Catchment-scale Water Balance Research Articles

Response of climate change and land use land cover change on catchment-scale water balance components: a multi-site calibration approach

Abstract The present study focused on evaluating the separate and combined response of land use land cover and climate change (CC) on future water balance components of a Subarnarekha River basin, spanning between the latitudes 21°33′N–23°18′N and longitudes 85°11′E–87°23′E, situated in the eastern India. The Soil and Water Assessment Tool is used for single-site calibration and multi-site calibration (MSC) of the model to characterize the future water balance components of the basin using the Cellular Automata-Markov model and climate projections under two representative concentration pathway (RCP) scenarios (4.5 and 8.5). The findings indicate that the model parameters obtained through MSC better represent spatial heterogeneity, making it the preferred calibration approach for model simulations. In the middle region of the basin, future annual water yield, groundwater recharge (GWR), and streamflow showed a reduction, respectively, by 46–47%, 29–30%, and 13–15%, while evapotranspiration showed an increase by 5–7% following projected CC under both RCP scenarios. The findings are relevant for policy-makers to mitigate the adverse effects of reduced GWR for sustainable water resources management. Future research may integrate reservoir operation frameworks to effectively address the water management issues of the basin.

Forest canopy interception can reduce flood discharge: Inferences from model assumption analysis

Some interception (IC) models assume proportionality between the IC and rainfall amount during heavy rainfall, while others do not. Although differences in the assumptions of the IC model can significantly impact water balance and runoff prediction, quantitative analysis has not yet been conducted. In this study, we investigated the influence of IC model selection on runoff prediction and its parameter estimation in aforested catchment in Japan. First, an IC model was presented as a function that can continuously represent the case where the IC increases with rainfall but asymptotically reaches a flat plateau and the case where IC is completely proportional to rainfall. The IC estimation using the model was compared with the IC data observed in 2020–2021 at three coniferous plantation plots within the catchment. Second, the IC model was incorporated into a catchment-scale water balance model (TOPMODEL-based tank model), and then the IC model that optimally reproduced the runoff observed in 2010–2015 was determined. The results showed that the observed IC and runoff were optimally reproduced when the IC was completely proportional to rainfall. However, it was observed that the assumption of the IC was not as influential on runoff predictability as the water retention capacity parameter of the catchment water balance model. Meanwhile, once the appropriate water retention capacity parameter was fixed, the impact of the difference in IC models on runoff prediction was significant at high-flow periods. Moreover, estimated high flow became large without the assumption that the IC was somehow proportional to rainfall, suggesting the importance of adequately representing the occurrence of IC, especially during heavy rainfall events. This implies that the IC by the forest canopy plays a significant role in controlling flood discharge from forested watersheds.

Climatic and anthropogenic drivers of a drying Himalayan river

Abstract. Streamflow regimes are rapidly changing in many regions of the world. Attribution of these changes to specific hydrological processes and their underlying climatic and anthropogenic drivers is essential to formulate an effective water policy. Traditional approaches to hydrologic attribution rely on the ability to infer hydrological processes through the development of catchment-scale hydrological models. However, such approaches are challenging to implement in practice due to limitations in using models to accurately associate changes in observed outcomes with corresponding drivers. Here we present an alternative approach that leverages the method of multiple hypotheses to attribute changes in streamflow in the Upper Jhelum watershed, an important tributary headwater region of the Indus basin, where a dramatic decline in streamflow since 2000 has yet to be adequately attributed to its corresponding drivers. We generate and empirically evaluate a series of alternative and complementary hypotheses concerning distinct components of the water balance. This process allows a holistic understanding of watershed-scale processes to be developed, even though the catchment-scale water balance remains open. Using remote sensing and secondary data, we explore changes in climate, surface water, and groundwater. The evidence reveals that climate, rather than land use, had a considerably stronger influence on reductions in streamflow, both through reduced precipitation and increased evapotranspiration. Baseflow analyses suggest different mechanisms affecting streamflow decline in upstream and downstream regions, respectively. These findings offer promising avenues for future research in the Upper Jhelum watershed, and an alternative approach to hydrological attribution in data-scarce regions.

