湿地水文研究的若干热点问题

湿地水文研究是认识湿地生态系统的结构、过程与功能,进行湿地生态保护和恢复重建以及水资源合理配置的前提与基础。重点对湿地水文研究的几个热点问题进行了评述。这些问题包括,全球气候变化与人类活动影响下的湿地水文响应;湿地水文过程对湿地生态系统的影响机制;湿地水循环过程与水文模型。在此基础上,提出了基于水循环模拟的流域湿地水资源合理配置的基本思路,并对目前中国湿地水文亟待研究的重要议题进行了阐述。     

Quality assessment from a hydrological perspective of a digital elevation model derived from WorldView-2 remote sensing data

Abstract

A digital elevation model (DEM) derived from a stereo pair of WorldView-2 (WV-2) images was assessed against ground-truth GPS point datasets. Two assessment methods were used: (a) vertical accuracy assessment and (b) hydrological assessment of surface runoff variables. Three agricultural plots with different topographic slopes were selected to perform a vertical accuracy assessment, followed by a comparative assessment of a set of hydrological variables. The results show an overall vertical accuracy of 0.45 m, confirming the potential of WV-2 stereo images to extract elevation information at high spatial resolution. Concerning plot-scale micro-topographic features, the WV-2 DEM performed better on the plot with rolling slopes (5–10%), extracting variables such as the total length and drainage area of flow paths with relative errors lower than 20%. However, some limitations were detected in the extraction of variables such as terrain slope, drainage points of flow paths and terrain depressions in areas of flatter slopes (<5%).

Editor Z.W. Kundzewicz; Associate editor D. Gerten     

Hydrogeologic framework and borehole yields in Ghana

In Ghana, 68% of the population live in rural communities, which are scattered and remote. Groundwater is the most feasible source of potable water supply for most of these dispersed and remote settlements. To meet the present and future challenges of population expansion vis-à-vis the observed declining rainfall in most parts of Africa including Ghana, it is necessary to assess, efficiently manage, and utilize the groundwater resources. The objective of this paper is therefore to describe the hydrogeologic framework and analyze borehole yields as part of the groundwater-resources assessment of Ghana. The hydrogeologic units are broadly categorized as: (1) the Basement Complex (crystalline rocks), which underlies about 54% of the country; (2) the Voltaian System, which underlies about 45%; and (3) the Cenozoic, Mesozoic, and Paleozoic sedimentary strata (Coastal Provinces), which underlie the remaining 1% of the country. The Basement Complex and the Coastal Provinces have higher groundwater potential than the Voltaian System. This is particularly significant, because the Basement Complex and the Coastal Provinces underlie the most densely populated areas of the country and can hence be tapped for human use. The average borehole yields of the Basement Complex, the Coastal Provinces and the Voltaian System range from 2.7–12.7, 3.9–15.6, and 6.2–8.5?m3/h, respectively.     

Measurements and Observations in the XXI century (MOXXI): innovation and multi-disciplinarity to sense the hydrological cycle

To promote the advancement of novel observation techniques that may lead to new sources of information to help better understand the hydrological cycle, the International Association of Hydrological Sciences (IAHS) established the Measurements and Observations in the XXI century (MOXXI) Working Group in July 2013. The group comprises a growing community of tech-enthusiastic hydrologists that design and develop their own sensing systems, adopt a multi-disciplinary perspective in tackling complex observations, often use low-cost equipment intended for other applications to build innovative sensors, or perform opportunistic measurements. This paper states the objectives of the group and reviews major advances carried out by MOXXI members toward the advancement of hydrological sciences. Challenges and opportunities are outlined to provide strategic guidance for advancement of measurement, and thus discovery.     

Characterizing ponds in a watershed simulation and evaluating their influence on streamflow in a Mississippi watershed

Small water bodies, such as ponds and wetlands, are common landscape features, but often are not simulated within a watershed modeling framework. The wetland modeling tool, AgWET, uses a GIS framework to characterize the features of ponds and wetlands so that they can be incorporated into watershed simulations using the Annualized Agricultural Non-Point Source (AnnAGNPS) pollution model. AgWET was used to characterize farm ponds on the Goodwin Creek Experimental Watershed in northwest Mississippi and AnnAGNPS simulated watershed hydrology. Monthly streamflow was validated at four watershed gauges with Nash-Sutcliffe efficiency values between 0.91 and 0.94. Ponds influenced watershed hydrology at various scales, with a decrease in average streamflow by 4% at the watershed outlet, 8% at the sub-watershed scale, and an average of 56% immediately downstream of the ponds. AgWET can be used to simulate ponds in watershed assessments for improved results and evaluation of future scenarios.     

Analytical solution of transient flow in a sloping soil layer with recharge

Abstract

An analytical solution of planar flow in a sloping soil layer described by the linearized extended Boussinesq equation is presented. The solution consists of the sum of steady-state and transient-series solutions, the latter in a separation-of-variables form, and can satisfy an arbitrary initial condition via collocation; this feature reduces the number of series terms, making the solution efficient. Key parameter is the dimensionless linearization depth η _o (R), R being the dimensionless recharge. The variable η _o (R), not the slope, characterizes the flow as kinematic or diffusive, and R ≈ 0.2 demarcates the two regimes. The transient series converges rapidly for large η _o (large R, near-diffusive flow) and slowly as η _o → 0 (kinematic flow). The quasi-steady (QS) state method of Verhoest & Troch is also analysed and it is shown that the QS depth profiles approximate the transient ones well, only if Δt exceeds a system-dependent transition time between flow states (possibly >>1 day). In an application example for a 30-day recharge series, the QS solution differs from the transient one by as much as 20% (RMSE = 15%), does not track recharge changes as well and fails to conserve mass.     

