Ecohydrological modelling in a deciduous boreal forest: Model evaluation for application in non-stationary climates

Soil moisture is an important driver of growth in boreal Alaska, but estimating soil hydraulic parameters can be challenging in this data-sparse region. Parameter estimation is further complicated in regions with rapidly warming climate, where there is a need to minimize model error dependence on interannual climate variations. To better identify soil hydraulic parameters and quantify energy and water balance and soil moisture dynamics, we applied the physically based, one-dimensional ecohydrological Simultaneous Heat and Water (SHAW) model, loosely coupled with the Geophysical Institute of Permafrost Laboratory (GIPL) model, to an upland deciduous forest stand in interior Alaska over a 13-year period. Using a Generalized Likelihood Uncertainty Estimation parameterisation, SHAW reproduced interannual and vertical spatial variability of soil moisture during a five-year validation period quite well, with root mean squared error (RMSE) of volumetric water content at 0.5 m as low as 0.020 cm³/cm³. Many parameter sets reproduced reasonable soil moisture dynamics, suggesting considerable equifinality. Model performance generally declined in the eight-year validation period, indicating some overfitting and demonstrating the importance of interannual variability in model evaluation. We compared the performance of parameter sets selected based on traditional performance measures such as the RMSE that minimize error in soil moisture simulation, with one that is designed to minimize the dependence of model error on interannual climate variability using a new diagnostic approach we call CSMP, which stands for Climate Sensitivity of Model Performance. Use of the CSMP approach moderately decreases traditional model performance but may be more suitable for climate change applications, for which it is important that model error is independent from climate variability. These findings illustrate (1) that the SHAW model, coupled with GIPL, can adequately simulate soil moisture dynamics in this boreal deciduous region, (2) the importance of interannual variability in model parameterisation, and (3) a novel objective function for parameter selection to improve applicability in non-stationary climates.     

河口生态需水研究进展

针对陆海交互作用下河口生态需水影响因素多且时空差异性显著的特点,在系统分析淡水输入对河口生态系统水环境过程和生物过程影响的基础上,按生态需水目标的差异,将河口生态需水分析方法划分为基于典型生物资源保护目标、基于生境保护目标以及基于多目标整合分析3种类型。从生态需水目标筛选、生态需水目标对淡水输入响应关系以及生态需水计算方法实用性方面探讨了河口生态需水研究中面临的主要问题。结论认为生态系统健康综合表征指标的确定应成为分析河口生态需水的关键科学基础,筛选控制性生态要素构建河口关键生态水文过程模型可成为有效提高生态需水计算方法实用性的主要技术手段。     

Ecohydrological modelling with EcH2O‐iso to quantify forest and grassland effects on water partitioning and flux ages

We used the new process‐based, tracer‐aided ecohydrological model EcH₂O‐iso to assess the effects of vegetation cover on water balance partitioning and associated flux ages under temperate deciduous beech forest (F) and grassland (G) at an intensively monitored site in Northern Germany. Unique, multicriteria calibration, based on measured components of energy balance, hydrological function and biomass accumulation, resulted in good simulations reproducing measured soil surface temperatures, soil water content, transpiration, and biomass production. Model results showed the forest “used” more water than the grassland; of 620 mm average annual precipitation, losses were higher through interception (29% under F, 16% for G) and combined soil evaporation and transpiration (59% F, 47% G). Consequently, groundwater (GW) recharge was enhanced under grassland at 37% (~225 mm) of precipitation compared with 12% (~73 mm) for forest. The model tracked the ages of water in different storage compartments and associated fluxes. In shallow soil horizons, the average ages of soil water fluxes and evaporation were similar in both plots (~1.5 months), though transpiration and GW recharge were older under forest (~6 months compared with ~3 months for transpiration, and ~12 months compared with ~10 months for GW). Flux tracking using measured chloride data as a conservative tracer provided independent support for the modelling results, though highlighted effects of uncertainties in forest partitioning of evaporation and transpiration. By tracking storage—flux—age interactions under different land covers, EcH₂O‐iso could quantify the effects of vegetation on water partitioning and age distributions. Given the likelihood of drier, warmer summers, such models can help assess the implications of land use for water resource availability to inform debates over building landscape resilience to climate change. Better conceptualization of soil water mixing processes and improved calibration data on leaf area index and root distribution appear obvious respective modelling and data needs for improved simulations.     

