Regional Hydro-Ecological Simulation System Research Articles

Balancing uncertainty and complexity to incorporate fire spread in an eco-hydrological model

Wildfire affects the ecosystem services of watersheds, and climate change will modify fire regimes and watershed dynamics. In many eco-hydrological simulations, fire is included as an exogenous force. Rarely are the bidirectional feedbacks between watersheds and fire regimes integrated in a simulation system because the eco-hydrological model predicts variables that are incompatible with the requirements of fire models. WMFire is a fire-spread model of intermediate complexity designed to be integrated with the Regional Hydro-ecological Simulation System (RHESSys). Spread in WMFire is based on four variables that (i) represent known influences on fire spread: litter load, relative moisture deficit, wind direction and topographic slope, and (ii) are derived directly from RHESSys outputs. The probability that a fire spreads from pixel to pixel depends on these variables as predicted by RHESSys. We tested a partial integration between WMFire and RHESSys on the Santa Fe (New Mexico) and the HJ Andrews (Oregon State) watersheds. Model assessment showed correspondence between expected spatial patterns of spread and seasonality in both watersheds. These results demonstrate the efficacy of an approach to link eco-hydrologic model outputs with a fire spread model. Future work will develop a fire effects module in RHESSys for a fully coupled, bidirectional model.

Nitrogen cycling and export in California chaparral: the role of climate in shaping ecosystem responses to fire

AbstractClimate change models predict that interannual rainfall variability will increase in California over the next several decades; these changes will likely influence how frequently California ecosystems burn and how they respond to fire. Fires uncouple N mobilization from uptake by destroying plant biomass and increasing nitrification. Following fire, autumn and winter rains can leach N into streams from slopes that have been denuded. The amount of N exported depends on how rapidly soil microbes metabolize it into mobile forms such as , and the rate that recovering plants take up available N. However, the long‐term effects of a changing climate on postfire N dynamics remain unknown. We used the ecohydrologic model RHESSys (regional hydro‐ecologic simulation system) to evaluate how interannual climate variability may affect the magnitude of N mineralization, nitrification, N export, and plant recovery following fire. N export was highest when fire was followed by drought; even though there was less water moving through the system, dry conditions prolonged the period during which N mobilization was decoupled from plant uptake. We also found that the effects of drought on N export were magnified in stands dominated by obligate seeders, which initially recovered more slowly than resprouters. These findings suggest that climate may regulate N balance most powerfully by influencing how quickly plants “turn on” and begin to immobilize N.

Effects of Model Spatial Resolution on Ecohydrologic Predictions and Their Sensitivity to Inter-Annual Climate Variability

The effect of fine-scale topographic variability on model estimates of ecohydrologic responses to climate variability in California’s Sierra Nevada watersheds has not been adequately quantified and may be important for supporting reliable climate-impact assessments. This study tested the effect of digital elevation model (DEM) resolution on model accuracy and estimates of the sensitivity of ecohydrologic responses to inter-annual climate variability. The Regional Hydro-Ecologic Simulation System (RHESSys) was applied to eight headwater, high-elevation watersheds located in the Kings River drainage basin. Each watershed was calibrated with measured snow depth (or snow water equivalent) and daily streamflow. Modeled streamflow estimates were sensitive to DEM resolution, even with resolution-specific calibration of soil drainage parameters. For model resolutions coarser than 10 m, the accuracy of streamflow estimates largely decreased. Reduced model accuracy was related to the reduction in spatial variance of a topographic wetness index with coarser DEM resolutions. This study also found that among the long-term average ecohydrologic estimates, summer flow estimates were the most sensitive to DEM resolution, and coarser resolution models overestimated the climatic sensitivity for evapotranspiration and net primary productivity. Therefore, accounting for fine-scale topographic variability in ecohydrologic modeling may be necessary for reliably assessing climate change effects on lower-order Sierra Nevada watersheds (≤2.3 km2).

