The effects of a dry sand layer on groundwater recharge in extremely arid areas: field study in the western Hexi Corridor of northwestern China

Evaporation capacity is an important factor that cannot be ignored when judging whether extreme precipitation events will produce groundwater recharge. The evaporation layer’s role in groundwater recharge was evaluated using a lysimeter simulation experiment in the desert area of Dunhuang, in the western part of the Hexi Corridor in northwestern China’s Gansu Province. The annual precipitation in the study area is extremely low, averaging 38.87 mm during the 60-year study period, and daily pan evaporation amounts to 2,486 mm. Three simulated precipitation regimes (normal, 10 mm; ordinary annual maximum, 21 mm; and extreme, 31 mm) were used in the lysimeter simulation to allow monitoring of water movement and weighing to detect evaporative losses. The differences in soil-water content to a depth of 50 cm in the soil profile significantly affected rainfall infiltration during the initial stages of rainfall events. It was found that the presence of a dry 50-cm-deep sand layer was the key factor for “potential recharge” after the three rainfall events. Daily precipitation events less than 20 mm did not produce groundwater recharge because of the barrier effect created by the dry sand. Infiltration totaled 0.68 mm and penetrated to a depth below 50 cm with 31 mm of rainfall, representing potential recharge equivalent to 1.7 % of the rainfall. This suggests that only extreme precipitation events offer the possibility of recharge of groundwater in this extremely arid area.     

Soil CO2 concentration in biological soil crusts and its driving factors in a revegetated area of the Tengger Desert,Northern China

Biological soil crusts (BSCs) are an important cover in arid desert landscapes, and have a profound effect on the CO₂ exchange in the desert system. Although a large number of studies have focused on the CO₂ flux at the soil–air interface, relatively few studies have examined the soil CO₂ concentration in individual layers of the soil profile. In this study, the spatiotemporal dynamics of CO₂ concentration throughout the soil profile under two typical BSCs (algae crusts and moss crusts) and its driving factors were examined in a revegetated sandy area of the Tengger Desert from Mar 2010 to Oct 2012. Our results showed that the mean values of the vertical soil CO₂ concentrations under algal crusts and moss crusts were 600–1,200 μmol/mol at the 0–40 cm soil profiles and increased linearly with soil depth. Daily CO₂ concentrations showed a single-peak curve and often had a 1–2 h time delay after the maximum soil temperature. During the rainy season, the mean soil CO₂ concentration profile was 1,200–2,000 μmol/mol, which was 2–5 times higher as compared to the dry season (400–800 μmol/mol). Annually, soil moisture content was the key limiting factor of the soil CO₂ concentration, but at the daily time scale, soil temperature was the main limiting factor. Combined with infiltration depth of crusted soils, we predicted that precipitation of 10–15 mm was the most effective driving factor in arid desert regions.     

Groundwater recharge estimation using chloride,stable isotopes and tritium profiles in the sands of northwestern Senegal

A study of environmental chloride, deuterium, oxygen-18, and tritium in deep sand profiles (35 m) has been carried out in order to estimate their relative value for measuring average groundwater recharge. The investigation was located at a 0.1-km² site in Quaternary sands near the northwestern coast of Senegal in a zone of rainfed agriculture. By using a steady-state model for duplicate unsaturated zone chloride profiles, the long-term average recharge at the site was estimated to be 30 mm yr^–1 or around 10% of the average precipitation (290 mm). The chloride concentration of adjacent shallow groundwater was relatively uniform and comparable to the unsaturated zone average, while the spatial variability in the depth distribution of Cl^– in the unsaturated zone was considerable. Stable isotope (deuterium and oxygen-18) data show that there is some isotopic enrichment due to direct evaporation through the soil surface. The degree of heavy isotope enrichment is proportional to the extent of evaporative loss and there is good correspondance with the chloride enrichment. Nevertheless, stable isotopes cannot be used quantitatively to estimate the recharge. The excellent preservation of the peak in thermonuclear tritium in precipitation in the unsaturated zone at depths between 12 and 20 m enables an estimated annual recharge of 24 mm yr^–1 in this area to be calculated, using the piston flow model. Agreement therefore between Cl and³H as tools for recharge measurement is reasonable over the site.     

