Optimization of water-urban-agricultural-ecological land use pattern:A case study of Guanzhong Basin in the southern Loess Plateau of Shaanxi Province,China

Extensive land use will cause many environmental problems.It is an urgent task to improve land use efficiency and optimize land use patterns.In recent years,due to the flow decrease,the Guanzhong Basin in Shaanxi Province is confronted with the problem of insufficient water resources reserve.Based on the Coupled Ground-Water and Surface-Water Flow Model(GSFLOW),this paper evaluates the response of water resources in the basin to changes in land use patterns,optimizes the land use pattern,improves the ecological and economic benefits,and the efficiency of various spatial development,providing a reference for ecological protection and high-quality development of the Yellow River Basin.The research shows that the land use pattern in the Guanzhong Basin should be further optimized.Under the condition of considering ecological and economic development,the percentage change of the optimum area of farmland,forest,grassland,water area,and urban area compared with the current land use area ratio is+2.3,+2.4,-6.1,+0.2,and+1.6,respectively.The economic and ecological value of land increases by14.1%and 3.1%,respectively,and the number of water resources can increase by 2.5%.

Assessment of Climate for Agricultural Suitability and Optimal Allocation of Agricultural Production in the Guanzhong Region, Shaanxi Province

Based on spatial climatic data of agriculture and the experiment data, the models of agro-ecological assessment of climate for agricultural suitability in this study were developed using the fuzzy mathematical method. Three coefficients, in- cluding the resource coefficient （Cr）, the efficiency coefficient （Ce）, and the utility co- efficient （K）, were used in the models, which were calculated based on temperature, moisture, and sunshine duration data of Guanzhong region, Shaanxi Province. The results indicated that resource coefficient was higher in west of the region than that in east, and higher in south （especially in the Central Shaanxi Plain） than that in the Weibei plateau. The value of Cr changed from 6.5 to 9.2 from north to plain area. Spatial change of efficiency coefficient was obvious, lower in the northeast than in the central plain, and the value of Ce changed from 2.3 to 6.5 from the northeast to the central plain. As for utility coefficient, it was lower in northeastern part of the Weibei plateau and in southern mountain areas than that in the central plain, showing significant latitudinal zonality. Furthermore, the value of K increased from 0.35 to 0.78 from northeast to the central plain, and decreased from 0.78 to 0.53 from the central plain to southern mountain areas. These indicated that climate resource in the central plain region was more abundant and potential, compared with other regions. GuanZhong region was classified into three larger agricultural zones and three small independent zones, according to agro-ecological assessment. Light, heat and water resources should be made use of in an efficient way in spatial allo- cation of agricultural production. For example, water facilities should also be im- proved in Weibei plateau region where highly-qualified fruit should be enhanced and fruit processing industrial chain should be shaped. Large-scale production area of wheat should be increased in central irrigation region and more vegetable bases should be developed around large and medium-scale cities. Thanks for outstanding water conservation function, the three-dimensional agriculture including medicine and other sideline production should be developed in Qinling Mountains and the special- ized commercial agriculture should be accelerated in independent small zones, ac- cording to local conditions. In the research, different crop varieties were developed in corresponding regions as per current eco-climatic conditions.

Micromorphological Features of Old Cultivated and Modern Soils in Guanzhong Areas, Shaanxi Province, China

Using the data obtained from the LGT soil profile, this article attempts to illustrate the process of modem soil formation in the Guanzhong areas and its micromorphological features. The micromorphology is observed under a petrographic microscope, and its image is quantitatively measured by LEICAL Qwin 2.6 software. Micromorphological observations of the thin sections show that the assemblage of minerals in different horizons is very similar, which is mainly composed of Q and P1. However, there are obvious differences in C/F15μm ratio, mineral content, and coarse features. The pedofeatures is mainly composed of clay, calcite, and amorphous Fe. Ap horizon is characterized by abundant needleshaped secondary calcite, secondary clay, and earthworm fecal pellet. BC horizon is characterized by a large quantity of secondary calcite with various shapes. Bt1and Bt2 horizons are characterized by abundant clay hypocoatings and a small quantity of secondary calcite. All the results of this research suggest that Earth-cumulic Orthic Anthrosols consist of both the upper Ap horizon, which cause loessal dung and eolian dust deposition, and cultivation occurs simultaneously during the process of Ap horizon-formation, and the lower BC horizon, which is aeolian sedimentary at the time of relative aridity during late Holocene.

