THE RELATIONSHIP BETWEEN THE TIME BASES OF SIMULATION MODELS AND THEIR STRUCTURE1

ABSTRACT: The time base of a simulation model can be defined as a combination of two time intervals. One is the interval used for input and internal computations. The second is the interval used for the output and calibration of the model. The time base of a model is related on the one hand to the type of applications for which the simulated data are used, and on the other hand to the structure and complexity of the model. The latter may be represented by the number of parameters employed to specify the operation of the model. Using data typical to relatively small watersheds in a semiarid climate, the interaction between the complexity of a series of models and the time bases used by them was studied. This included the effects of the two factors, time base and complexity, on the values of the optimal parameters, prediction of mean annual flow, and general performance of the models. The main conclusion is that if the acceptable time base is longer, the model can be less complex needing fewer parameters. There is also an advantage in using a time base comprising a shorter input time interval and a longer output time interval.     

Modeling and Measurement of Structural Changes at a Landscape Scale in Dryland Areas

We present a technique to quantify and model the intensity of structural changes produced by management of dry grazing lands at a landscape scale. The technique is illustrated with the analysis of digitized black–white (b/w) imagery and an application to the study of changes induced by grazing gradients. Structural changes in patchy vegetation canopies were studied in the Patagonian Monte (Chubut, Argentina) at two resolution scales by means of linear transects in the field (50 m) and others drawn on aerial b/w photographs (2–5 km) of grazed paddocks. Spatial series of plant cover values along transects in the field and on photographs were analyzed with standard techniques of spectral analyses, including auto-correlation spectra and Fourier transforms. In order to test the internal consistency of the techniques used, synthetic plant canopies with patches of varying cover and size were generated by means of a stochastic model of plant growth under different stocking rates or after varying periods of recovery. The behavior of the simulation model is consistent with the observed dynamics of plant canopies in semiarid environments. There is a consistent relation between the number and geometric properties of plant patches (patch number, patch size, patch connectivity) and the signal/noise ratios of the Fourier decomposition describing plant density data. Signal/noise ratios corresponding to plant cover data in paddocks with different grazing treatment are consistent with the assumptions derived from modeled canopies, as well as those estimated from optical density of b/w aerial photographs of paddocks. We tested the hypotheses that patch arrangements as quantified by the signal/noise ratios vary in accordance with grazing gradients in paddocks with a permanent corner-located watering point. The use of digitized b/w images allows inspecting permanent changes over time periods when other types of images were not yet available.     

An Integrated Approach for Impact Assessment of Water Harvesting Techniques in Dry Areas: The Case of Oued Oum Zessar Watershed (Tunisia)

In the arid regions of Tunisia, considerable investments are being made to maintain the old water harvesting techniques and introduce new ones to capture the scarce amount of rainwater (100 mm to 230 mm annually) for agricultural and domestic purposes. However, no detailed assessment of the multiple effects and the costs and benefits of these techniques have been made so far. This paper summarizes the results of an in depth investigation of the multiple impacts (runoff mobilization, ground water recharge, agro-socio-economic impacts) of the water harvesting works undertaken in the watershed of oued Oum Zessar (southeastern Tunisia). The importance of interdisciplinary and integrated approaches was revealed through this detailed impact assessment and economic evaluation. In fact, the profitability of the water harvesting works depends largely on the criteria chosen. However, further refinements are needed to better include all possible impacts (positive and negative) that occur as a result of the installation of the water harvesting structures.     

珠江三角洲的耕地及其持续利用

耕地的持续利用是农业持续发展的前提和基础,是人类经济社会持续发展的根本保证。本文论述了珠江三角洲耕地的现状和经济快速增长所引发的耕地问题,并指出耕地的持续利用需要全社会的共同努力。     

丽江地区中低产耕地及其开发利用

本义探讨了丽江地区中低产耕地的特征,并从自然因素和人文因素分析了低产的原因,进而提出合理改良利用低产耕地的对策。     

中国西北干旱区的地下水资源及其特征

分析了中国西北干旱区自然地理环境、水资源形成和分布及其利用状况,重点讨论了该区地下水资源分布的地区差异与赋存特征。该区平原地下水资源是山区水资源(包括降水和雪冰融水)在平原区内的转化;山区水资源是内陆河流和平原地下水的惟一补给来源;盆地平原地下水和地表水构成一个统一的水资源系统、独立的水文单元和完整的水生态体系。充分了解干旱区降水-地表水-地下水资源链的特征,正确认识干旱区“三水”转化自然规律,合理利用“三水”,对该区域社会经济持续发展具有重要意义。     

Influence of temperature on the heavy metals accumulation of five vegetable species in semiarid area of northwest China