Catchment processes can amplify the effect of increasing rainfall variability

By filtering the incoming climate signal when producing streamflow, river basins can attenuate—or amplify—projected increases in rainfall variability. A common perception is that river systems dampen rainfall variability by averaging spatial and temporal variations in their watersheds. However, by analyzing 671 watersheds throughout the United States, we find that many catchments actually amplify the coefficient of variation of rainfall, and that these catchments also likely amplify changes in rainfall variability. Based on catchment-scale water balance principles, we relate that faculty to the interplay between two fundamental hydrological processes: water uptake by vegetation and the storage and subsequent release of water as discharge. By increasing plant water uptake, warmer temperatures might exacerbate the amplifying effect of catchments. More variable precipitations associated with a warmer climate are therefore expected to lead to even more variable river flows—a significant potential challenge for river transportation, ecosystem sustainability and water supply reliability.

Assessing the impact of seasonal-rainfall anomalies on catchment-scale water balance components

Abstract. Although water balance components at the catchment scale are strongly related to annual rainfall, the availability of water resources in Mediterranean catchments also depends on rainfall seasonality. Observed seasonal anomalies in historical records are fairly episodic, but an increase in their frequency might exacerbate water deficit or water excess if the rainy season shortens or extends its duration, e.g., due to climate change. This study evaluates the sensitivity of water yield, evapotranspiration, and groundwater recharge to changes in rainfall seasonality by using the Soil Water Assessment Tool (SWAT) model applied to the upper Alento River catchment (UARC) in southern Italy, where a long time series of daily rainfall is available from 1920 to 2018. We compare two distinct approaches: (i) a “static” approach, where three seasonal features (namely rainy, dry, and transition fixed-duration 4-month seasons) are identified through the standardized precipitation index (SPI) and (ii) a “dynamic” approach based on a stochastic framework, where the duration of two seasons (rainy and dry seasons) varies from year to year according to a probability distribution. Seasonal anomalies occur when the transition season is replaced by the rainy or dry season in the first approach and when season duration occurs in the tails of its normal distribution in the second approach. Results are presented within a probabilistic framework. We also show that the Budyko curve is sensitive to the rainfall seasonality regime in UARC by questioning the implicit assumption of a temporal steady state between annual average dryness and the evaporative index. Although the duration of the rainy season does not exert a major control on water balance, we were able to identify season-dependent regression equations linking water yield to the dryness index in the rainy season.

Linking parametric and water-balance models of the Budyko and Turc spaces

The Budyko and Turc frameworks have become very popular tools for the estimation of catchment-scale water balance. In their original definition, these frameworks and the resulting equations, which are considered equivalent, apply in the long-term to very large catchments, whose water balance is predominately driven by climatic factors. Several equations similar to Budyko’s and Turc’s have subsequently been proposed to account also for the effect of catchment characteristics, including parametric formulations as well as equations resulting from a simplified physical representation of the water balance. After highlighting their advantages and disadvantages, we show that models based on the water balance, which account for rainfall variability and feedback between water availability and evapotranspiration, are more versatile to describe catchment-scale rainfall partitioning. They include parameters that have clearer physical meaning and, therefore, can be estimated independently of streamflow and evapotranspiration, thereby making them more amenable to practical use in un-gaged catchments. They show that Budyko’s and Turc’s point of view are equivalent only for large catchments. Additionally, water balance models have limiting conditions for extremely dry and wet climates that differ from those of the Budyko equations and its parametric formulations, as expected in catchments with a finite water storage capacity; in these models, Budyko’s and Turc’s points of view become equivalent only for large catchments.