Study of evaporation and recharge in desert soil using environmental tracers, New Mexico, USA

 The purpose of this study is to investigate the rates and mechanisms of recharge and evaporation in soils of a desert environment using two environmental tracers (chloride and oxygen-18). The profiles of chloride concentration and oxygen-18 enrichment in soil-water, together with the depth distribution of water content in soil, reveal information about long-term recharge and instantaneous evaporation processes without needing to know the physical properties of the soil. Three holes were hand-augured, in different desert settings in southeastern New Mexico. The chloride concentration profiles were used, with the chloride mass balance method, to estimate long-term recharge rates in these three holes as 0.5, 0.8, and 2.4 mm yr^–1. Analysis using a bimodal flow and transport model shows that possibly 85% of the recharge occurs via movement of water through preferred pathways in the root zone. Preferential flow was evident in all three sampling sites. Clay layers have a noticeable effect on the development of water content distribution and thus on oxygen-18 enrichment and chloride concentration profiles. The spatial variation in clay layering partly explains the variation in recharge rate estimates. Received: 13 October 1995 · Accepted: 15 November 1995     

西北干旱内陆盆地区裸土蒸发强度

鉴于包气带蒸发的水分是从地表散失,故查明裸土蒸发规律对研究包气带水分蒸发、潜水蒸发问题具有重要意义。为了查明西北干旱内陆盆地区裸土蒸发强度规律,通过开展裸土含水率观测、包气带负压观测等野外原位试验,结合气象资料,对包气带剖面流场特征进行分析,采用有限差分法对裸土蒸发强度进行计算。结果表明：依据包气带水分在垂向上迁移的不同状态,将裸土区包气带剖面划分为水分向上传输带和水分向下迁移带2个带;西北内陆干旱盆地裸土包气带极限蒸发深度为1.2 m;受外界蒸发能力和包气带向上输水能力交替控制作用,盆地内裸土蒸发出现在4月下半月至9月下半月,其中4月下半月至5月裸土蒸发强度最大,平均蒸发强度达0.56 mm/d;裸土蒸发强度平均仅占水面蒸发强度的3.5%,最大达6.7%。     

The surface energy budget on the debris-covered Koxkar Glacier in China

Energy fluxes were measured by using the eddy covariance system plus an automatic weather station at the debris-covered area on Koxkar Glacier from March to August, 2009. The coldest month of the glacier was January, and air temperature reached a maximum in July and August. Wind velocity at 2.0 m was higher in summer and lower in winter as a whole. Precipitation was concentrated from May to September and accounted for about 80 % of the total. Daily latent heat fluxes were higher than daily sensible heat fluxes during the observation period. The main reason for higher latent heat fluxes from March to April was snow cover. From June to August, latent heat fluxes during the daytime were limited by surface water content, and were lower than sensible heat fluxes, but latent heat fluxes were higher than sensible heat fluxes during summer nights because of air convection in the debris layer. Summer evaporation was higher than in the spring, and evaporation was 53.7 % of the precipitation from 19 June to 23 August. The Bowen ratio ranged from ?2.0 to 2.0 at the site.     

