FLOW PERSISTENCE AS A MODIFIER OF SEASONAL RUNOFF PATTERNS1

ABSTRACT: The effect of flow persistence on seasonal patterns of watershed runoff was modeled by using runoff of the immediate antecedent month as an index. Monthly runoff was expressed as a function of monthly rainfall, season of the year, and runoff of the antecedent month. The three independent variables were expressed functionally as sliding polynomials, thus producing a piece-wise, form-free, three-dimensional causative structure. A model form allowing complete interactivity of the three independent variables could not be optimized because of insufficient data with high values of both antecedent runoff and monthly rainfall. A model with reduced interactivity was successfully optimized. Data sets from five watersheds ranging from 0.14 to 398 square miles were analyzed. Results were presented as a series of contour maps that showed contours of monthly runoff in the data space of season and monthly rain. In the series of maps, the patterns of the runoff contours changed with changing values of antecedent runoff. During the wet season of the year the contours changed significantly with antecedent runoff, but changes in the dry season were minimal. The quantitative change of runoff was more readily portrayed with cross-sections through the contoured surfaces.     

盐池封育草场土壤种子库特征及其与植被的关系

封育是退化草场恢复的重要措施之一,封育的效果在一定程度上取决于土壤中种子库的状况。以毛乌素沙地西南缘宁夏盐池县的退化草场为研究对象,采用相似性分析和排序方法对封育和未封育草场土壤种子库的特征及其与地上植被间关系进行了研究,结果表明无论封育区还是未封育区,土壤种子库中的种子主要分布于0～5cm的土层,6～15cm土层的种子数很少;封育区土壤种子库的物种丰富度大于未封育区,物种多样性则表现为H值较大,而D值较小,总体上看封育措施可以提高地上植被和土壤中种子的种类和数量,一些一年生植物、中生植物和盐生植物开始出现在地面或土壤种子库中。地上植被和土壤种子库间的相似性分析结果则表明相同措施同一土层间种子库中等相似,而不同土层间还是存在着一些差异。由于封育措施导致了植被样方的相似性较低,然而土壤种子库间的相似性仍然较高,两者间是否存在着一定程度的滞后效应有待于进一步研究。     

川西高海拔河流中溶解性有机质(DOM)紫外-可见光吸收光谱特征

溶解性有机质(DOM)对全球碳循环及气候变化具有重要意义.本文选取川西北4条高海拔河流,即杂谷脑河、抚边河及岷江(高山峡谷河流)与白河(高原河流),对其天然水体中有色溶解性有机质(CDOM)的紫外-可见光吸收光谱特征进行了研究与比较.结果表明,川西高海拔河流DOC变化范围为1.55~5.66 mg·L~(-1),CDOM(以a(355)表征)变化范围为0.96~6.31 m-1,荧光溶解性有机质(FDOM)(以lg Fn(355)表征)变化保持在2.08~2.83之间.与高山峡谷区河流比较,高原河流芳香性、疏水性特征常数SUVA254、SUVA260较高,吸光度之比E2/E3、光谱斜率S275~295及光谱斜率之比SR较低,揭示了高原河流CDOM芳香性较强、疏水性组分丰富、分子结构较为复杂;SUVA254、SUVA260分别与SR、S275~295呈显著负相关,表明白河由于高通量陆源腐殖质输入影响了CDOM特征.川西高原4条高海拔河流中SUVA254、SUVA260存在显著相关关系,表明芳香性结构普遍存在于河流CDOM疏水组分中.川西高原天然河流水体中CDOM与DOM的浓度之间总体上无直接关联,因为不同天然水体在DOM形成过程中,陆源DOC输入组分和强度及转化机制存在差异.研究结果对于揭示高海拔区域在全球变化背景下有机碳循环的区域效应等方面具有重要意义.     

草场管理中的市场机制与习俗制度的关系及其影响:青藏高原案例研究

近年来,政府大力推动在完善承包到户基础上的草场经营权的流转,目的之一是通过市场手段来整合牧户个体的草场资源,促进畜牧业的规模化生产。市场机制与牧区传统管理草场的习俗制度,二者的关系及其对草场管理的影响是怎样的?论文选择位于青海省贵南县的元义村和查乃核村为案例地来研究上述问题。元义村反映市场机制取代习俗制度,查乃核村则反映了市场机制嵌套在习俗制度中。通过比较两个案例地的牧户生计、畜牧业生产以及贫富差距,研究发现:1)查乃核村的牧户年均畜牧业资产高于元义村;2)查乃核村的畜牧业生产量逐年增加、牲畜死亡率明显减少,畜牧业生产收益率高于元义村;3)查乃核村的贫富差距明显低于元义村。基于此,论文认为市场机制嵌套在社区习俗制度的制度安排在草场管理中更有效,因为这样的管理模式在满足牧户个体对于权属明晰和补偿需求的同时,能够维持社区共用草场以适应青藏高原多变的气候特征所导致的资源时空分布的异质性。随着市场化的发展,草场管理需要权属明晰及市场机制,但实现这样的目标不需要弱化甚至取代已有的社区习俗制度。     