Comparison of Micromorphological Features of Two Agricultural Cultivated Soils in Guanzhong Areas, Shaanxi Province, China

By comparing micromorphological features of irrigated and non-irrigated soils in Guanzhong areas, China, this paper tries to illustrate the influences of farming management methods on the soil-forming process. The micromorphology was observed under a petrographic microscope and its image was quantificationally measured by Nikon NISBR 2.2 software. Both irrigated and non-irrigated soils have the same soil profile pattern, Ap1- AP2- BC, but the former has a more obvious profile dissimilation. The minerals assemblage of soil profiles A and B are very similar, which is mainly composed of Q and P1. Compared with non-irrigated soil, grains of irrigated soil remarkably decrease in length, area, eqdiameter, perimeter, elongation, roundness, and C/F10μm ratio; voids are characterized by more regular void shape and more smooth void wall; there is more abundant residual clay and small amount of illuvial clay. All results in this study suggest that the farming management method has influences on soil profile dissimilation and micromorphology. Agricultural irrigation could strengthen the degree of weathering, make smaller and rounder soil grains, cause a significant increase of residual clay and appearance of illuvial clay. But no significant change has been observed in the minerals assemblage of coarse grains.

Arsenic Distribution Pattern in Different Sources of Drinking Water and their Geological Background in Guanzhong Basin, Shaanxi, China

To study arsenic （As） content and distribution patterns as well as the genesis of different kinds of water,especially the different sources of drinking water in Guanzhong Basin,Shaanxi province,China,139 water samples were collected at 62 sampling points from wells of different depths,from hot springs,and rivers.The As content of these samples was measured by the intermittent flowhydride generation atomic fluorescence spectrometry method （HG-AFS）.The As concentrations in the drinking water in Guanzhong Basin vary greatly （0.00-68.08 tg/L）,and the As concentration of groundwater in southern Guanzhong Basin is different from that in the northern Guanzhong Basin.Even within the same location in southern Guanzhong Basin,the As concentrations at different depths vary greatly.As concentration of groundwater from the shallow wells （〈50 m deep,0.56-3.87 μg/L） is much lower than from deep wells （110-360 m deep,19.34-62.91 μg/L）,whereas As concentration in water of any depth in northern Guanzhong Basin is 〈10 μg/L.Southern Guanzhong Basin is a newly discovered high-As groundwater area in China.The high-As groundwater is mainly distributed in areas between the Qinling Mountains and Weihe River; it has only been found at depths ranging from 110 to 360 m in confined aquifers,which store water in the Lishi and Wucheng Loess （Lower and Middle Pleistocene） in the southern Guanzhong Basin.As concentration of hot spring water is 6.47-11.94 μg/L; that of geothermal water between 1000 and 1500 m deep is 43.68-68.08 μg/L.The high-As well water at depths from 110 to 360 m in southern Guanzhong Basin has a very low fluorine （F） value,which is generally 〈0.10 mg/L.Otherwise,the hot springs of Lintong and Tangyu and the geothermal water in southern Guanzhong Basin have very high F values （8.07-14.96 mg/L）.The results indicate that high As groundwater in depths from 110 to 360 m is unlikely to have a direct relationship with the geothermal water in the same area.As concentration of all reservoirs and rivers （both contaminated and uncontaminated） in the Guanzhong Basin is 〈10 μg/L.This shows that pollution in the surface water is not the source of the high-As in the southern Guanzhong Basin.The partition boundaries of the high-and low-As groundwater area corresponds to the partition boundaries of the tectonic units in the Guanzhong Basin.This probably indicates that the high-As groundwater areas can be correlated to their geological underpinning and structural framework.In southern Guanzhong Basin,the main sources of drinking water for villages and small towns today are wells between 110-360 m deep.All of their As contents exceed the limit of the Chinese National Standard and the International Standard （〉10 μg/L） and so local residents should use other sources of clean water that are 〈50 m deep,instead of deep groundwater （110 to 360 m） for their drinking water supply.

