Canopy Rainfall Interception Research Articles

Old-Growth Forest Canopy Structure and Its Relationship to Throughfall Interception

Abstract Structural elements of the forest canopy such as foliage, branches, and epiphytes intercept, retain, and regulate inputs of matter and energy to the forest floor in spatially variable ways. Most studies of forest structure-function have analyzed relationships between canopy structure and the spatial distribution of inputs at the forest stand level. Few studies have focused on within-canopy spatial scales or identified the functional roles of particular structural components. Our objective was to quantitatively assess the amount of canopy structural material to a critical functional attribute: canopy interception of rainfall. We investigated forest canopy structure with a novel technique, “vertical canopy cylinder transects,” that uses access from a construction crane in an old-growth coniferous forest in Washington State. We characterized the diversity and composition of forest structural elements (tree foliage and branches by species, epiphytes by functional group) and found the majority of the foliage and epiphytes concentrated in the upper- to mid-canopy, with some vertical stratification of the epiphyte functional groups. We also quantified one aspect of forest function, the volume and variability of throughfall interception. We documented a strong relationship between the amount of structural elements and rainfall interception (R2 = 0.79). This approach of quantifying three-dimensional canopy structure at particular locations had higher predictive ability for throughfall volume than previous approaches that were not able to gain direct access to canopy components. FOR. SCI. 50(3):290–298.

MVP: a model to simulate the spatial patterns of photosynthetically active radiation under discrete forest canopies

The spatial patterns of photosynthetically active radiation (PAR) under forest canopies, including both its mean and spatial variation, are critical factors to numerous understory ecophysiological processes. Currently, Beer's law is the primary algorithm used in ecological models simulating PAR transmission through plant canopies, because more accurate models are too complicated to be used operationally. This study developed a simple and computationally efficient model at a stand scale to simulate both the mean and variation of PAR (MVP) under forest canopies. The model assumes that a forest canopy is composed of individual crowns distributed within upper and lower boundaries with two types of gaps: the between- and within-crown gaps. The between-crown gaps are simulated with geometric optics, and the within-crown gaps are described by Beer's law. The model accounts for the covariance of PAR in space through time, making it possible to simulate both instantaneous and daily accumulated variance of PAR. Validation with observed PAR from the boreal ecosystem–atmosphere study (BOREAS) indicates that the model captures the mean and variance of PAR under forest canopy reasonably well. MVP holds the potential to improve simulation of light interception by forest canopies as well as the treatment of canopy rainfall interception in ecological models.

Diagnosing a distributed hydrologic model for two high‐elevation forested catchments based on detailed stand‐ and basin‐scale data

This study evaluates the performance and internal structure of the distributed hydrology soil vegetation model (DHSVM) using 1998–2001 data collected at Upper Penticton Creek, British Columbia, Canada. It is shown that clear‐cut snowmelt rates calculated using data‐derived snow albedo curves are in agreement with observed lysimeter outflow. Measurements in a forest stand with 50% air crown closure suggest that the fraction of shortwave radiation transmitted through the canopy is 0.18–0.28 while the hemispherical canopy view factor controlling longwave radiation fluxes to the forest snowpack is estimated at 0.81 ± 0.07. DHSVM overestimates shortwave transmittance (0.50) and underestimates the view factor (0.50). An alternative forest radiation balance is formulated that is consistent with the measurements. This new formulation improves model efficiency in simulating streamflow from 0.84 to 0.91 due to greater early season melt that results from the enhanced importance of longwave radiation below the canopy. The model captures differences in canopy rainfall interception between small and large storms, tree transpiration measured over a 6‐day summer period, and differences in soil moisture between a dry and a wet summer. While the model was calibrated to 1999 snow water equivalent (SWE) and hydrograph data for the untreated control basin, it successfully simulates forest and clear‐cut SWE and streamflow for the 3 other years and 4 years of preharvesting and postharvesting streamflow for the second basin. Comparison of model states with the large array of observations suggests that the modified model provides a reliable tool for assessing forest management impacts in the region.

