Wim Klaassen

Water storage and evaporation as constituents of rainfall interception

Abstract Intercepted rainfall may be evaporated during or after the rain event. Intercepted rain is generally determined as the difference between rainfall measurements outside and inside the forest. Such measurements are often used to discriminate between water storage and evaporation during rain as well. Two well-accepted methods underestimate water storage by a factor two as compared to direct observations. The underestimation of storage is compensated by an overestimation of evaporation during rain by a factor of three. The direct observations of water storage and evaporation appear to agree with previous direct observations. Thus, it is concluded that these observations are representative. Also, our results based on methods using only rainfall measurements inside and outside the forest appear to agree with previous results. This would result in the conclusion that the common methods systematically underestimate water storage and overestimate evaporation during rain. Indeed, the systematic errors can be explained by the neglect of drainage before saturation. Water storage is better simulated assuming an exponential saturation of a larger storage capacity. A smaller evaporation can be simulated using an appropriate resistance to vapour transport. The observations in dense coniferous forest showed water storage to be the dominant process in rainfall interception, but this conclusion should not be generalized to other forests and climates. Direct observations of water storage and evaporation are recommended to build a realistic set of parameters for rainfall interception studies of the main vegetation types.

A comparison of models simulating rainfall interception of forests

Rainfall interception models have been validated using observations from a coniferous forest in France and a deciduous forest in the Netherlands. The models differ in level of complexity. This complexity, however, appears to have only a secondary effect on the results. The most sensitive factor in the models is shown to be the evaporation rate of the intercepted water. The evaporation rate is determined from the atmospheric vapour pressure deficit and the aerodynamic resistance using the Penman equation. In the models, the aerodynamic resistance is approximated from momentum transport. This approximation leads to an overestimation of interception. The models can be improved by using a simple and well-known correction for the aerodynamic resistance.

Relaxed eddy accumulation measurements of the sea-to-air transfer of dimethylsulfide over the northeastern Pacific

[1] Gas transfer rates were determined from relaxed eddy accumulation ( REA) measurements of the flux of dimethylsulfide (DMS) over the northeastern Pacific Ocean. This first application of the REA technique for the measurement of DMS fluxes over the open ocean produced estimates of the gas transfer rate that are on average higher than those calculated from commonly used parameterizations. The relationship between the total gas transfer rate and wind speed was found to be gas k(gas) = 0.53 (+/-0.05) U-10(2). Because of the effect of the airside resistance, the waterside transfer rate was up to 16% higher than kgas. Removal of the airside transfer component from the total transfer rate resulted in a relation between wind speed and waterside transfer of k(660) = 0.61 (+/-0.06) U-10(2). However, DMS fluxes showed a high degree of scatter that could not readily be accounted for by wind speed and atmospheric stability. It has to be concluded that these measurements do not permit an accurate parameterization of gas transfer as a function of wind speed.

Rainfall interception near a forest edge

The process of interception is studied by comparing observations of net rainfall near a wind exposed forest edge with simulations of evaporation from a wet canopy. The simulations show a strong enhancement of the evaporation rate from the wet forest canopy near the upwind edge. The increased evaporation rate should result in an increased interception loss and a decreased throughfall near the edge. Observed throughfall appears hardly dependent on fetch from the forest edge, in agreement with previously published results. The seeming discrepancy between model and observations is explained by effects of humidity and wind velocity. The model simulates interaction between the surface and the lower atmosphere. It is argued that this interaction is suppressed for humidity variations, as the atmospheric humidity is influenced by evaporation of falling rain. Even by prescribing a constant atmospheric humidity, the model simulates a higher evaporation rate near the edge as wind velocity is increased near the edge. However, observations show that throughfall is statistically independent of wind velocity during rain, which is explained by a decrease in water storage capacity of the forest. It is concluded that the concept of a constant water storage capacity is questionable. Direct observations of water storage are recommended to quantify possible sensitivities of water storage capacity to environmental factors. For aggregation studies it is concluded that rainfall interception is basically independent of patch size. However, it is argued that the forest edge dries more quickly and transpiration may start sooner after the rain has stopped. This would result in an aggregation problem for transpiration.

Simultaneous use of relaxed eddy accumulation and gradient flux techniques for the measurement of sea-to-air exchange of dimethyl sulphide

The sea-to-air flux of the biogenic volatile sulphur compound dimethyl sulphide was assessed with the relaxed eddy accumulation (REA) andthe grad ient flux (GF) techniques from a stationary platform in the coastal Atlantic Ocean. Fluxes variedbetween 2 and16 mmol m � 2 d � 1 . Fluxes derived from REA were on average 7.175.03mmol m � 2 d � 1 , not significantly different from the average flux of 5.372.3mmol m � 2 d � 1 derived from GF measurements. Gas transfer velocities were calculatedfrom the fluxes andseawater DMS concentrations. They were within the range of gas transfer rates derived from the commonly used parameterizations that relate gas transfer to wind speed. r 2002 Elsevier Science Ltd. All rights reserved.