Urban Spatial Form Optimization Strategies Based on the Principle of Climate Adaptability of Regional Architecture: A Case Study of Traditional Cave Dwelling Villages in Northern Shaanxi Province

A multitude of climate-adaptive design approaches are embedded in regional architecture,which have a positive impact on addressing the deformed development of urban spatial patterns and the dual challenges of climate and resources.This paper examines the principles of climate adaptability embedded in the site layout and spatial organization characteristics of traditional cave dwelling villages in northern Shaanxi Province.The extracted climate adaptability principles are summarized,and the resulting design strategies that are well-suited to the contemporary urban space form are presented.Through analysis,it can be observed that traditional cave dwelling villages in northern Shaanxi are predominantly situated on south-facing slopes in proximity to water at low altitudes.These villages are characterized by compact building groups and east-west development,which is constrained by the elements of mountains and rivers.A two-level street system is generated,comprising streets parallel to the contour line and roadways perpendicular to the contour line.This results in the formation of a courtyard form enclosed by mountains.Such site layout and spatial organization exhibit excellent climate adaptability with regard to heat,ventilation,and wind storage.In light of the aforementioned considerations,the following urban spatial form design strategies are put forth:①the topographic height difference can be exploited to obtain sufficient sunshine;②the group shape can be optimized in order to reduce building energy consumption;③the best orientation of the building can be chosen to take account of both winter and summer conditions;④the height and length can be combined in order to form natural masking;⑤the D/H ratio of streets and roadways should be controlled to achieve a balance between heat gain and cooling of groups;⑥vents should be set appropriately to optimize group ventilation;⑦climate buffers should be established to increase the level of climate response.

Analysis of the Current Situation of Drug Clinical Trial Institutions in Shaanxi Province

To understand the current situation of institutional registration in Shaanxi Province after the implementation ofregistration system management in drug clinical trial institutions.Relevant information was collected on the“Announcement on the Accreditation of Drug Clinical Trial Institutions”issued by the National Medical Products Administration from 2005 to August 2022,the record management information system of drug and medical device clinical trial institutions,and the drug clinical trial registration and information publicity platform.A retrospective analysis was carried out in terms of institutional development,regional distribution,registered majors,principal investigators,and the number of drug clinical trials.After the implementation of institution registration,the number of drug clinical trial institutions in Shaanxi Province increased by 47.4%,884 principal investigators were registered,the number of registered majors expanded from 58 qualified to 117,and the professional scope increased by 50.4%.The policy of institution registration is conducive to promoting the rational use of medical resources and the development of drug clinical trial institutions and improving the healthy development of the pharmaceutical industry in Shaanxi Province.

Thinking on Effective Methods of Successful Prediction of Geological Hazards in Shaanxi, China

In the process of human survival and development, it is inevitable to transform the original state of the world, thus forming a contradiction between the earth and the earth. The violent form of this contradiction is geological disasters. Geological disasters pose a threat to human life and property, and cause damage caused by natural or human factors, often causing casualties. The destruction process of geological disasters is usually a gradual process, showing many pre-disaster symptoms, such as local landslides, surface cracks, building deformation, tree skew, and ground sound. Evacuation can be avoided in advance according to the disaster precursors, so as to avoid casualties and achieve successful prediction. By reviewing the general situation of geological disasters in Shaanxi Province and the casualties in 2020, the difficulties in the prevention and control of geological disasters are summarized. In view of these difficulties, an on-site investigation, visit and analysis of geological disaster points and successful forecast points in Shaanxi Province in 2020 were conducted. In addition, combined with actual cases and years of work experience, the successful prediction experience of geological disasters was discussed from 8 aspects. Finally, the “Regulations on the Reward for Successful Geological Disaster Forecasting in Shaanxi Province” was revised in order to improve the successful prediction ability of geological disasters in Shaanxi Province and even the whole country, provide reference for future prevention and control of geological disasters, and effectively protect the safety of people’s lives and property.