Landfill Capping with Woodland Ecosystems

As the field of phytoremediation has evolved over the last 10 years, there has been a growing interest in the use of vegetative covers as alternative landfill caps. Consequently, a number of vegetative caps (grass, shrubs, and trees) have been installed at landfills across the United States, with tree caps being the focus of this paper. The two primary objectives of vegetative caps are to minimize water percolation into landfill waste via a “sponge and pump” mechanism and to prevent surface soil erosion. The soil pores hold precipitation like a sponge until plant roots can access the water; plants take up this water for growth and release it into the atmosphere by transpiration. The erosion control objective is achieved by canopy interception of rainfall, minimizing the “splash effect,” and the growing of a root matrix that interlaces through soil particles. Secondary benefits of vegetative caps include aesthetic appeal for the neighboring community, sequestration of greenhouse gases, reduced construction...

Comparative water use of wattle thickets and indigenous plant communities at riparian sites in the Western Cape and KwaZulu-Natal

Large-scale funding by both the Government and the private sector continues in support of the Working-for-Water Programme, which is active in many regions of the country. One justification for this programme of alien tree removal is the streamflow enhancement that is believed to follow the replacement of dense stands of invasive trees by indigenous, largely herbaceous or shrub dominated plant communities. Often the densest stands of invader trees occur within riparian zones, where removal of trees in close proximity to stream channels is believed to strongly enhance streamflow. Few data are available, however, to support this assumption. Results from a number of research catchments have consistently shown that afforestation significantly decreases streamflow where the pre-afforestation vegetation was seasonally dormant mountain grassland or fynbos (Versfeld, 1994). The net difference in evapotranspiration (ET) between riparian thickets of alien trees and riparian fynbos may be quite different, due to the yearlong availability of soil water and enhanced plant growth in riparian zones. The water use of alien invasive trees in South Africa remains largely unknown, adding further uncertainty to the effect of alien removal on streamflow. This paper describes the results of a comparative study of annual ET between indigenous riparian plant communities and riparian wattle thickets (Acacia mearnsii() at four sites in the Western Cape and KwaZulu-Natal. The Bowen ratio energy balance (BREB) technique was used to record a 12-month record of 20 min evaporation rates from a fynbos riparian plant community in the Jonkershoek valley (Western Cape), and a grassland riparian community on the property Gilboa in the KwaZulu-Natal midlands. Closed-canopy, mature stands of self-established (A. mearnsii( in the Wellington and Groot Drakenstein areas of the Western Cape were selected to provide comparative transpiration data. The heat pulse velocity (HPV) technique was used to record hourly sap-flow rates in six sample trees representing the range of tree sizes at both wattle sites. Total daily sap flow in all sample trees experiencing adequate soil water availability was found to be very closely correlated to tree size and an index defined as the product of mean daily vapour pressure deficit (VPD) of the air and the number of daylight hours. These relationships were used to predict the water use of wattle thickets at Jonkershoek and Gilboa, using VPD and day-length data recorded at these sites. Published estimates of canopy rainfall interception were added to the sap flow (transpiration) component to yield a combined annual ET to compare to the BREB ET data. Table 1 summarises the annual evapotranspiration at each site. (table***) We conclude that the removal of riparian wattle and its replacement by indigenous herbaceous plants may indeed result in significant reductions in annual ET, and could very likely lead to streamflow enhancement. However, this study has clearly shown that annual ET varies considerably in different riparian plant communities, and that one must consider the structural and physiological characteristics of both the pre-clearing and post-clearing vegetation in order to predict the net change in ET. This conclusion supports an earlier view (Versfeld et al., 1998) that an improved methodology of general applicability is required to enhance the accuracy of water use predictions for a wide range of alien and indigenous plant communities. Such predictions are important to prioritise clearing operations in areas invaded by alien trees. WaterSA Vol.27(4) 2001: 529-538

