Wet Canopy Evaporation Rate Research Articles

Comparisons of rainfall partitioning and canopy interception modelling in pure and mixed forests on the Chinese Loess Plateau

AbstractRainfall partitioning is a key process in forest ecosystems that affects the forest hydrological cycle and ecological services. However, little is known about the rainfall partitioning in Pinus tabuliformis (PT), Robinia pseudoacacia (RP), and Pinus tabuliformis×Robinia pseudoacacia (PT × RP) forests, which are widely distributed on the Chinese Loess Plateau (CLP) for controlling soil and water losses. The objectives of this study were to compare the rainfall partitioning and conduct interception modelling in PT, RP, and PT × RP forests, using data from the Ansai and Changwu sites on the CLP. The sites feature similar weather conductions and both pure and mixed forests of the same age. The tree density of PT × RP was moderate at the Ansai site but higher at the Changwu site compared to PT and RP. We quantitatively measured the throughfall (Tf), stemflow (Sf), and interception loss (Ic) for the forests at the two sites during the growing season (May–October, 2022), and simulated Ic using the Revised Gash Analytical Model (RGAM) and Liu model. The results showed that measured Ic for PT × RP (26.0% of the total gross rainfall, Pg) was larger than for RP (22.0% of Pg) but smaller than for PT (27.8% of Pg) at the Ansai site, and vice versa for the Tf and Sf (RP > PT × RP > PT), while the measured Ic for PT × RP (33.7% of Pg) at the Changwu site was larger than both for PT (31.1% of Pg) and RP (19.5% of Pg). The Tf and Sf for PT × RP was smaller than for PT and RP. Given the similar weather conditions, differences in Ic for the pure and mixed forests at each site were attributed to canopy storage capacity, wet canopy evaporation rate, and tree density. Additionally, the RGAM and the Liu model effectively simulated and captured the differences in Ic for PT, RP, and PT × RP. However, the RGAM performed better with a relative error less than −7.3%, Nash‐Sutcliffe model efficiency ranging from 0.82 to 0.95, and root mean square error less than 0.5 mm. According to the RGAM, evaporation from the wet canopy during rainfall events and after rainfall cessation were the two most influential components. The results of this study suggest that mixed forest can optimize rainfall partitioning compared to pure forests, but a reasonable density should be adopted to increase rainwater replenishment to the forest floor.

Estimating rainfall interception loss of three dominant shrub species in an oasis–desert ecotone using in situ measurements and the revised Gash analytical model

Canopy interception loss affects the local water budget by removing a non-negligible proportion of rainfall from the terrestrial surface. Thus, quantifying interception loss is essential for thoroughly understanding the role of vegetation in the local hydrological cycle, especially in dryland ecosystems. However, sparse shrubs in dryland ecosystems have not been sufficiently studied, owing to time- and labor-intensive field experiments and challenging model parameterization. In this work, 4-year growing season field experiments on rainfall partitioning were conducted for three dominant shrub species (Haloxylon ammodendron, Nitraria sphaerocarpa, and Calligonum mongolicum) in an oasis–desert ecotone in northwestern China. The revised Gash analytical model was well parameterized, which reliably simulated the cumulative interception loss for sparse shrubs, and the validated model performed better for H. ammodendron, followed by C. mongolicum and N. sphaerocarpa, with relative errors of 8.4%, 15.4%, and 23.9%, respectively. The mean individual interception loss percentage for H. ammodendron (28.4%) was significantly higher than that for C. mongolicum (11.0%) and N. sphaerocarpa (10.9%) (p < 0.05), which could be ascribed to the higher canopy storage capacity and wet-canopy evaporation rate of H. ammodendron. For all shrub species, the majority proportion of interception loss occurred during canopy saturation and drying-out periods, accounting for approximately 79–85% of the cumulative interception loss. Overall, the mean local interception loss of three dominant shrub species in the ecotone removed nearly 17% of the corresponding cumulative rainfall during the growing season. These results not only provide methodological references for estimating the interception loss of sparse vegetation in dryland ecosystems, but also provide scientific insights for water resource management and ecosystem restoration in water-limited regions similar to the experimental site.

