests ascertain the variation of GWL and recharge as well as ENSO impacts in the command area (area irrigated by reservoir using the canal networks) and non-command areas. A significant difference in recharge between El Niño and La Niña is observed in the command as well as in the non-command areas. doi: 10.2166/nh.2018.154 s://iwaponline.com/hr/article-pdf/50/2/793/549251/nh0500793.pdf C. Rishma (corresponding author) Yashwant B. Katpatal Visvesvaraya National Institute of Technology, Nagpur, Maharashtra 440010, India E-mail: rishma61@gmail.com


INTRODUCTION
In recent decades, interannual climate variability such as El Niño Southern Oscillation (ENSO) has become one of the major concerns for agriculturists and water resource managers globally. Imprints of ENSO on regional monsoon, groundwater levels (GWL), streamflow, droughts, flood and statistical techniques' to analyse the impacts of ENSO on groundwater resources. Also, the previous studies examined the influence of ENSO on groundwater by only observing the variations in GWL, which does not completely depict the hydrological impacts due to ENSO. To suffciently relate the hydrological changes to ENSO, the present study analyses other prerequisite impact parameters like deviation of GWL from long-term average (anomaly), change detection (year-to-year variation) of groundwater, net disharge, and net groundwater recharge in the area.
The study also analyses the different impacts of ENSO in the command area (CA) and non-command area (NCA). Therefore, the specific objectives of the present study are as follows: • To analyse the spatial annual variations in the groundwater occurrence using spatial interpolation techniques such as inverse distance weighted (IDW) and to generate groundwater anomaly maps using geographic information system (GIS) techniques to establish the relationship of groundwater occurrence to different ENSO phases.
• To estimate the net groundwater discharge and recharge in the study area during 1996 to 2015, and to understand its harmonisation with ENSO events.
• To observe the difference in GWL of El Niño and La Niña years using a two-sample Kolmogorov-Smirnov test to understand the deviation in the distribution of two GWL series.
• To perform trend analysis of GWL and temporal analysis of groundwater recharge in the CA and NCA using different statistical methods.
• To observe the significant variation in the impacts of ENSO in CA and NCA using the Mann-Whitney test.

Study area
The Venna River basin, a sub-basin of the Wardha catchment under the Godavari basin of Maharashtra, India, has Some of these wells are within the CA while the majority of the wells are present in the NCA. The geographical location of the study area, the spatial location of observation wells and the CA are depicted in Figure 1.    Table 2.

Methodology
The ENSO-GWL relationship has been examined using both GIS and statistical analyses. Spatially interpolated surfaces of depth to GWL data of the month of October analysis. Long-term average GWL data (

RESULTS AND DISCUSSION
The widespread influence of ENSO on regional rainfall is well known and it has diverse impacts over different parts of the world. Most of the ENSO events resulted in above or below normal rainfall in different countries including India. The main source of aquifer recharge is the regional rainfall and hence the fluctuation of rainfall with ENSO events has an impact on the regional groundwater as well.
Hence, to identify the impacts of ENSO on groundwater, several factors such as spatial distribution, anomaly maps, groundwater fluctuation, net groundwater storage and usage, etc., have been analysed separately as discussed below.

Temporal variation of rainfall
Average annual rainfall in the study area for the period 1985-2015 is plotted in Figure 3. A clear relationship between rainfall and ENSO can be seen. The rainfall in El Niño years is less than in normal years and greater than normal in La Niña years.  1998,1999,2007,2010 Weak La Niña 2000, 2011 Groundwater levels Figure 4 shows maps of the depth to groundwater in the study area in October (postmonsoon Kharif) for the years 1996-2015. They were prepared by spatial interpolation of water level data in observation wells using IDW.

Groundwater level anomaly maps
Apart from understanding the year-to-year variation, longterm variation of GWL is also important for climate variability studies. Hence, the groundwater anomaly maps have been generated and related to ENSO events. Figure 6 shows GWL anomaly maps classified into six classes ranging from much below normal (>2.5 m) to much above normal (>À 2.5 m). The groundwater table in the observation wells is 'much below the normal' for El Niño years with a higher decline in GWL of 1.5 to 2.5 m and >2.5 m during strong El Niño years (1997 and 2015).
Similarly, strong and medium La Niña years show a higher rise (much above normal) in GWL. Also, maximum deviation can be observed in the wells in the subwatersheds WRWP-1,WRWBD-3 and WRWBD-2 (Figure 2(a)). Hence, these subwatersheds may be considered as groundwater vulnerable/management zones for the ENSO impacts.

Seasonal fluctuation of groundwater level
The

Comparison of GWL of El Niño and La Niña using
Kolmogorov-Smirnov test Figure 9 shows the variation in the cumulative distribution of depth to groundwater levels during the El Niño and La Niña years generated using the two-sample Kolmogorov-Smirnov test.

The depth to GWL of each well has been averaged for El
Niño and La Niña years separately, and the discrepancies

Variation in impacts of ENSO within CA and NCA
The variation in the impact of ENSO on CA and NCA has been studied separately by conducting the temporal analysis of net groundwater storage. Figure 11 shows the average of net groundwater storage estimated from GWL data of all observation wells.
In Figure 11, it may be observed that the net groundwater storage of CA exceeds the net groundwater storage of NCA which may be due to the reservoir seepage and irrigation water. It was also observed that during ENSO warm events (1997, 2002, 2006, 2009 and 2015), there is less groundwater storage (negative values indicate a net deficit in groundwater storage and overexploitation of groundwater due to low rainfall) in both CA and NCA. The groundwater storage is always more than 3 m and 2.5 m in CA and NCA, respectively, during ENSO cold events (1998, 1999, 2000, 2007, 2010 and 2011). Maximum groundwater storage of 8.23 m and 6.9 m in CA and NCA, respectively, is obtained during 2000, which was a consecutive La Niña year.

Mann-Whitney U test
The Mann-Whitney test at a 5% significance level has been performed to understand the significant difference between the net groundwater storage in CA and NCA in the El Niño and La Niña phases. Table 5 shows the results of the analysis. In command areas, the difference in maximum and minimum groundwater storage between El Niño and La Niña phases is 5.4 m and 4.5 m, respectively. Whereas, in the non-command areas, the difference in maximum and minimum groundwater storage between El Niño and La Niña phases is 6.7 m and 8.2 m, respectively. Similar integrated approaches may be adopted to achieve clarity in analysing the impacts of interannual and interdecadal climate variability on different hydrological and vegetation parameters at regional or global levels.  Minimum -El Niño À2.9 À5.4 Minimum -La Niña 1.6 2.8 P-value 0.016 0.008