Annual and seasonal variability of climate in South Saurashtra Agroclimatic zone

Climate is the most dominating factor influencing the suitability of a crop in a particular region. Inter seasonal climatic variability in terms of temperature and rainfall in Gujarat was studied by Parmar et al (2005) using daily historical weather data. Dixit et al (2005) studied the rainfall variability in Kankan region In this paper annual and the inter seasonal variability of all weather parameters using 41 years weather data (1965-2005) has been studied for Junagadh under south Saurashtra Agroclimatic zone. Junagadh (21o 31’N, 70o33’ E 61.0 m amsl) is located on the lap of mount Girnar at the middle of Saurashtra region of Gujarat. The climate of the region is characterized as semi-arid high rainfall zone with dry winter and humid summer.

Climate is the most dominating factor influencing the suitability of a crop in a particular region. Inter seasonal climatic variability in terms of temperature and rainfall in Gujarat was studied by Parmar et al (2005) using daily historical weather data. Dixit et al (2005) studied the rainfall variability in Kankan region In this paper annual and the inter seasonal variability of all weather parameters using 41 years weather data  has been studied for Junagadh under south Saurashtra Agroclimatic zone. Junagadh (21 o 31'N, 70 o 33 ' E 61.0 m amsl) is located on the lap of mount Girnar at the middle of Saurashtra region of Gujarat. The climate of the region is characterized as semi-arid high rainfall zone with dry winter and humid summer.
The daily weather data from 1965 to 2005 recorded at Agrometeorological Observatory, Junagadh Agricultural University were transformed into standard weekly (1 to 52 weak) means and totals. As per IMD standards the year was divided into four seasons as summer (10 th to 22 nd week), monsoon (23 rd to 39 th week), post monsoon (40 th to 49 th week), and winter (49 th to 9 th week) in order to study the annual and seasonal climatic behavior in terms of statistical properties like mean, median, standard deviation (SD) and coefficient of variation (CV%). From the weekly values of 52 weeks of 41 years annual and seasonal values were derived.

Annual variability
The variability of weather parameters on annual basis is presented in Table 1.
The annual mean maximum and minimum temperature at Junagadh are 34.4 0 C and 19.9 0 C respectively. The CV% of minimum temperature is higher than the maximum temperature. The average temperature is 27.1 0 C with 3% variability. The annual mean daily BSS is 7.6 hrs with a variability of 6.4%.
The annual average evaporation is 6.6 mm per day with 12% coefficient of variation. The annual average rainfall is 876.2 mm with 50.5% variability and 443mm standard deviation. The highest annual rainfall of 2793.7 mm was received in the year 1983 in 52 rainy days. The annual average rainy day is 37 days with a standard deviation of 10 days. In 1987 the lowest rainfall of 145.1mm was recorded in 13 rainy days. The highest rainfall of 1390.0 mm was recorded on 21 st June 1983, the rarest event in the region.

Variability during summer seaso
The climatic variability for summer season (Table 2) reveals that the mean maximum temperature is 38.3 0 Cwith standard deviation of 1.4 o C and variability of 3.7 percent. The highest maximum temperature of 42.3 0 C was observed in 1973 and the lowest (32.5 0 C) in the year 1967. The mean wind speed for summer season is 9.2 kmph with variability of 14.4 percent. The mean BSS for the season is 9.7 hrs with 7.3% variability. Highest BSS during the season was in 1978 and the lowest in 1997. The mean daily evaporation rate for summer season is 10.1 mm with 12.4% variability. The rainfall during summer was very low i.e. 11.8 mm with one rainy day.

Variability during monsoon season
The statistical properties of climatic parameters are given in Table 3. The mean maximum temperature for monsoon season is 32.9 0 C ranging from 30.6 0 C in 1994 to 37.7 0 C in 1974. The mean minimum temperature is 24.3 0 C with a range of 11.9 to 26.3 0 C and higher variability of 10.9 percent. The bright sun shine hours are the lowest during this season with higher variability. The rate of evaporation is also low (5.0 mm) with a variability of 21.4 percent.
The mean rainfall for monsoon season is 814.3 Table 1: Annual variability in weather parameters at Junagadh

Variability during post-monsoon season
This is a transition period for harvesting Kharif crops and sowing of rabi crops. Rainfall in this period The mean maximum temperature for post monsoon season is 35.3 0 C (Table 4) with a variability of 3.7 percent. The mean minimum temperature is 18.4 o C with a variability of 10.3 percent. The minimum temperature ranged from 13.6 0 C in 1974 (13.8 0 C in 2000) to 22.1 0 C in the year 1976. The morning and afternoon relative humidity respectively are 70% and 34%. The mean RH is 53 percent. The mean wind speed for the season is 4.7 kmph with a high variability of 31.1 percent, and a range of 2.6 to 7.5 kmph. The mean bright sun shine is 8.8 hr with a narrow range of 7.9 to 9.9 hrs. The rate of evaporation is 5.4 mm for the season. The mean rainfall for post-monsoon season is 47.3 mm which is only 5.5% of annual rainfall and 6% of monsoon rainfall, with two rainy days.

Variability during winter season
The mean maximum temperature for winter season (Table 5) is 31.4 0 C, ranging from 29.2 0 Cin 1967 to 35.4 0 Cin 1996. The mean minimum temperature is 12.4 0 C with a variability of 9 percent in a range of 10.3 to 15.3 0 C. The lowest minimum was recorded in 1975 and the highest minimum in 2002. The morning relative humidity is 63% and the afternoon RH is 28%, the lowest among the seasons. The mean evaporation rate is 5.8 mm, Rainfall during the season is very negligible.

Rainfall variability
The rainfall analysis of 41 year  indicated that the region received normal annual rainfall of 876.2 mm in 37 days, out of which the monsoon season received 814.3mm (95 per cent) in 33 rainy days.
The median value of annual rainfall is 856.4 and for monsoon season it is 775.4 mm. The onset and withdrawal of monsoon (Table 6) rainfall have been computed with a criterion of 25 mm rainfall in a week in 3 rainy days (with rainfall during subsequent weeks) and 10 mm of rainfall in a week with no rainfall in subsequent weeks respectively. Normal onset and withdrawal of monsoon weeks in the region are 24 th (11-17 June) and 39 th week (24-30 Sep). The average duration of the season is about 15 weeks (25 th to 39 th week) or 108 days. The shortest monsoon period of four weeks was recorded in the year 1972 and the longest period was 23 weeks in 1998.
The study revealed that in the lowest rainfall year like 1987 the annual minimum and mean temperature, annual wind speed and BSS were the highest and during monsoon season of the year the minimum temperature, wind speed, BSS and evaporation were the highest; during post-monsoon season maximum and mean temperature, wind speed and evaporation were highest. At Junagadh in the last 41 years 1973 was the warmest year with 37.6 0 C maximum temperature and 1967 was the coldest year with 32.2 0 C.The coldest winter occurred in 1975 with 10.3 0 C minimum temperature and the hottest summer was 1973 with 42.3 0 C.