Abstract:

Aims Caragana liouana has been widely planted in northwest China for ecological restoration which has induced the planted shrub landscape in the desert steppe zone and changed the structure and function of the original ecosystem. These plantations would affect the water vapor interactions between land surface and atmosphere eventually. Therefore, studying the evapotranspiration characteristics of the planted shrub communities in the desert steppe zone is significant for us to understand its ecohydrological mechanism and guide local ecological restoration.
Methods The transpiration of shrubs and the evapotranspiration under shrub canopy in the desert steppe zone in Yanchi County, Ningxia, were measured by a coupled method of sap flow sensors and lysimeter from May to August in 2018. The effects of environmental factors on the evapotranspiration of the planted shrub communities were analyzed by path analysis.
Important findings (1) The evapotranspiration obtained by the coupled method of sap flow sensors and lysimeter was consistent with the estimations based on the water balance method and the eddy covariance method. Thus, the coupled method of sap flow and lysimeter can be used to measure the evapotranspiration and its components of the planted shrub communities in the desert steppe zone. (2) The diurnal variations of transpiration rate of shrubs and the evapotranspiration rate under shrub canopy were consistent with each other, and both presented an obvious single peak at noon. The evapotranspiration of the planted shrub communities mainly occurred in day time. However, the time of the maximum transpiration rate of shrubs was 1 h later than that of the maximum evapotranspiration rate under shrub canopy. (3) The cumulative transpiration of shrubs was 83.6 mm, the daily average transpiration was 0.7 mm·d^-1, and the series curve of daily transpiration had a parabolic shape from May to August. The cumulative evapotranspiration under shrub canopy was 182.5 mm, and the daily average evapotranspiration was 1.5 mm·d^-1 during the same period. The evapotranspiration under shrub canopy was higher than the transpiration of shrubs. It can be concluded that the water consumption in the planted shrub communities is mainly caused by evapotranspiration under shrub canopy. (4) The total evapotranspiration of the planted shrub communities from May to August was 266.1 mm, and the total precipitation in the same period was 222.6 mm. Therefore, the water balance in the planted shrub communities was in deficit. (5) Net radiation is the most important and direct driving factor that affected evapotranspiration of the planted shrub communities, and it coupled other environmental factors to drive the process of evapotranspiration. Therefore, the water deficit in the planted shrub communities encroaching desert steppe zone should be paid attention in ecological restoration and reconstruction.

Key words: desert steppe zone, planted shrubs community, transpiration, water cycle, Caragana liouana