运用溴离子示踪法评价玛纳斯河流域平原区潜水蒸发

潜水蒸发是干旱内陆盆地区地下水的主要排泄方式,但其定量评价存在很大的不确定性,是水均衡分析和水资源评价的难点.在新疆玛纳斯河流域平原区,以溴离子为示踪剂于2017年5月在不同土地利用类型区试验点投溴化钠,分别于2017年8月和2018年8月采样测定溴离子垂向分布,根据溴离子垂向运移速率确定潜水蒸发速率.试验点溴离子的浓度在垂向剖面上的分布呈现单峰形态且峰值上移,根据其峰值上移距离计算得出非沙漠区年平均潜水蒸发量为33.59 mm;不同土地利用类型潜水蒸发由强至弱依次为棉田、荒地、林地和沙漠,年平均蒸发量分别为41.71 mm、34.01 mm、11.28 mm、8.58 mm.溴离子示踪法评价潜水蒸发量的结果与前人相符;潜水蒸发速率与土地利用类型、包气带岩性、土壤体积含水量和潜水埋深有关,岩层粘粒含量越高、含水量越高、越靠近细土平原低地势区,潜水蒸发作用越强.      

Lead enrichment, adsorption and speciation in urban garden soils under long-term wastewater irrigation in northern Nigeria

Unsafe lead (Pb) concentrations in leafy vegetables raised in urban and peri-urban agricultural production systems have been reported across cities in Northern Nigeria, even though Pb concentrations in soils are within regulatory safe levels. This study examined the soil enrichment, adsorption and chemical species of Pb in urban garden fields irrigated with untreated wastewater at three industrial locations in Kano, northern Nigeria. Total Pb in the soil profiles ranged from 9 to 91 mg kg^?1 and decreased rapidly from the surface to the subsurface layer, but attaining nearly constant concentration at depth ≥1.2 m in the profiles. The potentially labile Pb maintained fairly constant concentration with depth up to 0.9 m, but decreased fairly rapidly with depth thereafter. There was a significant Pb enrichment of the soils, extending up to 30–60 cm depth in the soil profiles. The adsorption of Pb by the soils increased drastically with pH, and attained maximum adsorption at pH ≥ 7.0 in the surface layer, and at pH ≥ 6 in the subsurface layer. The surface soils adsorbed between 85 and 97 % of added Pb at pH ≤ 5. Free Pb²⁺ activities in soil solution accounted for between 46 and 87 % at pH 5–7 of total dissolved Pb (Pb_T). The quantifiable chemical species of Pb in solution consisted mainly of PbOH⁺, PbSO ₄ ^· , PbCl⁺ and PbOH ₂ ^· which accounted for between 0.9 and 26 % of Pb_T in soil solution at pH ≥ 5.0, but declining to between 0.1 and 2.1 % at pH ≥ 7.5. There was no apparent equilibrium between Pb²⁺ activities and known Pb-compounds in the soils. It was concluded from the data that reports of excess Pb concentrations in leafy vegetables raised in these soils are consistent with high free Pb²⁺ activities maintained in soil solution by these predominantly sandy-textured soils.     

Variations in dust-related PM<Subscript>10</Subscript> emission from an arid land due to surface composition and topsoil disturbance

Aeolian (wind) erosion is most common in arid regions. The resulted emission of PM₁₀ (particulate matter that is smaller than 10 μm in diameter) from the soil has many environmental and socioeconomic consequences such as soil degradation and air pollution. Topsoil resistance to aeolian transport highly depends on the surface composition. The study aim was to examine variations in PM₁₀ fluxes in a desert-dust source due to surface composition and topsoil disturbance. Aeolian field experiments using a boundary layer wind tunnel alongside soil composition analysis were integrated in this study. The results show variations in PM₁₀ fluxes (ranging from 9.5 to 524.6 mg m^?2 min^?1) in the studied area. Higher wind velocity increased significantly the PM₁₀ fluxes in all surface compositions. A short-term natural disturbance caused changes in the aggregate soil distribution (ASD) and increased significantly PM₁₀ emissions. Considering that PM₁₀ contains clays, organic matter, and absorbed elements, the recorded PM₁₀ fluxes are indicative of the potential soil loss and degradation by wind erosion in such resource-limited ecosystems. The findings have implications in modeling dust emission from a source area with complex surfaces.     