River recharge sources and the partitioning of catchment evapotranspiration fluxes as revealed by stable isotope signals in a typical high-elevation arid catchment

Catchment-scale hydrological cycles are expected to suffer more extremes under a background of climate change. Quantifying hydrological changes in high and remote areas is practically challenging. However, stable isotopes in river water can be seen to vary, dependent upon the combined influence exerted by recharge sources and local climatic conditions; the study of river water stable isotopes can therefore provide a meaningful method for delineating catchment-scale hydrological studies. In this study, we present high-resolution time series of river δ18O and d-excess values; additionally, we identify the seasonal dynamics of river recharge sources and major components of the catchment-scale water balance, together with precipitation and groundwater isotopes, and concurrent meteorological data recorded in Magazangbu catchment on the northwestern Tibetan Plateau (TP). Using isotopic analysis, and within a proportional framework, we partitioned the isotopic fractionation (E1) or non-fractionation (E2) from soil evaporation fluxes (Esoil) apparent in different processes, using NDVI (Normal Differential Vegetation Index) data collected by MODIS satellites to calculate the vegetation fractional coverage (VFC), and Global Land Data Assimilation System (GLDAS) records to determine evapotranspiration data (ET). Finally, the contributions made by each ET component (Esoil and plant transpiration) to total catchment ET were computed for the high and remote northwestern TP. Our results show that: (1) river δ18O values were high in summer and low in winter, while d-excess values displayed a contrary seasonal cycle; (2) for the monsoon period, precipitation contributed 60.6% to Magazangbu catchment runoff. Deeper groundwater was the main water source for the winter low base flow, and shallow groundwater or high elevation snowmelt was the principal component of the spring thaw and autumn freezing periods; and (3) a substantial proportion of Esoil (96.4% annually; 92.2% during monsoon) was consumed without isotopic fractionation (E2); plant transpiration (T) constituted less than half of total ET (41% annually, 29% during monsoon) in Magazangbu catchment. This calculation of river recharge sources and partitioning of catchment ET components using isotopic signals and MODIS NDVI data or GLDAS ET data provide new methods for hydrological studies in high and remote areas. These results provide important catchment-scale water-balance information which is very useful to climate models conducted in a high-elevation arid environment.

Glacier Mass Balance and Catchment-Scale Water Balance in Bolivian Andes

In investigating glacier mass balance and water balance at Huayna Potosi West, a glacierized basin in the Bolivian Andes (Cordillera Real), we used a remote sensing method with empirical area-volume relationships, a hydrological method with runoff coefficients, and water balance method. Results suggest that remote sensing method based on the glacier area from satellite images and area-volume relationships is too imprecise to use in performing analysis in short time intervals. Glacier mass balance obtained using a new area-volume relationship was, however, similar to that obtained by the water balance method, thus proving that the new area-volume relationship is reasonable to use for analyzing glaciers within a certain size range. The hydrological method with a runoff coefficient considered glacier as the only storage for saving or contributing to runoff and nonglacier area as the only source of evaporation. We applied a fixed runoff coefficient of 0.8 without considering wet or dry seasons in nonglacier areas – a method thus sensitive to meteorological and hydrological data. We also did not consider glacier sublimation. The water balance method is applicable to the study region and excelled other methods in terms of resolution, having no empirical coefficients, and considering sublimation and evaporation. Among its few limitations are possibly underestimating evaporation and runoff over nonglacier areas during wet months and thus possibly overestimating glacier contribution at mean time.

A hydrograph separation method based on information from rainfall and runoff records

Summary Hydrograph separation is considered as the first step to catchment-scale water balance analysis. A wide variety of hydrograph separation methods exists ranging from empirical to analytical and physical. This study discusses a physically-based approach that combines baseflow separation and event identification with minimal data requirement. The input datasets are basin-average rainfall and discharge time series. Outputs are baseflow time series, the timing of the runoff events, differentiated as single- or multi-peak, and the associated rainfall event time series. To assess the method’s feasibility, hydrograph properties are evaluated for both long-term (annual and monthly) and event-scale time series. Results show that the long-term derived baseflow indices are positive (negative) correlated with basin area (runoff coefficient). The event scale analysis shows that the timing-related parameters (i.e. durations of rainfall and flow events and time lag between rainfall to flow events) increase with basin area in terms of magnitude and variability. Similar dependence on basin scale is shown for the water balance-related parameters determined from this analysis, namely event rainfall and baseflow volumes and baseflow index. Water balance parameters are shown to be characterized with less degree of variability for single-peak events relative to multi-peak events.

