Trend analysis of frost period, growing season, absolute minimum and maximum temperature in Fars province

Agricultural productions and management strategies can be highly affected by long term changes and variability in temperature. Agrometeorological indices are commonly used to evaluate how weather and climate conditions affect crop production. The extreme temperature-based indices are important tools for monitoring agrometeorological indices. In this study, trend analysis of extreme temperature-based agrometeorological indices for five stations including Abadeh, Fasa, Shiraz, Badjgah and Kooshkak in Fars province was studied. The Abadeh, Fasa, Shiraz stations are synoptic stations and Badjgah and Kooshkak stations are agrometeorological stations. Ten indices including frost indices (last spring frost, first fall frost, length of frost period, number of frost days and chilling hours), growing season indices (start, end and length of growing season) and annual absolute temperature indices (minimum and maximum temperature) were investigated in the present study. Using daily minimum and maximum temperature data, time series of these ten indices were separately produced over a long period. The autocorrelation function for these constructed time series were plotted and used to check serially correlated time series indices. For each station, those time series indices, which were serially correlated, were transformed to produce an uncorrelated pre-whitened time series. For example, last spring frost in Shiraz station, and start of growing season in Badjgah station were serially correlated and it is necessary to be pre-whitened. The autocorrelation function for all indices in Kooshkak station have indicated that all correlations were statistically insignificant and therefore no necessary to pre-whitening. The nonparametric Mann-Kendal test which is commonly used to trend analysis was used to determine the existence of trend in time series agrometeorological indices. The linear trend which is commonly applied to quantify time series changes was applied to determine the amount (slope) of time series agrometeorological indices. The sequential Mann-Kendal test was also applied to determine initiation of abrupt trend changes and explore in sub-periods of time series agrometeorological indices. The applied sequential Mann-Kendal test indicated a significant jump in the air temperature at Fasa station in 1984. This station had a re-location in 1984 (personal communication with Fasa station). Therefore, the Fasa station was removed for further analysis in this study. This was also applied to determine initiation of abrupt trend changes and explore in sub-periods of time the results of both Mann-Kendall and linear trend indicated that the trends of time series agrometeorological indices were different for various stations.A significant decreasing trend was observed for most frost indices in Badjgah station, which is an agrometeorological station at agriculture collage of Shiraz University. A significant decreasing trend, at 5% significance level, was observed for length of period and number of frost days, respectively, with -9 and -12 Day/Decade slope in Badjgah from 1986 such that the length of frost period and number of frost days had largest decreasing trend among frost indices. Also, a significant decreasing linear trend were observed for chilling hours in Badjgah and Shiraz stations with -68 and -66 Hour/ Decade slope. For all growing season indices trend existence was observed in Badjgah and Shiraz stations, except start of growing season in Badjgah. An earlier start, delay in end, and increasing in the length of growing season were observed in both Badjgah and Shiraz stations. For most indices, a change was occurred in 1970s and 1980s in Badjgah and Shiraz, respectively. A significant trend in the annual absolute maximum and minimum temperature was observed in Kooshkak station (near to Marvdasht) using the Mann-Kendall analysis at 5% significance level. In this station, a change had occurred in the absolute annual minimum temperature in 1992. In the other hand, after 1979 a change had occurred in the absolute annual maximum temperature in Kooshkak station. A significant downward trend for Absolute minimum temperature has been started from 1999 and this trend has continued up to the end of studied year.For each station, the studied indices were indicated different trends and beginning time. A significant trend was observed in all of frost period and growing season indices at Badjgah station. The information of first fall frost and last spring frost is vital for harvest and planning crops. Decreasing trend of chilling hours in Shiraz and Badjgah stations have indicated that dormancy period is longer and plants need more time to exit from dormancy period.