Revisit of event-based rainfall characteristics at Shapotou area in northern China

Distribution of rainfall event sizes and interval lengths between events are important characteristics of arid and semi-arid climates. Understanding their importance will contribute to our ability to understand ecosystem dynamics in these regions. Rainfall event timing and magnitude are important drivers of ecosystem processes and are instrumental in creating landscape heterogeneity in arid and semi-arid regions. Rainfall event characteristics were analyzed using an automatic tipping-bucket rain-gauge record across the entire summer monsoon season from 2008 to 2015 at the arid desert area of Shapotou in the Tengger Desert, China. Changing the minimum inter-event time (MIT) from 30 min to 24 h alters the number of rainfall events from 64 to 25 for the event depth larger than 0.1 mm. The mean rainfall intensity declined from 0.95 mm/h to 0.53 mm/h, and the geometric mean event duration rose from 0.55 h to 4.4 h. The number of rainfall events, mean rainfall intensity, and geometric mean event duration differed under different criteria of individual rainfall depths, except that for an individual rainfall depth of 0.5, 1.0, and 5.0 mm. The aforementioned features differed only at the lowest range of the mean rainfall intensity and depth for MIT=3 and 6 h. These findings suggest that identification of event-based rainfall in this specific arid region can be better achieved by setting the MIT at six hours. The wide variation in rainfall event properties indicate the need for paying more attention to the proper selection and reporting of event criteria in studies that adopt event-based data analysis. This is especially true in quantifying effective rainfall for soil water replenishment in terms of rainfall depth and intensity with infrequent rainfall events.     

塔克拉玛干沙漠石英氧同位素与粒级关系的研究

沙漠与黄土的物源联系是黄土研究关注的问题之一。石英氧同位素可示踪物源,粒度被用以分析沉积环境,但在沙漠物源研究中将二者结合应用的报道尚不多。本文测定了塔克拉玛干沙漠沙丘沙和沙丘剖面中河湖相沉积物的粒度及不同粒级的石英氧同位素,结果显示:1沙丘沙以极细砂和细砂为主,粉砂和黏土含量很低;河湖相沉积物以粉砂和黏土为主,砂含量低于5%。2各粒级石英δ~(18)OSMOW值介于13.8‰~19.7‰,其中沙丘沙为13.8‰~19.7‰,河湖相沉积物为15.6‰~17.6‰;沙丘沙石英δ~(18)O值随粒级增大未呈现减小趋势,同一粒级石英的δ~(18)O值存在一定变异。3经比较,塔克拉玛干沙漠砂和粉砂粒级的石英δ~(18)O值均高于中国其他沙漠,且20μm的石英δ~(18)O值与黄土高原的马兰黄土、洛川黄土—古土壤序列20μm的石英δ~(18)O值并无显著性差异。     

IMPACT OF DESERT DUST ON THE EAST ASIAN SUMMER MONSOON

The radiative forcing (RF) of Asian desert dust and its regional feedbacks to the East Asian summer monsoon (EASM) system are investigated with a coupled regional climate-desert dust model. The statistical significance of desert dust effects are analyzed through 20 summer seasons (1990-2009). In order to estimate the dust effects reasonably, some improvement has been achieved for the coupled model, including the updates of optical properties and desert source area distribution. We find that the desert dust can result in a roughly weakened monsoon in eastern China, Korean Peninsula, Japan and Indian Peninsula and a strengthened monsoon in Indochina Peninsula in the lower troposphere. Moreover, the precipitation comparisons between observational data and simulated patterns are also suggestive of the desert dust effect on the EASM. In the upper troposphere, the southward shift of the westerly jet (WJ) by the dust effect can be seen as an indicator of the weakened monsoon in great part of the monsoon areas. The change of the moist static energy (MSE) contrast between land and ocean is the main reason for the EASM variations.     

