Variability of Soil Moisture and Its Relationship with Surface Albedo and Soil Thermal Parameters over the Loess Plateau

Data from July 2006 to June 2008 observed at SACOL （Semi-Arid Climate and Environment Observatory of Lanzhou University, 35.946°N, 104.137°E, elev. 1961 m）, a semi-arid site in Northwest China, are used to study seasonal variability of soil moisture, along with surface albedo and other soil thermal parameters, such as heat capacity, thermal conductivity and thermal diffusivity, and their relationships to soil moisture content. The results indicate that surface albedo decreases with increases in soil moisture content, showing a typical exponential relation between the surface albedo and the soil moisture. The heat capacity, the soil thermal diffusivity, and soil thermal conductivity show large variations between Julian day 90-212 and 450-578. The soil thermal conductivity is found to increase as a power function of soil moisture. Soil heat capacity and soil thermal diffusivity increase with increases in soil moisture. The SACOL observed soil moisture are also used to validate the AMSR-E/AQUA retrieved soil moisture and there is good agreement between them. The analysis of the relationship between satellite retrieved soil moisture and precipitation suggests that the variability of soil moisture depends on the variation of precipitation over the Loess Plateau.

Evapotranspiration and Soil Moisture Balance for Vegetative Restoration in a Gully Catchment on the Loess Plateau, China

Evapotranspiration, soil moisture balance and the dynamics in a gully catchment of the Loess Plateau in China were determined with 6 land use treatments including natural grassland, shrubs （Caragana rnicrophylla）, two woodlands （Prunus armeniaca var. ansu and Pinus tabulaeformis）, cultivated fallow, and farmland （Triticum aestiuum L.） in order to obtain a better understanding of soil moisture balance principles and to improve vegetation restoration efficiency for ecological rebuilding on the plateau. Average runoff from cultivated fallow was very high, reaching 10.3% of the seasonal rainfall. Evapotranspiration under T. aestivurn was not significantly different from natural grasslands. Compared with natural grass, evapotranspiration was significantly greater （P 〈 0.05） in 2002 and there was an increase in soil moisture depleted in the 1-3 m soil under P. armeniaca, P. tabulaeformis and C. microphylla. During the two years of the study the average soil moisture （0-100 cm soil profile） of T. aestivurn was generally the highest, with P. armeniaca, P. tabulaeformis and C. rnicrophylla usually the lowest. Thus, according to the soil moisture balance principle for this area the planned reforestation project was not ecologically reasonable. Reducing human disturbance and restoration with grass could be more effective.

Response of soil nutrients to terracing and environmental factors in the Loess Plateau of China

Terracing is a widely adopted agricultural practice in mountainous regions around the world that aims to conserve soil and water resources.Soil nutrients play a crucial role in determining soil quality,particularly in landscapes prone to drought.They are influenced by factors such as land-use type,slope aspect,and altitude.In this study,we sought to examine the impact of terracing on soil nutrients(soil organic content(SOC),total nitrogen(TN),nitrate-nitrogen(NO_(3)^(-)-N),ammonium nitrogen(NH_(4)^(+)-N),total phosphorus(TP),available phosphorus(AP),total potassium(TK),and available potassium(AK))and how they vary with environmental factors in the Chinese Loess Plateau.During the growing season,we collected 540 soil samples from the 0 to 100 cm soil layer across five major land-use types,different slope aspects,and varying altitudes.Additionally,a meta-analysis of literature data further corroborated the effective accumulation of soil nutrients through terracing in the Loess Plateau.Our findings are as follows:(1)Terraced fields,regardless of land-use type,showed a significant improvement in SOC and TN content.(2)Soil nutrient contents within terraced fields were predominantly higher on sunny slopes.(3)Terraces at lower altitudes are characterized by elevated SOC concentrations.(4)A meta-analysis of literature data pertaining to terracing and soil nutrients in this region confirmed the effective accumulation of soil nutri-ents through terracing.The elucidated outcomes of this study offer a profound theoretical underpinning for the accurate planning and management of terraces,the scientific utilization of land resources,and the enhancement of land productivity.

