Plant‐compositional effects on surface runoff and sediment yield in subalpine grassland

Soil erosion such as sheet erosion is frequently encountered in subalpine grassland in the Urseren Valley (Swiss Central Alps). Erosion damages have increased enormously in this region during the last 50 y, most likely due to changes in land‐use practices and due to the impact of climatic changes. In order to estimate the effect of vegetation characteristics on surface runoff and sediment loss, we irrigated 22 pasture plots of 1 m² during 1 h at an intense rain rate of 50 mm h^–1 in two field campaigns using a portable rain simulator. The rain‐simulation plots differed in plant composition (herb versus grass dominance) and land‐use intensity but not in plant cover (>90%) nor in soil conditions. Prior to the second rain‐simulation campaign, aboveground vegetation was clipped in order to simulate intense grazing. The generated surface runoffs, sediment loss, relative water retention in the aboveground vegetation, and changes in soil moisture were quantified. Runoff coefficient varied between 0.1% and 25%, and sediment loss ranged between 0 and 0.053 g m^–2. Thus, high infiltration rates and full vegetation cover resulted in very low erosion rates even under such extreme rain events. Surface runoff did not differ significantly between herb‐ and grass‐dominated plots. However, clipping had a notable effect on surface runoff in the test plots under different land‐use intensity. In plots without or with intensive use, surface runoff decreased after clipping whereas in extensively used plots, surface runoff increased after the clipping. This opposite effect was mainly explained by higher necromass and litter presence at the extensively used plots after the clipping treatment. The results obtained here contribute to a better understanding of the importance of vegetation characteristics on surface‐runoff formation, thus, on soil‐erosion control. Overall, we delineate vegetation parameters to be crucial in soil‐erosion control which are directly modified by the land‐use management.     

Assessing land degradation in the Monduli District,northern Tanzania

One of the many contentious issues facing the appropriate and accurate assessment of land degradation is the varying emphasis placed on vegetation degradation and soil degradation processes. This has led to the compartmentalization of land degradation assessment methods, depending on the particular perspective adopted. The land degradation assessment method presented here attempts to take into account both vegetation and soil degradation. This methodology is applied to the southern part of the Monduli District of northeast Tanzania, an area typifying the so‐called ‘affected drylands’ of Africa. Three sets of land cover maps synchronized against long‐term rainfall data (1960s, 1991 and 1999) were used to assess land degradation in the area. Utilizing these three sets of land cover maps as a basis for change detection, it is possible to distinguish areas that experienced changes in vegetation due to rainfall variability from those areas that were subject to changes consequent upon anthropological factors. All areas that displayed overall depletion of natural and semi‐natural vegetation due to human factors were deemed to have undergone land degradation, whereas areas that experienced inter‐annual land cover changes due to rainfall variability were classified as experiencing cover change due to ecosystem dynamics. This method provides a complete and appropriate assessment of land degradation in the study area and can be used to improve degradation assessment in other semiarid areas. Copyright © 2006 John Wiley & Sons, Ltd.     

Controls of infiltration–runoff processes in Mediterranean karst rangelands in SE Spain