Simulated watershed responses to land cover changes using the Regional Hydro-ecological Simulation System

Simulated watershed responses to land cover changes using the Regional Hydro-ecological Simulation System

An Eco-Hydrological Model-Based Assessment of the Impacts of Soil and Water Conservation Management in the Jinghe River Basin, China

Many soil and water conservation (SWC) measures have been applied in the Jinghe River Basin to decrease soil erosion and restore degraded vegetation cover. Analysis of historical streamflow records suggests that SWC measures may have led to declines in streamflow, although climate and human water use may have contributed to observed changes. This paper presents an application of a watershed-scale, physically-based eco-hydrological model—the Regional Hydro-Ecological Simulation System (RHESSys)—in the Jinghe River Basin to study the impacts of SWC measures on streamflow. Several extensions to the watershed-scale RHESSys model were made in this paper to support the model application at larger scales (>10,000 km2) of the Loess Plateau. The extensions include the implementation of in-stream routing, reservoir sub-models and representation of soil and water construction engineering (SWCE). Field observation data, literature values and remote sensing data were used to calibrate and verify the model parameters. Three scenarios were simulated and the results were compared to quantify both vegetation recovery and SWCE impacts on streamflow. Three scenarios respectively represent no SWC, vegetation recovery only and both vegetation recovery and SWCE. The model results demonstrate that the SWC decreased annual streamflow by 8% (0.1 billion m3), with the largest decrease occurring in the 2000s. Model estimates also suggest that SWCE has greater impacts than vegetation recovery. Our study provides a useful tool for SWC planning and management in this region.

Evapotranspiration deficit controls net primary production and growth of silver fir: Implications for Circum-Mediterranean forests under forecasted warmer and drier conditions

Warming-induced drought stress has been hypothesized as a major driver of forest net primary production (NPP) reduction, but we lack reliable field data to assess if higher temperatures lead to forest NPP reduction, particularly in humid sites and at basin to landscape spatial scales. The use of a landscape approach would allow considering the feedbacks operating between climate, topography, soil vegetation and water resources. Here we follow that approach by simulating NPP using the regional hydro-ecologic simulation system (RHESSys) model and by comparing the results with radial growth data (tree-ring widths and intrinsic water-use efficiency – iWUE). We evaluate the relationships between climate, growth, NPP, atmospheric CO2 concentrations (ca) and iWUE in xeric and mesic silver fir forests subjected to contrasting water balances. The growth data successfully validated the 11-month NPP cumulated until spring. The main negative climatic driver of growth and NPP was the summer evapotranspiration deficit, which shows a negative association with tree-ring width indices. Sensitivity analyses indicate that rising ca do not compensate the severe NPP reduction associated to warmer and drier conditions. The positive effect of rising ca on NPP is mediated by climatic site conditions being detected only in mesic sites, whereas the negative effects of drought on NPP override any ca-related enhancement of NPP in xeric sites. Future warmer and drier conditions causing a higher evaporative demand by the atmosphere could lead to a NPP decline in temperate conifer forests subjected to episodic droughts.

The use of CMIP5 data to simulate climate change impacts on flow regime within the Lake Champlain Basin

Abstract Study region Lake Champlain Basin, northwestern New England, USA. Study focus Our study uses regional hydrologic analyses and modeling to examine alternative possibilities that might emerge in the Lake Champlain Basin streamflow regime for various climate scenarios. Climate data as well as spatial data were processed to calibrate the Regional Hydro-Ecological Simulation System (RHESSys) model runoff simulations. The 21st century runoff simulations were obtained by driving the RHESSys model with climate data from the Coupled Model Intercomparison Project phase 5 (CMIP5) for representative concentration pathways RCP 4.5 and 8.5. New hydrological insights for the region Our analyses suggest that most of CMIP5 ensembles fail to capture both the trends and variability observed in historical precipitation when run in hindcast. This raises concerns of using such products in driving hydrologic models for the purpose of obtaining reliable runoff projections that can aid researchers in regional planning. A subset of five climate models among the CMIP5 ensembles have shown statistically significant trends in precipitation, but the magnitude of these trends is not adequately representative of those seen in observed annual precipitation. Adjusted precipitation forecasts project a streamflow regime described by an increase of about 30% in seven-day maximum flow, a four days increase in flooded days, a three orders of magnitude increase in base flow index, and a 60% increase in runoff predictability (Colwell index).