Tree-ring record of hillslope erosion and valley floor dynamics: Landscape responses to climate variation during the last 400 yr in the Colorado Plateau, northeastern Arizona

Dendrogeomorphologic approaches were used to study hillslope erosion and valley floor dynamics in a small drainage basin in the Colorado Plateau of northeastern Arizona, U.S.A. Root exposure in pinyon pines indicated hillslope erosion averaged 1.9 mm/yr over the last 400 yr, but erosion has been highly episodic. Negative increment growth anomalies in hillslope trees are interpreted as the consequence of rapid aerial exposure of roots by erosion. During the last 300 yr, two of three major episodes of these growth anomalies occurred after abrupt transitions from prolonged, multi-year droughts to sustained, lengthy periods of above-average precipitation. The most recent episode of these growth anomalies began within a few years after 1905 and was associated with the largest precipitation shift (drought to wet interval) in the last 400 yr. In contrast to trees on eroding hillslopes, increment growth of trees in more geomorphically stable landscape positions closely tracked the regional precipitation signal. Two major alluvial fills on the adjacent valley floor are also linked to the abrupt changes in precipitation regimes and the associated increases in delivery of runoff and sediments from slopes. The clay-cemented sandstones weather rapidly; rapid weathering and sediment production make slopes highly responsive to decadal precipitation changes. Significant vegetation declines on slopes during extreme drought make hillslope soils more prone to erosion if heavy precipitation follows soon thereafter.     

LISEM: A SINGLE-EVENT PHYSICALLY BASED HYDROLOGICAL AND SOIL EROSION MODEL FOR DRAINAGE BASINS. I: THEORY,INPUT AND OUTPUT

A new physically based hydrological and soil erosion model has been developed, which can be used for planning and conservation purposes: the LImburg Soil Erosion Model (LISEM). The LISEM model is one of the first examples of a physically based model that is completely incorporated in a raster Geographical Information System. This incorporation facilitates easy application in larger catchments, improves the user friendliness by avoiding conversion routines and allows remotely sensed data to be used. Processes incorporated in the model are rainfall, interception, surface storage in micro-depressions, infiltration and vertical movement of water in the soil, overland flow, channel flow, detachment by rainfall and throughfall, detachment by overland flow and transport capacity of the flow. Special attention has been given to the influence of tractor wheelings, small roads and surface sealing. Vertical movement of water in the soil is simulated using the Richard's equation. Optionally, the user can choose the Holtan or the Green–Ampt infiltration model. For the distribution flow routing, a four-point finite-difference solution of the kinematic wave is used together with Manning's equation.     

陆面水文—气候耦合模拟研究进展

陆面水文过程是全球/区域气候模式十分重要而又十分薄弱的环节。本文通过科学文献计量法,分析了陆面水文—气候耦合研究的发展状况及研究热点和趋势,并进一步对研究中存在的问题和挑战进行综述和探讨。现有多数气候模式中的陆面模式主要基于一维垂向结构设计,缺乏对流域尺度水文过程的精细描述,尤其缺乏下垫面人类活动影响的描述。因此,为了科学认识水文过程与气候变化的相互反馈作用机制,大量研究主要通过耦合流域水文模型与气候模式,研究不同时空尺度下水文过程变化的气候反馈效应。陆面过程模式中水文过程的改进和大尺度水文模型发展为陆面水文—气候耦合模拟奠定基础,在此基础上,陆面水文—气候耦合研究正从传统的单向耦合研究逐步发展为考虑气候—水文反馈的双向耦合研究。然而,双向耦合研究远未成熟,其问题集中表现为陆面水文—气候模型耦合过程如何匹配并提高系统稳定性、研制有效的尺度转换方案、完善参数化方案并评估参数不确定性、研制有效参数移植方法并提高模型适用性以及高分辨率甚至超分辨率模拟等方面,逐步解决上述问题并提高模拟精度是未来水文—气候耦合模拟研究的重要发展方向。     

Estimating historic lake stages from one‐time snapshot,the Shuttle Radar Topography Mission of 2000

Lake studies play an important role in water management, ecology and other environmental issues. Monitoring lake stages is typically the first step on the lake studies. However, for the Prairie Pothole Region (PPR) of North America with millions of small lakes and potholes, onsite measurements for lake stages are almost impossible with the conventional gage stations due to the limited accessibility to lakes and the requirement of economical and human resources. To overcome this limitation, we employed the remote‐sensing approach to extract lake stages in PPR for bulk processing. To estimate lake stages, several image‐processing techniques were used with the Shuttle Radar Topography Mission (SRTM) data in January, 2000, a one‐time snapshot useful in historic lake level reconstruction. In this research, image processing techniques, for example, averaging, Fast Fourier Transform (FFT), Lee‐Sigma filtering were applied and masking with Canny Edge Detector (CED) and semi‐automated contouring were developed for lake stage estimation. In terms of simplicity, averaging might be useful if 1·5 m of the local bias for the study area in stage estimates. However, the FFT and Lee‐Sigma methods were slightly better than averaging. Masking with CED and semi‐automated contouring provided outstanding accuracy in the estimation. The masking and contouring methods provided ± 0·5 m of stage differences for relatively large lakes greater than 10 km² when compared with stage reading in spite of their calculation complexities and was shown as the best approaches among implemented methods in this study. Copyright © 2010 John Wiley & Sons, Ltd.