Hydroclimatic Variability across Tapi Basin, India: Issues and Implications

Prem Lai Patel and Priyank J. Sharma

Sardar Vallabhbhai National Institute of Technology (SVNIT)

Introduction

The changing climate has impacted many dimensions of human life. The impacts of climate change on water resources systems have shown regional variations and have become a thrust area of research for hydro-climatologists. Climate change has manifested modifications in the global and regional hydrologic cycles, which have led to erraticism in the spatio-temporal rainfall distribution (Groisman et al., 2005). A paradigm shift can be observed with an escalation in the frequency of weather-related hazards, such as floods, droughts, heatwaves, cyclones, bush fires, etc., in the past two decades (Hoeppe, 2016). Hence, studies on the river basin scale would enable us to understand the specific impacts of climate change on water resources of the basin for water and food sustainability (Panda et al., 2013; Sharma et al., 2019b). The present chapter deals with the analysis of hydroclimatic variability across the Tapi basin in India. Further, the prominent changes in hydrological and morphological characteristics of the Tapi basin have also been highlighted.

Study Area

Climatic Characteristics of the Tapi Basin

The Tapi River, originating from Multai in the Betul district of Madhya Pradesh, drains a total area of 65,145 km2. The Tapi basin is further demarcated into three basins, see Figure 5.1. The Upper Tapi basin (UTB) extends from Multai (origin of the Tapi River) to the Hathnur Dam, encompassing an area of 29,430 km2, whereas the Middle Tapi basin (MTB) is demarcated by the area drained between the Hathnur Dam and the Ukai Dam (i.e., 32,925 km2), and the Lower Tapi basin (LTB) outspreads from the Ukai Dam to the Arabian Sea, draining an area of 2795 km2. As per the Koppen-Geiger climate classification, the Tapi basin exhibits three climatic types, viz., tropical wet savanna climate (Aw), tropical dry savanna (As) and hot semi-arid (BSh) climate types, which encompass around 15.2%, 47.1% and 37.7% of the basin area, respectively (Rubel & Kottek, 2010). The monthly average air temperature in the basin is found to vary in the range of 20.7°C-33.0°C. The Tapi basin receives an average annual rainfall of around 830 mm, about 90% of which is recorded from June to September months of the southwest monsoon (Sharma et al., 2018a). The southwest monsoon passing over the Arabian Sea and Bay of Bengal brings rainfall to the basin.

Index map of the Tapi basin

Figure 5.1 Index map of the Tapi basin.

The rainfall variability across the basin is governed by the geographical location, its east-west orientation and the relief pattern. The LTB receives higher rainfall (around 1000-1800 mm) due to its proximity to the Arabian Sea, followed by the narrow stretch between Gwaligarh hills and Satpura ranges in the northern part of the UTB (around 1000-1500 mm). The southern part of the UTB and the central part of the MTB receive a moderate rainfall of around 600-800 mm, while the south-west part of the MTB, falling in the leeward side of the Western Ghats, receives a meager rainfall of less than 600 mm.

Physiographic characteristics of the Tapi basin

The Tapi basin is a major river basin of the Indian Peninsular region and is bounded by mountain ranges on three sides and a large water body on one side. The Tapi basin is bounded by hill ranges from three sides, namely the Satpura ranges, Mahadeo hills and Ajanta-Satmala ranges in the north, east and south directions, respectively, while the Western Ghats lie in the south-west side. The narrow western part of the Tapi basin is adjacent to a large water mass, viz., the Arabian Sea (see Figure 5.1). Thus, a wide variation in physiography is observed across the basin, which varies from 1563 m in the Western Ghats to around 0 m (above the mean sea level) near Surat City. The major land use land cover in the Tapi basin, assessed during the year 2005-2006, comprises agricultural land (66.19%) followed by forest cover (24.41%), wasteland (5.16%), water- bodies (2.99%) and built-up land (1.26%) (CWC, 2014).

Key water resources projects in the Tapi basin

The Tapi River has fourteen major tributaries of length greater than 50 km, out of which only four tributaries are gauged, while the remaining are ungauged. Out of these, four right bank tributaries, namely, Aner, Arunavati, Gomai and Vaki, originate from the Satpura ranges, draining small areas and flow in the south-west direction to join the Tapi River. Ten other left bank tributaries, namely, Amravati, Bori, Buray, Girna, Mona, Nesu, Panjhara, Purna, Sipna and Waghur, originate from the Gwaligarh hills, Ajanta hills, Western Ghats and Satmalas. The important water resources projects in the Tapi basin are the Hathnur and Ukai dams across the Tapi River, while the Girna Dam is located across the Girna River (see Figure 5.1). Other than the above, the Prakasha, Sarangkheda and Sulwada barrages are constructed across the Tapi River, while the Dahigaon weir is located across the Girna River in the MTB. The storage of these barrages is used for the withdrawal of water for lift irrigation schemes. The Kakrapar weir is built across the Tapi River, downstream of the Ukai Dam. The Kakrapar right and left bank main canals take off from the Kakrapar weir, whereas the Ukai left bank main canal takes off directly from the Ukai Dam. The Ukai-Kakrapar irrigation project irrigates a command area of 331,557 hectares (Sharma et al., 2016). The salient features of Ukai, Hathnur, and Girna dams are summarized in Table 5.1. The Hathnur and Girna reservoirs serve conservation demands, whereas the Ukai reservoir is a multi-purpose reservoir catering to conservation demands, hydropower generation and partial flood control.