Ecohydrology in semiarid urban ecosystems: Modeling the relationship between connected impervious area and ecosystem productivity

In water-stressed, semiarid urban environments, connections between impervious surfaces and drainage networks may strongly impact the water use and ecosystem productivity of neighboring vegetated areas. We use an ecohydrologic model, the Regional Hydro-Ecological Simulation System (RHESSys), to quantify the sensitivity of vegetation water use and net primary productivity (NPP) to fine-scale impervious surface connectivity. We develop a set of very fine-scale (2 m2) scenarios that vary both the percentage of impervious surface and fraction of this impervious surface with direct hydrologic connections to urban drainage systems for a small hillslope. When driven by Mediterranean climate forcing, model estimates suggest that total vegetation water use declines with increasing impervious area. However, when impervious area is hydrologically disconnected from the urban drainage network, declines in water and carbon fluxes with decreased vegetated area can be partially, or in some cases even completely, offset by increased transpiration and NPP in the remaining vegetation. Relative increases in water use and NPP of remaining vegetation are much greater for deeply rooted shrubs and trees and negligible for shallow rooted grasses. We extrapolate our findings to the catchment scale by developing a first-order approximation of fine-scale impervious connection impacts on aggregate watershed water and carbon flux estimates. Our approach offers a computationally and data-efficient method for estimating the impact of impervious area connectivity on these ecohydrologic fluxes. For our only partially urbanized Santa Barbara watershed, estimates of water use and NPP that account for fine-scale impervious connection differed by more than 10% from those that did not.

Hydrological impacts of climate and land‐use changes in a mountain watershed: uncertainty estimation based on model comparison

AbstractThis paper evaluates the response of streamflow and other components of the water balance to changes in climate and land use in a Pyrenean watershed. It further provides a measure of uncertainty in water resources forecasts by comparing the performance of two hydrological models: the Soil and Water Assessment Tool (SWAT) and Regional Hydro‐ecological Simulation System (RHESSys). Regional climate model outputs for the 2021–2050 time frame and hypothetical (but plausible) land‐use scenarios considering re‐vegetation and wildfire processes were used as inputs to the models. Results indicate an overall decrease in river flows (up to 30%, depending on the combination of scenarios) when the scenarios are considered, except for the post‐fire vegetation scenario, in which streamflows are simulated to increase (between 2% and 10%). However, the magnitude of these projections varies between the two models used, as SWAT tends to produce larger hydrological changes under climate change scenarios and RHESSys shows more sensitivity to changes in land cover. The final prediction will therefore depend largely on the combination of the land‐use and climate scenarios and on the model used. Copyright © 2014 John Wiley & Sons, Ltd.

Respuesta hidrológica del Pirineo central al cambio ambiental proyectado para el siglo XXI

Streamflows in five Mediterranean mountain headwaters in the central Spanish Pyrenees were projected under various climate and land use change scenarios. Streamflows were simulated using the Regional Hydro-Ecologic Simulation System (RHESSys). The results show that changes in precipitation and temperature could cause a decline of annual streamflow between 13% and 23%, depending on the considered catchment. When the effect of increased forest cover in the basins is added to climate change effects, the decrease in annual streamflow is enhanced up to 19% and 32%. The largest hydrological changes resulting from environmental change are projected mainly in early spring, summer and autumn, when the decline may exceed 40%. Winter is the least affected season by environmental change because of increased runoff as a consequence of reduced storage of water in the snowpack and an earlier onset of the snowmelt, and the lower consumption of water by vegetation during the cold season. The magnitude of hydrological change as a result of the assumed environmental change scenarios may lead to serious impacts on water management and ecology of the studied region, as well as the water availability in the Ebro basin.

Does consideration of water routing affect simulated water and carbon dynamics in terrestrial ecosystems?

Abstract. The cycling of carbon (C) in terrestrial ecosystems is closely coupled with the cycling of water. An important mechanism connecting ecological and hydrological processes in terrestrial ecosystems is lateral flow of water along landscapes. Few studies, however, have examined explicitly how consideration of water routing affects simulated water and C dynamics in terrestrial ecosystems. The objective of this study is to explore how consideration of water routing in a process-based hydro-ecological model affects simulated water and C dynamics. To achieve that end, we rasterized the regional hydro-ecological simulation system (RHESSys) and employed the rasterized RHESSys (R-RHESSys) in a forested watershed. We performed and compared two contrasting simulations, one with and another without water routing. We found that R-RHESSys was able to correctly simulate major hydrological and ecological variables regardless of whether water routing was considered. When water routing was considered, however, soil water table depth and saturation deficit were simulated to be greater and spatially more heterogeneous. As a result, water (evaporation, transpiration, and evapotranspiration) and C (forest productivity, soil autotrophic and heterotrophic respiration) fluxes also were simulated to be spatially more heterogeneous compared to the simulation without water routing. When averaged for the entire watershed, the three simulated water fluxes were greater while C fluxes were smaller under simulation considering water routing compared to that ignoring water routing. In addition, the effects of consideration of water routing on simulated C and water dynamics were more apparent in dry conditions. Overall, the study demonstrated that consideration of water routing enabled R-RHESSys to better capture our preconception of the spatial patterns of water table depth and saturation deficit across the watershed. Because soil moisture is fundamental to the exchange of water and C fluxes among soil, vegetation and the atmosphere, ecosystem and C cycle models therefore need to explicitly represent water routing in order to accurately quantify the magnitude and patterns of water and C fluxes in terrestrial ecosystems.