Soil moisture of different vegetation types on the Loess Plateau

Water stored in deep loess soil is one of the most important resources regulating vegetation growth in the semi-arid area of the Loess Plateau, but planted shrub and forest often disrupt the natural water cycle and in turn influence plant growth. The purpose of this study was to examine the effects of main vegetation types on soil moisture and its inter-annual change. Soil moisture in 0-10 m depth of six vegetation types, i.e., crop, grass, planted shrub of caragana, planted forests of arborvitae, pine and the mixture of pine and arborvitae were measured in 2001,2005 and 2006. Soil moisture in about 0-3 m of cropland and about 0-2 m of other vegetation types varied inter-annually dependent on annual precipitation, but was stable inter-annually below these depths. In 0-2 m, soil moisture of cropland was significantly greater than those of all other vegetation types, and there were no si nificant differences among other vegetation types. In 2-10 m, there was no significant mois- ture difference between cropland and grassland, but the soil moistures under both of them were significantly higher than those of planted shrub and forests. The planted shrub and forests had depleted soil moisture below 2 m to or near permanent wilting point, and there were no significant moisture differences among forest types. The soil moisture of caragana shrub was significantly lower than those of forests, but the absolute difference was very small. The results of this study implicated that the planted shrub and forests had depleted deep soil moisture to the lowest limits to which they could extract and they lived mainly on present year precipitation for transpiration.

The effects of land use and its patterns on soil properties in a small catchment of the Loess Plateau

Due to relatively strong human activities in the hilly area of Loess Plateau, the natural vegetation has been destroyed, and landscape pattern based on agricultural land matrix was land use mosaic composing of shrub land, grassland, woodland and orchard. This pattern has an important effect on soil moisture and soil nutrients. The Danangou catchment, a typical small catchment, was selected to study the effects of land use and its patterns on soil moisture and nutrients in this paper. The results are as follows: The comparisons of soil moisture among seven land uses for wet year and dry year were performed: (1) the average of soil moisture content for whole catchment was 12.11% in wet year, while it was 9.37% in dry year; (2) soil moisture among seven land uses was significantly different in dry year, but not in wet year; (3) from wet year to dry year, the profile type of soil moisture changed from decreasing type to fluctuation-type and from fluctuant type to increasing type; (4) the increasing trend in soil moisture from the top to foot of hillslope occurred in simple land use along slope, while complicated distribution of soil moisture was observed in multiple land uses along slope. The relationships between soil nutrients and land uses and landscape positions were analysed: (1) five nutrient contents of soil organic matter (SOM), total N (TN), available N (AN), total P (TP) and available P (AP) in hilly area were lower than that in other areas. SOM content was less than 1%, TN content less than 0.07%, and TP content between 0.05% and 0.06%; (2) SOM and TN contents in woodland, shrub land and grassland were significantly higher than that in fallow land and cropland, and higher level in soil fertility was found in crop-fruit intercropping land among croplands; (3) soil nutrient distribution and responses to landscape positions were variable depending on slope and the location of land use types.

Responses of soil moisture to vegetation restoration type and slope length on the loess hillslope

Soil moisture, a critical variable in the hydrologic cycle, is highly influenced by vegetation restoration type. However, the relationship between spatial variation of soil moisture, vegetation restoration type and slope length is controversial. Therefore, soil moisture across soil layers（0-400 cm depth） was measured before and after the rainy season in severe drought（2015） and normal hydrological year（2016） in three vegetation restoration areas（artificial forestland, natural forestland and grassland）, on the hillslopes of the Caijiachuan Catchment in the Loess area, China. The results showed that artificial forestland had the lowest soil moisture and most severe water deficit in 100-200 cm soil layers. Water depletion was higher in artificial and natural forestlands than in natural grassland. Moreover, soil moisture in the shallow soil layers（0-100 cm） under the three vegetation restoration types did not significantly vary with slope length, but a significant increase with slope length was observed in deep soil layers（below 100 cm）. In2015, a severe drought hydrological year, higher water depletion was observed at lower slope positions under three vegetation restoration types due to higher transpiration and evapotranspiration and unlikely recharge from upslope runoff. However, in 2016, a normal hydrological year, there was lower water depletion, even infiltration recharge at lower slope positions, indicating receiving a large amount of water from upslope. Vegetation restoration type, precipitation, slope length and soil depth during a rainy season, in descending order of influence, had significant effects on soil moisture. Generally, natural grassland is more beneficial for vegetation restoration than natural and artificial forestlands, and the results can provide useful information for understanding hydrological processes and improving vegetation restoration practices on the Loess Plateau