Spatio-temporal rainfall interception loss at the catchment scale from earth observation in a data-scarce area, Northern Ethiopia

Rainfall interception loss (EI) is an important spatio-temporal variable in hydrological studies. The EI is required as input of integrated hydrological models, which nowadays are increasingly applied in water resources management. However, the EI is typically not considered or arbitrarily simplified, which causes wrong model parameterization and typical overestimation of water resources. Such problem can be particularly consequential in water-limited environments, where EI represents a substantial percentage of rainfall. The EI is rarely quantified experimentally because field data acquisition for its direct estimation is cumbersome and costly, especially at large catchment/basin scales. Hence, Earth Observation Satellites (EOS) can be used as an alternative source of data acquisition to quantify EI, especially at large spatial scales. The main aim of this study was to estimate the daily variability of rainfall interception loss in the data-scarce Zamra catchment (1588 km2) in Ethiopia, applying the EOS solution of the revised Gash model (RGM). Next to EOS, this study utilized four automated weather station data. To test the RGM assumption permitting the use of rainfall as one storm per rainy day (instead of the event-based rainfall), high temporal resolution (15 min) automated weather-station data was used. The test confirmed the validity of that assumption in the Zamra catchment (ZC), so the daily bias-corrected by station data rainfall was used as input. The weather station data was also used to define the event-based ratio of the mean wet-canopy evaporation rate to the mean rainfall rate (Ec¯/R¯) ranging from ∼0.037 to 0.041 at the low-elevated stations to ∼0.063 at the high-elevated station. As the variability of the Ec¯/R¯ of the three low-elevated stations was pretty uniform, the mean 0.04 was assumed for areas below 2000 m a.s.l., but for the areas above, the Ec¯/R¯ was assigned as altitude-dependent applying geographical weighted regression method. Other biophysical parameters, the fractional canopy cover and the canopy storage capacity obtained from leaf area index, were derived from Sentinel 2. The final EI was highly spatially and temporally variable, ranging from zero at bare lands, to 30% of annual rainfall in forested areas and with ∼4% mean of annual rainfall for the whole ZC. While applying EOS solution of RGM, facilitated with limited amount of ground-based data, the presented study offers an efficient and realistic way of spatio-temporal quantification of EI, particularly suitable for large data-scarce areas such as the ZC.

Global canopy rainfall interception loss derived from satellite earth observations

AbstractInformation on global canopy rainfall interception loss is essential for understanding the dynamics of land surface processes and the water cycle. In addition, large uncertainty remains regarding the global variation in this factor. The development of satellite earth observations has provided a great opportunity for the estimation of global rainfall interception loss to reveal the spatiotemporal variations. In the current study, the analytical Gash model was adapted and applied to estimate global rainfall interception loss from 2001 to 2015 on the basis of satellite remote‐sensing products, for example, gross rainfall amount and rate and leaf area index. The Dalton‐type equation was adopted to estimate the wet canopy evaporation rate in the revised Gash model. The estimation on the basis of the Dalton‐type equation showed better agreement with the ground observations than the classical Penman–Monteith equation in terms of estimated rainfall interception loss and its ratio to gross rainfall. Large spatial variations in global rainfall interception loss were found, and high values appeared in the regions with dense vegetation cover and high gross rainfall amounts, for example, tropical rainforests, whereas high rainfall interception to gross rainfall ratios occurred in the regions with low rain rates and high vegetation cover. This study constitutes a significant step forward in understanding global hydrological cycle on the basis of earth observation products.