Design and Implementation of Kepler Workflows for BioEarth

BioEarth is an ongoing research initiative for the development of a regional-scale Earth System Model (EaSM) for the U.S. Pacific Northwest. Our project seeks to couple and integrate multiple stand-alone EaSMs developed through independent efforts for capturing natural and human processes in various realms of the biosphere: atmosphere (weather and air quality), terrestrial biota (crop, rangeland, and forest agro-ecosystems) and aquatic (river flows, water quality, and reservoirs); hydrology links all these realms. Due to the need to manage numerous complex simulations,an application of automated workflows was essential. In this paper, we present a case study of workflow design for the BioEarth project using the Kepler system to manage applications of the Regional Hydro-Ecologic Simulation System (RHESSys) model. In particular, we report on the design of Kepler workflows to support: 1) standalone executions of the RHESSys model under serial and parallel applications, and 2) a more complex case of performing calibration runs involving multiple preprocessing modules, iterative exploration of parameters and parallel RHESSys executions. We exploited various Kepler features including a user-friendly design interface and support for parallel execution on a cluster. Our experiments show a performance speedup between 7–12x, using 16 cores of a Linux cluster, and demonstrate the general effectiveness of our Kepler workflows in managing RHESSys runs. This study shows the potential of Kepler to serve as the primary integration platform for the BioEarth project, with implications for other data- and compute-intensive Earth systems modeling projects.

Impact of climate and land use change on water availability and reservoir management: Scenarios in the Upper Aragón River, Spanish Pyrenees

Streamflows in a Mediterranean mountain basin in the central Spanish Pyrenees were projected under various climate and land use change scenarios. Streamflow series projected for 2021–2050 were used to simulate the management of the Yesa reservoir, which is critical to the downstream supply of irrigation and domestic water. Streamflows were simulated using the Regional Hydro-Ecologic Simulation System (RHESSys). The results show that increased forest cover in the basin could decrease annual streamflow by 16%, mainly in early spring, summer and autumn. Regional climate models (RCMs) project a trend of warming and drying in the basin for the period 2021–2050, which will cause a 13.8% decrease in annual streamflow, mainly in late spring and summer. The combined effects of forest regeneration and climate change are expected to reduce annual streamflows by 29.6%, with marked decreases affecting all months with the exception of January and February, when the decline will be moderate. Under these streamflow reduction scenarios it is expected that it will be difficult for the Yesa reservoir to meet the current water demand, based on its current storage capacity (476hm3). If the current project to enlarge the reservoir to a capacity of 1059hm3 is completed, the potential to apply multi-annual streamflow management, which will increase the feasibility of maintaining the current water supply. However, under future climate and land cover scenarios, reservoir storage will rarely exceed half of the expected capacity, and the river flows downstream of the reservoir is projected to be dramatically reduced.

Simulated watershed responses to land cover changes using the Regional Hydro-Ecological Simulation System

In this work, we used the Regional Hydro-Ecological Simulation System (RHESSys) model to examine runoff sensitivity to land cover changes in a mountain environment. Two independent experiments were evaluated where we conducted simulations with multiple vegetation cover changes that include conversion to grass, no vegetation cover and deciduous/coniferous cover scenarios. The model experiments were performed at two hillslopes within the Weber River near Oakley, Utah watershed (USGS gauge # 10128500). Daily precipitation, air temperature and wind speed data as well as spatial data that include a digital elevation model with 30 m grid resolution, soil texture map and vegetation and land use maps were processed to drive RHESSys simulations. Observed runoff data at the watershed outlet were used for calibration and verification. Our runoff sensitivity results suggest that during winter, reduced leaf area index (LAI) decreases canopy interception resulting in increased snow accumulations and hence snow available for runoff during the early spring melt season. Increased LAI during the spring melt season tends to delay the snow melting process. This delay in snow melting process is due to reduced radiation beneath high LAI surfaces relative to low LAI surfaces. The model results suggest that annual runoff yield after removing deciduous vegetation is on average about 7% higher than with deciduous vegetation cover, while annual runoff yield after removing coniferous vegetation is on average as about 2% higher than that produced with coniferous vegetation cover. These simulations thus help quantify the sensitivity of water yield to vegetation change. Copyright © 2013 John Wiley & Sons, Ltd.