A mathematical model of soil moisture spatial distribution on the hill slopes of the Loess Plateau

Based on important factors that affect soil moisture spatialdistribution, such as the slope gradients, land use, vegetation cover, and surface water diffusion characteristics together with field measurements of soil moisture data obtained from the surface soil under different land use struc-tures, a soil moisture spatial distribution model was established. The diffusion degree coefficient of surface water for different vegetations was estimated from soil moisture values obtained from field measurements. The model can be solved using the finite unit method. The soil moisture spatial distribution on the hill slopes in the Loess Plateau were simulated by the model. A comparison of the simulated values with measurement data shows that the model is a good fit.

Review on Soil Moisture of Plantation Land in the Loess Plateau

Water is a crucial factor influencing eco-environment conservation in the Loess Area in China. Soilmoisture is also an indispensable factor to plant growth because of limited water supply. In this paper,previous studies of soil moisture are summarized from methods and contents. Meanwhile, some problems inthe relevant researches are pointed out and discussed. Some solutions are brought forward.

Seasonal Variations of the Surface Fluxes and Surface Parameters over the Loess Plateau in China

Turbulent fluxes were measured by an eddy covariance system at three levels over an intricate land surface on the southern part of the Loess Plateau, consisting of heterogeneous flat terrain and a large valley 500 maway from the observation site to the southeast. The surface roughness length, the seasonal variation of bulk transfer coefficient for sensible heat (CH), and the seasonal variation of surface moisture availability (β) were also analyzed based on the observation. The flux footprint was carefully considered in this study. A relatively dry period of the experimental area existed from June to the first week of July 2004 when the land surface offered turbulent energy to the atmospheric surface layer mainly by sensible heat flux with a maximum value of around 230 Wm-2. A wet duration lasted from the second week of July to the end of September 2004 with very frequent rainfall events in conditions when the winds were mainly from the southeast;latent heat flux was dominant during the wet season and reached a peak value of around 280 Wm-2. The surface parameters of CH and β were calculated when the mean winds coming from the flat terrain, i.e., from the northwest direction. The values of CH ranged between 0.004 and 0.006 during the observational year of June 2004 to June 2005. The surface moisture availability β changed with seasons as anticipated with high values during June and July 2004 and lowest values around0.03 inFebruary 2005. Its peak value of 0.91 occurred in July;the mean value of β during the wet season was 0.29. Furthermore, the relationship between the surface soil water content and β indicated that changes in soil water content contributed much to variations of surface moisture availability β.

Soil water depletion depth by planted vegetation on the Loess Plateau

Evapotranspiration of much planted vegetation exceeds precipitation, and this can deplete soil water and cause a deep dry layer in the soil profile, which is a serious obstacle to sustainable land use on the Loess Plateau, China. This study aimed to determine water depletion depth of planted grassland, shrub, and forest in a semiarid area on the Loess Plateau. Soil moisture of five vegetation types was measured to >20 m in depth. The vegetation types were crop, natural grasse, seven-year-old planted alfalfa (Medicago sativa L.), 23-year-old planted caragana (Caragana microphylla Lam.) shrub, and 23-year-old planted pine (Pinus tabulaeformis L) forest land. Through comparing moisture of planted alfalfa grass, caragana shrub, and pine forest to crop and natural grassland, the depth and amount of soil water consumed by grassland, caragana brush and pine forest was determined. The depth of soil water depleted by alfalfa, caragana brush, and pine forest reached 15.5, 22.4 and 21.5 m, respectively.