Semiarid karst landscapes represent an important ecosystem surrounding the Mediterranean Basin for which little is known on runoff generation. Knowledge of the sources and patterns of variation in infiltration–runoff processes and their controls is important for understanding and modelling the hydrological functions of such ecosystems. The objectives of this paper are to determine the infiltration rates and their controls in a representative mountain karst area (Sierra de Gádor, SE Spain) at micro-plots and to investigate the integrated response of rainfall on a typical hillslope. Rainfall simulations in micro-plots and natural rainfall-runoff monitoring on a hillslope were carried out complementarily. We investigated the role of soil surface components (vegetation, rock outcrop, fracture, and soil crust), topographic position, antecedent soil moisture, and rainfall characteristics in regulating infiltration–runoff processes. Results of rainfall simulation revealed the importance of vegetation cover and the presence of rock fractures in promoting the infiltration in the limestone karst landscape, while bare patches and rock outcrops acted as sources for runoff. All plots with > 50% vegetation cover had no runoff with up to 55 mm h^− 1 of simulated rain. In contrast, nearly all bare plots had runoff under the same simulated rain, with runoff coefficients ranging from 3.1 to 20.6% on dry soil surface conditions, and from 2.0 to 65.4% on wet soil surfaces. Runoff coefficients amounted to 59.0–79.5% for rock outcrops without cracks, but were drastically reduced by the presence of cracks. The surfaces with rock fragments resting on the soil (generally located in the middle of the slopes) prevented more effectively the runoff generation than those surfaces where rock fragments were embedded in the top soil. Antecedent soil moisture had significant impact on runoff generation, with wet soil having doubled runoff coefficient, shortened time to runoff, and increased runoff rate compared to the same but dry soil. Linear regressions indicated that the main controls for constant infiltration rate were the cover percentages of vegetation and litter, plus rainfall intensity; while the major controls for runoff coefficient were the bare soil and vegetation coverage, plus rainfall intensity. High infiltration rates measured at the micro-plots agreed with low intra-event runoff coefficients (mostly below 1%) observed under natural rainfalls at the hillslope. Runoff depth and coefficient at the hillslope was significantly correlated with rainfall depth, maximum hourly rainfall intensity and antecedent precipitation over 20 days (AP₂₀). During the 1.5-year monitoring period from Sep-2003 to Mar-2005, the overall infiltration was 41% of the total rainfall amount and the maximum infiltration rate was almost 94% of the largest single rainfall event. The results from this study contribute to improved understanding of the magnitude and controls of the surface runoff in semiarid karst mountain areas.     

Runoff generation in a degraded Andean ecosystem: Interaction of vegetation cover and land use

Tropical mountain regions are affected by rapid land use/-cover change, which may threaten their (eco-)hydrological functions. Although there is a growing interest in evaluating the effect of land use/-cover change on mountain hydrology, quantitative assessments of the impact of land use/-cover on hydrological processes are hampered by the lack of field measurements characterizing runoff generation processes. In this paper, we present results from field experiments of rainfall runoff mechanisms in the southern Ecuadorian Andes. A rainfall simulator was used to quantify the hydrological response of distinct land use/-cover types to intense rainfall (about 40 mm/h). The rainfall runoff experiments indicate that degraded and abandoned land generate surface runoff within a few minutes after the start of the rainfall event. These lands have a very rapid and sharp hillslope hydrological response, as Hortonian overland flow is the dominant runoff generation mechanism. In contrast, surface runoff on arable and rangelands is rare, as their soils are characterized by a high infiltration capacity (i.e. > 29 mm/h). Our experiments provide evidence that runoff generation in degraded Andean ecosystems is mainly controlled by the surface vegetation cover and land management. When reducing the surface vegetation cover, the soil is increasingly affected by rapid hillslope runoff as the presence of large amounts of smectites in the outcropping soft rocks makes the material very prone to sealing and crusting, thereby enhancing runoff generation.     

我国南方亚热带丘陵山地土地退化研究

我国南方土地退化的形成是由其生态环境的脆弱性决定的。土地退化不仅表现为土壤退化,还表现为植被退化和土地状况恶化,具有明显的坡面分带性。从50年代到80年代,不同人类活动地区土地退化变化的特点不同。土地退化的动态变化主要是由于土地的开垦及人类活动对植被的破坏。土地退化严重的地区既不是人口较少和经济落后的基岩山区,也不是人口较密、经济发达的低丘岗地区,而是人口增长较快、经济较落后的低山丘陵区。     

Land degradation assessment by farmers in Bolivian mountain valleys

A methodology is presented for assessing the seriousness and impact of land degradation, from a historical and a farmer perspective, in regions where data are not available. Farmers have been directly involved in the assessment of soil, productivity and vegetation cover loss over the past decades, both on cropland and on rangeland. The results for the Bolivian mountain valleys in north Chuquisaca show that the intensive use of small cropland fields has increased soil degradation and consequently led to less water and nutrient availability. Due to the subsequent productivity loss many cropland fields have been abandoned. Rangeland is also severely degraded as a result of the excessive removal of vegetation and subsequent soil loss. Farmers are very concerned, but consider the impact of changed rainfall and less available cropland beyond their control. Consequently, no serious initiatives are taken to solve the land degradation problem. This research adduces evidence of increased land degradation in the Bolivian mountain valleys over recent decades, and stresses the need for urgent (integrated) action with regard to soil and water conservation. Copyright © 2005 John Wiley & Sons, Ltd.     

