The variation characteristics of evapotranspiration of soil moisture in dry farmland in Southeast Gansu

Using the Peman formula,value of potential evapotranspiration in dry farmland in the southeast Gansu Province in 1960s～1990s was calculated,and the spatial and temporal variation characteristics of potential evapotranspiration were analyzed,and the calculated value was compared with that measured by evaporation pan.The result showed that the value of potential evapotranspiration was mainly influenced by temperature and precipitation.The minimum value appeared in 1980s,while the maximum increasing value appeared in 1990s,especially in Guanshan area.The potential evapotranspiration reached the maximum in every season in 1990s,and summer was the highest season,while winter the smallest.The variation tendency of potential evapotranspiration and the measured one is basically consistent.The value of potential evapotranspiration was higher than the measured one by 30% during 1960s～1990s,especially in summer which was higher than the measured one by about 50%,while smaller than the measured one in winter.The shortage of soil moisture increased continuously since 1990s in dry farmland,especially in spring,which had great influence on crops'growth.