Formation and development of salt crusts on soil surfaces

The salt concentration gradually increases at the soil free surface when the evaporation rate exceeds the diffusive counter transport. Eventually, salt precipitates and crystals form a porous sodium chloride crust with a porosity of 0.43 ± 0.14. After detaching from soils, the salt crust still experiences water condensation and salt deliquescence at the bottom, brine transport across the crust driven by the humidity gradient, and continued air-side precipitation. This transport mechanism allows salt crust migration away from the soil surface at a rate of 5 μm/h forming salt domes above soil surfaces. The surface characteristics of mineral substrates and the evaporation rate affect the morphology and the crystal size of precipitated salt. In particular, substrate hydrophobicity and low evaporation rate suppress salt spreading.     

黑河流域山区植被生态水文功能的研究

依据土壤-植被-大气系统的结构特性,从林冠层、苔藓-枯枝落叶层、土壤层剖面结构分析了黑河流域山区水源涵养林在水文过程中的作用.观测试验表明,林冠截留大气降水的32.7%,使到达林地的水分相对减少而养分增加,而林冠遮荫使林内土壤蒸发仅为林外草地的34.2%.苔藓-枯枝落叶层疏松多孔,最大持水量可达12.5mm水层深,加上表层较高的体积含水量和较小的水分变差系数,使其在涵蓄一部分大气降水的同时具有良好的保水性能.林地土壤具有良好的渗透性和涵蓄大气降水的能力,从而减少了地表径流量.森林的蒸散发使林区空气湿度高于周边地区17%,形成山区独特的森林小气候,从而进一步影响着山区的水文过程.     

Heavy metal pollution in the soil surrounding a thermal power plant in Playas de Rosarito,Mexico

The generation of electricity has been identified as one of the main pollutant activities, and some studies have established an increment of heavy metals in soil in the areas surrounding these plants. The aim of this study was to evaluate the soil concentrations of heavy metals in the zone surrounding a thermoelectric power in Mexico. Thirty-two top soil samples (0–5 cm) were collected; additionally, four depth profiles (1 m) were investigated. Median concentrations for chromium, vanadium, nickel, mercury, and cadmium were 47, 47, 73, 0.02, and 0.01 mg/kg, respectively. Higher Cr, Ni, and V concentrations were observed in the soil depth profiles located closer to the plant in comparison with the concentrations found in the soil depth profile located further away from the plant; these results may indicate a possible accumulation of these metals. The geoaccumulation index results indicated that most of the sites were in the classifications of unpolluted and unpolluted to moderately polluted (classes 1 and 2). The statistical results showed that downwind of the plant in relation to the prevailing winds, there was a strong correlation between soil concentrations of chromium, copper, nickel, and vanadium. Based on the results of this study, it can be concluded that the use of fuel oil at the thermoelectric plant contributed to the accumulation of vanadium and nickel in the soil of the surrounding areas, as well as chromium and copper.     

阿拉善沙漠湿沙层水分来源同位素示踪

为弄清阿拉善沙漠湿沙层的水分来源,在该地区进行了人工模拟降水入渗的示踪试验。模拟单次降水量为59 mm,观察剖面最大入渗深度仅为46 cm,这一结果表明该地区的降水几乎不能通过沙层入渗到地下水中。对4个沙丘湿沙层剖面中不同深度的含水率、Cl-、δD与δ18O进行了分析,数据显示在蒸发能力极强的阿拉善地区,地下水是以薄膜水的形式,通过蒸发、凝结向地表运动,最终蒸发排泄。泉水、井水、湖水与土壤水中的同位素特征表明具有相同的补给源,均来自于地下水。推断横穿阿拉善地块的杂多-雅布赖断裂带与狼山-日喀则断裂带中可能存在地下水深循环通道,青藏高原河流、湖泊的渗漏水可能是阿拉善地下水的主要补给源。     