LAND USE AND CLIMATE CHANGES AND THEIR IMPACTS ON RUNOFF IN THE YARLUNG ZANGBO RIVER BASIN,CHINA

Impacts of land use and climate change on runoff were investigated by studying the runoff in the Yarlung Zangbo River basin, China. Trends in precipitation, mean air temperature, and runoff were analysed by non‐parametric Mann‐Kendall tests. Land‐use changes were examined with land‐use transition matrix and geographic information system tools. Land‐use and climate changes showed several characteristics, including increased reforestation, decreased grassland, retreat of glaciers and increased desertification. Human activity caused great impact, especially within densely populated regions and cities. Reforestation and degradation of grasslands were more frequent than deforestation and cultivation of grasslands. Annual mean air temperature, precipitation and runoff showed increasing trends between 1974 and 2000. The impacts of land use and climate change on runoff had different effects depending on region and season. In the season of freezing, climate change clearly affected runoff within regions that experienced precipitation. Altered evapotranspiration accounted for about 80 per cent of runoff changes, whereas land‐use changes appear to have had greatest impact on runoff changes within regions that have inconsistent relationships between runoff and climate change. It was demonstrated that afforestation leads to increased runoff in dry seasons. It was estimated that glacier snow melt has caused annual runoff to increase at least 6·0 mm/10yr, 2·1 mm/10yr and 1·7 mm/10yr in Regions 1, 3 and 4, respectively, whereas evapotranspiration caused annual runoff to decrease at least 7·4 mm/10yr in Region 2. Copyright © 2012 John Wiley & Sons, Ltd.     

2000—2020年白尼罗河上游地区植被动态及其驱动因子

[目的] 明确21世纪白尼罗河上游的植被动态及变化的驱动因子,为该区域生产活动、环境政策的制定与调整提供科学指导。 [方法] 以白尼罗河上游地区为研究区,基于降水、温度和人口数据,利用趋势分析、偏相关分析及残差趋势法确定了该地区2000—2020年植被(NDVI)变化特征及其主导因子的空间差异。 [结果] 白尼罗河上游地区NDVI平均以0.105/10 a的速率上升,且温度变化对于NDVI的影响强于降雨;人类活动总体对植被造成负面影响,但是这一负面影响的趋势正在逐渐减弱;在5种土地类型中,灌木地的植被为气候变化及人类活动变化背景下最为脆弱的(所受正面影响小,负面影响大);流域内15.01%陆地范围植被变化主要受人类活动主导,另外84.99%受气候变化主导。 [结论] 虽然流域内整体植被呈现增长趋势,但是个别地区植被发生了严重退化,尤其是城镇的扩张以及农田开垦的扰动对植被造成了破坏,当地在寻求增加粮食产量及旅游业收入的前提下应当做好植被的监测与管理工作。     

Effect of vegetation cover on the hydrology of reclaimed mining soils under Mediterranean-Continental climate

Vegetation cover plays a major role in the restoration and stabilization of disturbed systems. The analysis of relationships between restored vegetation and soil hydrology has special relevance for the evaluation and operation of mining reclamation, particularly in Mediterranean-Continental environments, where climatic conditions restrict the development of continuous vegetation cover. The effect of herbaceous vegetation cover on soil hydrology was analysed by means of rainfall simulation (63 mm h^− 1; 0.24 m²) in reclaimed soils derived from opencast coal mining (a non-saline and clay-loam textured spoil) in central-eastern Spain. A total of 75 simulation experiments were conducted at three different times throughout the year (late winter, summer and autumn) to control the influence of seasonal climatic fluctuations. Sediment concentrations in runoff and the runoff coefficient decreased exponentially with vegetation cover, while increases in steady infiltration rates were obtained with vegetation cover. Additional delays in runoff responses (longer time to runoff start and stabilization) and increases in the wetting front depth were observed with vegetation cover. Seasonal variations in soil surface state and moisture strongly influenced hydrological responses; although the influence of season on the analysed hydrological responses was attenuated by vegetation cover, especially in the case of infiltration rates. We also determined a practical ground cover threshold for site restoration and evaluation of over 50% vegetation cover, which could help achieve an optimum biological control of hydrological soil responses in the studied environment.     