Bird Species Assemblages as Indicators of Biological Integrity in Great Basin Rangeland

The study evaluates the potential for bird species assemblages to serve as indicators of biological integrity of rangelands in the Great Basin in much the same way that fish and invertebrate assemblages have been used as indicators in aquatic environments. Our approach was to identify metrics of the bird community using relatively simple sampling methods that reflect the degree of rangeland degradation and are consistent over a variety of vegetation types and geographic areas. We conducted the study in three range types (i.e., potential natural plant community types) in each of two widely separated areas of the Great Basin: south-eastern Idaho (sagebrush steppe range types) and west-central Utah (salt-desert shrub range types). Sites were selected in each range type to represent three levels of grazing impact, and in Idaho included sites modified for crested wheatgrass production. Birds were sampled by point counts on 9 100-m radius plots at 250-m spacing on each of 20 sites in each area during the breeding season. In sagebrush-steppe, 964 individuals in 8 species of passerine birds were used in analyses. Five metrics were significantly related to impact class, both when analyzed within range type and when analyzed with all range types combined. Species richness, relative abundance of shrub obligate species, and relative abundance of Brewer's sparrow were generally lower for the higher impact classes, whereas the reverse was true for dominance by a single species and for relative abundance of horned larks. In contrast, total number of individuals did not differ significantly as a function of impact class. In salt-desert shrub, a total of 843 birds in 4 species were included in analyses, 98% of which were horned larks. None of the metrics identified above was significantly related to impact class. Two metrics for breeding birds in sagebrush steppe (species richness and dominance) showed little overlap between values for the extremes of impact class, and thus they have potential as indicators of biological integrity. However, the sensitivity of these metrics appears to be greatest at the high impact end of the spectrum, which suggests they may have limited utility in distinguishing between sites having light and moderate impact.     

A CALIBRATION PROCEDURE USING TOPMODEL TO DETERMINE SUITABILITY FOR EVALUATING POTENTIAL CLIMATE CHANGE EFFECTS ON WATER YIELD1

ABSTRACT: An evaluation was conducted on three forested upland watersheds in the northeastern U.S. to test the suitability of TOPMODEL for predicting water yield over a wide range of climatic scenarios. The analysis provides insight of the usefulness of TOPMODEL as a predictive tool for future assessments of potential long-term changes in water yield as a result of changes in global climate. The evaluation was conducted by developing a calibration procedure to simulate a range of climatic extremes using historical temperature, precipitation, and streamfiow records for years having wet, average, and dry precipitation amounts from the Leading Ridge (Pennsylvania), Fernow (West Virginia), and Hubbard Brook (New Hampshire) Experimental Watersheds. This strategy was chosen to determine whether the model could be successfully calibrated over a broad range of soil moisture conditions with the assumption that this would be representative of the sensitivity necessary to predict changes in streamfiow under a variety of climate change scenarios. The model calibration was limited to a daily time step, yet performed reasonably well for each watershed. Model efficiency, a least squares measure of how well a model performs, averaged between 0.64 and 0.78. A simple test of the model whereby daily temperatures were increased by 1.7°C, resulted in annual water yield decreases of 4 to 15 percent on the three watersheds. Although these results makes the assumption that the model components adequately describe the system, this version of TOPMODEL is capable to predict water yield impacts given subtle changes in the temperature regime. This suggests that adequate representations of the effects of climate change on water yield for regional assessment purposes can be expected using the TOPMODEL concept.     

Do Polynomials Adequately Describe the Hypsometry of Monadnock Phase Watersheds?

Hypsometry has been shown to be a useful tool in geomorphic analysis of watersheds with the use of third‐degree polynomial equations to express the hypsometric curve. Despite its usefulness with watersheds in the equilibrium stage, the third‐degree polynomial has been found to be inadequate to describe the hypsometry of Monadnock phase watersheds. Three other equations — a modified third‐degree polynomial with a rational term, a sigmoidal model, and a double exponential — were used to determine hypsometric attributes of 32 Monadnock phase watersheds and compared to the third‐degree polynomial form. The three other equations were found to be better fits for Monadnock phase watersheds than the third‐degree polynomial equation, regardless of which ratio — area or elevation — was plotted as the independent variable. Due to the occasional failure of each functional form to give logical values for hypsometric attributes, the importance of using more than one form equation is discussed. After determining the best‐fit equation for each watershed, the usefulness of hypsometric attributes is discussed in relation to erosion processes within Monadnock phase watersheds.     