관측자료와 RHESSys 모형을 이용한 산림유역의 생태수문 적용성 평가 -설마천유역을 대상으로-

본 연구에서는 RHESSys (Regional Hydro-Ecologic Simulation System) 모형과 관측 자료를 이용하여 유출량, 증발산량, 토양수분량, 총일차생산량과 순광합성량을 평가하고자 한다. 수문생태모형인 RHESSys는물, 탄소 및 질소순환과 지형 공간적 변화에 따른 물질이동을 모의할 수 있다. 대상유역은 설마천 유역(8.5 <TEX>$km^2$</TEX>)으로 우리나라 북서쪽에 위치하고 있다. 유역의 90%이상이 산림유역이고, 토양은 대부분 사양토이다. 2007~2009년의 관측 일유출량을 이용하여 유출량을 검 보정하였고, 증발산량은 에디 공분산 방법에 의한 플럭스 타워로부터 관측되었으며 토양수분은 신뢰할 만한 실측자료를 바탕으로 모형의 보정(2007~2008) 및 검증(2009)을 실시하였다. 또한 지구의 탄소순환을 규명할 수 있는 식생의 순광합성량과 총일차생산량에 대한 모형의 검 보정은 Terra 위성의 MODIS (Moderate Resolution Imaging Spectroradiometer) 센서를 이용한 산출물인 순광합성량과 총일차생산량 자료를 바탕으로 모형의 보정(2007) 및 검증(2008)을 실시하였다. 모의 결과 보정기간 동안의 상관계수와 Nash-Sutcliffe 모형 효율은 유출량은 0.74와 0.63이었고, 증발산량과 총일차생산량의 상관계수는 각각 0.54와 0.93이었다. 모형 검정결과 유출량의 상관계수와 Nash-Sutcliffe 모형효율 각각 0.92와 0.84였으며, 총일차 생산량의 상관계수는 0.93이었다. This study is to evaluate the RHESSys (Regional Hydro-Ecological Simulation System) simulated streamflow (Q), evapotranspiration (ET), soil moisture (SM), gross primary productivity (GPP) and photosynthetic productivity (PSNnet) with the measured data. The RHESSys is a hydro-ecological model designed to simulate integrated water, carbon, and nutrient cycling and transport over spatially variable terrain. A 8.5 <TEX>$km^2$</TEX> Seolma-cheon catchment located in the northwest of South Korea was adopted. The catchment covers 90.0% forest and the dominant soil is sandy loam. The model was calibrated with 2 years (2007-2008) daily Q at the watershed outlet and MODIS (Moderate Resolution Imaging Spectroradiometer) GPP, PSNnet and 3 year (2007～2009) daily ET data measured at flux tower using the eddy-covariance technique. The coefficient of determination (<TEX>$R^2$</TEX>) and the Nash-Sutcliffe model efficiency (ME) for Q were 0.74 and 0.63, and the average <TEX>$R^2$</TEX> for ET and GPP were 0.54 and 0.93 respectively. The model was validated with 1 year (2009) Q and GPP. The <TEX>$R^2$</TEX> and the ME for Q were 0.92 and 0.84, the <TEX>$R^2$</TEX> for GPP were 0.93.