Impacts of elevated CO2, climate change and their interactions on water budgets in four different catchments in Australia

Summary Future water availability is affected directly by climate change mainly through changes in precipitation and indirectly by the biological effects of climate change and elevated atmospheric CO 2 concentration (eCO 2 ) through changes in vegetation water use. Previous studies of climate change impact on hydrology have focused on the direct impact and little has been reported in the literature on catchment-scale the indirect impact. In this study, we calibrated an ecohydrological model (WAVES) and used this model to estimate the direct and indirect effects and the interactive effect between climate change and eCO 2 on water availability in four different catchments in Australia with contrasting climate regime and vegetation cover. These catchments were: a water-limited forest catchment and an energy-limited forest catchment, a water-limited grass catchment and an energy-limited grass catchment. The future meteorological forcing was projected from 12 GCMs representing a period centred on 2050s and future CO 2 concentration was set as 550 ppm. Modelling experiments show that impacts of eCO 2 and projected climate change on vegetation growth, evapotranspiration (ET) and runoff were in the same magnitude but opposite directions in all four catchments, except for the effects on runoff in the energy-limited grass catchment. Predicted responses of runoff to eCO 2 indicate that eCO 2 increased runoff in the energy-limited forest catchment by ∼2% but decreased runoff in other three catchments from 1% to 18%. This study indicates that rising CO 2 increases ecosystem water use efficiency but it does not necessarily result in increased runoff because elevated CO 2 also stimulates vegetation growth and increases ET. Elevated CO 2 was proved to have greater impacts on runoff than climate change in the forest catchments. Modelling experiments also suggest that interactive effects between climate and CO 2 are important, especially for predicting leaf area index (LAI) and ET in grassland catchments or runoff in water-limited catchments, where interactive effects were 1–6%. It implies that the assumption that linear combination of individual effects in most of previous studies is not appropriate. This study highlights the importance of considering elevated CO 2 in assessing climate change impacts on catchment-scale water balance and failure to account for direct eCO 2 effect or its interactive effects can lead to large bias in the predictions of future water budgets, especially for the water-limited catchments in Australia.

Modelling annual evapotranspiration in a semi-arid, African savanna: functional convergence theory, MODIS LAI and the Penman–Monteith equation

Accurately measuring evapotranspiration (ET) is essential if we are to derive reasonable estimates of production and water use for semi-arid savannas. Estimates of ET are also important in defining the health of an ecosystem and the quantity of water used by the vegetation when preparing a catchment-scale water balance. We derived ET0 from an automatic weather station 30 km west of Skukuza, Kruger National Park, South Africa using the Penman– Monteith equation, and then used the MODIS LAI to inform the model of canopy phenological dynamics. This result was compared with 173 days of ET measurements from the eddy covariance (ETec) system near Skukuza in 2007 as well as from the ET recorded by a large-aperture scintillometer at the same site in 2005. The model compared favourably with both sets of measured data and, when used independently of the eddy covariance data, ETMODIS predicted an annual ET of 378 mm in 2007 for the semi-arid savanna around the Skukuza flux site.

On the need for bias correction in regional climate scenarios to assess climate change impacts on river runoff

