Dew formation and its variation in Haloxylon ammodendron plantations at the edge of a desert oasis, northwestern China

Dewfall may be a critical source of moisture in desert environments and may determine sustainability of sand-stabilizing planted vegetation. However, little is known about factors responsible for dew formation, the relative importance of dew as a source of water, and its variability in plantations. During June and October of 2013, the dew amounts and duration were estimated by using the Bowen ratio energy budget technique (BREB), and the dew variability on sand dunes planted with Haloxylon ammodendron 5, 20, and 40 years before were measured by microlysimeter. We quantified dew formation characteristics in a sand-stabilizing H. ammodendron plantation at the edge of a desert oasis, northwestern China. The results indicated that the average daily amount of dew in the H. ammodendron plantations during the observation period based on BREB was 0.13 mm, and the dew duration lasted from 1 to 9.5 h. Dew occurred on 77% of growing season days, the number of days with dew amounts of > 0.03 mm comprised 95% of the total dewfall days, and the cumulative amount of dew for those days was 16.1 mm. Air temperature, relative humidity, the difference between air temperature and dew point, and wind speed had significant effects on dew formation. The thresholds of the dew formation were RH > 50% and wind speed < 4.27 m/s. As a result of larger canopy area and lower Sky View Factors to 20- and 40-year-old H. ammodendron, the accumulated amount of dew was always significantly greater, and its night-time variability was almost 3 times greater for 5-year-old than for 20- and 40-year-old shrubs. In addition, near-ground dew amounts at the inter-space of three ages of H. ammodendron exhibited higher values than that under the canopy, while dew formation lagged and the maximum cumulative amount of dew was observed 2 h later under the canopy of shrubs. The Bowen ratio method estimated actual dew reasonably well. It is concluded that dew may be a frequent and stable water resources in H. arnmodendron plantations at the edge of a desert oasis, and there is a mutually reinforcing effect between dewfall and the sand-fixing vegetation system.