Integration of hillslope hydrology and 2D hydraulic modelling for natural flood management

Abstract Natural flood management (NFM) has recently invigorated the hydrological community into redeploying its process understanding of hydrology and hydraulics to try to quantify the impacts of many distributed, ‘nature-based’ measures on the whole-catchment response. Advances in spatial data analysis, distributed hydrological modelling and fast numerical flow equation solvers mean that whole-catchment modelling including computationally intensive uncertainty analyses are now possible, although perhaps the community has not yet converged on the best overall parsimonious framework. To model the effects of tree-planting, we need to understand changes to wet canopy evaporation, surface roughness and infiltration rates; to model inline storage created by ‘leaky barriers’ or offline storage, we need accurate channel hydraulics to understand the changes to attenuation; to model the complex behaviour of the whole network of NFM measures, and the possibility of flood peak synchronisation effects, we need efficient realistic routing models, linked to key flow pathways that take into account the main physical processes in soils and the antecedent moisture conditions for a range of different rainfall events. This paper presents a new framework to achieve this, based on a cascade of the Dynamic Topmodel runoff generation model and the JFlow or HEC-RAS 2D hydraulic models, with an application to the Swindale Catchment in Cumbria, UK. We demonstrate the approach to quantify both the effectiveness of a relatively large ‘runoff attenuation feature’ in the landscape and the uncertainty in the calculation given model parameter uncertainty.

The modelling of rainfall interception in growing and dormant seasons for a pine plantation and a black locust plantation in semi-arid Northwest China

Interception loss can remove a significant portion of rainwater from forested ecosystems. Therefore, the quantification and modelling of interception loss are of significant importance if human and ecosystem water demands are to be balanced under a future changing climate. This is particularly true for semi-arid/arid regions, where afforestation has become an important ecological restoration measure to tackle desertification, poverty and climate change. However, quantification and modelling of interception loss of plantations in these regions have rarely been reported. In the present study, rainfall interception loss was quantified and modelled over a one-year period (January-December 2016) for a deciduous broad-leafed R. pseudoacacia plantation and an evergreen needle-leaf P. tabuliformis plantation (common afforestation tree species) situated in the semi-arid Loess Plateau of China. The stand age, density, canopy cover and leaf area index of R. pseudoacacia during the study period were 15 years, 2000 tree ha−1, 0.48 and 1.41 m2 m−2, respectively. The corresponding values for Pinus tabuliformis were 17 years, 1200 tree ha−1, 0.62 and 2.53 m2 m−2. The measured throughfall, stemflow and derived estimates of interception loss for R. pseudoacacia were 81.1%, 1.3% and 17.6%, respectively. The corresponding values for P. tabuliformis were 75.4%, 0.7% and 23.9%. Given that the weather conditions experienced by the two forest stands were similar, the observed higher interception loss for P. tabuliformis can be explained by the higher canopy storage capacity and wet canopy evaporation rate of this species. The revised Gash analytical model of rainfall interception was well calibrated and validated against field measurements and was able to simulate the cumulative interception loss at two forest stands accurately, and it also effectively captured the seasonal variation (leafed growing and leafless dormant seasons), provided that the optimized wet-canopy evaporation rates were used. The revised model was highly sensitive to the canopy storage capacity and changes in the ratio of mean wet canopy evaporation to mean rainfall intensity and less sensitive to canopy cover, but it was found to be fairly insensitive to the trunk storage capacity.

Typhoon-induced changes in rainfall interception loss from a tropical multi-species ‘reforest’