Abstract. In climate change impact research, the assessment of future river runoff as well as the catchment-scale water balance is impeded by different sources of modeling uncertainty. Some research has already been done in order to quantify the uncertainty of climate projections originating from the climate models and the downscaling techniques, as well as from the internal variability evaluated from climate model member ensembles. Yet, the use of hydrological models adds another layer of uncertainty. Within the QBic3 project (Québec–Bavarian International Collaboration on Climate Change), the relative contributions to the overall uncertainty from the whole model chain (from global climate models to water management models) are investigated using an ensemble of multiple climate and hydrological models. Although there are many options to downscale global climate projections to the regional scale, recent impact studies tend to use regional climate models (RCMs). One reason for that is that the physical coherence between atmospheric and land-surface variables is preserved. The coherence between temperature and precipitation is of particular interest in hydrology. However, the regional climate model outputs often are biased compared to the observed climatology of a given region. Therefore, biases in those outputs are often corrected to facilitate the reproduction of historic runoff conditions when used in hydrological models, even if those corrections alter the relationship between temperature and precipitation. So, as bias correction may affect the consistency between RCM output variables, the use of correction techniques and even the use of (biased) climate model data itself is sometimes disputed among scientists. For these reasons, the effect of bias correction on simulated runoff regimes and the relative change in selected runoff indicators is explored. If it affects the conclusion of climate change analysis in hydrology, we should consider it as a source of uncertainty. If not, the application of bias correction methods is either unnecessary to obtain the change signal in hydro-climatic projections, or safe to use for the production of present and future river runoff scenarios as it does not alter the change signal. The results of the present paper highlight the analysis of daily runoff simulated with four different hydrological models in two natural-flow catchments, driven by different regional climate models for a reference and a future period. As expected, bias correction of climate model outputs is important for the reproduction of the runoff regime of the past, regardless of the hydrological model used. Then again, its impact on the relative change of flow indicators between reference and future periods is weak for most indicators, with the exception of the timing of the spring flood peak. Still, our results indicate that the impact of bias correction on runoff indicators increases with bias in the climate simulations.

Analysis of Near-surface Soil Moisture Spatial and Temporal Dynamics in an Experimental Catchment in Southern Italy

Management of water resources in basins under Mediterranean climate is based on t e knowledge of the complex interaction among rainfall, evapotranspira ion, streamflow discharge, and changes in water storage. Understanding the spatial and temporal evolution of such relations is a key issue for obtaining reliable applications of hydrologica models. In this study we identified the water dynamics involved in the catchment-scale water balance within a sub- humid area in Southern Italy. To meet this objective, precipitation and evapotranspirati n were retrieved through data recorded by a weather station whereas water discharge was monitored at the water reservoir delimited by an earth dam in the outlet of the Alento River 102.5 km2 catchment. The landuse is dominated by pasture, orchard and grassland and the main terrain attributes have been calculated from a 5 m DEM. Six hillslope transects have been delineated along hillslopes in order to cap ure soil variability. Surface soil water content has been monitored with a portable TDR device along the aforementioned transects with a spatial interval of 50 m in 10 field campaigns, from October 2004 to January 2005. Mediterranean climate is characterized by dry summers with strong water deficit and wet winters with replenishment of the water reservoir. In the dry season, soil water content has high tempora instability and is mainly influenced by the aspect and tangential curvature. On the other hand during the wet season near saturation conditions of soil surface are able to diagnose rapid streamflow response since rainfall events generate runoff and subsurface later flow. Soil water content is more stable and shows a signifi ant correlation with the slope and the wetness index.

WATER BALANCE OF GLACIERIZED CATCHMENTS IN TROPICS: A CASE STUDY IN BOLIVIAN ANDES

On outer-tropical glaciers, mostly in Peru and Bolivia, ablation and accumulation occur all year round whereas maximum in wet seasons. This study investigated glacier mass balance and catchment scale water balance of Glacier Tuni and Huayna Potosi, which locate at the west side of Bolivian Andes. Remote sensing and hydrological methods have been applied to conduct glacier mass balances monthly and annually in recent years. With remote sensing method, glacier volume change has been derived based on glacier area change obtained utilizing cloud free Landsat Thematic Mapper (TM) scenes. Results of commonly negative glacier mass balance have revealed that glaciers in this region are retreating. Despite of interests on glaciological aspects, water balances of glacierized catchments have been carried out. Except for in a few months glaciers got positive mass balance, glaciers continuously contributed to discharge.