Revegetation with artificial plants improves topsoil hydrological properties but intensifies deep-soil drying in northern Loess Plateau,China

Knowledge about the effects of vegetation types on soil properties and on water dynamics in the soil profile is critical for revegetation strategies in water-scarce regions, especially the choice of vegetation type and human management measures. We focused on the analysis of the effects of vegetation type on soil hydrological properties and soil moisture variation in the 0–400 cm soil layer based on a long-term（2004―2016） experimental data in the northern Loess Plateau region, China. Soil bulk density（BD）, saturated soil hydraulic conductivity（Ks）, field capacity（FC） and soil organic carbon（SOC） in 2016, as well as the volumetric soil moisture content during 2004–2016, were measured in four vegetation types, i.e., shrubland（korshinsk peashrub）, artificial grassland（alfalfa）, fallow land and cropland（millet or potato）. Compared with cropland, revegetation with peashrub and alfalfa significantly decreased BD and increased Ks, FC, and SOC in the 0–40 cm soil layer, and fallow land significantly increased FC and SOC in the 0–10 cm soil layer. Soil water storage（SWS） significantly declined in shrubland and grassland in the 40–400 cm soil layer, causing severe soil drought in the deep soil layers. The study suggested that converting cropland to grassland（alfalfa） and shrubland（peashrub） improved soil-hydrological properties, but worsened water conditions in the deep soil profile. However, natural restoration did not intensify deep-soil drying. The results imply that natural restoration could be better than revegetation with peashrub and alfalfa in terms of good soil hydrological processes in the semi-arid Loess Plateau region.

Afforestation using micro-catchment water harvesting system with microphytic crust treatment on semi-arid Loess Plateau： A preliminary result

Water harvesting is one of main measures to solve water shortage resulting from less precipitation and erratically seasonal dis- tribution in arid and semi-arid areas. Different types of anti-infiltration treatments including mechanical and chemical to micro-catchment and their runoff efficiencies had been reported. This paper, through 5 years experiment from 1992 to 1996, is aimed at studying the im- pacts of microcatchment water-harvesting system (MCWHS) with microphytic crust treatment on afforestation on semi-arid Loess Plateau. The results showed that after 3 years of crust inoculation, crust had covered majority of MCWHS and the function of water harvesting had also been demonstrated partially, there were significant difference in soil moisture of shallow soil layer in three typical spring stages be- tween crust cover and control treatments (0.05 level), and about 0.9%-6.04% increase of monthly mean soil moisture within 1m soil layer in spring of late 3 years. The impact of severe spring drought can be alleviated effectively. In the meanwhile, as crust developed on the treated surface, there are significant differences (0.05 level) for tree height (H), diameter at breast height (DBH) and diameter at ground level (DGL) at the end of the study period (1996) with the increases by 22.38%, 17.34%, and 20.49% respectively compared with the con- trol treatment. Microphytic crust, as one of biological infiltration-proof materials, may become the optimized option for revegetation in Chinese Great West Development Strategy due to its self-propagation, non-pollution to water qualities, long use duration and relatively cost effective. Further work should be focused on the selection of endemic crust species and their batch-culture in arid environment.

Soil temperature and moisture sensitivities of soil CO_2 efflux before and after tillage in a wheat field of Loess Plateau,China

As a conventional farming practice, tillage has lasted for thousands of years in Loess Plateau, China. Although recent studies show that tillage is a prominent culprit to soil carbon loss in croplands, few studies have investigated the influences of tillage on the responses of soil CO2 efflux （SCE） to soil temperature and moisture. Using a multi-channel automated CO2 efflux chamber system, we measured SCE in situ continuously before and after the conventional tillage in a rain fed wheat field of Loess Plateau, China. The changes in soil temperature and moisture sensitivities of SCE, denoted by the Q10 value and linear regression slope respectively, were compared in the same range of soil temperature and moisture before and after the tillage. The results showed that, after the tillage, SCE increased by 1.2-2.2 times; the soil temperature sensitivity increased by 36.1%-37.5%; and the soil moisture sensitivity increased by 140%-166%. Thus, the tillage-induced increase in SCE might partially be attributed to the increases in temperature and moisture sensitivity of SCE.