Typhoon Haiyan made landfall in the central Philippines on 8 November 2013 as one of the strongest tropical cyclones ever. It also affected a 23-year-old multi-species ‘reforest’ at Manobo near Tacloban City on Leyte Island. As part of a larger hydrological investigation of the impacts of reforestation on streamflow response in the Tacloban area, gross rainfall (P), throughfall (TF; 24 roving collectors) and stemflow (SF; 12 trees) were monitored at Manobo between June 2013 and May 2014. Leaf Area Index (LAI) above each TF collector was measured regularly. Total rainfall interception losses (I) were determined using Gash’s revised analytical model for three consecutive periods:(i) pre-Haiyan (baseline), (ii) post–Haiyan (damaged canopy), and (iii) after initial canopy recovery. Modeled I was 18% of P before disturbance, 12% for the period with the most extensive canopy damage, and 17.5% after initial canopy recovery. Stemflow was low, and weighted mean values accounted for 2.7%, 1.3% and 2.0% of P for the respective periods. Contrasts in period-average values of I reflected changes in LAI as well as wet-canopy evaporation rates. Storm-based TF fractions at the 5 m × 5 m sub-plot scale were inversely related to LAI, especially for small storms and low rainfall intensities. Inferred hourly rates of wet-canopy evaporation showed a strong positive relationship to hourly rainfall intensity during large storms. The revised analytical model was also run using pre-disturbance parameter values for the entire year to assess the overall effect of canopy damage on I. Estimated annual losses with and without canopy disturbance were 514 mm (15% of P) and 572 mm (17%), respectively. Thus, observed and inferred changes in rainfall partitioning after canopy disturbance and initial recovery were comparatively modest, likely because the measurement site was relatively sheltered from the winds during typhoon passage and re-foliation relatively rapid. However, given the predicted increase in occurrence of ‘super-typhoons’ due to continued global warming and oceanic freshening, the structure of forests in affected regions can be expected to be modified, with potential consequences for rainfall partitioning and hydrological response.

Importance of transitional leaf states in canopy rainfall partitioning dynamics

The temporal dynamics of forest canopy rainfall partitioning are important to forest ecology and management as it influences all subsequent hydrological processes along the rainfall-to-discharge flow path. Despite a growing body of literature on the importance of coupled hydrological–ecological interactions during periodic forest life cycle events, little work has examined how canopy rainfall partitioning varies across transitional leaf states (between the leafed vs. leafless states). This study analyzed a 3 year field monitoring campaign for two tree species in semiarid Iran (Robinia pseudoacacia and Platanus orientalis) to describe rainfall partitioning dynamics across the full-leaf, senescence, leafless, and leafing states. Crown saturation point, canopy storage capacity, free throughfall coefficient and the ratio of wet canopy evaporation rate to mean rainfall intensity were related to decreases/increases in plant area index and canopy closure. The high variability of rainfall partitioning observed in this study highlights the importance of transitional leaf states in the temporal characterization of water inputs to forest surfaces and boundary layer.

Remote sensing upscaling of interception loss from isolated oaks: Sardon catchment case study, Spain

Tree interception loss from two Mediterranean oak species, Quercus ilex (Q.i.) and Quercus pyrenaica (Q.p.), was estimated during 2-year period (1 October 2011 to 30 September 2013) in sparsely vegetated Sardon catchment (∼80km2, Spain) by: i) rainfall, throughfall and stemflow measurements; ii) Gash model temporal extrapolation; and iii) remote-sensing spatial upscaling. The annual, measured tree interception losses (Im) of Q.i. and Q.p. in the first year were 51% and 16% of P (335mm) and in the second, 46% and 10% of P (672mm), respectively. The revised Gash analytical model of rainfall interception loss, extrapolated well the Im temporal variability of Q.i. and Q.p., provided the throughfall-based, and not Pennman-Monteith-based, average wet canopy evaporation rates were used. Finally, a novel method of spatial upscaling of a tree-based interception loss into plot- and catchment-scale, using per-species, reference tree interception loss and object-attributes derived from satellite imagery, was proposed. The interception losses from Q.i. and Q.p. were upscaled first into two homogeneous plots (1-ha each and both with ∼20% canopy cover), one with Q.i. and the other with Q.p. oaks and then into the entire Sardon catchment with ∼7% canopy cover. The obtained annual-mean, plot interception losses were 9.5% of P in evergreen Q.i. and 2.5% of P in deciduous Q.p. plot. The annual-mean catchment interception loss was 1.4% of P. The proposed upscaling method is expected to improve catchment water balances, replacing common arbitrary or literature based tree interception loss estimates.

