Reiji Kimura

Climate of the Loess Plateau

The Loess Plateau in China covers an area of ca. 640,000 km2, extending between 34° N and 40° N/100° E and 115° E, which is the middle region of China’s Yellow River. The climate of Loess Plateau is strongly influenced by latitude, longitude, and topography. It has a typical continental monsoon climate. Winters are cold and dry, and most rainfall occurs during the summer (June to September). Annual precipitation is approximately 400 mm (minimum 150 mm, maximum 750 mm). Under China’s physical geography classification, the Loess Plateau is classified as the Loess Plateau sub-region in the North China region. The most significant aspect of its climatological characteristics is the distinct seasonality of temperature and precipitation distribution. According to the aridity index (the value obtained when dividing annual precipitation by potential evapotranspiration) (Millennium Ecosystem Assessment (2005) Ecosystems and human well-being: desertification synthesis. World Resource Institute, Washington, DC), the Loess Plateau belongs to a semiarid area.

Satellite-based mapping of dust erodibility in northeast Asia

Windblown dust originating in China and Mongolia causes health effects and agricultural damage in its source areas and causes Asian dust events in Japan. An early warning system that could be combined with weather forecasts would be helpful in preventing serious damage. However, it is difficult to specify source areas of dust with current dust modeling systems because land surface information, including vegetation coverage and land surface soil water content, is inadequate. To find and monitor dust source regions, a semi-real-time dust erodibility map was developed based on MODIS satellite data that focuses particularly on the threshold wind speed in a target area of northeast Asia including China and Mongolia (35°–50°N, 75°–120°E). The mapping system incorporates satellite data on snow cover, areas of frozen soil, surface soil water content, and vegetation cover.

Effect of flexible and rigid roughness elements on aeolian sand transport

ABSTRACT In developing countermeasures to reduce the negative effects of strong dust events, the fundamental relationship between surface conditions and sand transport remains problematic. We conducted field observations and wind tunnel experiments to examine the effect on sand transport efficiency of actual frontal area when using flexible roughness elements (artificial grass) and rigid roughness elements (tufts of stiff wire). In the field observations, the sand trap ratio approached a limit as the frontal area of artificial grass, measured as the Actual Frontal Area Index (AFAI), approached 25%, equivalent to a vegetation cover of 20%. The wind tunnel experiments showed that the height of deposited sediment decreased downwind with both roughness elements, due to their reduction of both shear velocity and sand transport rates. The sediment flux decreased as the AFAI increased, and the rigid roughness element trapped more sand particles than the flexible one except at higher wind speeds and lower AFAI values. The sand trapping efficiency was greater for rigid roughness elements than for flexible ones, probably due to their high aerodynamic resistance, and thus wire tufts were more effective for trapping sand than artificial grass at higher AFAI values.

Validation and application of the monitoring method for degraded land area based on a dust erodibility in eastern Asia

ABSTRACT Arid regions are very sensitive to climate change and human activity, two critical drivers of change that are degrading environmental conditions. Part of the world’s drylands lie in eastern Asia, including China and Mongolia, where the problems of desertification, drought, and Asian dust events (ADEs) are frequent. To help prevent economic damage from these problems, an early warning and monitoring system based on numerical models, remote sensing, and weather forecasts is needed. I define a degraded land area as ‘the area where dust can easily occur’ and make exclusive use of satellite data to identify land that meets certain conditions of vegetation and aridity. I then validate this definition against a dust erodibility map and occurrences of ADEs over Japan, which was closely related to the extent and severity of dust areas in Mongolia and parts of China, especially in March (coefficient of determination R2 = 0.856). The yearly change of degraded land area indicates a clear decreasing trend in China (R2 = 0.210), but an overall negative trend in Mongolia (R2 = 0.010). Years of major droughts in China and Mongolia correspond well to large positive deviations in degraded land area.

Monitoring Regional Desertification

The monitoring of desertification requires important biological and physical methods. In this chapter, diagnostic methods for regional desertification of the Loess Plateau will be introduced using water balance and water-use efficiency obtained by actual field observation and remote sensing techniques. A wetness index using a numerical simulation soil model (see Chap. 2) and meteorological data from 43 observatories was developed for diagnostic methods at the macro scale (across the Loess Plateau). Potential distribution of vegetation cover in the plateau was explored by comparing the distribution as determined by the wetness index with the present-day vegetation cover from satellite imagery.