Oxygen Consumption in Permeable and Cohesive Sediments of the Gulf of Aqaba

Oxygen profiles were measured in the sediments of the Gulf of Aqaba (Red Sea), an oligotrophic marine system affected by episodic seasonal flash floods and intense aeolian dry deposition. Sediment cores were retrieved from shallow (15–45 m), intermediate (250–561 m) and deep (700 m) water sites of south–north and east–west transects. Dissolved oxygen concentrations were measured simultaneously by using microelectrodes and microoptodes immediately after sampling and after transportation. Oxygen penetration depths were found to increase from 2 to 5 mm at the shallow water sites with sandy permeable sediments to 10–21 mm at the deeper sites with cohesive muddy sediments. This increase corresponds to decrease in oxygen diffusive fluxes at the sediment–water interface and oxygen consumption rates with depth. Oxygen consumption rates exhibit local maxima at the oxic–anoxic sediment boundary, which may be attributed to oxygen reduction coupled to oxidation of dissolved Fe(II) and Mn(II) at deep and intermediate water sites and of hydrogen sulfide at shallow water sites. Microelectrodes and microoptodes measurements of cohesive sediments from deep and intermediate water sites yielded similar results. By comparison, the microoptodes displayed more robust measurements than microelectrodes in sandy near-shore sediments. This was attributed to their flexible fiber structure that is less likely to break or to abruptly displace sand particles. After transportation of sediment cores from Eilat to Beer Sheva followed by ≤?24-h storage, no changes in oxygen fluxes and consumption rates were detected.     

基于蒸渗仪和解析法估算毛乌素沙地潜水蒸发量

地下水蒸发是旱区地下水均衡计算中重要的排泄项之一。由于包气带水分运移高度非线性且大气—地表界面动力学过程复杂,估算潜水蒸发量一直是地下水资源评价的难题之一。利用内蒙古乌审旗河南乡均衡试验场E601型蒸渗仪,建立了毛乌素沙地水面蒸发及4种典型岩性（风化砂岩K₁、萨拉乌苏组砂Qpal+l、砂质壤土Qhl、风积沙Qheol）的饱和土蒸发原位试验,结合长期观测获取的大量数据,开展了地下水蒸发与水面蒸发、埋深的关系和地下水蒸发量计算方法研究。结果表明:（1）4种典型岩性（风化砂岩、萨拉乌苏组砂、砂质壤土、风积沙）饱和蒸发量与水面蒸发量比值分别为0.60,0.77,0.47,0.88,表明不同岩性的饱和裸土的蒸发强度不等于自由水面的蒸发强度;实际计算裸土蒸发强度时,不能以自由水面蒸发强度作为参考点,如果运用,必须校正。（2）利用蒸渗仪观测数据和土壤水运动方程稳态解析解,获得4种典型岩性（风化砂岩、萨拉乌苏组砂、砂质壤土、风积沙）潜水稳定蒸发计算的关键经验系数c,分别为628932.63,165058.71,48948.21,1525104.031 m?2。（3）利用稳定蒸发公式确定鄂尔多斯盆地风沙滩区四种典型包气带岩性（风化砂岩、萨拉乌苏组砂、砂质壤土、风积沙）潜水极限蒸发深度约为60 cm,结果得到了室内非稳态蒸发试验的佐证,为研究区水资源评价提供了重要的参数依据。     

Spatial and seasonal distributions of soil phosphorus in a typical seasonal flooding wetland of the Yellow River Delta,China