The aim of the present study was to investigate the possible effects of temperature on cadmium (Cd), lead (Pb), copper (Cu) and zinc (Zn) accumulation in five vegetable species collected at different sites (Shuichuan, Beiwan, Dongwan and Wufe) in northwest China. The meteorological data of air and soil temperatures were recorded daily during the period from sowing to harvest for the five vegetables. The air and soil temperatures affected the capacity of pumpkin, cabbage, brassica napus and Chinese cabbage to accumulate Zn, Cd, Cu and Pb. Principal component analysis showed that temperature, photosynthetic and physiological factors all contributed to the soil–plant transfer properties of DTPA-extractable heavy metals. Temperature played a more important role in Cd, Pb, Cu and Zn accumulation in four vegetables in this semiarid area. However, the enormous surface area of spinach was likely to elevate heavy metal loads owing to atmospheric deposits. For most vegetables studied, there was a striking dissimilarity in the uptake and translocation ability of Cd, Zn and Cu in soil, but similar accumulation to translocation for Pb in soil.     

城市绿地对周边热环境影响遥感研究--以北京为例

城市绿地是缓解城市热环境效应问题的主要因素之一,它不仅在宏观上影响城市区域尺度气候条件,而且在小区尺度直接影响到周边的热环境条件。本文从绿地景观格局的角度出发,利用遥感技术和地理信息技术,以北京市主城区的城市绿地作为研究对象,以绿地周边建筑物作为热环境影响承载体中介,分析了影响建筑热环境的绿地的主要景观因子,开展了城市绿地对周边热环境的影响范围、降温幅度以及绿地景观参数与降温幅度的相关关系研究。首先,从TM遥感影像上人工数字化选取了26个城市绿地斑块,同时提取了其周边建筑物像元;然后,基于定量遥感理论反演了绿地和建筑物的温度,并利用GIS工具统计了绿地和建筑的景观格局信息;最后,基于空间统计分析方法和等温线周长-温度曲线变点方法确定了城市绿地对周围建筑物热环境的影响范围,通过相关性分析探讨了城市绿地景观参数与其降温幅度之间的相关性。研究结果显示：（1）在100 m空间分辨率的尺度下,北京大部分城市绿地斑块对周边100 m范围内的建筑具有降温效应;面积在0.5 km2以上的绿地斑块,对周边100 m范围内建筑物具有明显降温效应,降温幅度在0.46~0.83℃之间,平均降温幅度为0.72℃;面积在0.5 km2以下的,具有较高植被覆盖度的绿地斑块有一定的降温效应,不具有较高植被覆盖度的绿地斑块降温效应不明显;（2）绿地斑块的周长、面积、形状指数和植被覆盖度与其周边建筑物的降温幅度没有显著的相关性。该结果表明,绿地的面积无论多大,其对周边环境的降温效应都限制在一定空间范围内;在布设城市绿地时分散型绿地比集中式大绿地对周边环境的总体降温效应更好。研究结果揭示了城市绿地对周边热环境影响的空间范围、降温幅度以及绿地景观参数与降温幅度的相关关系,可为城市规划建设及环境评价等提供科学参考。     

冀西北半干旱区等高植物篱面源污染控制机理

坡面养分流失是造成面源污染的主要形式.本研究利用冀西北黄土丘陵区的试验小区的长年观测资料,并结合野外全坡面大型人工模拟降雨试验及室外实验数据,分析了冀西北温带及半干旱地区等高植物篱对面源污染控制机理,得出在暴雨条件下细沟产生并导致侵蚀剧烈增加的临界坡长为10～15m;研究揭示出冀西北半干旱区面源污染形态以泥沙携带养分流失为主,等高植物篱控制面源污染的途径是控制土壤颗粒中的养分流失.     

Using spatially explicit,time-dependent analysis to understand how social factors influence conservation outcomes

Conservation across human-dominated landscapes requires an understanding of the social and ecological factors driving outcomes. Studies that link conservation outcomes to social and ecological factors have examined temporally static patterns. However, there may be different social and ecological processes driving increases and decreases in conservation outcomes that can only be revealed through temporal analyses. Through a case study of the invasion of Falcataria moluccana in Hawaii, we examined the association of social factors with increases and decreases in invader distributions over time and space. Over 7 years, rates of invader decrease varied substantially (66–100%) relative to social factors, such as building value, whether land was privately or publically owned, and primary residence by a homeowner, whereas rates of increase varied only slightly (<0.1–3.6%) relative to such factors. These findings suggest that links between social factors and invasion in the study system may be driven more by landowners controlling existing invasive species, rather than by landowners preventing the spread of invasive species. We suggest that spatially explicit, time-dependent analyses provide a more nuanced understanding of the way social factors influence conservation outcomes. Such an understanding can help managers develop outreach programs and policies targeted at different types of landowners in human-dominated landscapes.