Table 5.1 Salient features of the key water resources projects in the Tapi basin

Sr. no.

Description

Ukai Dam

Hathnur Dam

Girna Dam

1.

Catchment area (km2)

62,225

29,430

4729

2.

Year of construction

1972

1982

1969

3.

Gross Storage Capacity (I06 m3)

7414.29

388.00

608.45

4.

Live Storage Capacity (I06 m3^

6730.00

255.00

523.55

5.

Dead Storage Capacity (I06 m3)

684.39

133.00

84.90

6.

Maximum Water Level [MWL] (m)

106.99

215.50

-

7.

Full Reservoir Level [FRL] (m)

105.15

214.00

398.07

8.

Spillway Crest Level [SCL] (m)

91.14

207.50

389.84

9.

Dead Storage Level [DSL] (m)

82.29

200.00

381.81

10.

Culturable Command Area (ha)

3,31,557

47,350

79,293

II.

Rule levels (m)

a.

1st July

97.84

-

388.57

b.

15th July

99.54

-

393.65

C.

1st August

101.50

-

397.10

d.

15th August

101.77

209.50 (21st Aug)

397.15

e.

1st September

102.10

-

397.46

f.

15th September

103.63

212.00

397.74

g-

1st October

105.15

213.50

398.07

h.

15th October

-

214.00

398.07

Materials and Methods

Data Sources

The daily station rainfall of 54 rain gauge stations in the Tapi basin for the period 1973-2013 was collected from India Meteorological Department (IMD), Pune. Out of these rain gauge stations, 24,25, and 5 stations are located in the UTB, MTB and LTB, respectively. The rainfall data have been tested for homogeneity and consistency. The missing rainfall records were found to be less than 5% in the entire data set. The missing rainfall records, wherever detected, were filled using the inverse distance weighting method (Chen & Liu, 2012). The daily streamflows for 12 stream gauging stations for the period 1973-2013 were collected from Tapi Division, Central Water Commission (CWC), Surat. Also, the daily inflows into the Hathnur reservoir (1987-2013) and monthly flows into the Girna reservoir (1973-2013) were collected from Tapi Irrigation Development Corporation (TIDC), Jalgaon, Government of Maharashtra. The daily reservoir levels, storage volume and inflows into the Ukai reservoir for the concurrent period were collected from Ukai Civil Circle, Ukai, Government of Gujarat.

Hydroclimatic Indices

The rainfall and streamflow data are further analyzed to derive the time series of hydroclimatic indices mentioned in Table 5.2. The analysis of gradual trends in hydroclimatic indices is further undertaken using a non-parametric Modified Mann-Kendall (MMK) test (Hamed & Rao, 1998), and their results are discussed in subsequent sections.

Table 5.2 Definition of hydroclimatic indices used in the present study

Indicator

Indicator definition

Mathematical formulation

Units

PRCPTOT (annual total rainfall)

Annual total rainfall from days with rainfall > 2.5 mm

mm

RD (rainy days)

Number of days when rainfall > 2.5 mm

days

Rxlday (maximum l-day rainfall)

Annual maximum 1 -day rainfall

mm

QTOT (annual total runoff)

Annual total runoff volume

m3

Qxl day (maximum l-day flow)

Annual maximum 1 -day streamflow

m3/s

Here, P, and Q, are the daily rainfall (in mm) and streamflow values (in m3/s), respectively, in a particular year; D, is the rainy day and n is the total number of days in a year. Also, dt refers to the number of seconds in a year.

Spatio-Temporal Variations of Trends in Rainfall Indices

Upper Tapi Basin

The UTB is further delineated into two sub-catchments, viz., Burhanpur (area» 10,940 km2) and Purna (area» 18,490 km2) sub-catchments. The Burhanpur sub-catchment exhibits a sub-humid climate, whereas the Purna sub-catchment exhibits a semi-arid climate (Sharma et al., 2018b). There are 24 rain gauge stations in the UTB. A wide variation in the total annual rainfall (PRCPTOT) is observed for both sub-catchments, see Figure 5.2a. The maximum (minimum) value of PRCPTOT observed across the UTB is