Soil degradation in Kabd area,southwestern Kuwait City

Adverse environmental impacts of human activities are the main causes of soil degradation in the desert of Kuwait in general, and in Kabd area in particular. In this study, assessment of soil degradation in open and protected sites has been carried out using field measurements and laboratory investigations. The overall status of vegetation is nearly twice as low in vegetation cover in the open sites than in the protected ones due to overgrazing and off‐road transport. Compaction of soil due to pressure exerted on the soil by vehicles led to a significant reduction in its porosity, permeability and infiltration capacity. The average infiltration rate of the compacted soils is 51 per cent lower than that of the non‐compacted soils. The bulk density of the non‐compacted soils is 3.4 per cent lower than that of compacted soil. The average topsoil resistance of compacted soils has increased by 83 per cent in comparison with non‐compacted soil. Using the least squares method a relation between infiltration rate (IR) and penetration resistance (PR) of the topsoil for the study areas is found (i.e. IR = −0.148 PR + 1.85 with R² = 25 per cent). Soil strength within the soil profile shows maximum penetration resistance readings at 11.5 cm depth in average in compacted soils, while it shows maximum readings at 34.6 cm depth in average in non‐compacted soils. The adverse changes in the chemical properties due to soil compaction is also investigated. A restoration plan is needed in order to reduce land degradation. Copyright © 2002 John Wiley & Sons, Ltd.     

Do subtropical seasonal forests in the Gran Chaco, Argentina, have a future?

While much information is available about tropical and temperate ecosystems, there is a remarkably little information as to land cover and land use changes in the subtropical biomes of the world. Here, we quantify changes in the spatial patterns of land cover types at the southern edge of the seasonally dry, subtropical Chaco forest of South America during the second half of the 20th century using a vegetation map printed in 1969 and a Landsat TM based digital map produced 30 years later. Results show a massive contraction of forest; ca. 1.2 million ha of original lowland and mountain subtropical dry forests and woodlands, 85% of the total, have been cleared in only 30 years. This loss of Chaco forests of 2.2% year⁻¹ is consistent with or even exceeds, global trends. Forest vegetation now persists as fragments where there was formerly continuous cover. Most of undisturbed Chaco forest has now been converted to pasture or is undergoing secondary succession. Today, these new vegetation types, resulting mainly from agricultural expansion, have increased 10-fold in cover and now represent the commonest land cover types. The increased intensity of agricultural usage, possibly triggered by an increase in annual rainfall during the last decades, has been accompanied by changes in agricultural practices and a relative decline in the rural population.     

基于SWAT模型的长江源土地利用/土地覆被情景变化对径流影响研究

土地利用/土地覆被变化对流域水资源循环过程及配置影响显著。为了研究土地利用/土地覆被情景变化对径流影响情况,以长江源流域为例,构建了适合该流域的分布式水文模型(SWAT),并模拟了不同土地利用/土地覆被情景下的流域年均径流量和径流深。模拟结果如下:当流域林地和草地面积增加到最大,径流量减少了16.7%,达到304.12m3/s,径流深减少14.02mm;当林地和草地面积减少,并逐步变为沙地、裸地,流域径流量增加16.1%,达到424.32m3/s,径流深也增加了13.49mm;当林地和草地面积减少到无植被覆盖,将使流域径流量增加28.4%,达到469.67m3/s,径流深增加23.88mm;在草地覆被最佳状况下,径流量有所增加,但增加幅度不大,只有5.6%,达到385.98m3/s,径流深增加4.72mm。综上所述,长江源流域林地和草地面积增加,导致径流减少,而沙地和裸地面积增加导致径流量增加。     

