Chapter- Three GEOMORPHOLOGY AND RAINFALL PATTERN 3.1 INTRODUCTION Geomorphology and its slope as a basic and applied science in general and as tool for searching groundwater resources in efficient geomorphological land forms plays an important role in the water resources study. Geomorphology is the science of the landforms and their systematic study is important so as to interpret them as signatures of the past and ongoing geological processes. These geomorphic features have great bearing on water resources of any area. Certain agents like rivers, streams, glaciers and winds, etc relentlessly operate on the earth crust to bring about the changes of degradation and aggradation and these features are important from the point of understanding surface and subsurface water movement. Landforms like plateaus, residual hill, pediment, pediplain, piedmont, valleys and other landscape features are critical for evaluation of the water resources of any basin or region, because accumulation and movement of surface and ground water is depending on them. Similarly, rainfall pattern is a strongly influencing factor in the landform evolution and as well in hydro geomorphology. Rainfall pattern is also a key for evaluating the terrain for water replenishment, storage and losses. Obviously critical and intelligent rainfall pattern analysis is vital for predicting droughts, cycles of surplus rainfall and runoff pattern, which are all ultimately necessary for comprehensive planning of any watershed. 28
3.2 GEOMORPHOLOGY OF THE STUDY AREA 3.2.1 Residual hills When the process of erosion takes place gigantic wave of water flows on the mountains and other highland areas this causes boulders, sediments and other solid materials are washed away and when these rocks re-situated in a large size on other parts of land then called residual hills. We can also say that the hard rocks left behind after erosion are then called residual hills. The structure which controlled their formations are joints and fractures. The lithology in residual hill area is dominant with granodiorite, tonalite and migmatitic gneiss. They are the prominent and elevated features in the study area. Such hills are noticed in north-west and adjacent areas of Hanumanapura. They are also noticed around Karya Village.(Plate 5.3.1) 3.2.2 Pediments Gently sloping surface of low relief eroded in bedrock at the base of a much steeper mountain slope. It is commonly covered by a thin, patchy veneer of alluvial sand and gravel that thickens down slope to merge with the alluvial fill of the valley. Pediments are prominent landscape features in arid and semiarid regions with veneer of detritus lithologies like granodiorite, tonalites and migmatitic gneisses constitute pediments. Fractures have played important role in their formation. These pediments are situated at south-east of Hanumanapura and also North West of kembal. (Plate 5.3.2) 3.2.3 Pediment inselbergs Isolated low relief hills surrounded by gently sloping, smooth, erosional bed rocks constitute pediment inselbergs. Bedrocks like granodiorites and migmatitic gneisses are the main lithology controlling the pediment inselbergs and joints fracture, 29
lineaments have controlled their formation. Prominent pediment inselbergs are situated in north east of study area. (Plate 5.3.3) 3.2.4 Pediment Inselberg complex This complex consists of small isolated like standing out prominently in a form because of their resistance to weathering. The pediments dotted with a number of inselbergs which cannot be separated and mapped as individual units are referred to as pediment inselbergs complex having moderate to strong slope. As the dominant lithology in the area is granitoids, these pediment inselberg complexes are also encompassed by granodiorite, tonalitic and migmatitic gneisses. These are controlled by structure like joints, fracture and lineaments. The difference between the pediment inselberg complex and pediment inselberg is that in case of pediment inselberg, it is a single isolated low relief hill but in the case of pediment inselberg complex it is more than one isolated low relief hill but occurring closely. These land forms are observed in north east of Karya village. (Plate 5.3.4) 3.2.5 Pediplain shallow They are formed by coalescence of buried pediments, where a thick overburden of weathered materials accumulates. The intensely weathered areas of granitoids constitute these landforms. Varying thickness of shallow over burden can be observed in such areas. Weathering of the bedrocks has been initiated by fractures, joints and minor lineaments. These land forms are spread in madanahalli, Hosahalli, Dasanuru. (Plate 6.3.5) 3.2.6 Pediplain moderate Flat and smooth buried pediplain and pediment with moderately thick overburden are called pediplain moderate. Thickness of weathered material is high 30