Catchment-scale water balance modelling in a Carboniferous Limestone aquifer

Abstract Groundwater models are used to manage water resources in carbonate aquifers throughout the UK. These models often utilize non-linear modifications of equivalent porous media (EPM) assumptions to simulate the observed head-flow relationships. This paper describes the application of similar principles to time-variant water balance modelling of a karstified aquifer: the Carboniferous Limestone in SW England. The non-linear model successfully simulates the large changes in bulk hydraulic conductivity required to adequately model flows from two karstic spring catchments. Model results illustrate that the rate of drainage from the vadose zone is affected by the extent and thickness of soil cover. In a catchment with mixed soil cover, vadose zone drainage is shown not to be significant in sustaining summer low flows at the spring. The performance of the model is limited in some cases: summer storm flows in particular are poorly simulated. The limitations suggest that the true response of the system is dependent on recharge intensity (rate) as well as antecedent water levels and that the EPM assumption of good connection between conduits and the diffuse flow zone is not always valid in this aquifer. This may have implications for groundwater model design in other carbonate aquifers.

A comparison of sap flux-based evapotranspiration estimates with catchment-scale water balance

Many researchers are using sap flux to estimate tree-level transpiration, and to scale to stand- and catchment-level transpiration; yet studies evaluating the comparability of sap flux-based estimates of transpiration ( E t) with alternative methods for estimating E t at this spatial scale are rare. Our ability to accurately scale from the probe to the tree to the watershed has not yet been demonstrated, nor do we know the relative impact of the main sources of variability on our scaled estimates. Accounting for the variability in the radial distribution of sap flux within the sapwood, the variability of transpiration among trees and between plots within the catchment, and the variability in stand density, sapwood area, and leaf area are critical for making landscape inferences about transpiration. During 2004 and 2005, we continuously monitored 40 trees in three plots within a 13.5-ha gauged watershed comprising a 50-year-old eastern white pine plantation within the Coweeta Basin in western North Carolina, USA. We scaled sap flux-based estimates of stand transpiration ( E t) and surface area-based estimates of stand interception ( E i) to the catchment and compared these with water balance estimates of evapotranspiration ( E, precipitation minus runoff, P − R o). For both years, the sum of sap flux scaled E t and E i were 14 and 7% lower than evapotranspiration estimated from P − R o. Our results show that a considerable amount of variation exists at each scaling step encountered; however, a simple scaling exercise revealed that omitting among plot variation affected the sap flux scaled E t estimate by 48%. Thus, the largest source of variability in scaling to the landscape was landscape variation in stand density and sapwood area.

A nonparametric stochastic downscaling framework for daily rainfall at multiple locations

Use of General Circulation Models (GCMs) for climate change impact assessment is often limited by their incapability at representing local features and dynamics at spatial scales finer than the in‐built GCM grid scale. This has led to the development of downscaling techniques for transfer of coarse GCM simulated weather output to finer spatial resolutions. This paper presents a nonparametric stochastic spatial downscaling framework for multisite daily rainfall occurrence and amount. At site rainfall occurrences are downscaled using a nonparametric nonhomogeneous hidden Markov model (NNHMM) that represents spatial dependence across the rainfall occurrence field using a dynamic weather state indicative of the centroid and average wetness fraction of the rainfall occurrence field. The rainfall amounts on the wet days are downscaled using a nonparametric kernel density approach that accommodates variations in the rainfall downscaling model at individual locations. Spatial dependence in the rainfall amounts is simulated by driving each of the single‐site amounts model with spatially correlated random numbers. The proposed framework is applied for downscaling of rainfall at a network of 30 rain gauge stations around Sydney in Australia, and its performance is evaluated. The analyses of the results show that the logic of providing separate treatments for rainfall occurrence and amounts at individual locations imparts considerable accuracy in the representation of characteristics of interest in hydrologic studies. These characteristics include representation of rainfall spell patterns, spatial distribution of the rainfall occurrence and amount fields, representation of low and high rainfall extremes at individual stations and across the field, as well as common indicators of water balance and variability that are of importance in a catchment scale water balance simulation.