Soil samples from 0 to 100 cm depth were collected in four sampling sites (Sites A, B, C and D) along a 250-m length of sampling zone from the Yellow River channel to a tidal creek in a seasonal flooding wetland of the Yellow River Delta of China in fall of 2007 and spring of 2008 to investigate spatial and seasonal distribution patterns of total phosphorous (TP) and available phosphorus (AP) and their influencing factors. Our results showed that TP contents in spring and AP contents in both seasons in surface soils increased with increasing distances away from the Yellow River channel. TP contents in surface soils (0–10 cm) followed the order Site A (698.6 mg/kg) > Site B (688.0 mg/kg) > Site C (638.8 mg/kg) > Site D (599.2 mg/kg) in fall, while Site C (699.6 mg/kg) > Site D (651.7 mg/kg) > Site B (593.6 mg/kg) > Site A (577.5 mg/kg) in spring. Generally, lower TP content (630.6 mg/kg) and higher AP level (6.2 mg/kg) in surface soils were observed in spring compared to fall (656.2 mg/kg for TP and 5.2 mg/kg for AP). Both TP and AP exhibited similar profile distribution patterns and decreased with depth along soil profiles with one or two accumulation peaks at the depth of 40–80 cm. Although the mean TP content in soil profiles was slightly higher in spring (635.7 mg/kg) than that in fall (628.0 mg/kg), the mean TP stock was obviously lower in spring (959.9 g/m²) with an obvious accumulation at the 60–80 cm soil depth compared to fall (1124.6 g/m²). Topsoil concentration factors also indicated that TP and AP had shallower distribution in soil profiles. Correlation analysis showed that AP had significant and positive correlation with these soil properties such as soil organic matter, salinity, total nitrogen and Al (p < 0.01), but TP was just significantly correlated with TN and Al (p < 0.05).     

The impact of stormwater on a soil profile in an infiltration basin

The effects of urban stormwater on the soil of an infiltration/ holding basin were studied within the framework of research carried out by OTHU (Urban Hydrology Field Observatory of Lyon, France). The biophysicochemical impacts of stormwater from an industrial watershed on the local soil (to a depth of 4 m in the unsaturated zone) were measured. Several types of measurement (pH, organic matter, particle size, heavy metals content, and heterotrophic viable bacterial counts) for three vertical soil profiles were carried out. High concentrations of heavy metals and significant variations in pH and silt to a depth of 1.5 m were observed. The concentrations decreased as a function of distance from the stormwater discharge pipe. Changes in the bacterial population were also observed, varying in accordance with the depth and location of the profile.

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Resumen Se estudiaron los efectos de tormentas en zonas urbanas sobre el suelo de una cuenca retenedora/de infiltración dentro del marco de investigación llevado a cabo por OTHU (Laboratorio de Observación de Campo de Hidrología Urbana, Lyon, Francia). Se midieron los impactos biofisicoquímicos de la tormenta en una cuenca industrial sobre el suelo local (a una profundidad de 4m en la zona no saturada). Se llevaron a cabo varios tipos de mediciones (pH, materia org′nica, tama?o de partículas, contenido de metales pesados, y conteo de bacterias viables heterotróficas) en tres perfiles ed′ficos. Se observaron altas concentraciones de metales pesados y variaciones significativas en pH y limo a una profundidad de 1.5m. Las concentraciones disminuyeron en función de la distancia del tubo de descarga de las aguas de la tormenta. También se observaron cambios en las poblaciones de bacterias los cuales variaban en función de la profundidad y localización del perfil.

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Résumé Dans le cadre de la fédération de recherche OTHU (Observatoire de Terrain en Hydrologie Urbaine de Lyon) une étude sur l’impact des eaux pluviales sur le sol d’un bassin d’infiltration / rétention est présentée. Il s’agit d’évaluer l’impact biophysicochimique des eaux pluviales issues d’un bassin versant industriel sur une profondeur d’environ 4 m de zone non saturée. Pour cela plusieurs types de mesures ont été effectuées (pH, perte au feu, teneur en Cu, Pb, Cd, dénombrement bactérien) suivant trois profils verticaux. Les résultats montrent des concentrations importantes de métaux et des variations de pH jusqu’à 1.5 m de profondeur. De plus, les résultats montrent une variation suivant la position des points de prélèvements. Des modifications de la population bactérienne sont également observées suivant la profondeur et l’emplacement des profils.