Canopy hydrometeorological dynamics across a chronosequence of a globally invasive species, Ailanthus altissima (Mill., tree of heaven)

Many mechanisms aid invasive plants’ competitive interactions. Yet, the extent to which invasive plants alter canopy hydrometeorological mechanisms determining the quantity and distribution of rainwater resources to soils have never been assessed. We examine these mechanisms for a global invader, Ailanthus altissima, across an invasion chronosequence (stands aged 20, 30, 40 years) for 99 storms, each with ∼200 net rainfall observations. With age woody area index, ratio of wet canopy evaporation and rainfall rates, and stem drainage coefficient increased; while leaf area index, canopy water storage, and gap fraction declined. This corresponded to increased stemflow and decreased throughfall across annual, seasonal, and interstorm scales. Promotion of stemflow may be advantageous to the invasive species as it increases water supply to roots and may help distribute allelopathic chemicals through the soil. Further research is needed on relationships between canopy hydrometeorological changes during invasion and redistribution of water to soils.

Measurement and modeling of rainfall interception by two differently aged secondary forests in upland eastern Madagascar

Secondary forests occupy a larger area than old-growth rain forests in many tropical regions but their hydrological functioning is still poorly understood. In particular, little is known about the various components of evapotranspiration in these possibly vigorously regenerating forests. This paper reports on a comparison of measured and modeled canopy interception losses (I) from a semi-mature (ca. 20years) and a young (5–7years) secondary forest in the lower montane rain forest zone of eastern Madagascar. Measurements of gross rainfall (P), throughfall (Tf), and stemflow (Sf) were made in both forests for one year (October 2014–September 2015) and the revised analytical model of Gash et al. (1995) was tested for the first time in a tropical secondary forest setting. Overall measured Tf, Sf and derived I in the semi-mature forest were 71.0%, 1.7% and 27.3% of incident P, respectively. Corresponding values for the young forest were 75.8%, 6.2% and 18.0%. The high Sf for the young forest reflects the strongly upward thrusting habit of the branches of the dominant species (Psiadia altissima), which favours funneling of P. The value of I for the semi-mature forest is similar to values reported for old-growth tropical lower montane rain forests elsewhere but I for the younger forest is higher than reported for similarly aged tropical lowland forests. These findings can be explained largely by the prevailing low rainfall intensities and the frequent occurrence of small rainfall events. The revised analytical model was able to reproduce measured cumulative I at the two sites accurately and succeeded in capturing the variability in I associated with the seasonal variability in rainfall intensity, provided that Tf-based values for the average wet-canopy evaporation rates were used instead of values derived with the Penman-Monteith equation.

Evaluation of forest interception estimation in the continental scale Australian Water Resources Assessment – Landscape (AWRA-L) model

summary The AWRA-L model is a core component of the joint Bureau of Meteorology (BOM) and CSIRO Australian Water Resources Assessment (AWRA) system which integrates hydrological models and a variety of observations and satellite products to produce a National Water Accounting system for Australia. AWRA-L is a continental scale sub-model which provides surface water balance component estimates for all landscape types, including forests ranging from sparse woody vegetation in dry climates to dense tropical rainforest in wet climates. This paper describes the AWRA-L interception sub-routine, which is based on the widely used Gash model, and considers how its predictions compare with measurements of interception in tropical rainforests in north Queensland and sparse jarrah forests in Western Australia. The results demonstrate the importance of having accurate estimates of the three parameters which dominate the interception loss (as a fraction of rainfall), i.e. canopy water holding capacity, S, the average wet canopy evaporation rate, E, and the average rainfall rate, R. The current ‘default’ values of these parameters used in AWRA-L lead to significant errors in interception, which will also be reflected in the AWRA stream flow estimates. Analysis of how E and R values vary across the Australian continent have led to recommendations for how the interception calculations made by the AWRA-L model can be improved using values of E and R calculated from forest height and latitude. Revisions of the canopy storage parameters based on regressions derived from measured values at the rainforest and jarrah sites are also evaluated. Overall, the adoption of both updated canopy storage and E/R ratios outlined in this paper does provide a measurable improvement to the performance of the AWRA-L interception model. The approach developed in this study also has direct relevance to other applications of interception models for water balance modelling at large spatial scales.