Deriving catchment-scale water and energy balance parameters using data assimilation based on extended Kalman filtering

Important catchment-scale water and energy balance parameters are derived for a small catchment in southeastern Australia by assimilation in a catchment-scale soil-vegetation-atmosphere transfer (SVAT) model of subcatchment-scale soil water content observations and land surface temperature measurements. In order to incorporate the subcatchment-scale soil moisture variability and its time evolution in a data assimilation scheme, an extended Kalman filter (EKF) method is used in combination with a cost function minimization approach to derive effective parameters for the catchment as a whole. These parameters are the minimum surface resistance to evaporation and the soil hydrodynamic parameters. This method provides a balanced assessment of all the uncertainties regarding the description of the catchment hydrological behaviour. Moreover, these uncertainties are propagated forward in time in a single framework, which combines soil moisture correction with effective parameter estimation. Two issues are addressed in this paper: (a) the applicability of the method for scaling purposes (effective parameterization) and (b) the relationship between effective parameters obtained by this method and parameters obtained by a classical minimization routine that ignores soil moisture correction and observation uncertainty. Effective parameters are found to be consistent with the available local parameter measurements. Derived parameter sets with and without EKF have been found to be very similar and this has been explained in a number of ways.

Forest canopy hydraulic properties and catchment water balance: observations and modeling

We present observations and simulations examining plant–water relations in a forested catchment characterized by strong topographic control over surface hydrology and stand structure. The system is dominated by xeric and mesic ecotypes of Quercus rubra L. Soil depths are typically very thin and the severity of seasonal water stress in these trees is determined largely by position along the local topographic gradient. Water balance related ecotypic differences in Q. rubra were measured during the 2000 growing season at an upland and lowland site. Significant site differences in stomatal conductance, sap flux density, and leaf to sapwood area ratios were observed. However, mid-day leaf water potentials and the leaf area-specific canopy hydraulic conductance were not significantly different despite a 20% increase in soil moisture content, an average 10 m increase in tree height, and a higher leaf area index at the lowland site. These results suggest a close coordination of tree morphology, stand structure, and the hydraulic conductance of the combined soil–root–leaf pathway that governs leaf-level water vapor exchange rates similarly across the topographic gradient. To place these stand-level observations in the context of catchment-scale water balance we linked the SPA canopy model with a 1-D soil column model and TOPMODEL hydrologic formulations. The SPA model was used to represent the canopy because of its specific inclusion of hydraulic constraints on transpiration and leaf water potential. The combined model is spatially explicit in the distribution of ecotype morphology, and calculates transpiration rates for TOPMODEL-derived saturated and unsaturated area fractions within the watershed separately. Model predictions of stomatal conductance, upland latent energy flux, stream discharge, and soil moisture content compared favorably with observations. Sensitivity analyses of canopy model parameters indicate stream discharge in this system is most sensitive to changes in the maximum leaf area index, the minimum leaf water potential, and belowground resistance. Discharge was least sensitive to changes in stem hydraulic conductivity and capacitance. Model results and observations are discussed with respect to adaptations to water stress, hydraulic controls on canopy water use, and ecosystem water use.

Estimation of catchment-scale water-balance with a soil-vegetation-atmosphere transfer model

Catchments with a small elevation range and relatively long dry periods in high radiation conditions may be described as an array of vertical one-dimensional pathways for water and energy. Such a representation enhances the ability of SVAT modeling to simulate mass exchanges across the catchment. This note reports on a comparison of a Soil-Vegetation-Atmosphere Transfer (SVAT) model (Braud et al., 1995), a deterministic hydrological model (Dawes and Hatton, 1993) and a stochastic hydrological model (Sivapalan and Woods, 1995; Kalma et al., 1995). The original version of the SVAT model only considers vertical transport and this one-dimensional representation must be aggregated to describe the entire catchment. Therefore, two new versions have been developed: a deterministic SVAT model which sub-divides the catchment into 40 sub-regions linked by surface flow, and a stochastic model which provides a distribution of the output fluxes as related to the spatial distribution of initial water content and/or soil properties. All simulations have been made for a 60-day period.