| Frost as a natural phenomenon is controlled by temperature, which have close impact on agriculture. The spatial and temporal distribution of forst can reflect temperature change in a certain time, especially the extreme minimum temperature. Frost injury is one of the hazards of a serious threat to agricultural production, the annual economic losses caused by frost is very serious. Based on this, the present study using 60 meteorological stations' s ground 0cm minimum temperature(T0cm) date is to analyze frost index, which include first frost day(FFD), last frost day(LFD), frost day(FD) and not frost day(NFD), in the Loess Plateau from 1980 to 2014. Selecting this frost index analyze the interannual variability, the spatial distribution and the altitude and latitude influenced on this frost index. Based on the winter wheat jointing stage, divided frost level to statistics of each grade occurrence frequency and accumulated frost, we divide frost hazard zoning of different levels in the Loess Plateau. The may conclusions are as follows:(1) Over the past 34 year of frost index value in Loess Plateau shows different trend, the annual average of first frost date were 3.15/10 a of the rate postpone, last frost day in a-2.89/10 a rate to advanced, frost day in a-5.95/10 a rate to decreases, while frost-free day in a 6/10 a rate to increases. In different regions of the Loess Plateau, valley plain area is the fastest region that the first frost day postponed, frost day decreasesing and frost-free day increasesing, but the last frost day in advance the slowest area. On the contrary, loess gully region is the slowest area that the change of first frost day, frost day and frost-free day but last frost day in advance the fastest area. In the 0.05 confidence level, the mutation of first frost day in valley plains area and loess gully area occurred in 2004, while in loess hilly area and rocky mountain area the mutation occurred in 2003; the mutation of last frost day in different rigions are inconsistent, valley plain area is not a mutation, the year of occurred mutation in loess gully area, loess hilly area and rocky mountain area is 2005, 1998, 1999, respectively; the mutation of frost day and forst-free day in the Loess Plateau occurred in 2001. In terms of decadal variations, first frost day postponed fastests since 2000, followed by 80s; last frost day postponed slowly in the 1980 s but advanced rapidly in 90s; with the increase of the year, frost day reduced faster while frost-free day increased faster.(2) Nearly 34 years, the probability of occurrence earlier or extremely early first frost in the Loess Plateau were 14.71% and 2.94%, and the absolute variability of first frost is 4.92/day, Valley plains change range of the first frost is the largest, followed by rocky mountain area, the range in the loess gully region is the smallest; an exception occurs first frost year more concentrated in the 90 s. the probability of occurrence later or extremely late last frost in the Loess Plateau were 20.59% and 2.94%, the absolute variability of last frost is 4.44/day, Valley plains change range of the last frost is the largest, followed by loess gully region, the range in the rocky mountain area is the smallest, and the year occurring abnormal last frost are most concentrate in the 80 s.(3) In terms of spatial distribution shows the first frost day in the Loess Plateau north early than south, west early than east, while is contrary with the distribution of last frost day. The distribution of frost day in the Loess Plateau north more than south, west more than east, while is contrary with frost-free day. There is a negative correlation between first frost day and frost-free day with altitude and latitude, but a positive correlation between last frost day and frost day with altitude and latitude.in terms of geographical location, the effect of latitude on the frost index greater than altitude.(4) In general, frequency of each grade frost after winter wheat jointing in the Loess Plateaug gradually decreases from north to south, particularly the frequency of severe frost shows a saddle-shaped in the western Loess Plateau, but increasing from south to north in the east. Except valley plain area, the frequency of slight frost in the Loess Plateau is more than 80%. The range of high-frequency region of Moderate, and severe frost is reduced. Overall, the Guanzhong basin is the smallest-frequency region of frost, while Ordos Plateau, north Shanxi plateau and Qinghai mountain is the highestfrequency region to occurrence frost. Compared the different levels of frost, slight frost has large range of high-frequency region, severe frost is main low- frequency region.(5) The distribution of accumulated frost of each grade in the Loess Plateau is becoming higher from south to north, and in the south-west Loess Plateau shows highlow-high change trends. The hightest value of accumulated frost located in mountain but the lowest value was in basin. Low latitude regions are mainly slight and moderate frost, and high latitude regions is mainly severe frost. Nearly 34 years, severe, moderate and slight frost were-7.61/10a?-0.34/10a?-0.25/10 a of the rate decreases. The largest accumulated frost mainly focused on 90 s. The greatest impact on loess gully regions is slight frost, influenced by moderate and severa frost is loess hilly region.(6) The distribution of last frost hazard risk index of winter wheat in the Loess Plateau shows north higher than south and west higher than east, have an increase trends from the plains to the mountains. With the frost level rises, the differences of the frost hazard risk index between east with west are smaller, but this difference more obvious from north to south. The frost high-risk areas move to north while the low-risk areas locate in south Loess Plateau and the range is gradually increases. The high-risk areas of slight frost are Qinhai mountain and liupanshan mountain, the low-risk areas are basin that located in east-south Plateau, the most widely distributed is moderate-risk areas. The high-risk areas of moderate frost are mainly located in liupanshan mountain, followed by Qinhai mountain, the low-risk areas are located in basin, river valley and watershed, which the range larger than slight frost. The high-risk areas of Severa frost are distribution in north taihangshan mountain, the low-risk areas located in south of 39°N in the Loess Plateau. Compared with slight and moderate frost, the center of highrisk areas of severa frost move to north and the proportion is reduced, but the range of low-risk is significant expansion. |
PDF Full Text Request