The water and energy exchange of a shaded coffee plantation in the lower montane cloud forest zone of central Veracruz, Mexico

The water and energy fluxes of a shaded coffee plantation in the lower montane cloud forest (LMCF) zone of central Veracruz, Mexico, were measured over a two-year period (September 2006–August 2008) using the eddy covariance method. Complementary measurements of throughfall and stemflow were made to study rainfall interception. The sum of the observed sensible (H) and latent (λE) heat fluxes was almost 95% of the net radiation (Rn) minus the canopy heat storage fluxes, indicating very good energy balance closure. The mean annual evapotranspiration was 1066mm, and 95% of the corresponding FAO Penman-Monteith reference evapotranspiration (ET0) of 1117mm yr−1. Interception loss was 8% of annual rainfall (1386mm). Both the eddy covariance, and the throughfall and stemflow measurements showed average wet-canopy evaporation rate to be very low (0.05mmh−1) compared to the corresponding rainfall rate (3.06mmh−1). As a result, and despite the low canopy storage capacity of the coffee plantation (Cm, 0.50mm), interception was dominated by post-event evaporation of intercepted water rather than by within-event evaporation. Comparing the results for the coffee plantation with interception data from mature and secondary LMCFs in the study area suggests that the conversion of LMCF to shade-coffee may lead to a decrease in interception loss of 8−18% of incident rainfall. This decrease is caused by a three- to seven-fold decrease in Cm, probably due to the lower leaf area and smaller epiphyte biomass of the coffee plantation. The mean annual dry-canopy evaporation was 992mm, and 89% of ET0. Comparing the eddy covariance-based estimate of dry-canopy evaporation for the coffee plantation with sapflow-based estimates of transpiration for the LMCFs did not show any clear differences.

Rainfall interception by natural and planted forests in the Middle Mountains of Central Nepal

► Rainfall interception loss was measured and modelled for natural and planted forest. ► The difference in rainfall interception loss between the two forests was examined. ► The model simulates interception loss well under monsoonal montane conditions. ► The difference in interception loss is due to differences in LAI and tree density. ► The high annual interception loss from planted forest could affect dry season flows. Measurements of gross rainfall, throughfall and stemflow were made during the 2011 rainy season in an evergreen natural forest and in a planted pine forest located in complex topography in the Middle Mountains of Central Nepal. For the period of observation (20 June–9 September, 2011), measured throughfall, stemflow and derived interception loss in the natural forest were 76.2%, 1.4% and 22.4% of incident rainfall, respectively. Corresponding values for the pine plantation were 83.0%, 0.5% and 16.5%. The revised version of Gash’s analytical model of rainfall interception was calibrated and validated for the two stands, representing a first for montane sub-tropical monsoonal conditions. The results of the modelling corresponded well with observed values, provided an optimised value was used for the average wet-canopy evaporation rate. The model was subsequently used to calculate annual interception losses from the two forests. Modelled estimates of annual interception loss for the planted and natural forest were 291 mm and 319 mm and represented 19.4% and 22.6% of incident rainfall, respectively. The high interception loss from the planted forest is considered a contributory factor towards the observed decline in dry season streamflow following the reforestation of degraded hillsides in the Middle Mountains of Nepal.