Dry Intrusion Analysis of A Heavy Rainstorm in Southern Shaanxi

[Objective] One regional rainstorm weather in the south area of Shaanxi was expounded.[Method] By employing the NCEP/NCAR 1°×1°reanalysis data,routine observation and automatic precipitation station data,the large scale circulation background field of regional precipitation in the south area of Shaanxi province from July 16-18 in 2010 was expounded.By dint of physical quantity such as the meridional wind,relative humidity and false relative temperature,the characteristics of dry intrusion and its role in rainstorm were expounded.[Result] The mild and high latitude at 500 hPa was stable.The continental high pressure and subtropical high pressure were stable.The 700 and 850 hPa shear lines or low vortexes were the main influence system that resulted into large scale of rainstorm in south Shaanxi.The landing typhoon had distinct influences on the importance of storm.The influence of typhoon and subtropical high was mutual.It not only formed and strengthened the torrent in the low air,also transmitted water vapor and energy incessantly to the south area of Shaanxi,playing an important role in augment of rainstorm.There were two distinct dry intrusions in the rainstorm process,showing a dense dry layer in the high layer of convective layer.The existence of dry layer was conductive to the accumulation of convective unstable energy in the low layer.The release of convective unstable energy could transmit warm and wet airstream in the low level to the higher level,being conductive to the occurrence of precipitation.The analysis of θse in the warp direction vertical profile suggested that there was frontal zone in this rainstorm process and the dry air activity in this rainstorm process was very strong.[Conclusion] The study provided theoretical reference to the forecast in future.

Analysis of Meteorological Conditions of Freeze Damage to Citrus in Southern Shaanxi in the Winter of 2010 and Defensive Countermeasures

In order to take advantage of the climate resources more effectively ac- cording to the local circumstances and to plan and develop the citrus industry in Southern Shaanxi more reasonably. On the basis of the investigation of freeze dam- age to citrus occurring in Southern Shaanxi in the winter of 2010, the climatic back- ground for the formation of this freeze damage was analyzed. In combination with the freeze damage indicators during the overwintering period and the harmful accu- mulated cold during the cold wave, indexes for grading the freeze damage in southern Shaanxi were analyzed and verified, and the perspective of grading the freeze damage using the harmful accumulated cold during the cold wave was also presented. Through analyzing the extremely lowest temperature and the harmful ac- cumulated cold in the winter of 2010 and in history at 12 citrus growing counties （districts） in Ankang area and Hanzhong area, the reasons why the freeze damage to citrus during the overwintering period was severer in the west than in the east of Southern Shaanxi were discussed, and the results obtained were basically consistent with the actual situation observed from investigation. Finally, defensive countermea- sures against the freeze damage to citrus during the overwintering period were put forward from several aspects.

Effect of Different Land Use Patterns on Physical Properties of Soil Water in Yellow River Wetland in Shaanxi Province

[Objective] This study was to reveal the effect of different land use patterns on physical characteristics of soil water in the Yellow River wetland in Shaanxi Province.[Method]Taking Yellow River wetland in Shaanxi Province as experimental plot,we compared the physical properties of the soil water under different land use patterns and studied the physical properties and the change law of soil water during the wetland degeneration process.[Result]Under different land use patterns,soil bulk density rose with the increase of soil depth.During the degeneration process of from river wetland to reclaimed wetland（paddy field）,finally to abandoned land owing to salinization,the mean soil bulk density reduced correspondingly from 1.474 to 1.522 g/cm3,finally to 1.593 g/cm3 when abandoned.Accompanying wetland degeneration,soil became compact increasingly,and the indicators of soil porosity（total porosity,capillary porosity,non-capillary porosity）were also reduced with the change of land use patterns,in which,capillary porosity and total porosity reached the extremely significant level with the change of land use patterns,and non-capillary porosity reached significant level.The changes of soil porosity condition accelerated the deterioration of wetland.Under different land use patterns,the maximum soil moisture capacity,capillary moisture capacity and minimum moisture capacity all showed a similar change law.Compared with wetland,the maximum soil moisture capacity of reclaimed land（paddy field）and salinized land respectively decreased by 5.7% and 22.3%,capillary moisture capacity by 0.2% and 19.4%,minimum moisture capacity by 2.7% and 15.9%.Of the three land use patterns,wetland displayed both higher water holding capacity and water drainage capacity over reclaimed land（paddy field）and salinized land.By comparison with wetland,the reclaimed land（paddy field）and salinized land respectively decreased by 12.4% and 15.2% in total water holding capacity,and by 2.7% and 15.9% in total water drainage capacity.[Conclusion]To conserve the water resource in Yellow River wetland,regulate the hydrological cycle and enhance drought and water logging resistances,it should be noted that reasonable countermeasures be taken to exploit the state-owned forest land and paddy field around the wetland and the related resources.

