دانلود رایگان مقاله اثرات محافظتی نبخا بر روی واحه

abstract

Oases are of great importance to the sustainable development of economies and societies in desert regions. Thus, it is important to understand how many oases have survived for hundreds and even thousands of years. Oases in Northwest China include nearly all of the types found across the world. Among these oases, the Minqin Oasis, surrounded by mobile sandy deserts, is a stereotypical natural oasis that has been developed for more than 2000 years, making it ideal for a case study. Our investigation indicates that the nebkhas belt at the fringe of the Minqin Oasis, which consists of three sub-belts semi-mobile, semi-fixed and fixed nebkhas from desert to the oasis, plays a key role in its stability. The fractional speed-up ratio (ds) of wind generally fell within the range of
0.15 to
0.55 below the height of 0.6 m. Above the height of 0.6 m, wind velocity in the semi-mobile nebkhas was 5–27% weaker with a maximum reduction of 65% in the other two sub-belts. When wind carrying sand and dust from the desert passes through each sub-belt, its velocity is weakened by vegetation and dunes, and almost all sand and dust is deposited in the nebkha belt incrementally by grain size, ranging from coarse to fine. As such, nebkhas serve as a protection belt and ensure the existence of oases

5. Discussion and conclusions

When the ratio (L/l) of downwind distance (L) to the height (l) of a dune reaches 8–10, wind velocity is gradually restored to that of free wind, and an internal boundary layer begins to form. When L/l reaches 25–30, the internal boundary layer develops stably, and the wind profile is close to that calculated for sites upwind (Walker and Nickling, 2002). Site 10 of the observation section was located in cropland of the Minqin Oasis (Fig. 3), downwind of site 9 in the fixed nebkha sub-belt. The L/l value for site 10 was 13.3, within the range of 10–25, indicating that the wind velocity at this position was gradually returning to that of winds beyond the protective scope of the nebkha belt. However, the nebkha belt protects the oasis not only by reducing wind velocity but also by intercepting highly concentrated windblown sand from mobile dunes in the desert. When sand-laden wind passed through the nebkha belt, almost all of the particles were deposited in the belt. Thus, even though wind velocity was gradually restored within the oasis, the damage caused by windblown sand to the inner ecosystem of the oasis was minimized. Although the average height of Tangut Nitraria shrubs, T. ramosissima shrubs and Songory Reaumuria shrubs was less than 0.6 m, 1.5 m and 2.3 m, respectively, these shrubs played the most important role in the interception of windblown sand. Below the height of 0.6 m, except at site 2 on the mobile dune, ds values at the observation sites were negative. With the increase of VC, ds values showed a decreasing trend, and at the height of 0.2 m, the minimum value of ds was below
0.95 (Fig. 3a). In the typical nebkha observation area, with the exception of a small number of bare inter-dunes, ds values below the height of 0.6 m were negative and generally fell within the range of
0.15 to
0.55; at the height of 0.2 m, the minimum ds fell below
0.6 (Figs. 7 and 8a). This result indicated that vegetation has significant effects on reducing wind velocity below canopy height. Previous research has shown that approximately 80% of total windblown sand flux occurs below the height of 0.3 m, and more than 90% of total windblown sand flux occurs below the height of 2.0 m (Wu, 2010). Based on the finding that dunes weaken near surface wind velocity, we conclude that shrub vegetation intercepts the vast majority of windblown sand originating from mobile dunes.