The performance of the reformulated Gash's interception loss model in Mexico's northeastern temperate forests

AbstractThe quantitative importance of rainfall interception loss and the performance of the reformulated Gash model were evaluated as a function of basal area in Mexico's northeastern temperate forest communities. A sensitivity analysis as well as an iterative search of parameters matched interception loss measurements and assessments and isolated coefficient values that drive the model performance. Set hypothesis was tested with a total of 73 rainfalls recorded on four forest stands with different canopy cover for model fitting (39) and validation (34). The reformulated Gash model predicted well rainfall interception loss because mean deviations between recorded and modelled interception loss as a function of gross rainfall, MD, were &lt;2.6% and 5.3% for fitting and validating parameter data sets, respectively. Basal area was negatively related to the model performance, but maximum projected MD range values can be found in most interception loss studies, for example, &lt;7% when basal area is &lt;5 m2 ha−1. The wet canopy evaporation rate and the canopy storage coefficient drive interception loss and the iterative parameter search showed that high wet canopy evaporation rates were expected in these forests. These parameters must be further studied to physically explain drivers of high wet canopy evaporation rates. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Seasonal variability of interception evaporation from the canopy of a mixed deciduous forest

Gross rainfall, net rainfall and stemflow were measured in a mixed deciduous woodland in southern England over a period of 14 months. Continuous measurements of standard weather data and momentum and sensible heat fluxes between the forest canopy and the atmosphere accompanied the investigation. The gross rainfall was corrected for catch losses due to high turbulence. Reliable net rainfall data were obtained from a combined application of simple storage gauges and troughs connected to automatic tipping bucket gauges. The evaporation rates from the wet canopy were calculated with the Penman–Monteith equation using the measured aerodynamic conductance to the momentum flux and, additionally, with the eddy covariance energy balance approach. Both methods agreed in the observation that the average wet canopy evaporation rate was slightly higher in the leafless period, due to higher wind speeds and the different aerodynamic properties of the canopy. Together with the lower average rainfall rate this counterbalanced the reduced storage capacity of the leafless canopy and maintained a relatively high interception loss throughout the year being 29% of the gross rainfall in the leafed period and 20% in the leafless period. The analytical sparse canopy rainfall interception model of Gash et al. [Gash, J.H.C., Lloyd, C.R., Lachaud, G., 1995. Estimating sparse forest rainfall interception with an analytical model. J. Hydrol. 170, 79–86] was parameterised, for the first time, for a mixed deciduous woodland. Separate parameters were derived for the leafed and the leafless canopy. The model explained the seasonal variability in the interception loss very well and is a suitable tool to analyse and predict this important component of the annual water balance of deciduous forests.

Modelling interception in coastal and montane rainforests in northern Queensland, Australia

Summary This paper reports a comparison of measured and modelled interception for three different forest types at six rainforest locations in northern Queensland. The Gash interception model was able to reproduce cumulative interception at the sites accurately, provided an appropriate value of canopy storage capacity ( S ) was used, 2.0–3.6 mm. These values are significantly higher than S values generally reported in other rainforest studies (∼1 mm) and the reason may be that Australian rainforests contain many epiphytes and mosses, which can trap significant quantities of water within the canopy. There is also some evidence of a seasonal variation in S and wet canopy evaporation rate ( E ), both being lower in the dry season than the wet season. However, although the rainfall rate ( R ), S and E all affect the seasonal value of interception, the changes in these three parameters tend to compensate and so the biggest factor affecting seasonal variations in interception is the number of small storms. The consequence of this is that it is still possible to get good estimates of seasonal and annual interception using R, S and E values that are fixed for the entire year. Values of E fell in the range 0.35–0.81 mm h −1 , which are 1.4–9 times the concurrent rates estimated using the Penman–Monteith equation. This implies that either our rainforests received very large amounts of advected energy during rain storms, or the Penman–Monteith E values are too low. Some advection of energy to our sites is quite feasible given their proximity to the ocean and generally well exposed locations. However, most of the above discrepancy is probably due to underestimation of the Penman–Monteith values of E , because of errors in the estimation of the above canopy relative humidity, due to the use of weather data adjacent to rather than above the forests and inherent difficulties of measuring the very high humidity’s that occur during rainfall.