Crop Yield Response to Water and Fertilizer in Loess Tableland of China：A Field Research

This paper describes a field research on crop yield response to water and fertilizer in Changwu tablelandof the Loess Plateau. Under the current level of land management in this areai low fertility of soil is thefirst yield-limiting factor and water deficit is the second. At a given fertility level, the relationship betweencrop yield and its water consumption is parabolic. Within a certain range of soil moisture, the soil wateravailability to crop is not sharply decreased with its markedly decreasing content. Because of this relativelyhigh availability and the high reserve of soil water at the early stage of crop growth, drought due to inadequateor poorly distributed rainfall becomes less serious. High fertilizer input, improved crop variety and optimumcnltivation techniqne can give a 50-100% yield increment of dryland crops in the loess tableland.

黄土高原近40年梯田建设的土壤固碳效益初步估算

[目的]为揭示黄土高原梯田土壤有机碳含量的时空规律和梯田土壤固碳时空分布特征。[方法]基于META分析的方法,研究梯田土壤有机碳含量随土层深度和梯田修建年限的变化特征,分析黄土高原各区域梯田土壤固碳的时空分布特征。[结果](1)SOC含量和固碳效益随土壤深度的增加而减少,80—100 cm土层的SOC含量为0-20 cm土层的60.74%,0—20 cm土层的固碳效益为23.1%,80—100 cm土层为5.7%。(2)梯田固碳效益随修建年限呈非线性增加,梯田0—20 cm土层固碳效益由修建第1年的-8.29%逐步增加,在第24年时达到最大值34%,此后保持稳定。(3)1979-2023年黄土高原梯田总固碳约为38.29 Mt C,平均固碳38.49 t C/(km^(2)·a)。泾河上中游流域平均固碳速率最高,为50.14 t C/(km^(2)·a),河龙区间平均固碳速率最低,为29.04 t C/(km^(2)·a)。[结论]在黄土高原兴修梯田可以提高SOC含量,并且梯田的固碳是一个相对漫长的过程,需要科学的设计及管护才能更好地发挥其固碳效益。在黄土高原开展梯田建设加之合理规划、设计、科学的管护是提高区域土壤固碳效益的有效措施。研究结果可为黄土高原地区水土保持固碳效益核算提供理论与方法支持。

大佛寺煤矿开采区土壤水分变化遥感反演研究

土壤水分是驱动矿区生态环境变化的关键要素。为研究黄土高原煤矿区开采沉陷对土壤水分变化的扰动影响,以彬长矿区大佛寺煤矿为例,采用主动微波和光学遥感影像反演开采沉陷区土壤水分的空间变化,用水云模型消除植被对主动微波影像的后向散射系数影响,通过实地采样数据构建支持向量机回归模型,获取矿区地表土壤水分的分布特征,分析黄土沟壑区地形因子和开采沉陷变形对矿区土壤水分变化的影响。结果表明:SVM构建的回归分析模型可以有效反演黄土高原煤矿开采区土壤水分的空间分布,地形特征对土壤水分的影响最大,沟壑区域土壤含水量更高;在未受开采影响的区域,土壤水分变化受地形坡度影响最大,坡向次之,高程影响最小;在40201工作面开采沉陷变形区域,土壤水分分布的变异系数显著增大,尤其在采动裂缝发育区更为明显。研究结果为揭示黄土高原煤矿开采区土壤水分变化的时空特征及机理提供了技术支持。

人工林种植和生长对黄土高原生态系统固碳和水文调节功能的影响——基于遥感时序分析证据

随着生态恢复工程的进展,黄土高原人工林种植面积不断增加,黄土高原人工林生态系统服务功能的响应变化仍存在争议。基于黄土高原人工林的时空分布和种植年份信息,从生态系统净初级生产力(NPP)、蒸散发(ET)以及土壤含水率(SMC)三个维度评估黄土高原地区人工林生态系统固碳和水文调节服务功能的动态变化及其对天然林和整个区域生态系统功能的影响,并进一步研究2000—2020年间人工林林龄对生态系统服务功能的影响。天然林的NPP相对于整个区域的百分比基本保持不变,维持在21%左右;而人工林的NPP相对于整个区域的百分比显著增加,并在2009年超过了天然林,成为黄土高原NPP增加的主要来源。人工林占整个区域的ET百分比和SMC百分比均显著增加,分别由不到2%增长至13.9%、1.5%上升至11.8%。至2015年,黄土高原人工林的ET和SMC已经逐步趋于稳定。随着林龄增大,SMC变化速率和变化量不断下降。ET与林龄成正相关,而SMC与林龄负相关,表明具有不同年龄结构的人工林将有效减弱树木生长所引起的土壤干旱的影响。尽管黄土高原人工林林龄与NPP、ET和SMC变化速率和变化量之间存在显著相关性(P<0.01),但由于现阶段此地区人工林多为幼龄林和中龄林,未来这种显著相关关系是否持续有待进一步研究。