野外模拟降雨条件下坡面降雨入渗、产流试验研究

利用野外模拟降雨试验,研究了植被类型、降雨强度、地表结皮、覆盖杂草等对荒草地和裸地降雨入渗、产流的影响。结果表明:裸地因降雨易产生地表结皮,产流时间主要取决于降雨强度,两者的关系可用幂函数描述,达显著水平;荒草地由于植被覆盖度较高,产流时间主要取决于土壤初始含水量,两者的关系可用线性公式描述,达极显著水平;与裸地相比,荒草地能延缓产流,并有效拦蓄径流;裸地形成地表结皮后,产流提前,平均入渗率降低,但坡面覆盖杂草(尤其是坡中下部覆盖杂草)能有效拦蓄径流、延缓产流、增加入渗,并促进土壤水分向深层的运移,而雨后采用抑制蒸发的措施能起到类似的效果     

Rangeland development of the Mu Us Sandy Land in semiarid China: an analysis using Landsat and NOAA remote sensing data

Degradation of the dry semiarid ecosystems in the Mu Us Sandy Land of north central China was explored using high‐resolution satellite images from 1978, 1987 and 1996. This study monitored both changes in grassland biomass production and reclamation activities to detect the nature and scale of land degradation since major economic reforms were introduced in 1978. The position of the high‐resolution images within the vegetation cycles was inspected from National Oceanographic and Atmospheric Administration (NOAA) NDVI images at 10‐day repetition and seasonal precipitation patterns. A model was developed to categorize changes in the vegetation signal activity from 30 × 30 m pixels into vegetation cover development and land‐use changes between 1987 and 1996. A general increase of biomass production was evident despite the rapid increase in numbers of grazing animals. This increase in biomass was confirmed by the NOAA time series, which also revealed annual variability related to the amount and pattern of the seasonal rains. Rangeland conversion to farmland was detected, and this process has increased the area of cultivation almost fivefold. The classified area of cultivation corresponds with reported statistical records, also showing that irrigation features in virtually 100 per cent of the sown area. Signs of declining biological production, indicating land degradation processes, are few. Biomass production has increased, with a gain in the economic output from both crop and animal production. The early start of active measures to halt desertification has increased vegetation cover and lowered wind erosion potential and grasslands seems to be managing the high levels of grazing pressure. Copyright © 2003 John Wiley & Sons, Ltd.     

黄丘区野外坡面产流产沙过程对不同植被覆盖结构的响应

植被覆盖结构是影响坡面侵蚀过程的重要因素,通过在黄丘区罗玉沟流域开展自然植被覆盖坡面野外模拟降雨试验,分析了90 mm/h雨强条件下地下根系、地下根系+地表结皮层和地下根系+地表结皮层+植被冠层3种不同覆盖结构对坡面产流产沙过程影响。结果表明:植被根系层次对增强土壤入渗起主导作用,使土壤入渗量增加67%,稳定入渗率增加39.7%;3种植被覆盖结构中,地表结皮层对产流和产沙具有最好的调控作用,减流贡献率和减沙贡献率分别为43%和67%;根系小区和裸地小区累积产流量与累积产沙量之间呈幂函数关系,随着植被覆盖结构的叠加,累积产流量与累积产沙量之间呈线性函数的关系;多层次植被覆盖结构能够延长坡面侵蚀过程的发育期,缩短活跃期。研究结果对定量评价植被措施的减水减沙作用具有重要意义。     

Runoff Trends Driven by Climate and Afforestation in a Pyrenean Basin

The abandonment of traditional rural life in mountain areas has favoured the expansion of forest in the headwaters of Pyrenean rivers. In this paper, we (i) analyse hydro‐climatic trends at the annual and monthly scales in three nested sub‐catchments in a central Pyrenean basin and (ii) quantify the relative contribution of climate change and forest cover on the observed changes in runoff. Land use maps indicate an increase in the forest cover in all sub‐basins for the period 1987–2009. Non‐parametric Mann–Kendall statistic on annual and monthly hydro‐climatic data from 1965 to 2009 was used to identify trends in climate and runoff data. To assess the potential influence of the period length, trends were also examined for the 1941–2009 stretch. Upward trends were detected for temperature and potential evapotranspiration, particularly during summer and winter months. Precipitation trends and their significance depended on the length of the period considered, although overall results indicated a decrease, particularly for February and July. The effects of climate change and land cover change on annual runoff were identified by means of a change point analysis. Results indicated that a change in annual runoff took place in the 1980s, suggesting the non‐linearity in the relationship between precipitation and runoff, and thus the influence of land use factors. The influence of forest cover was confirmed by comparing observed runoff trends with those resulting from a hydro‐climatic model that does not take into account land use changes. Divergence between both trends revealed that forest cover can account for ~40% of the observed decrease in annual runoff. Copyright © 2015 John Wiley & Sons, Ltd.     

