CHAPTER-11 CLIMATE AND RAINFALL

Transcription

1 CHAPTER-11 CLIMATE AND RAINFALL 2.1 Climate Climate in a narrow sense is usually defined as the "average weather", or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period ranging from months to thousands or millions of years. These quantities are often surface variables such as temperature, precipitation and wind. The climate of the area has been classified based on the Thornthwaite's climatic classification (Thornthwaite, 1948). He used the terms precipitation effectiveness (P-E index) and temperature efficiency (T-E index) in his classification and developed the following formulae to express the P-E and T-E indices in terms of precipitation and temperature. P-E Index: I = Sum of twelve monthly values of 115 (P/T-10)10/9 (2.1) where, P = mean monthly precipitation in inches T = mean monthly temperature in F T-E Index: I = Sum of twelve monthly values of (T-32/4) (2.2) where, T = mean monthly temperature in F In the present study, the annual P-E and T-E indices have been calculated by using the above Thornthwaite's index formulae from the data (Table. 2.1) collected from IMD. 8

2 Table.2.1 Summary of monthly mean temperature and precipitation of Barak Valley ( ) Month Cachar District Karimganj District Hailakandi District Mean (T) in C Mean (P) Mean (T) in C Mean (P) Mean (T) Mean (P) in mm in mm in C in mm January February March April May June July August September October November December Source : IMD (Hydromet Division) Table.2.2 Precipitation Effective Index (PE) classes as per Thornthwaite's classification Symbol Humidity Province Characteristic Vegetation A Wet Rainforest >= 128 B Humid Forest C Sub-Humid Grassland D Semi-Arid Steppe E Arid Desert <=15 Annual Precipitation Effective Index (PE) Table.2.2(continued) Temperature Efficiency Index (TE) classes as per Thornthwaite's classification Symbol Temperature Provinces T/E Index A' Tropical 127 B' Mesothermal C Microthermal D' Taiga E' Tundra 1-15 F' Frost 0 9

3 The P-E and T-E indices are found to be 140 and 134 respectively for Cachar district, 159 and 131 respectively for Karimganj district and 112 and 137 respectively for Hailakandi district. As per classification criteria (Thornthwaite, 1948), these values indicate that the climate of the area is megathermal wet climate with rainforest type of vegetation (Table.2.2). 2.2 Rainfall To study the rainfall characteristics of Barak valley, rainfall data for the period were collected from IMD. The area receives fairly high amount of precipitation, average annual rainfall being 2607 mm. The annual and mean monthly rainfall of the rain gauge stations are given in Table: 2.3 The highest average annual rainfall was recorded at Karimganj district, being 2925 mm and the lowest recorded at Hailakandi district, being 2229 mm. Sixty five percent of annual rainfall is received during the monsoon season, i.e. June to September. The highest mean monthly rainfall is 682mm, which occurs during the month of June. The distribution of mean monthly rainfall is almost same every year i.e. amount of rainfall gradually increases from March onwards, reaches maximum during June or July, and thereafter decreases gradually and finally becomes scanty to almost negligible during December to February. The Co-efficient of rainfall variation and standard deviation of the stations have been worked out and are shown in the Table.2.3. The coefficient of variation is low in Cachar district (12.5) and high in Karimganj district (31.08). From the table it is also observed that rainfall is quite stable as the periods of above and below average rainfall are almost equal. 10

4 Table.2.3 Rainfall Characteristics of Barak Valley ( ) Co-Efficient of variation Standard Deviation Number of years above Number of years below Average Rainfall Minimum recorded average average on L'LZ 98'9T9 j m m CM CM CM ZV6ZZZ Station Maximum recorded Cachar Hailakandi Karimganj I Annual ZT6ZZZ Dec o Nov Oct Sep &D 3 < "5 <N O d us oon Jan Jun Feb May Apr Table.2.3 (Continued) Mar 1' ZVS ZVO Station Cachar Hailakandi Karimganj Average % Distrbtn

5 evaporation from soils. Conceptually, potential evapotarnspiration (PET) is the upper limit of evapotranspiration. Estimation of PET is important to understand the water budget. PET of an area is estimated by Thornthwaite's method. Thornthwaite (1948) provided the following formula for computing monthly potential evapotranspiration: e = 1.7 x (10xT/f)a --(23) where, e is unadjusted PET (in cm) per month, considering 30 days a month and 12 hour of possible sunshine per day. T = Mean Air Temperature ( C) / = Annual Heat Index (summation of /for 12 months) / = (T/5)1514 (2.4) a = An empirical exponent computed by the following formula a = / / (2.5) Michael (1978) suggested a latitude adjustment factor, K, so that adjusted PET (in mm) can be expressed as PET = k x e x 10 (2.6) Considering normal temperature values (Table:2.1), / and a are computed to be and 5.41 respectively for Cachar district, and 4.76 respectively for Karimganj district and and 6.35 respectively for Hailakandi district. Considering k as 0.9 (for latitude) the results are summarized in Table

6 Table.2.5 Computation of Potential Evapotranspiration (Thornthwaite's method) for Cachar District Month Mean T C T/5 i e PET (mm) January February March April May June July August September October November December Moisture deficit Soil Moisture Accretion Soil Moisture Utilisation Fig.2.2 PET - Rainfall relationship, Cachar District 15

7 Table.2.6 Computation of Potential Evapotranspiration (Thornthwaite's method) for Karimganj District Month Mean T C T/5 i e PET (mm) January February March April May June July August September October November December <;<n 700 «TmO 400 Rainfall'mm > PETimmj 100 Moisture Deficit Soil Moisture Accretion Soil Moisture Utilization Fig.2.3 PET - Rainfall relationship, Karimganj District 16

8 Table.2.7 Computation of Potential Evapotranspiration (Thornthwaite's method) for Hailakandi District Month Mean T C T/5 i e PET (mm) January February March April May June July August September October November December P ai nfall* m n > PET-inmi Moisture Deficit Soil Moisture Accretion Soil Moisture Utilisation Fig.2.4 PET - Rainfall relationship, Hailakandi District 17

9 From the PET - Rainfall relationship of Cachar, Karimganj and Hailakandi district it is observed that precipitation exceeds potential evapotranspiration from the end of March till the end of November, indicating surplus moisture for surface run-off and ground water recharge. From the beginning of December to the middle of March PET exceeds precipitation. During this period plants use water stored in the soil. When precipitation exceeds PET for the first time during the end of March, rain water is used mainly to recharge the soil water to field capacity. Only after recharge of soil water by mid April significant run-off is observed. Therefore, it can be concluded that there is much scope for ground water recharge during April to November. 18