Distributed Hydrologic Modeling in the Suburban Landscape: Assessing Parameter Transferability from Gauged Reference Catchments1

Mittman, Tamara, Lawrence E. Band, Taehee Hwang, and Monica Lipscomb Smith, 2012. Distributed Hydrologic Modeling in the Suburban Landscape: Assessing Parameter Transferability from Gauged Reference Catchments. Journal of the American Water Resources Association (JAWRA) 48(3): 546‐557. DOI: 10.1111/j.1752‐1688.2011.00636.xAbstract: Distributed, process‐based models of catchment hydrologic response are potentially useful tools for the assessment of Low Impact Development (LID) techniques in urbanized catchments. Their application is often limited, however, by the lack of continuous streamflow records to calibrate poorly constrained parameters. This article examines the transferability of soil and groundwater parameters from a forested reference catchment to a nearby suburban catchment. We use the Regional Hydro‐Ecologic Simulation System (RHESSys) to develop hydrologic models of one gauged forested and one ungauged suburban catchment within the Baltimore Ecosystem Study (BES) study area. We use a parameter uncertainty framework to calibrate soil and groundwater parameters for the forested catchment, and discrete measurements of streamflow from the suburban catchment to assess parameter transferability. Results indicate that the transfer of soil and groundwater parameters from forested reference to nearby suburban catchments is viable, with performance measures for the suburban catchment often exceeding those for the forested catchment. We propose that the simplification of hydrologic processes in urbanized catchments may account for the increase in model performance in the suburban catchment.

Modeling hydrologic controls on denitrification: sensitivity to parameter uncertainty and landscape representation

This paper presents a general discussion of the interplay between model structure and hydrologic parameters in the context of denitrification estimation using coupled hydro-ecosystem models at a watershed scale. Given the key role played by hydrology in denitrification models, sensitivity analysis of hydrologic parameters is needed to determine both uncertainty in denitrification estimates and to suggest how measured data, such as streamflow, can be effectively used to reduce this uncertainty. This paper contributes to the broad goal of sensitivity analysis by examining the linkage between landscape tessellation, calibration, and the ability of models to capture hot-spot contributions to watershed scale denitrification across a range of N-loading. For a small mid-Atlantic forested watershed, denitrification estimates using RHESSys (regional hydro-ecologic simulation system) are compared across different strategies for calibration and landscape tessellation. Results demonstrate the utility of several potential approaches to account for hydrologically mediated hot-spots within landscapes.

The Potential Utility of Physically Based Hydrologic Modeling in Ungauged Urban Streams

A wide range of hydrologic modeling tools based on geographic information systems are available to aid urban planning, design, and environmental assessment by providing an understanding of how urban development impacts water quality and water quantity. These tools range from complex, data-intensive, process-based simulation models to simpler, empirical, coefficient-based approaches. We use a case study watershed in San Diego, California, to illustrate the potential contribution of physically based models as a means to evolve conceptual understanding of geographic pattern and process, rather than the prediction of aggregate watershed behavior. We use the Regional Hydro-Ecologic Simulation System to examine interactions between internal watershed processes that lead to runoff production. Through this case study, we address the issue of limited streamflow data for model calibration, a requirement that is often difficult to meet in smaller, ungauged watersheds. We illustrate how flow data from a similar watershed, combined with sensitivity analysis, can serve to compensate for this data limitation in the context of understanding urban impacts on hydrologic behavior. We then use this modeling approach to demonstrate that the conclusions made about the effects of urbanization on streamflow are a complex function of interactions between the watershed drainage network configuration and antecedent soil moisture conditions. Further we show that these qualitative, but potentially useful conclusions do not depend on precise knowledge of uncertain parameters such as soil hydraulic conductivity. We discuss the overall “ease of use” of our modeling approach and the implications for environmental assessment and design.

Spatial patterns of simulated transpiration response to climate variability in a snow dominated mountain ecosystem

AbstractTranspiration is an important component of soil water storage and stream‐flow and is linked with ecosystem productivity, species distribution, and ecosystem health. In mountain environments, complex topography creates heterogeneity in key controls on transpiration as well as logistical challenges for collecting representative measurements. In these settings, ecosystem models can be used to account for variation in space and time of the dominant controls on transpiration and provide estimates of transpiration patterns and their sensitivity to climate variability and change. The Regional Hydro‐Ecological Simulation System (RHESSys) model was used to assess elevational differences in sensitivity of transpiration rates to the spatiotemporal variability of climate variables across the Upper Merced River watershed, Yosemite Valley, California, USA. At the basin scale, predicted annual transpiration was lowest in driest and wettest years, and greatest in moderate precipitation years (R2 = 0·32 and 0·29, based on polynomial regression of maximum snow depth and annual precipitation, respectively). At finer spatial scales, responsiveness of transpiration rates to climate differed along an elevational gradient. Low elevations (1200–1800 m) showed little interannual variation in transpiration due to topographically controlled high soil moistures along the river corridor. Annual conifer stand transpiration at intermediate elevations (1800–2150 m) responded more strongly to precipitation, resulting in a unimodal relationship between transpiration and precipitation where highest transpiration occurred during moderate precipitation levels, regardless of annual air temperatures. Higher elevations (2150–2600 m) maintained this trend, but air temperature sensitivities were greater. At these elevations, snowfall provides enough moisture for growth, and increased temperatures influenced transpiration. Transpiration at the highest elevations (2600–4000 m) showed strong sensitivity to air temperature, little sensitivity to precipitation. Model results suggest elevational differences in vegetation water use and sensitivity to climate were significant and will likely play a key role in controlling responses and vulnerability of Sierra Nevada ecosystems to climate change. Copyright © 2008 John Wiley &amp; Sons, Ltd.

