Study on the regional evapotranspiration over different surface conditions of the Longzici spring drainage

The Longzici spring system, one of the largest karst spring in North China, is located in the western margin of the Linfen basin, covering an area of 2,250 km2. The springs discharge at an annually average rate of 4.81 m3/s derived from the records from 1955 to 2013. The karst water has been an important source for local water supply. But now the spring flow dramatically degrades because of climate change and human activities. Precipitation is the main source of the spring water recharge. This recharge occurs in two ways, namely the infiltration of precipitation over the carbonate rock areas and linear leakage of stream and river water. So it is very important to evaluate the recharge of karst water through an afforestation project. Based on NOAA/AVHRR data, the daily and monthly evapotranspiration (ET) of the Longzici spring are estimated during the period of 2014.4-2014.10, using surface energy balance system (SEBS) and the data of meteorological stations in Linfen. The ET has a monthly average value of 99.21 mm, with the maximum monthly value of 197.26 mm in June and the minimum one of 40.49 mm in April. In addition, the regional ET over different lithology and vegetation coverage rates are analyzed through spatial overlay. The results indicate that the ET increases with the increasing vegetation coverage rates in the carbonate rock outcrop regions, while it is the lowest in the medium vegetation coverage rate of clastic rock regions. Overall, the ET in the clastic rock regions is higher than that in the carbonate rock outcrop regions. If only ET is taken into consideration, keeping the medium vegetation coverage rate in clastic rocks will be favorable to increasing of recharge of karst water and afforestation in carbonate rock areas will enhance ET but not favorable to recharge from rainfall infiltration.