How much does dry‐season fog matter? Quantifying fog contributions to water balance in a coastal California watershed

The seasonally‐dry climate of Northern California imposes significant water stress on ecosystems and water resources during the dry summer months. Frequently during summer, the only water inputs occur as non‐rainfall water, in the form of fog and dew. However, due to spatially heterogeneous fog interaction within a watershed, estimating fog water fluxes to understand watershed‐scale hydrologic effects remains challenging. In this study, we characterized the role of coastal fog, a dominant feature of Northern Californian coastal ecosystems, in a San Francisco Peninsula watershed. To monitor fog occurrence, intensity, and spatial extent, we focused on the mechanisms through which fog can affect the water balance: throughfall following canopy interception of fog, soil moisture, streamflow, and meteorological variables. A stratified sampling design was used to capture the watershed's spatial heterogeneities in relation to fog events. We developed a novel spatial averaging scheme to upscale local observations of throughfall inputs and evapotranspiration suppression and make watershed‐scale estimates of fog water fluxes. Inputs from fog water throughfall (10–30 mm/year) and fog suppression of evapotranspiration (125 mm/year) reduced dry‐season water deficits by 25% at watershed scales. Evapotranspiration suppression was much more important for this reduction in water deficit than were direct inputs of fog water. The new upscaling scheme was analyzed to explore the sensitivity of its results to the methodology (data type and interpolation method) employed. This evaluation suggests that our combination of sensors and remote sensing allows an improved incorporation of spatially‐averaged fog fluxes into the water balance than traditional interpolation approaches.     

The role of recharge and evapotranspiration as hydraulic drivers of ion concentrations in shallow groundwater on Everglades tree islands,Florida (USA)

Recently, evapotranspiration has been hypothesized to promote the secondary formation of calcium carbonate year‐round on tree islands in the Everglades by influencing groundwater ions concentrations. However, the role of recharge and evapotranspiration as drivers of shallow groundwater ion accumulation has not been investigated. The goal of this study is to develop a hydrologic model that predicts the chloride concentrations of shallow tree island groundwater and to determine the influence of overlying biomass and underlying geologic material on these concentrations. Groundwater and surface water levels and chloride concentrations were monitored on eight constructed tree islands at the Loxahatchee Impoundment Landscape Assessment (LILA) from 2007 to 2010. The tree islands at LILA were constructed predominately of peat, or of peat and limestone, and were planted with saplings of native tree species in 2006 and 2007. The model predicted low shallow groundwater chloride concentrations when inputs of regional groundwater and evapotranspiration‐to‐recharge rates were elevated, while low evapotranspiration‐to‐recharge rates resulted in a substantial increase of the chloride concentrations of the shallow groundwater. Modeling results indicated that evapotranspiration typically exceeded recharge on the older tree islands and those with a limestone lithology, which resulted in greater inputs of regional groundwater. A sensitivity analysis indicated the shallow groundwater chloride concentrations were most sensitive to alterations in specific yield during the wet season and hydraulic conductivity in the dry season. In conclusion, the inputs of rainfall, underlying hydrologic properties of tree islands sediments and forest structure may explain the variation in ion concentration seen across Everglades tree islands. Copyright © 2012 John Wiley & Sons, Ltd.     

Optimizing environmental flow and macrophyte management for restoring a large eutrophic lake-marsh system

With increasing environmental degradation from eutrophication, management strategies associated with eutrophic water restoration initiatives are necessary and have garnered wide interest. Macrophytes contribute to water quality improvement in lake-marsh systems through nutrient uptake, while causing considerable water loss through transpiration. In reverse, hydrological variations affect macrophyte growth status and thus phytoremediation performance. The interactions between hydrological and ecological processes in lake-marsh systems are complex, but environmental flow management strategies accounting for these interactions have been rarely proposed. Accordingly, this study combines environmental flow and macrophyte management in the restoration of a large eutrophic lake-marsh system, accounting for interactions between hydrological and nutrient removal processes. We consider both nutrient uptake and transpiration-driven water loss of macrophytes as well as the impacts of hydrological conditions on macrophyte growth status and sediment biological denitrification. Regulating upstream dam operation is an effective measure for environmental flow management. Thus, we develop an optimization model to guide upstream water release while simultaneously regulating macrophyte area in a lake-marsh system, aiming to minimize the ratio of environmental flow demand to reservoir water availability and maintain satisfactory water quality in the system throughout the year. A genetic algorithm is adopted for solving the optimization model. By applying the method in a typical lake-marsh system (Baiyangdian) in China as a case, we introduce a new macrophyte management regime (for area cover and harvesting regimes) and an optimal environmental flow management schedule. Results show that appropriate monthly fluctuations in wetland water level are beneficial to phytoremediation performance, and harvesting macrophytes in stages is effective in reducing transpiration-driven water loss without inordinately damaging nutrient uptake by macrophytes. This study offers a useful tool for lake-marsh system restoration management and highlights the significance of regulating hydrological processes in water quality restoration.     

