Spatial and Temporal Characteristics of Dry/Wet Land Surface in Anhui Province with Their Impacts on Agriculture in Recent 30 Years

The reference evapotranspiration (E_T0) in Anhui for the period from 1971 to 2000 is calculated using FAO Penman-Monteith equation, in which the term of net radiation is modified utilizing the observation data of radiation in Anhui. Aridity index (I_a) is calculated by E_T0 and precipitation of the same period, and variation of area land surface arid and humid condition and its impact on agriculture are analyzed based on aridity index in different time scales. The results indicate that the isoline of I_a=1 is the boundary of the humid and semi-humid zones, and it has a good consistency with the isoline of 1000 mm annual precipitation. Also, the isoline has clear agricultural meaning. The isoline (I_a=1) fluctuates in the direction of south and north during 1970s, 1980s and 1990s, and this fluctuant zone is just Jianghuai watershed where is apt to drought and waterlog. Temporal variation of land surface arid and humid condition in Anhui shows obvious regional characteristics. On the one hand, because of temporal variation trends of the precipitation and the reference evapotranspiration, aridity indices present significant increasing trend in fluctuant zone, and significant decreasing trend in semi-humid and humid zones. On the other hand, influenced by great interannual variability of precipitation, interannual variability of aridity indices is also great, which is most remarkable in the fluctuant zone, and second remarkable in semi-humid zone. Therefore, drought is the most serious menace in the fluctuant zone; and drought and waterlog both exist in the semi-humid and humid zones, which are remarkable in 1990s. Temporal variations of land surface arid and humid condition root mostly in summer half year in each region. Temporal variation of land surface arid and humid condition affects increasingly on agriculture in each district. In semi-humid zone, affected by the water shortage and high rainfall variability, the frequencies of summer waterlog, summer-autumn and autumn-winter drought are quite high. In Jianghuai watershed liabledrought fluctuant zone the variation trends of precipitation and aridity index are opposite to those of the other two areas, the rice and autumn harvested crops are threatened by drought evidently. In humid zone, although the average condition is humid, variations of precipitation and aridity index are even high than those of the semihumid area in summer half year and the frequencies of drought are increasing. The normal growth of rice and semi-tropical fruit trees is threatened increasingly by seasonal droughts. Anhui belongs to the transition area of semi-humid and humid zones. The spatial and temporal characteristics of land surface arid and humid condition in Anhui Province are thoroughly analyzed. The results are useful to understand the variation of semi-humid and humid climate and its influences, as well as to guide the agriculture. Limited by data, the variation of semi-humid and humid is only discussed in the recent 30 years. In fact, the variation of land surface arid and humid condition status is a gradual process. The analysis on data of a longer time period helps to get the more scientific and objective results. Therefore, it is very necessary to keep on complementing the data for a long-time research.