compare to pediplain shallow. The weathered materials are chiefly constituted by gneisses and migmatites. They are extended towards south-west of Dasanur, Karya, Devanuru and Hanumanapura villages. (Plate 6.3.6) 3.2.7 Pediplain shallow command area They are the pediplain shallow land forms but occurring in command area. Like in other areas, they are formed by coalescence of buried pediments where a thick overburden of weathered materials occurs to form pediplain shallow..here the overburden is weathered material of granodiorite, tonalitic and migmatitic gneisses with varying thickness. The weathering is controlled by fracture, faults and lineaments. These land forms are spread in Madanahalli, Suttur. (Plate 6.3.7) 3.2.8 Pediplain moderate command area Flat and smooth buried pediplain and pediment with moderate thickness of overburden occurring in command area are termed as pediplain moderate command area. The lithology here is granodiorite, tonalitic and migmatitic gneisses. They are controlled by fracture and lineaments. Thickness of the weathered material is high compared to Pediplain shallow command area and extends up to Hanumanapura. They are also observed close to karapura, Dasanuru. Beside mapping and delineating the above land forms the water bodies have also been delineated using toposheets updated with imageries. These surface water bodies influence the surrounding bore wells and ground water recharge conditions. (Plate 6.3.8) 31
3.3 CLIMATE AND RAINFALL: The study area, like the rest of the Southern Peninsula is characterized by four seasons of distinct climatic characteristics viz. (1) Winter, (2) summer, (3) Advancing monsoon and (4) Retreating monsoon.(map.3.1) 3.3.1.Winter: December to February is the winter time. At this time days are cold with average temperature of 10-15 ºC, but it can drop down at some times. Normally winters are dry, but the temperature difference is not so marked due to moderating effect of Indian Ocean, Bay of Bengal and Arabian Sea. From the water replenishment point of view this is the period where zero rainfall is received. 3.3.2 Summer: March, April, and May are the summer months. It is a time period when sun rays fall vertically on Indian subcontinent. The average temperature in summer is around 34 C but at times the maximum temperature can be far above the average some times reaching up to 42 C. Rainfall during this period is zero to scanty, but at some periods substantial precipitation also occurs. Also this is a period of higher water consumption both for domestic and agricultural purposes. Many lakes also go dry during this period. Withdrawal of groundwater is also maximum which causes severe depletion and many shallow wells go dry. As this is a critical period of water management, surface water management through storage and articificial recharge assumes greater importance. 32
3.3.3 Advancing monsoon: It is the time period when major part of rain is received in the months of June, July, August and September. This period forms the core of advancing south western monsoon, like in almost all parts of the Southern Peninsula. The monsoon approaches with moisture laden winds and invariably associated with violent thunderstorms and lightening. This is the critical period of water replenishment for both surface water bodies and ground water. The failure of these monsoons causes severe imbalance in the water supply of any region. And the present study area has also witnessed such situation in the past. 3.3.4 Retreating monsoon: This season normally starts after the month of September after the retreat of the regular monsoon. In this period the rainfall gradually decreases towards November through October and comes to cessation normally at the end of November. This phenomenon is normally related to retreating monsoon. However, some times during November formation of cyclones in Bay of Bengal takes place and bring incessant rains. The unusual rains and other climatic disorders are like droughts are linked to climate change largely brought about by industrialization and excess carbon emissions which is a regional phenomenon. Such disorders and adverse effects are also recorded for the study area. 3.3.5 Temperature: The average temperature of the air at any place mainly depends on latitude, longitude and altitude of the area and it is basically controlled by radiation. December and January are coldest months in the area and temperature ranges between 12-18 0 C. The mean minimum temperature is 13 0 C in December. April is the hottest month in 33
the area. The maximum temperature in this period is 39 0 C. There is a complete sympathetic behavior between temperature and diffused solar radiation. (Table.3.2) In the period from March to May, there is a continuous rise in temperature. On normal days, the day temperatures during summer may exceed 39 0 C there is welcome relief from the heat when thunder showers occur during April and May. With the advance of the south west monsoon about the beginning of June, the day temperatures drop appreciably and throughout the southwest monsoon period, the weather is pleasant. After mid-november, both day and night temperatures decrease progressively. January is the coldest month with mean daily maximum at 13.20 0 C.On some days during the period November to January; the minimum temperature may go below 16 o C. The temperature remains nearly the same for several months but begins to rise in February and touches the peak in either April or May, in both maximum and minimum. Minimum is near about 19 0 C and the maximum is near about 38 0 C for several months. 