수문생태모형 RHESSys의 평가: 두 복잡지형 유역에서의 모수화와 적용

전지구적인 생태시스템 안에서 탄소와 물의 흐름은 아주 밀접하게 관련되어 있다. 생태계 물질순환과정을 모사해 주는 모형들은 연구자들이 직접 측정하기 어려운 복잡한 생태계에서의 물질순환 과정들의 상호작용과 그 변화를 예측할 수 있는 도구이다. 이 연구에서는 생태수문모형 RHESSys(Regional Hydro Ecological Simulation System)를 광릉시험림 유역에서 모수화한 후, 다른 지형 조건과 기후 및 식생을 가진 점봉산 강선리 유역에 적용함으로써 RHESSys모수화의 다유역으로의 확장가능성과 문제점 등을 조사하였다. RHESSys는 Geographic Information System(GIS)를 바탕으로 공간적인 탄소와 물 및 영양분의 흐름을 모사하는 생태수문모형이다. 모수화는 광릉시험림 유역의 1982-1999년간 일유량과 나이테 자료로 추정한 임목생장량을 사용하였다. 수직 및 수평 방향의 수리전도도에 관련된 중요 모수들과 임목 생장과 관련된 분배 모수들을 Monte-Carlo 접근법을 사용하여 최적화하였다. 모형결과를 광릉연구지의 실측 토양수분과 비교해 본 결과 RHESSys는 토양수분을 다소 과소평가하는 경향이 있으나, 일변화를 유의하게 잘 모사하였으며, 임목생장량의 연변화를 잘 모사하였다. 광릉연구지에서 개발한 모수를 점봉산 강선리 유역에 적용한 결과, 광릉과 마찬가지로 RHESSys는 토양수분을 다소 과소평가하나 여름철의 중요한 시계열 경향을 유의하게 잘 모사하였다. 또한 실측 공극률의 사용이 모형의 토양수분예측력에 중요함을 확인하였다. 반면에 강선리 유역의 임목생장량에 대해선 모형의 예측력이 높지 않았다. 수문과정 모수에 비해 식생성장 및 분배관련 모수의 경우 단일 유역의 모수화를 타 유역으로 확장하는 데에 상당한 불확실성이 있음을 확인하였다. 각 유역의 임목생장 및 분배 특성을 반영하는 독립적인 모수화 과정이 필요하며 이를 뒷받침해 줄 최소한의 현장측정항목을 발굴하는 후속연구가 필요하다. In this study, we examined the flux of carbon and water using an eco-hydrological model, Regional Hydro-Ecologic Simulation System (RHESSys). Our purposes were to develop a set of parameters optimized for a well-designed experimental watershed (Gwangneung Research Watershed, GN) and then, to test suitability of the parameters for predicting carbon and water fluxes of other watershed with different regimes of climate, topography, and vegetation structure (i.e Gangseonry Watershed in Mt. Jumbong, GS). Field datasets of stream flow, soil water content (SWC), and wood biomass product (WBP) were utilized for model parameterization and validation. After laborious parameterization processes, RHESSys was validated with the field observations from the GN watershed. The parameter set identified at the GN watershed was then applied to the GS watershed in Mt. Jumbong, which resulted in good agreement for SWC but poor predictability for WBP. Our study showed that RHESSys simulated reliable SWC at the GS by adjusting site-specific porosity only. In contrast, vegetation productivity would require more rigorous site-specific parameterization and hence, further study is necessary to identify primary field ecophysiological variables for enhancing model parameterization and application to multiple watersheds.