Results and Discussion

Hydrological integrity of a river basin ensures the maximum amount of water available naturally as stream flow, soil moisture etc., to meet ecological and social (domestic, irrigation and livestock) demands in a river basin. Monthly monitoring of hydrological parameters reveal that stream in the catchments with good forest (evergreen to semi-evergreen and moist deciduous forests) cover have reduced runoff as compared to catchments with poor forest covers. Runoff and thus erosion from plantation forests was higher from that of natural forests. Forested catchment have higher rates of infiltration as soil are more permeable due to enhanced microbial activities with higher amounts of organic matter in the forest floor (Ramachandra et al. 2013a). Streams with good native forest cover in the catchment showed good amount of dry season flow for all 12 months. While strems in the catchment dominated by agricultural and monoculture plantations (of Eucalyptus sp. and Acacia auriculiformis) are sesonal with water availability ranging between 4-6 months. This highlights the impacts of land use changes in tropical forests on dry season flows as the infiltration properties of the forest are critical on the available water partitioned between runoff and recharge (leading to increased dry season flows). This emphises the need for integrated watershed conservation approaches to ensure the sustained water yield in the streams.

Land use change:

Sharavathi river basin in the Western Ghats has been a subject of research and debates due to its ecologically rich environment (Ramachandra et al. 2013b; Ramachandra et al. 2014b). The 1940 SOI toposheet show that the entire river basin was covered with forests. Table 3 gives the percentage change analysis of forest cover (reserved forests) based on 1940 toposheets (48K/13, 48J/16 and 48O/1), and 1989 (Landsat TM) and 1999 (LISS III) imageries. This shows that more than 50% of the forests have decreased/removed by human activities and submergence due to damming the Sharavathi River.

Classified remote sensing data corresponding to the upstream of Sharavathi river basin (upstream) for 1989 and 1999 are given in Figure 10 and 11 respectively. Table 4 gives the error matrix generated for Landsat TM 1989 classified data, which indicates an overall accuracy of classification as 96.4%. Similarly, Table 5 lists the confusion matrix (error matrix) for IRS LISS III data with an accuracy of 97.6%. Low accuracy for settlements from both the imageries is partly due to spectral signature overlap with vegetation as most of the houses are surrounded by vegetation inside and outside the compound. Townships are few and houses are sparsely distributed separated by hundreds of meters. Since the building material is also different as almost all houses have tiled roof than concrete, overlap also occurred between open fields. Overlap of spectral signatures was also observed between evergreen/semi-evergreen forest and moist deciduous forests in full leaf.

Table 3: Changes in Reserve Forests Area (1940-1999)

Table 4: Error Matrix (Landsat TM, 1989)

Note: Evg/SE -  evergreen/semievergreen forests; MD – moist deciduous forests; Plant – plantations; Grass – grasslands and scrubs; Agri – agricultural lands; Open – open fields; Sett- settlements

Table 5: Error Matrix (IRS LISS III, 1999)

Note: Evg/SE -  evergreen/semievergreen forests; MD – moist deciduous forests; Plant – plantations; Grass – grasslands and scrubs; Agri – agricultural lands; Open – open fields; Sett- settlements

Figure 10: Land use Classification (Landsat TM, 1989)

Figure 11: Land use Classification (IRS LISS III, 1999)

Table 6 compares the percentage change in the land use in Sharavathi upstream river basin based on 1989 and 1999 data. It is seen that natural forests such as evergreen/semi-evergreen and moist deciduous forests have decreased by 28.2% whereas monoculture plantations (due to afforestation work of the forest department) have increased by 17.3%. Grasslands and scrubs have decreased by 28.4% in 1999 but this can be attributed to the season as in summer, grasses dry out leaving only the scrubs. The main anthropogenic activity apart from plantation is paddy cultivation, which has increased by 5% in 1999. Paddy cultivation is the most common agricultural activity in the basin and is usually grown in valleys.
Human population is increased in the basin and there has been immigration of people from adjoining districts of Karnataka and also from neighbouring States. Although the dam has provided electricity, water and other benefits to the surrounding areas, it has proved to be at the cost of the valuable ecosystem. Evergreen forests are being cleared to yield timber, which are used for electric transmission poles and railway sleepers. The felled areas are sometimes tended for getting the natural regeneration of valuable species. Deciduous forests supply timber, firewood, charcoal, bamboo, matchwood and plywood. Monoculture plantations of teak, silver oak (Gravillea robusta), matchwood, Acacia sp., etc. are planted by the forest department in clear felled forest areas. Table 7 give the sub-basin wise area under different land uses.

Table 6: Changes in Land use in Upstream (1989-1999)

Table 7 Land use in the upstream of Sharavati (Sub basinwise in sq.km)

Note: Evg/SE -  evergreen/semievergreen forests; MD – moist deciduous forests; Plant – plantations; Grass – grasslands and scrubs; Agri – agricultural lands; Open – open fields; Sett- settlements

Rainfall analysis
Yearly data for hundred years were available for 7 taluks in and around the river basin viz. Hosanagara, Sagara, Soraba and Tirthahalli (Shimoga district) and Honnavar, Kumta and Siddapur (Uttara Kannada district). Since these are the taluks surrounding the river basin, rainfall analysis is done to study any variation in rainfall for 100 years. The rainfall periods were divided into 1901-1964 and 1964-2001, which represents respectively the periods before construction and after construction of Linganamakki dam. Table 8 gives the mean and standard deviation of rainfall in Shimoga district. Hosanagara, Sagara, Tirthahalli and Siddapur taluks showed reduction in the mean annual rainfall with a significant reduction in Tirthahalli taluk. Sagara and Hosanagara were selected for further studies as these districts cover the study area.
The sub-basins were further classified into clusters viz. west, east, south and central depending on their geographical locations. Rainfall data were available for 18 rain gauge stations (within the upstream) from 1989-1999. The mean rainfall (1989-1999) was determined for each sub basin in order to observe the rainfall variation and distribution within the river basin. They were classified as follows:

Figure 12 illustrates the mean annual precipitation variability in western and eastern sub-basins.
Regression analysis was carried out for each rain gauge station considering rainfall as dependent variable and latitude, longitude, altitude and land cover as independent variables. The area of influence of each rain gauge station in the upstream was delineated with respect to contours and drainage and the land cover expressed as NDVI was determined using the imagery for each area around the gauge. Regression analysis showed rainfall having significant relationship with variables such as land cover, latitude, longitude, and altitude. At 5% level of significance rainfall showed good relationship between land cover, latitude, altitude and longitude. The probable relationships are given in Table 9. From the regression relationship, it is evident that the rainfall increases with land cover (NDVI) and decrease with latitude, longitude and altitude. Sensitivity analysis show the relationship holds good for all sub basins in the upstream region of the river basin.

Table 8: Variation of Rainfall in Shimoga

Table 9: Probable Relationships of Rainfall

Figure 12:  Annual rainfall pattern in last two decades in the sub-basins of Sharavathi river upper catchment.

Interception