On interception modelling of a lowland coastal rainforest in northern Queensland, Australia

Recent studies of the water balance of tropical rainforests in northern Queensland have revealed that large fractions of rainfall, up to 30%, are intercepted by the canopy and lost as evaporation. These loss rates are much higher than those reported for continental rainforests, for example, in the Amazon basin, where interception is around 9% of rainfall. Higher interception losses have been found in coastal and mountain rainforests and substantial advection of energy during rainfall is proposed to account for these results. This paper uses a process based model of interception to analyse the interception losses at Oliver Creek, a lowland coastal rainforest site in northern Queensland with a mean annual rainfall of 3952 mm. The observed interception loss of 25% of rainfall for the period August 2001 to January 2004 can be reproduced by the model with a suitable choice of the three key controlling variables, the canopy storage capacity, mean rainfall rate and mean wet canopy evaporation rate. Our analyses suggest that the canopy storage capacity of the Oliver Creek rainforest is between 3.0 and 3.5 mm, higher than reported for most other rainforests. Despite the high canopy capacity at our site, the interception losses can only be accounted for with energy advection during rainfall in the range 40–70% of the incident energy.

Wet canopy evaporation rate of three stands in Western Sichuan, China

The wet canopy evaporation rate (Er) was calculated by Penman-Monteith combination model based on three assumptions and with meteorological variables 2 m above the canopy in three stands, dominated by spruce (SF), fir (FF) and birch (BF) trees, respectively, in the subalpine forests in western Sichuan, China over a growing season. The total amount of theE was 44.5 mm for SF, 88.5 mm for FF and 57.8 mm for BF, accounting for 9.2%, 16.6% and 10.2% of the gross rainfall, respectively, in the measuring period. There was the highest average monthlyEr and percentage ofE to gross rainfall for FF compared with SF and BF. MeanEr was 0.097 mm h−1 (ranging from 0.028 to 0.487 mm h−1), 0.242 mm h−1 (from 0.068 to 0.711 mm h−1) and 0.149 mm h−1 (from 0.060 to 0.576 mm h−1) for SF, FF and BF, respectively. The highest average monthlyEr occurred in June was 0.120 mm h−1 for SF, 0.317 mm h−1 for FF and 0.169 mm h−1 for BF, and the lowest value in October was 0.083 mm h−1 for SF, 0.187 mm h−1 for FF and 0.101 mm h−1 for BF, respectively. The averages ofEr from 8:00 to 16:00 were significantly higher than those from 0:00 to 8:00 and from 16:00 to 0:00 for the three stands. The marked daily and monthly differences ofEr were contributable to the variations of solar radiation, air temperature and relative humidity above the canopy.

Average wet canopy evaporation for a Sitka spruce forest derived using the eddy correlation-energy balance technique

Hourly measurements of evaporation of intercepted rainfall were made for six months in a Sitka spruce forest in Central Wales (UK) with an eddy-correlation system. Sensible heat flux was measured with a three-dimensional sonic anemometer mounted on a mast above the forest, and evaporation was solved as the residual in the energy balance equation. Hourly evaporation was also estimated with the Penman–Monteith equation, using the eddy-correlation measurements to estimate the aerodynamic conductance. Wind direction dependent differences in surface roughness caused the aerodynamic conductance to vary from 0.09 to 0.29 m s −1 at the average wind speed. Average measured (0.12 mm h −1) and estimated (0.09 mm h −1) wet canopy evaporation rates agreed reasonably well with a previously estimated Penman–Monteith value of 0.13 mm h −1 for the same forest [J. Hydrol. 48 (1980) 89].