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
Table.2.1 Summary of monthly mean temperature and precipitation of Barak Valley (2005-09) 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 18.25 5.84 18.05 0.42 18.95 4.68 February 20.25 56.6 20.1 26.6 21.1 40.3 March 23.65 137.8 23.75 151.9 25.05 110.6 April 25.8 201.7 25.3 229.7 26.7 174.8 May 27.0 282.06 26.6 375.78 27.7 301.04 June 28.15 495.4 27.45 682.3 28.65 396.9 July 28.35 494.36 27.65 568.02 29.0 360.2 August 28.6 487.96 28.15 412.8 29.25 371.3 September 28.0 321 27.2 310.4 28.8 286.1 October 26.85 154.72 26.25 148.2 27.4 167.5 November 23.75 27.64 22.95 18.98 24.15 15.54 December 19.7 1.48 19.15 0 19.35 0.2 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 64-127 C Sub-Humid Grassland 31-46 D Semi-Arid Steppe 16-31 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 64-127 C Microthermal 32-63 D' Taiga 16-31 E' Tundra 1-15 F' Frost 0 9
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 2005-2009 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
Table.2.3 Rainfall Characteristics of Barak Valley (2005-2009) Co-Efficient of variation Standard Deviation Number of years above Number of years below Average Rainfall Minimum recorded average average 12.5 334.84 on L'LZ 98'9T9 j 31.08 909.12 m m CM CM CM 2666.58 ZV6ZZZ Station Maximum recorded Cachar Hailakandi Karimganj I 2954.2 2876.5 4106.5 Annual 2666.58 ZT6ZZZ Dec 1.48 0.2 2925.14 2606.95 o Nov 27.64 15.54 18.98 0.56 Oct 154.72 167.5 148.2 156.81 Sep 321 286.1 310.4 &D 3 < "5 <N O d 20.72 0.79 00 us oon Jan 5.84 4.68 2925.14 6.01 11.73 487.96 371.3 412.8 424.02 16.26 494.36 360.2 568.02 474.19 Jun 495.4 396.9 682.3 524.9 18.19 2079.9 1096.5 1524.8 Feb 20.13 May 282.06 301.04 375.78 319.63 12.26 Apr 201.7 174.8 229.7 202.1 7.75 Table.2.3 (Continued) Mar 1'37.8 110.6 151.9 133.4 ZVS 56.6 40.3 26.6 41.1 1.58 ZVO Station Cachar Hailakandi Karimganj Average % Distrbtn 3.647 0.14
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 = 0.000000675 /3-0.000077112 + 0.0172921 / + 0.49239 (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 298.35 and 5.41 respectively for Cachar district, 292.6 and 4.76 respectively for Karimganj district and 306.1 and 6.35 respectively for Hailakandi district. Considering k as 0.9 (for 25-30 latitude) the results are summarized in Table.2.5. 14
Table.2.5 Computation of Potential Evapotranspiration (Thornthwaite's method) for Cachar District Month Mean T C T/5 i e PET (mm) January 18.25 3.65 7.100867 4.178887 37.60998 February 20.25 4.05 8.311643 5.798578 52.1872 March 23.65 4.73 10.51332 9.454765 85.09289 April 25.8 5.16 11.99366 12.43611 111.925 May 27.00 5.4 12.84826 14.35088 129.1579 June 28.15 5.63 13.68578 16.36589 147.293 July 28.35 5.67 13.83327 16.73496 150.6146 August 28.6 5.72 14.01837 17.20425 154.8383 September 28.00 5.6 13.57553 16.02976 144.2678 October 26.85 5.37 12.74034 14.10123 126.9111 November 23.75 4.75 10.58069 9.581269 86.23142 December 19.7 3.94 7.972257 5.316811 47.8513 Moisture deficit Soil Moisture Accretion Soil Moisture Utilisation Fig.2.2 PET - Rainfall relationship, Cachar District 15
Table.2.6 Computation of Potential Evapotranspiration (Thornthwaite's method) for Karimganj District Month Mean T C T/5 i e PET (mm) January 18.05 3.61 6.983384 4.260884 38.34796 February 20.1 4.02 8.218608 5.896451 53.06806 March 23.75 4.75 10.58069 9.75984 87.83856 April 25.3 5.06 11.64351 11.81306 106.3175 May 26.6 5.32 12.56117 13.74299 123.6869 June 27.45 5.49 13.17384 15.11251 136.0126 July 27.65 5.53 13.31944 15.44749 139.0274 August 28.15 5.63 13.68578 16.3066 146.7594 September 27.2 5.44 12.99262 14.70065 132.3059 October 26.25 5.25 12.31179 13.20411 118.837 November 22.95 4.59 10.0458 8.800398 79.20358 December 19.15 3.83 7.637706 5.094344 45.8491 <;<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
Table.2.7 Computation of Potential Evapotranspiration (Thornthwaite's method) for Hailakandi District Month Mean T C T/5 i e PET (mm) January 18.95 3.79 7.517263 4.405928 39.65335 February 21.1 4.22 8.845514 6.315282 56.83754 March 25.05 5.01 11.46976 11.22154 100.9939 April 26.7 5.34 12.63274 13.89502 125.0552 May 27.7 5.54 13.35592 15.71642 141.4478 June 28.65 5.73 14.05549 17.596 158.364 July 29.00 5.8 14.31627 18.32651 164.9386 August 29.25 5.85 14.50354 18.86115 169.7504 September 28.8 5.76 14.16706 17.90653 161.1588 October 27.4 5.48 13.13753 15.15343 136.3809 November 24.15 4.83 10.85165 9.92702 89.34318 December 19.35 3.87 7.758797 4.725282 45.52754 400 ------ P ai nfall* m n > PET-inmi Moisture Deficit Soil Moisture Accretion Soil Moisture Utilisation Fig.2.4 PET - Rainfall relationship, Hailakandi District 17
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