1470.6 mm (637.6 mm) at the Chikhalda (Edalabad) station. The climatology and trends in PRCPTOT, rainy days (RD) and Rxlday across the UTB are shown in Figure 5.2. Heterogeneity in the trends of PRCPTOT is seen across the UTB, wherein at 50% of stations, an increasing trend is obvious, while across another 50% of stations, a decreasing trend is observed, see Figure 5.2a. The maximum (minimum) number of rainy days observed across the UTB is 66.6 days (37.2 days) at the Chikhalda (Edalabad) station. On the other hand, the trends in RD also show similar heterogeneity, as for PRCPTOT, across the UTB, see Figure 5.2b. Further, the stations experiencing higher rainy days, viz., Bhainsdehi, Chikhalda and Dharni, exhibit a decreasing trend in RD. On the other hand, the stations experiencing lower rainy days, viz., Burhanpur and Edalabad, depict significant increasing trends in RD. The maximum (minimum) value of Rxlday across the UTB is found to be 135.3 mm (80.9 mm) at the Chikhalda (Nandura) station. The analysis of Rxlday shows increasing (decreasing) trends at 11 (13) stations across the UTB, out of which 2 stations, namely, Bhainsdehi and Anjangaon, exhibit significant increasing trends at the 5% significance level, see Figure 5.2c. Out of the major urban centers in the UTB, only Buldana shows an increasing trend in Rxlday, whereas Akola, Amravati and Burhanpur show a decreasing trend in Rxlday. Due to sub-humid climatic characteristics, the magnitudes of PRCPTOT, RD and Rxlday are higher for the Burhanpur sub-catchment vis-a-vis the Purna sub-catchment. Overall, it can be seen that a spatially heterogeneous nature of trends in rainfall indices is observed across the UTB, wherein, specifically, the eastern part of the Purna sub-catchment has a dominant increasing trend in PRCPTOT, RD and Rxlday, whereas the western part of the Purna sub-catchment displays predominantly decreasing trends.

Middle Tapi Basin

The Girna sub-catchment (area» 10,061 km2) is a major hydrological unit in the MTB, covering almost one-third of the area of the MTB. The MTB is mostly characterized by a semi-arid climate with high temperature and comparatively lower rainfall. The temperature during the summer occasionally surpasses even 45°C and intense heatwave conditions are witnessed in the MTB. However, the northwestern portion of the MTB, predominantly forest area, experiences higher rainfall. There are a total of 25 rain gauge stations across the MTB.

The maximum (minimum) value of PRCPTOT in the MTB is observed to be 1111.6 mm (540.6 mm) at the Navapur (Sakri) station. The PRCPTOT exhibits increasing trends at 56% of stations while decreasing trends are observed at the remaining 44% of stations, see Figure 5.3a. The increasing trends are mainly concentrated in the central

and western portion of the basin. The regions of low rainfall have shown a rising tendency in PRCPTOT, which may augment water availability in the region in the near future. The maximum (minimum) value of RD in the MTB is estimated to be 51.6 days (32.0 days) at the Navapur (Sakri) station. From Figure 5.3b, it is seen that increasing (decreasing) trends in RD are observed at 72% (20%) of stations, while the remaining 8% of stations exhibit no trend in RD, see Figure 5.3b. Also, the Parola and Bhusaval stations exhibit a significant increasing trend in RD. Thus, a coherent increase in the number of rainy days is observed across the MTB, which would be beneficial for the Kharif crops through consistent moisture supply during their active growth phases. The maximum (minimum) value of Rxlday for the MTB is reported to be 129.2 mm (69.5 mm) at the Navapur (Sakri) station. The trend analysis of Rxlday reveals an increasing trend at 11 (44%) stations, a decreasing trend at 13 (52%) stations and no trend at one station (Shahada), see Figure 5.3c. Moreover, three stations, viz., Amal- ner, Pachora and Kalvan, have shown significant decreasing trends in Rxlday. Hence, a distinct regional divide in trend is evident, wherein the southeastern part of the MTB exhibit decreasing trends, while the northwestern part of the MTB shows an increasing trend in Rxlday.

Lower Tapi Basin

Surat City in the LTB is located on coastal-urban floodplains of the Tapi River. Surat City is a highly urbanized and industrialized region possessing a relatively flat topography compared to the UTB and MTB. The LTB experiences Aw’ climate type, characterized by higher rainfall and lower diurnal temperature fluctuations, due to its proximity to the Arabian Sea. There are five rain gauge stations in the LTB. The maximum (minimum) value of PRCPTOT in the LTB is found to be 1795.4 mm (953.3 mm) at the Songadh (Olpad) station. The PRCPTOT exhibits increasing trends at four stations, while a significantly increasing trend is observed at the Olpad station (see Figure 5.4a). However, no trend in PRCPTOT was detected for the Songadh station. The maximum (minimum) value of RD in the LTB is recorded to be 59.6 days (41.3 days) at the Songadh (Olpad) station. From Figure 5.4b, an increasing trend in RD is observed at four stations, while a decreasing trend is observed for the Songadh station. The maximum (minimum) value of Rxlday in the LTB is recorded to be 197.9 mm (133.2 mm) at the Songadh (Olpad) station. From Figure 5.4c, increasing trends in Rxlday are observed at three stations, with a significantly increasing trend at the Olpad station. On the other hand, a decreasing trend in Rxlday is observed for the Mangrol and Songadh stations. Thus, extreme rainfall events are increasing over the urbanized areas of the LTB (Surat, Kamrej and Olpad), while decreasing trends are observed over the forest dominated areas (Mangrol and Sogadh). This increase in extreme rainfall in urban areas could potentially escalate the risk of urban floods in Surat City in the LTB.

 
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