Suburban Food Security: Walkability and Nutritional Access in Metropolitan Detroit

This article explored the relationship between the local food environment and walkability in the socioeconomically diverse, inner-ring suburbs of metropolitan Detroit. The availability and cost differentials of food were surveyed using a modified version of the Nutrition Environment Measures Survey for Stores (NEMS–S), and a geographic information system–based method was designed to map walkability relative to licensed food retail establishments. Results showed that minority communities lack access to fresh produce and nutritionally adequate foods, which is compounded by limited mobility. By incorporating local-based economic incentives, low-income communities can better align neighborhood goals of obtaining nutritionally adequate food (and other services) with economic opportunity.     

The Effects of Spatial Scale and Aggregation on Food Access Assessment: A Case Study of Tucson,Arizona

Adequate access to healthy food has become a social issue due to the recent Great Recession and heightened levels of unemployment. Geographers have focused their attention on how to accurately evaluate food access and how to identify and delineate food deserts; that is, low-income neighborhoods where affordable and healthy food is lacking or limited. Findings of recent food access studies are, however, dramatically inconsistent. We argue that spatial scale and the level of aggregation used in constructing food access measures could account for a major portion of the varying results. We draw on an empirical study in the Tucson, Arizona, metropolitan area, to examine how varying geographic scales and aggregation methods affect food access assessment. We also provide an analysis to show how spatial scale and aggregation practices lead to inconsistent conclusions about food access and designation of food deserts.     

Fully coupled free‐surface flow and sediment transport modelling of flash floods in a desert stream in the Mojave Desert,California

For most of the year, a dry‐bed desert wash is void of water flow. Intensive rain events, however, could trigger significant flash floods that bring about highly complicated hydrodynamics and morphodynamics processes within a desert stream. We present a fully coupled three‐phase flow model of air, water, and sediment to simulate numerically the propagation of a flash flood in a field‐scale fluvial desert stream, the so‐called Tex Wash located in the Mojave Desert, California, United States. The turbulent flow of the flash flood is computed using the three‐dimensional unsteady Reynolds‐averaged Navier–Stokes equations closed with the shear stress transport k ? ω model. The free surface of the flash flood at the interface of air and water phases is computed with the level‐set method, which enables instantaneous tracking of the water surface as the flash flood propagates over the dry bed of the desert stream. The evolution of the desert fluvial stream's morphology, due to the action of the propagating flash flood on the mobile bed, is calculated using a Eulerian morphodynamics model based on the curvilinear immersed boundary method. The capabilities of the proposed numerical framework are demonstrated by applying it to simulate a flash flood event in a 0.65‐km ‐long reach of the Tex Wash, the intricate channel morphology of which is obtained using light imaging detection and ranging technology. The simulated region of the stream includes a number of bridge foundations. The simulation results of the model for the flash flood event revealed the formation of a highly complex flow field and scour patterns within the stream. Moreover, our simulation results showed that most scour processes take place during the steady phase of the flash flood, that is, after the flash flood fills the stream. The transient phase of the flash flood is rather short and contributes to a very limited amount of erosion within the desert stream.     

Adsorption of phosphate from the Yellow River water by desert particles: Kinetics and equilibrium studies

The equilibrium and kinetic adsorption of phosphate (P) from the Yellow River water onto desert particles taken from the Ulan Buh and Kubuqi deserts were investigated. Effect of the initial concentration of P and particulate matter on the sorption was explored. The kinetic studies show that adsorption rate was faster within the 0–8th hour period, then gradually decreased at 8th–48th hour period and basically tended to a dynamic equilibrium at the 48th hour, but there exists an adsorption difference of P onto different desert particles. The uptake process of P followed the pseudo-second-order rate model and the Morris-Weber diffusion equation which indicates that the adsorption process was mainly controlled by P diffusion inside the particles. The equilibrium investigation obtained an adsorption-desorption equilibrium mass concentration (EPC₀) of P from the Yellow River water on desert particles with the range of 0.010–0.042 mg/L, which is higher than present P concentration in the Yellow River. This study indicates that desert particles entering the river might release P to the Yellow River.     

