Climatic Change of the Green Grass Seasons in Northern Tibet from 1971 to 2000

The climatic change trend of the green grass seasons in northern Tibet is analyzed by calculating the data of mean tem perature, ≥5 ℃ critical temperature and precipitation from 1971 to 2000 in Nakchu district of Tibet. And the maximum potential evaporation is obtained through Penman-Monteith Model in terms of monthly maximum temperature, minimum temperature, wind speed, relative humidity and sunshine duration. The surface humid index is the ratio of rainfall to maximum potential evaporation. The features of seasonal and annual surface humid index are discussed. Main results are as follows: ① The green grass duration is about 80~164 days in Nakchu district of Tibet. The green grass duration is longer in the eastern pasture (over 130 days), where the heat and water conditions are more sufficient. Whereas it is shorter in the middle pasture (less 100 days) where the heat resources are deficient. The mean temperature is 7.5—9.6 ℃ and precipitation is 270—550 mm during the green grass seasons. ② In past 30 years, the mean temperature trend is warming and its increase is (0.18—0.36)℃/10a, especially in mid-western of Nakchu district. The increasing trend of rainfall is insignificant in most part of pastoral areas, but it is significant in Chali. Also, results show that the trend of green grass duration prolongs and ≥5 ℃ accumulated temperature increases obviously in pastoral areas. In general, the climatic trend behaves prolonging duration, warming temperature and increasing precipitation, that is favorable for grass growing. ③ The inter-decadal variations increase in mean temperature. The precipitation is less in the 1970s in most of pastoral areas, more in the 1980s, and it increases in the 1990s compared with that in the former 20 years. In the 1970s, there are later starting date, earlier ending date, shorter duration and lower accumulated temperature (≥5 ℃) during the green grass seasons. But to the contrary in the 1990s, the phase shows earlier starting date, later ending date, longer duration and higher accumulated temperature (≥5 ℃), there are advantageous for grass growth and husbandry production. ④ There are no anomalous years in western Nakchu district in terms of temperature, the anomalous cold years occurres in 1976 in mid-eastern Nakchu, and the anomalous years of precipitation appeared in 1970s. In addition, the accumulated temperature displays anomalous lower in the duration of green grass in most of husbandry districts in middle of the 1970s, whereas anomalous higher years appear in end of the 1990s. ⑤ During the past 30 years, the decrease of annual potential evaporation is significant in the pastoral areas, and the trend indicates a decrease of (10.1—37.8) mm/10a. The potential evaporation decreases more strongly in winter. As for surface humid index, the trend behaves more significant increase. The rate of increasing is (0.02—0.06)/10a.The increase displays strongly in autumn in most of Nakchu district. ⑥ The tendency of warm and humid climate caused by air temperature increasing, potential evaporation decreasing and surface humid index increasing benefits environmental improvement.