Land Use and Cover Dynamics Since 1964 in the Afro‐Alpine Vegetation Belt: Lib Amba Mountain in North Ethiopia

Human‐induced land use and land cover (LUC) changes threaten the ecosystem services of the vulnerable tropical afro‐alpine vegetation. Several LUC change studies are available for the Ethiopian highlands, but relatively little is known about LUC change in the afro‐alpine zones. In this study, LUC changes between 1964 and 2012 were mapped for the afro‐alpine zone of Lib Amba Mountain, part of the Abune Yosef Mountains in North Ethiopia. Historical LUC was derived from georeferenced aerial photographs of 1964 and 1982, and the present LUC (2012) from Bing Map satellite imagery. Based on these successive LUC maps a time‐depth map, LUC proportions, LUC transition matrices and LUC change trajectories were calculated. Two main phases of LUC change could be distinguished linked to the neo‐Boserupian perspective. (i) Between 1964 and 1982, there were large‐scale deforestation and general degradation of the vegetation above 3500 m, in a period of low population pressure; (ii) Between 1982 and 2012, an intensification of land use prevailed accompanied with a slight regeneration of the vegetation and the Erica arborea L. forest, under increased population pressure. Depth interviews indicated that local and governmental land management measures are very important for the protection against vegetation depletion and soil degradation. Quick recovery of the forest on Lib Amba provides confidence that degraded afro‐alpine areas would benefit in a short time from complete protection, given the vicinity of remaining patches of afro‐alpine vegetation. Management interventions are thus vital to restore the important ecosystem services of the afro‐alpine vegetation belt. Copyright © 2015 John Wiley & Sons, Ltd.     

不同时间尺度径流对植被变化的响应

 植被覆盖对陆地生态系统的水分循环起着重要作用。径流对植被变化在不同时间尺度的响应研究,对区域生态环境建设及水资源管理具有重要的参考价值。该文介绍基于对比流域法,在多年平均、年际、季节等不同时间尺度径流对植被变化响应的研究结果。多年平均尺度上,由植被类型更替引起的径流绝对变化量,湿润区较干旱区大,其相对变化量在干旱区严重;年际尺度上,因植被变化,径流从一种平衡状态到另一种平衡状态的响应时间因抚育措施和地区不同而不同,造林情况下,湿润地区一般需要15～20a,干旱地区约需25a甚至更长,森林自然恢复情况下则需要上百年;季节尺度上,径流对植被变化的响应在季风影响区较四季湿润地区强烈。流量历时曲线研究表明,干旱地区造林,其基流无论在量或发生概率的变化上均较湿润区强烈。     

基于改进径流曲线数模型的北京密云坡地径流估算

密云区是北京重要的地表饮用水源地,准确模拟地表径流量,对于分析泥沙和污染物的运移十分重要。近年来,学者们运用径流曲线数(soil conservation service curve number,SCS-CN)模型计算本区地表径流量,但预报精度不理想;未考虑降雨过程和雨强对于产流过程的影响,可能是造成预报误差的重要原因。该文利用密云石匣小流域5个坡面径流小区共201场降雨产流资料,提出次产流径流曲线数计算方法,以改进SCS-CN模型并分析改进后模型模拟效果。结果表明,次产流径流曲线数与多年平均径流曲线数的比值和最大30 min降雨量与次雨量的比值之间呈显著幂函数递增关系,据此提出计算次产流径流曲线数的幂函数方程,以改进SCS-CN模型。当曲线数为0.02时,改进后模型模拟效果最好,效率系数为0.693,明显高于未改进的SCS-CN模型。改进后模型对裸地和耕地的产流模拟效果较好,但对林地的产流模拟效果不理想。今后需在深入分析产流机理的基础上,进一步提出与土壤特性有关的模型参数优化方法。