Characteristics of soil evaporation, plant transpiration and water budget of Nitraria dune in the arid Northwest China

Nitraria, a widely growing shrub plant in the dry desert area of China, can fix moving sand along its canopy and form a large number of sand dunes. The Nitraria dune is one of the most effective ways to fix moving sand and protect oasis in Northwest China. However, after dune formation, Nitraria plants gradually die and then release the fixed sand which causes damage to the oasis again. A decrease in the ratio of transpiration (T) to evapotranspiration (ET) was assumed to be the main reason for Nitraria dune degradation because more water was used for soil evaporation, however, this assumption has remained untested because of the difficulty in measuring the Nitraria dune transpiration rate. To overcome this challenge, an intensive field experiment was carried out in 2008-2012 in the Minqin, a typical desert-oasis region in Northwest China. Four measurement sites (early growth stage, rapid growth stage, peak growth stage, and senescence stage) represent the different succession stages of a Nitraria dune. Meteorological parameters were measured by Bowen ratio system, vegetation features and soil physical properties were measured by conventional methods, soil evaporation and transpiration by thermal remote sensing and three-temperature model (3T model), soil moisture by gravimetric and neutron probe method, and evapotranspiration (ET) by Bowen ratio energy balance and water balance methods. Results showed that in a wet year (2008), annual ET was 121, 108, 114, and 126 mm, for the four stages, respectively. The ratio of ET to precipitation (P) was 103, 92, 97, and 107%, respectively. In a dry year (2010), ET was 75, 89, 79, and 79 mm, respectively, while the ET/P was 106, 126, 112, and 112%, respectively. ET accounted for 92-107% of the precipitation in the wet years and 106-126% in the dry years. ET was nearly equal to precipitation in the wet years and greater than precipitation in the dry years, indicating almost all water from precipitation evaporated in all sites. Our results also showed that vegetation coverage in the four stages was 0.15, 0.35, 0.74, and 0.23, respectively. Instantaneous value of T was 0.021, 0.014, 0.033, and 0.003 mm h−1, respectively for the four stages. Instantaneous value of soil evaporation (E) was 0.054, 0.013, 0.004, and 0.009 mm h−1, respectively. The corresponding ET was 0.075, 0.027, 0.037, and 0.012 mm h−1 for the four stages, respectively. The ratio of T/ET was 0.28, 0.52, 0.89, and 0.25, respectively for the four stages. The ratio reached to its maximum value of 0.89 and then began to decrease, which clearly proved our hypothesis. It concluded that the Nitraria plant not only consumes all the rainfall in the growing season, but also some of the water stored in the soil, which gradually consumes all the soil water storage and finally causes the Nitraria plant death. It also concluded that increasing T, decreasing E and keeping a high T/ET ratio is crucial for desert plants to survive. These results show that the above hypothesis is true and it will be useful for vegetation rehabilitation in the desert area.