Exploration of Eco-economic Development in Ecological Fragile Region——A Case Study on Wuqi County in Shaanxi Province

[Objective] The aim was to explore eco-economy in ecological fragile region with Wuqi County in Shaanxi Province as an example. [Method] Eco-economy development and major environmental problems in Wuqi County in Shaanxi Province were researched based on local social and economic conditions, population of impoverished people and environment vulnerability. [Result] The areas with vulnerable environment and backward economy should establish a virtuous circle between economy and environment and a sustainable model of economic development. [Conclusion] It is significant to change the severe situation of environment deterioration and to promote ecological civilization.

A New Species of the Genus Chorthippus (Orthoptera: Arcypteridae) from North of Shaanxi, China

In this paper, a new species of the genus Chorthippus is described from North of Shaanxi Province, China, namely： Chorthippus wuqiensis sp. nov. The type specimens are deposited in the Institute of Zoology, Shaanxi Normal University.

Rationality Research on Micro-evolution of Rural Courtyard Landscapes in Guanzhong Area of China

In response to the needs of landscape construction in the new countryside of west China,this paper takes Sanyi Village in Guanzhong area for example,from the combined perspective of modern landscape and traditional local culture,explores micro-evolution laws of rural courtyard landscapes in Guanzhong Area.By finding out the reasonable aspects as land use,layout and decoration,and the unreasonable aspects as material,texture and green cultivation,in the landscape micro-evolution of rural settlement courtyard,it tries to guide the landscape planning,provide some brief construction methods,focus on the"step by step"safeguarding and introductory development mode,explore tangible and intangible elements of landscape culture in rural settlements,and play the potentials of rural dwellings in courtyard landscape construction,so as to solve the conflict between conservation and development in rural settlements.

Biological records of paleoclimate and paleoenvironment changes from Guanzhong area, Shaanxi Province during the last glacial maximum

We have collected a suite of Paleoloxodon naumanni-Coelodonta antiquitatis fossils from a river terrace profile between Xi’an and Xianyang cities in Guanzhong Area, Shaanxi Province. A detailed examination of the fossils, together with 14C determinations and pollen analyses, have enabled us to reconstruct preliminary features of climate and environment changes and their evolutionary processes, within the last glacial maximum (LGM): about 20 ka ago, the climate was basically cool and wet, and was favourable for the existence of Paleoloxodon naumanni-Coelodonta antiquitatis fauna. This was followed by a cold, dry phase which was no longer suitable for this type of faunal suite, causing the death of a large number of mammalian assemblages. The available evidence indicates the existence of cold-dry and cool-wet climate and environment fluctuations during the LGM. The cool-wet stage within the LGM reflects a Heinrich event occurring in the high latitude areas, proving the existence of a teleconnection

Assessment on heavy metals pollution of agricultural soil in Guanzhong District

The monitored soil samples were collected from Heihui irrigated area, Jiaokou irrigated area, Qianhe river valley and Jinghe river valley （hereafter Heihui, Jiaokou, Qianhe and Jinghe for short respectively） of Guanzhong District. According to the Environment Quality Standard for Soil （GB 15618-1995II）, we evaluated the pollution status of heavy metals （Cd, As, Cr and Pb） that could seriously endanger soil environment and human health by using single-factor index and synthetic pollution index methods. The results indicate that the synthetic pollution indices P of soil heavy metals are less than 0.7 in Heihui, Jiaokou, Qianhe and Jinghe of Guanzhong, the single-factor indices Pi of soil heavy metals of most soil samples are less than 0.7, so the soil environmental quality is in a good condition in Guanzhong on the whole; the enrichment degree of soil heavy metals is in the order of Heihui, Jinghe, Qianhe and Jiaokou; the contaminated degree of soil heavy metals has the feature of Cd 〉 As 〉 Cr 〉 Pb; heavy metals contents in the cultivated horizon soil are generally higher than those in its underlayer soil, heavy metals contents of soil have the characteristic of enriching towards the cultivated horizon; Cd exceeds standard in the soil samples HS07a, b and HS08a, b at the Yangtao orchard in Heihui and in the soil sample QHS01a at the suburban vegetable plot in Qianhe, which was mainly caused by the long-term irrational use of chemical fertilizer and pesticide.