全球气候变化对湿地生态水文的影响研究综述

近百年来全球气候呈现以变暖为主要特征的显著变化,并且未来气温将继续上升,降水模式也会发生改变。从气候变化对湿地水文水资源的影响、气候变化影响下湿地水文与生态的相互作用过程以及湿地生态水文模型等3个方面,对国内外相关研究动态和发展趋势进行了总结分析。从中发现,当前全球气候背景下的湿地生态水文学正在从单一湿地生态水文过程为主要对象,发展成为以研究气候-水文-生态三者相互作用机制为主要内容的综合性、交叉性学科。现关于气候变化影响下水文-生态之间的关系多集中于单向作用的研究,特别是水文过程对植被的影响研究较多,缺乏对气候变化影响下湿地水文过程与生态过程相互作用机理的全面认识。气候变化对湿地生态水文的影响机制研究已经成为水文学研究亟待解决的科学问题,而基于物理机制的湿地生态水文模型,逐渐成为预测未来气候变化下湿地生态水文响应的重要工具。     

Using isotopes to understand landscape-scale connectivity in a groundwater-dominated,lowland catchment under drought conditions

The Demnitzer Millcreek catchment (DMC), is a 66 km² long-term experimental catchment located 50 km SE of Berlin. Monitoring over the past 30 years has focused on hydrological and biogeochemical changes associated with de-intensification of farming and riparian restoration in the low-lying landscape dominated by rain-fed farming and forestry. However, the hydrological function of the catchment, which is closely linked to nutrient fluxes and highly sensitive to climatic variability, is still poorly understood. In the last 3 years, a prolonged drought period with below-average rainfall and above-average temperatures has resulted in marked hydrological change. This caused low soil moisture storage in the growing season, agricultural yield losses, reduced groundwater recharge, and intermittent streamflows in parts of an increasingly disconnected channel network. This paper focuses on a two-year long isotope study that sought to understand how different parts of the catchment affect ecohydrological partitioning, hydrological connectivity and streamflow generation during drought conditions. The work has shown the critical importance of groundwater storage in sustaining flows, basic in-stream ecosystem services and the dominant influence of vegetation on groundwater recharge. Recharge was much lower and occurred during a shorter window of time in winter under forests compared to grasslands. Conversely, groundwater recharge was locally enhanced by the restoration of riparian wetlands and storage-dependent water losses from the stream to the subsurface. The isotopic variability displayed complex emerging spatio-temporal patterns of stream connectivity and flow duration during droughts that may have implications for in-stream solute transport and future ecohydrological interactions between landscapes and riverscapes. Given climate projections for drier and warmer summers, reduced and increasingly intermittent streamflows are very likely not just in the study region, but in similar lowland areas across Europe. An integrated land and water management strategy will be essential to sustaining catchment ecosystem services in such catchment systems in future.     

Pseudo-hysteretic double-front hiatus-stage soil water parcels supplying a plant–root continuum: the Green-Ampt-Youngs model revisited

Abstract

A tension-saturated water slug descends through a homogenous soil after a rainfall (irrigation) event and shrinks due to transpiration by a distributed root-sink and evaporation. The upper (drainage) and lower (imbibition) sharp fronts of the slug separate it from the superjacent and subjacent vadose zones, where water is immobile. In the slug, the hydraulic conductivity is constant according to the Green-Ampt model. The capillary pressures as well as effective porosities on the fronts are given (generally, different) constants that can be viewed as a kind of hysteresis. A volumetric sink models mild (no desaturation of the slug) soil water withdrawal by the plant roots. The sink intensity varies with the depth from the soil surface and with time. Mathematically, the hydraulic head is immediately expressed by double integration of a governing 1-D flow equation. The pressure and kinematic conditions on the fronts result in a Cauchy problem for a system of two ODEs, which is solved by computer algebra routines.

Editor D. Koutsoyiannis

Citation Kacimov, A. and Obnosov, U., 2013. Pseudo-hysteretic double-front hiatus-stage soil water parcels supplying a plant–root continuum: the Green-Ampt-Youngs model revisited. Hydrological Sciences Journal, 58 (1), 1–12.     

水文地理研究发展若干问题商榷

水问题研究一直是我们国家最重要的课题之一,关系到社会稳定、民生福祉、经济发展等一系列国家重大需求。水文地理是水文学与地理学交叉学科,主要采用地理学原理、观点和方法。陆地表层系统的提出对水文地理的系统性研究具有重要的启发作用。首先,对水文地理学的交叉关联、研究内容、全球水系统和生态水文研究的兴起进行阐述;其次,在总结陆地表层系统论思想的主要倡导者及其观点后,讨论了地理研究综合性和区域性的关系,提出综合性、区域性和时空动态过程机理等“三性”应是水文地理系统的研究思想;再次,基于学科基础理论的扩展,给出深化水文地理时空动态过程机理的系统科学技术的几个案例;最后,讨论了中国水文地理研究的几个重点发展方向,主要包括气候变化与人类活动对水循环过程的影响、生态水文四大平衡动态演变过程机理、水资源可再生性维护、“蓝绿水”的认知意义、水文地理区划与制图技术及智能技术等。