     

Effects of straw mulching on soil evaporation during the soil thawing period in a cold region in northeastern China

To study the effect of straw mulching on soil water evaporation, it is necessary to measure soil water evaporation under different conditions of straw mulching during the soil thawing period. A field experiment was conducted in winter, and soil evaporation was measured using a microlysimeter on bare land (LD) and 4500 (GF4500), 9000 (GF9000) and 13500 \(\hbox {kg/hm}^{2}\) (GF13500) straw mulch. The influence of different quantities of straw mulch on soil water evaporation during the thawing period was analyzed using the Mallat algorithm, statistical analysis and information cost function. The results showed that straw mulching could delay the thawing of the surface soil by 3–6 d, decrease the speed at which the surface soil thaws by 0.40–0.80 cm/d, delay the peak soil liquid water content, increase the soil liquid water content, reduce the cumulative evaporation by 2.70–7.40 mm in the thawing period, increase the range of soil evaporation by 0.04–0.10 mm in the early stage of the thawing period, and reduce the range of soil evaporation by 0.25–0.90 mm in the late stage of the thawing period. Straw mulching could reduce the range of and variation in soil evaporation and can reduce the effect of random factors on soil evaporation. When the amount of straw mulch exceeded 9000 \(\hbox {kg/hm}^{2}\), the effect of increasing the amount of straw mulch on daily soil water evaporation was small.     

Comparative analysis of groundwater formation in arid and super-arid deserts (with examples from Central Asia and northeastern Arabian Peninsula)

Groundwater formation within arid and super-arid deserts is discussed through comparative analysis. Although fresh groundwater is being formed in both types of deserts, mainly from infiltrated surface runoff, in arid deserts, infiltration is currently taking place, whereas in super-arid areas, it occurred mainly in the Pleistocene pluvial epoch. Groundwater discharge conditions are also different. In arid deserts, the discharge occurs mainly through transpiration by phreatophytes, which form phyto-hydrogeological ecosystems where the dominant factor is an interaction between groundwater and vegetation, with transpiration reaching over hundreds of mm/year and the thickness of the groundwater evaporation zone extending over 10 m. Active subsurface water exchange does not favor the preservation of Pleistocene fossil groundwater. Super-arid deserts, however, have an extremely scarce vegetative cover (low, if any), physical evaporation, a thin zone of evaporation influence, and slow water outflow. Favorable conditions therefore exist for the preservation of relict low-mineralized waters. Furthermore, arid and super-arid deserts differ by types of groundwater accumulation and horizontal and vertical zonalities. In arid deserts, hydrogeological problems are connected with the anthropogenic influence upon groundwater, and with control on rational use of surface water and groundwater. In super-arid deserts, the basic problem is depletion of fossil groundwater.     

NDVI dynamic changes and their relationship with meteorological factors and soil moisture

Normalized difference vegetation index (NDVI) is an important indicator for measuring vegetation coverage, which is of great significance for evaluating vegetation dynamics and vegetation restoration. It can clearly analyze the suitable growth condition of vegetation by studying the relationship between meteorological factors, soil moisture and NDVI. Based on MODIS/NDVI data, the spatio-temporal characteristics of vegetation coverage in the Weihe River Basin (WRB) were analyzed by the trend analysis method. The relationship of NDVI with meteorological factors and NDVI with soil moisture simulated by the soil and water assessment tool (SWAT) model was analyzed in this paper. The results show that NDVI values gradually change with an increase from north to south in the WRB. The maximum of the average monthly NDVI is 0.702 (August) and the minimum is 0.288 in February from 2000 to 2015. The results of the seven grades of NDVI trend line slope indicate that the improvement area of vegetation coverage accounts for 30.93% of the total basin, and the degradation area and basically unchanged area account for 23% and 42.9%, respectively. The annual mean soil moisture is 19.37% in the WRB. There was a strong correlation between NDVI and precipitation, temperature, evaporation and soil moisture, and the correlation coefficients were 0.78, 0.89, 0.71 and 0.65, respectively. The ranges of the most suitable growth conditions for vegetation are 80–145 mm (precipitation), 13–23 °C (temperature), 94–144 mm (evaporation) and 25–33% (soil moisture), respectively.