A new approach to modeling tree rainfall interception

A three‐dimensional physically based stochastic model was developed to describe canopy rainfall interception processes at desired spatial and temporal resolutions. Such model development is important to understand these processes because forest canopy interception may exceed 59% of annual precipitation in old growth trees. The model describes the interception process from a single leaf, to a branch segment, and then up to the individual tree level. It takes into account rainfall, meteorology, and canopy architecture factors as explicit variables. Leaf and stem surface roughness, architecture, and geometric shape control both leaf drip and stemflow. Model predictions were evaluated using actual interception data collected for two mature open grown trees, a 9‐year‐old broadleaf deciduous pear tree (Pyrus calleryana “Bradford” or Callery pear) and an 8‐year‐old broadleaf evergreen oak tree (Quercus suber or cork oak). When simulating 18 rainfall events for the oak tree and 16 rainfall events for the pear tree, the model over estimated interception loss by 4.5% and 3.0%, respectively, while stemflow was under estimated by 0.8% and 3.3%, and throughfall was under estimated by 3.7% for the oak tree and over estimated by 0.3% for the pear tree. A model sensitivity analysis indicates that canopy surface storage capacity had the greatest influence on interception, and interception losses were sensitive to leaf and stem surface area indices. Among rainfall factors, interception losses relative to gross precipitation were most sensitive to rainfall amount. Rainfall incident angle had a significant effect on total precipitation intercepting the projected surface area. Stemflow was sensitive to stem segment and leaf zenith angle distributions. Enhanced understanding of interception loss dynamics should lead to improved urban forest ecosystem management.

Latent and Sensible Energy Flux Over Deforested Land Surfaces in the Eastern Amazon and Northern Thailand

Land cover change may impact watershed hydrology and regional climate by altering land‐atmosphere exchanges of energy and water. Conversion of forest to pasture has previously been shown to decrease the rate of evaporation (including transpiration) because of reduced aerodynamic roughness, less canopy rainfall interception and greater canopy resistance during dry periods. However, less is known about the effects of forest replacement land covers other than pasture. In this study, field measurements of meteorological processes, stomatal resistance, and Leaf Area Index (LAI) were taken over various deforested land surfaces in the eastern Amazon Basin and in northern Thailand. Based on these measurements, evaporation and sensible heat flux were estimated over each site. Results show the degree to which different land covers replacing tropical forest affect energy partitioning. In comparison with forest, non‐irrigated, actively and recently cultivated sites had sharply lower rates of evaporation. Secondary vegetation generally had higher rates of evaporation than actively or recently used sites. The proportion of energy used for evaporation increased rapidly with age in secondary vegetation. Evaporation rates at sites eight and 25 years after abandonment in the Thailand study area were enhanced by positive sensible energy advection from nearby cultivated areas, and were similar to those of primary forest. Differences in dry season evaporation rates of 3‐, 8‐ and 25‐year secondary vegetation are explained, in part, by the differences in canopy resistance. Minimum canopy resistance was 223, 61 and 58 s m −1 , respectively at the 3‐, 8‐ and 25‐year sites.

Stem Flow, Throughfall, and Canopy Interception of Rainfall by Citrus Tree Canopies

It is generally believed that the interception of rain by the citrus tree canopy can substantially decrease the throughfall under the canopy as compared to that along the dripline or outside the canopy (incident rainfall). Therefore, the position of placement of soil-applied agrichemicals in relation to the tree canopy may be an important consideration to minimize their leaching during rain events. In this study, the distributions of rainfall under the tree canopies of three citrus cultivars, `Marsh' grapefruit (Citrus paradisi Macf.), `Hamlin' orange (Citrus sinensis L. Osbeck), and `Temple' orange (Citrus hybrid), were evaluated at four directions (north, south, east, west), two positions (dripline and under the canopy), and stem flow. There was not a significant canopy effect on rainfall amounts from stem flow or dripline, compared with outside canopy, for any citrus cultivar or storm event. However, throughfall varied significantly among the four cardinal directions under the canopy of all three citrus cultivars and was highly related to the wind direction. Among the three citrus cultivars evaluated in this study, throughfall, stem flow, and canopy interception accounted for 89.5% to 92.7%, 0.5% to 4.7%, and 5.8% to 9.3% of the incident rainfall, respectively.