黄土高原表层土壤湿度与降水关系的分析

使用黄土高原气象台站的土壤湿度和降水观测资料以及GLDAS和CMFD再分析资料,分析黄土高原地区土壤湿度与降水量的时空分布及变化特征,通过回归分析、格兰杰因果检验和奇异值分解(Singular value decomposition,SVD),研究土壤湿度与降水之间的关系,分析初始土壤湿度影响随后降水的时间尺度与空间范围。结果显示:黄土高原的土壤湿度与随后1~2个月降水回归分析的解释方差相对较高,较大值在夏秋季节(7-10月),黄土高原不同区域(Ⅰ区、Ⅱ区和Ⅲ区)的土壤湿度与随后21天降水相关的时间较全区域的多,时间较集中,说明黄土高原土壤湿度分布不均匀,不同区域差别较大,较大的滞后降水时间尺度适用于较大空间范围的分析。格兰杰因果检验表明黄土高原全区域秋季(10月、11月)的初始土壤湿度对随后1个月或2个月的降水有显著影响,在Ⅲ区8月土壤湿度对10月的降水也有显著影响,这与回归分析的结果一致。再分析资料的SVD分解的结果显示,1979-2014年7月黄土高原中部、北部和东部土壤较湿润时,8月西部和北部边缘的降水偏多;9月东部的土壤偏湿润,则10月黄土高原西部以及南北部的一些地区降水偏多。土壤湿度与降水的显著相关区域重叠部分较少,说明黄土高原土壤湿度对降水的影响存在一定程度的时空不对称性。

陕北黄土区山地苹果园土壤水分动态变化特征研究

以八年生矮化红富士果树为试材,采用野外定位观测法,对陕西省延安地区苹果园0~100 cm不同深度土壤水分与温度及相应气象要素进行连续1年的定位观测分析,对土壤水分的动态变化规律、时空变化及影响因子进行研究。采用方差分析、相关性和主成分分析方法探讨土壤水分的主导因素及其相互关系。研究表明:观测期土壤墒情的季节变化可划分为水分快速消耗期(4—6月)、水分恢复期(7—10月)、稳定期(11—次年3月);土壤体积含水量由浅到深呈先增后减、稳定性增强、垂直变化季节差异显著等特点;主成分分析结果表明,第一主成分中土壤温度及电导率所占比重较大,第二主成分中大气温度和湿度指标权系数的绝对值在所有主成分指标系数中是最大的,降雨量是第三主成分中标权系数最大的;研究区降雨量季节分配极不均匀,主要集中在生长季7—9月,占全年降雨量的73.5%,不同月份、深度的土壤水分变异系数均属于中等程度变异,土壤水分与大气温度、电导率降雨量、相对湿度、降雨量表现出极显著正相关关系,而与风速呈极显著负相关(P<0.01)。该研究揭示了山地苹果园土壤水分的变化规律,可为黄土高原地区果园的建设和管理提供理论依据。

间作模式下不同带宽对土壤含水量的影响

为探究间作模式下不同带宽对台塬坡耕地土壤含水量的影响,本研究选取在小偉河流域彭家底村,连续两年在不同坡度(5°、10°、15°)下的坡耕地布设不同宽度的玉米与苜蓿间作试验,使用中子水分测定仪和便携式时域反射仪(TDR)相结合,对不同带宽下的苜蓿条带、玉米条带的土壤含水量进行监测及分析。结果表明:玉米苜蓿间作下,地表含水量均小于深层土壤含水量;苜蓿条带内表层储水量略大于玉米,但深层土壤上,玉米的储水量大于苜蓿;在各带宽中,以5m带宽储水量较高,利于作物生长。研究结果对探求坡耕地粮草间作套种的最优模式,以及坡耕地管理技术提供参考。