3.3.6 Evapotranspiration It is the loss of water in the form of water vapours through evaporation from surface water bodies and soil, besides loss due to transpiration from vegetation. It is an important parameter for agro-climatic study. Evapotranspiration is greatly influenced by the surface temperature. However the other influencing/controlling factors on evapotranspiration are wind speed, availability of soil moisture and solar radiation. Obviously the evapotranspiration in the study is very high during March to May because of the high temperature phenomenon. 34
3.3.7 Potential evapotranspiration It is defined as the amount of water loss from the land surface by evaporation and transpiration processes, if sufficient water is available in the soil to meet the demand (Freez and Cherry, 1979). It depends on the evaporation capacity of the climatic condition of the atmosphere. The potential evapotranspiration in the study area is maximum during July to September due to high soil moisture content and significant storage of water in surface water bodies. 3.3.8 Relative humidity The percentage of relative humidity of the area is maximum during February, March, and April and high in the months of June to October throughout the day (Table 3.2, Fig 3.1) Further, it is seen that relative humidity and evaporation are antipathic in their behaviour as usual. 3.3.9 Wind velocity Monthly wind velocity measurements (both minimum and maximum) have been plotted in Fig. 3.1. The minimum wind velocity although the year is less than 4 Kmph, it is higher during August and September. In contrast to this the maximum wind velocity curves shows a maximum of 18 Kmph during August and minimum during October i.e. 8 Kmph wind velocity and evapotranspiration has direct relationship. 3.3.10 Sun shine The average distribution of mean daily hours of sunshine over the study area is shown in Fig. 3.1, Table 3.2. With the onset of monsoon during July there will be a significant decrease in sunshine and it is lower (up to 5 hrs/day). In July and August 35
after the rainy season the number of hours of sunshine increases (6-8 hours per day) and finally around 10-11 hours a day in February and March. 3.3.11 Diffused Solar Radiation During January with clear sky the mean daily radiation is about 1.5 kwh/m.sq./d and with an increase of clouds there is a general increase in diffused solar radiation to 20-30 kwh/m.sq./d in May. It is of interest to note the inverse relation between sunshine and diffused solar radiation. 3.3.12 Water deficit On the maps annual potential evapo transpiration (water needed) of Rao et al., 1976, (Maps 3.3, 3.4, 3.5). The study area map is overlain and the following conclusions are drawn. Potential evapotranspiration from the area = 140 cms. Actual evapotranspiration from the area = 80 cms. Water deficit of the area = 60 cms. To overcome this water deficit, groundwater management is required, storage capacity of the surface water bodies should be improved, and artificial methods should be adopted to improve the groundwater conditions. 3.3.13 SPECIAL WEATHER PHENOMENA This sub head deals with fog, dust storms, thunder and cyclonic effect caused due to depression.fog in the morning is a common feature during October and November with an average of 7 fog days annually.dust storms are common during the 36
summer especially during May and are minimum during November and December on an average 1.1 days of dust storms are observed annually.in the advanced stages of summer, thunder is increased and is maximum during April, May and October and is less from November to February with annual average days of thunder being 37 days.cyclonic storms due to depression in the Bay of Bengal are very common during October, November and December. 3.4 RAINFALL PATTERN OF THE STUDY AREA To assess the climatic condition and rainfall pattern, 30 years annual average rainfall data has been taken for Nanjangud taluk (which fall within the study area) to evaluate the precipitation potential and hydrological conditions. Other meteorological data of Mysore districts have also been collected for assessing hydrogeomorphological conditions. In the study area the premonsoon starts from January and ends at May and southwest monsoon starts from June and ends at September. Northeast monsoon starts from October and stretch up to December. The annual rainfall data accounting is from January to December. The rainfall data collected was from 1980 to 2009 and is presented in Table 3.1 its graphical presentation is shown in Map.3.1 Annual average for 30 years in Nanjangud taluk is 713.68 mm. From the climatic studies it can be inferred that annual maximum temp. is 39 C and average annual extreme temp is 27.5 C. Average rainfall of the study area is 59.47 mm. These factors are critical for agriculture productivity in the study area. (Map.3.2) 37