Factors Affecting Stream Nutrient Loads: A Synthesis of Regional SPARROW Model Results for the Continental United States1

Preston, Stephen D., Richard B. Alexander, Gregory E. Schwarz, and Charles G. Crawford, 2011. Factors Affecting Stream Nutrient Loads: A Synthesis of Regional SPARROW Model Results for the Continental United States. Journal of the American Water Resources Association (JAWRA) 47(5):891‐915. DOI: 10.1111/j.1752‐1688.2011.00577.x Abstract: We compared the results of 12 recently calibrated regional SPARROW (SPAtially Referenced Regressions On Watershed attributes) models covering most of the continental United States to evaluate the consistency and regional differences in factors affecting stream nutrient loads. The models – 6 for total nitrogen and 6 for total phosphorus – all provide similar levels of prediction accuracy, but those for major river basins in the eastern half of the country were somewhat more accurate. The models simulate long‐term mean annual stream nutrient loads as a function of a wide range of known sources and climatic (precipitation, temperature), landscape (e.g., soils, geology), and aquatic factors affecting nutrient fate and transport. The results confirm the dominant effects of urban and agricultural sources on stream nutrient loads nationally and regionally, but reveal considerable spatial variability in the specific types of sources that control water quality. These include regional differences in the relative importance of different types of urban (municipal and industrial point vs. diffuse urban runoff) and agriculture (crop cultivation vs. animal waste) sources, as well as the effects of atmospheric deposition, mining, and background (e.g., soil phosphorus) sources on stream nutrients. Overall, we found that the SPARROW model results provide a consistent set of information for identifying the major sources and environmental factors affecting nutrient fate and transport in United States watersheds at regional and subregional scales.     

A SIMPLE METHOD FOR ESTIMATING BASEFLOW AT UNGAGED LOCATIONS1

ABSTRACT: Baseflow, or water that enters a stream from slowly varying sources such as ground water, can be critical to humans and ecosystems. We evaluate a simple method for estimating base‐flow parameters at ungaged sites. The method uses one or more baseflow discharge measurements at the ungaged site and longterm streamflow data from a nearby gaged site. A given baseflow parameter, such as the median, is estimated as the product of the corresponding gage site parameter and the geometric mean of the ratios of the measured baseflow discharges and the concurrent discharges at the gage site. If baseflows at gaged and ungaged sites have a bivariate lognormal distribution with high correlation and nearly equal log variances, the estimated baseflow parameters are very accurate. We tested the proposed method using long‐term streamflow data from two watershed pairs in the Driftless Area of southwestern Wisconsin. For one watershed pair, the theoretical assumptions are well met; for the other the log‐variances are substantially different. In the first case, the method performs well for estimating both annual and long‐term baseflow parameters. In the second, the method performs remarkably well for estimating annual mean and annual median baseflow discharge, but less well for estimating the annual lower decile and the long‐term mean, median, and lower decile. In general, the use of four measurements in a year is not substantially better than the use of two.     

NUTRIENT CONCENTRATIONS AND YIELDS IN UNDEVELOPED STREAM BASINS OF THE UNITED STATES1

ABSTRACT: Data from 85 sites across the United States were used to estimate concentrations and yields of selected nutrients in streams draining relatively undeveloped basins. Flow‐weighted concentrations during 1990–1995 were generally low with median basin concentrations of 0.020, 0.087, 0.26, 0.010, and 0.022 milligrams per liter (mg/L) for ammonia as N, nitrate as N, total nitrogen, orthophosphate as P, and total phosphorus, respectively. The flow‐weighted concentration of nitrate exceeded 0.6 mg/L in only three basins. Total nitrogen exceeded 1 mg/L in only four basins, and total phosphorus exceeded 0.1 mg/L in only four basins. The median annual basin yield of ammonia as N, nitrate as N, total nitrogen, orthophosphate as P, and total phosphorus was 8.1, 26, 86, 2.8, and 8.5 kilograms per square kilometer, respectively. Concentrations and yields of nitrate tended to be highest in northeastern and mid‐Atlantic coastal states and correlated well with areas of high atmospheric nitrogen deposition. Concentrations and yields of total nitrogen were highest in the southeastern part of the nation and in parts of the upper Midwest. In the northeast, nitrate was generally the predominant form of nitrogen, and in the southeast and parts of the upper Midwest, organic nitrogen was the dominant form. Concentrations of total phosphorus were generally highest in the Rocky Mountain and Central Plain states.