Rural Tourism Development Strategies of Shaanxi Province in North China

Rural tourism development in Shaanxi Province is introduced:largely improved with a fast speed; large quantity of rural tourist sites covering a great range, a complete product system; more and more investment; new progress in the construction of management system and guarantee system; progress in the construction of new countryside. Development advantages and opportunities of rural tourism industry in Shaanxi Province are analyzed: rich humanistic, natural and agricultural resources are inborn advantages of its rural tourism development; excellent location and increasing GDP per capita provides more market opportunities for rural tourism development; prospering tourism industry provides an excellent exterior environment for rural tourism development; a complete traffic network facilitates rural tourism development. Then development strategies for the rural tourism industry of Shaanxi Province are given:government should be a guider, planning should be accelerated, layouts reasonably designed, rural tourism development concepts clarified; brands should be created to drive the overall development by using several prominent scenic spots and promote the scale-development of rural tourism industry; the core strategy is to launch concrete programs and promote rural tourism development; publicity should be enhanced to make the good reputation of rural tourism more popular; standard regulations should be made, special staff trained for rural tourism services and the system of "farm-stay" association improved.

Analysis of Heavy Metal Content in Food Crops Planted Along the Shaanxi Inner Mongolia Expressway

To understand the effect of the running of Shaanxi Inner Mongolia Expressway on food crops planted along the sides, we determined the heavy metal contents(Zn, Pb, Cu and Cb) in the digestion solutions of the root, stem and leaf from rice and corn. The results showed that highway promoted the accumulation of heavy metal elements in corn and rice. Compared with the CKs(crops produced where no highway extend), contents of heavy metal elements in two crops tested from two sampling sites were both over standard. This indicates that food crops planted within 100 m away from the highway are been polluted by heavy metals, which is directly related with the running of Shaanxi Inner Mongolia Expressway. Based on this, the areas within 100 m away from the highway are not suggested to cultivate food crops.

Changes in Ecosystem Service Values on the Loess Plateau in Northern Shaanxi Province,China

The Loess Plateau in northern Shaanxi Province of China covers Yulin, Yan＇an, and Tongchuan cities mainly. The kind of physiognomy is complicated and various, including the transitional region of the sand and wind, the hilly-gully fragile region, the pimple mound region from North to South. Large-scale land reclamation projects that are severely affecting these ecosystems have been implemented. In this paper, we reported an investigation to the changes in land use and ecosystem services on the Loess Plateau in northern Shaanxi Province from 1978 to 2000. We used three LANDSAT TM and/or ETM data sets to estimate the changes in the size of five land-cover/land-use categories, and we also used previously published value coefficients to estimate the changes in the value of ecosystem services delivered by each land category. Finally, we ranked the contribution of various ecosystem functions to the overall value of the ecosystem services, we have estimated that the annual value of the ecosystem services is 56.95 billon RMB yuan in the Loess Plateau in northern Shaanxi Province in 2000. In the region, from 1978 to 2000, the economic value of the fixing carbon was higher, the economic value of water conservation was the lowest, but the economic value of the NPP, fixing carbon, and supplying oxygen accounted for above ninety percent of the total value, obviously the vegetation created the biggest ecosystem service value. We can conclude that future land-use policy formulation should give precedence to the conservation of these ecosystems over uncontrolled reclamation, and that further land reclamation should be based on rigorous environmental impact analyses.

Construction of Monastery Garden Landscapes in Northern Shaanxi Province of China

In view of the present construction of monastery garden landscapes in northern Shaanxi,landscape features were pointed out as:harsh natural environment of monastery garden,dual functions of religion and tourism,large spatial capacity of architectural complex,long history and profound cultures,and also working as botanic gardens.Problems in these monastery gardens were proposed as:poor overall landscaping design,insufficient plants of religious cultures,lacking in artistic plant furnishings showing ornamental effects and seasonal aspects of plants,plant furnishings failed to show cultural characteristics of northern Shaanxi,poor diversity and improper ratio of plant furnishings,attaching insufficient importance to vertical greening,similar plant communities.On the basis of investigating characteristics and problems of monastery garden landscapes in northern Shaanxi,layouts and plant furnishings of incense roads,religious sectors and living sectors in those gardens were analyzed,suitable plant species were given as references,so as to explore new landscape modes of "human-nature oneness" and provide useful experience for the landscape construction of monastery gardens in Shaanxi.