南疆沙漠腹地夏季晴天与沙尘日小气候观测对比分析

利用塔克拉玛干沙漠大气环境观测试验站2006年8月13-31日近地层微气象资料以及常规地面观测资料,选取晴天与沙尘日个例,采用波文比能量平衡方法,对比分析了晴天与沙尘日沙漠腹地的小气候和地表能量平衡特征.结果表明,晴天气温、地温日变化幅度均大于沙尘日,各深度地温极值出现时间滞后于沙尘日;晴天夜间近地层存在逆温,沙尘日具有等温性;晴天夜间比湿较大,白天较小,沙尘日夜间比湿变化平缓,在沙尘暴发生时,比湿急剧增大,并有弱的逆湿现象存在;沙尘日总辐射、反射辐射、净辐射和感热通量比晴天明显偏小,向下长波辐射却大于晴天,但潜热变化差异不太明显.     

Estimation of the Threshold Friction Velocities over Various Dust Storm Source Areas in Northwest China

The emission of dust particles into the atmosphere is governed by the aerodynamic and resistant factors,which are quantified by the friction velocity u＊ and the threshold friction velocity u＊t, respectively. The threshold friction velocity u＊t influences the vertical dust flux and dust transport. Based on the micrometeorological data obtained in the springs of 2004 and 2006 over Hunshandake desert area, Loess Plateau,and Gobi desert area, the relationship between dust concentration and friction velocity for the dust events that occurred over Hunshandake desert area was investigated, and the threshold friction velocities over the three different dust source areas were estimated. The results show that the value of dust concentration is low during the preemission stage of a dust storm event, and the rapid increase of friction velocity provides favorable dynamic conditions for dust emission. During the dust emission stage, the dust concentration increases sharply due to mechanical and thermal turbulent mixing. At the calm-down stage, the dust concentration drops nearly linearly with the decreasing friction velocity, on account of the gravitational deposition of larger dust particles. When the dust concentration is higher than 200 μg m-3, it is considered as a dust emission process. According to the criteria, the values of threshold friction velocity over Hunshandake desert area and Gobi region are 0.6 and 0.45 m s-1, respectively. The threshold friction velocity over Loess Plateau depends on the wind direction, due to the complex terrain and inhomogeneous surface. The northwest wind shows the effects of the Mu Us desert in the northwest. The corresponding u＊t is 0.35 m s-1. The south wind exhibits the characteristics of the Loess hilly dunes in the south, and the u＊t is 0.7 m s-1.The large roughness length of the Loess hilly dunes and the large inter-particle cohesion for the clay soil texture increases the local friction velocity. Different threshold friction velocities and occurrence frequencies of strong wind account for different dust emission capabilities for various source areas.     

放牧制度对荒漠草原可萌发土壤种子库的影响

 以内蒙古苏尼特右旗短花针茅荒漠草原为研究对象,通过划区轮牧与自由放牧的比较试验,研究可萌发土壤种子库对不同放牧制度的响应。结果表明:①可萌发种子库物种组成在划区轮牧草地为11种,自由放牧草地为8种,封育禁牧区为9种,划区轮牧较自由放牧和禁牧有利于提高草地群落可萌发土壤种子库的植物种数以及多年生优良牧草;可萌发土壤种子库密度在封育禁牧区为(19 533.33±10 552.83)粒·m-2,划区轮牧区为(3 233.33±524.21)粒·m-2,自由放牧区为(2 553.60±2 152.48)粒·m-2,可萌发土壤种子库密度封育禁牧区显著高于划区轮牧区与自由放牧区,两放牧处理间无显著差异。②不同处理可萌发土壤种子库垂直分布有共同趋势, 有75.06%~83.19%分布在0~5 cm土层内,14.16%~21.68%分布在5~10 cm土层内,2.65%~4.90%分布在10~15 cm土层内,且不同土层可萌发土壤种子库密度均为封育禁牧区显著高于划区轮牧区与自由放牧区,两放牧处理间无显著差异。③划区轮牧区可萌发土壤种子库的丰富度指数和多样性指数高于自由放牧区。划区轮牧区和封育禁牧区可萌发土壤种子库组成的相似性系数最大,为0.857。