THE WEPPWATERSHED MODEL: I. HYDROLOGY AND EROSION

The Water Erosion Prediction Project (WEPP) watershed scale model is a continuous simulation tool thatextends the capability of the WEPP hillslope model to provide erosion prediction technology for small cropland andrangeland watersheds. The model is based on fundamentals of erosion theory, soil and plant science, channel flowhydraulics, and rainfall-runoff relationships, and contains hillslopes, channels, and impoundments as the primarycomponents. The hillslope and channel components can be further divided into hydrology and erosion components.Channel infiltration is calculated by a Green-Ampt Mein-Larson infiltration equation. A continuous channel waterbalance is maintained, including calculation of evapotranspiration, soil water percolation, canopy rainfall interception,and surface depressional storage. The channel peak runoff rate is calculated using either a modified Rational equation orthe equation used in the CREAMS model. Flow depth and hydraulic shear stress along the channel are computed byregression equations based on a numerical solution of the steady state spatially varied flow equations. Detachment,transport, and deposition within constructed channels or concentrated flow gullies are calculated by a steady statesolution to the sediment continuity equation. The impoundment component routes runoff and sediment through severaltypes of impoundment structures, including farm ponds, culverts, filter fences, and check dams. The purpose of this articleis to provide an overview of the model conceptual framework and structure. In addition, detailed mathematicalrepresentations of the processes simulated by the channel hydrology and erosion components are presented. The processessimulated by the impoundment component are not described in this article, but it does include impoundment effects onwatershed model channel peak discharge and time of concentration calculations.

Three-dimensional modeling of canopy tree interception of wind-driven rainfall

Field measurement of interception loss from rainforest vegetation in the cyclone-prone tropics is complicated by high wind speeds that cause incident rainfall to be inclined from vertical fall paths. Given the characteristic roughness of tropical rainforest canopies, we hypothesized that the more prominent canopy tree crowns create lateral rainshadows and intercept greater volumes of rainwater per unit projected crown area than less prominent neighboring canopy trees under inclined rainfall conditions. This hypothesis was tested by: (1) modeling the three-dimensional (3-D) geometry of a tropical rainforest canopy surface in northeast Queensland, Australia, using photogrammetrically derived crown elevation data; (2) computing the inclination angles and azimuths of wind-driven rainfall for raindays on which net rainfall was measured from selected canopy trees; (3) creating and applying a ray-tracing program to the 3-D canopy model to quantify lateral rainshadows and the effective rainfall-intercepting crown areas of the selected canopy trees; (4) calculating the C e C index (effective rainfall-intercepting crown area/projected crown area ratio) of each tree; (5) analyzing the relationship between the C e C index values and the measured net rainfall totals after correcting for differences in the trees' interception storage capacities. A significant correlation ( P < 0.05) was found for more than 80% of the raindays examined with rainfall inclination angles of more than 19° from a vertical fall path. These results suggest that the variation in net rainfall totals among neighboring canopy trees may be explained in part by the differential interception of inclined rainfall. Interception loss is generally measured as the difference between gross rainfall measured in the open and net rainfall reaching the forest floor. We conclude that more meaningful measures of interception loss could be obtained by accounting for differences in gross rainfall intercepted by canopy trees sampled for net rainfall.

Canopy Rainfall Interception and Throughfall in Burned and Unburned Tallgrass Prairie

Canopy Rainfall Interception and Throughfall in Burned and Unburned Tallgrass Prairie