Evaporation From rain‐wetted forest in relation to canopy wetness, canopy cover, and net radiation

Evaporation from wet canopies is commonly calculated using EPM, the Penman-Monteith equation with zero surface resistance. However, several observations show a lower evaporation from rain-wetted forest. Possible causes for the difference between EPM and experiments are evaluated to provide rules for the simulation of rainfall interception by forest canopies. The evaluation is executed using a micrometeorological model with a detailed representation of the forest canopy. Simulated results are compared with experimental results. In spite of theoretical reservations the evaporation of completely wet forest appears to agree with EPM. Evaporation from wet forest appears mainly dependent on net radiation. Rainfall interception is related to evaporation from the canopy. Evaporation from the canopy appears proportional with the square root of canopy cover and sensitive to canopy wetness. An accurate estimate of canopy wetness is needed to use EPM for the calculation of evaporation from rain-wetted forest.

LANDSCAPE VARIABILITY AND SURFACE FLUX PARAMETERIZATION IN CLIMATE MODELS

The Earths surface shows variability at the landscape scale (1-10 km); the influence of surface variability at this scale has been analysed to provide a parameterization for use in large-scale atmospheric models with a grid size unable to solve the landscape scale explicitly. Landscape variations are found to add drag to the atmosphere, owing to sudden changes in vegetation height. The drag increases momentum flux and indirectly influences the transfer of heat and gases between the landscape and the atmosphere. Consequently, the exchange between a variable landscape and the atmosphere deviates from a simple sum of the exchanges between landscape elements and the contiguous air layer, Strong influences are found for tree lines and forest edges. Most of the existing aggregation schemes for grid-averaged fluxes in large-scale models strongly underestimate the consequences of landscape variability owing to the neglect of drag at surface transitions. The supplementary drag can easily be incorporated in an aggregation scheme of surface fluxes in a large-scale model. New experiments on the landscape scale are recommended to improve the accuracy of the method.

Fluxes and gas transfer rates of the biogenic trace gas DMS derived from atmospheric gradients

[1] Gas transfer rates were determined from vertical profile measurements of atmospheric dimethylsulfide (DMS) gradients over the equatorial Pacific Ocean obtained during the GasEx-2001 cruise. A quadratic relationship between gas transfer velocity and wind speed was derived from the DMS flux measurements; this relationship was in close agreement with a parameterization derived from relaxed eddy accumulation measurements of DMS over the northeastern Pacific Ocean. However, the GasEx-2001 relationship results in gas transfer rates that are a factor 2 higher than gas transfer rates calculated from a parameterization that is based on coincident eddy correlation measurements of CO2 flux. The measurement precision of both the profiling and eddy correlation techniques applied during GasEx-2001 is comparable; the two gas transfer data sets are in agreement within their uncertainty. Differences in the number of samples and the wind speed range over which CO2 and DMS fluxes were measured are likely causes for the observed discrepancy.

Negligible direct radiative forcing of basin-scale climate by coccolithophore blooms

The water-leaving radiance, defined as radiation from the sun reflected off particles in water and exiting the ocean surface back into the atmosphere and space, is often used to derive ocean-colour information from remotely sensed data. However, it is in itself a measure of the amount of solar irradiance reflected by oceanic constituents and, therefore, not available to the Earths heat reservoir (changes in which can affect the Earths energy balance and climate). A strong influence on the water-leaving radiance is observed from coccolithophore blooms, owing to the highly reflective calcareous platelets or coccoliths covering these marine algae. We analysed remotely sensed water-leaving radiances (1998-1999) over the N. Atlantic, where the blooms are spatially and temporally most abundant, and found that the direct radiative forcing of climate between 402-565 nm (the major range of optical influence) by coccolithophores in this ocean is negligible (similar to0.05 W m(-2) mean annually). This is in contrast to what in situ or laboratory measurements on the immense local intensity of coccolithophore visible light scatter in the past two decades have led us to believe.

Influence of obstacles on the aerodynamic roughness of the Netherlands

The aim of this study was to analyse the influence of large- and small-scale obstacles (orography, tree lines, and dikes) on the effective aerodynamic roughness of the Netherlands, a relatively flat, small-scale landscape. The roughness averaging approach was based on drag coefficients. The effective roughness was locally dominated by small-scale obstacles such as tree lines and dikes. Even at a regional scale (40,000 km2), the small-scale obstacle drag was of the same order of magnitude as the shear stress due to landuse. The neglect of those obstacles on a regional scale would result in approximately 10% overestimated averaged windspeed at 10~m above the surface. It was concluded that small-scale obstacles need to be taken into account to calculate the aerodynamic roughness of flat landscapes. Orography was of minor importance in this lowland country.The aim of this study was to analyse the influence of large- and small-scale obstacles (orography, tree lines, and dikes) on the effective aerodynamic roughness of the Netherlands, a relatively flat, small-scale landscape. The roughness averaging approach was based on drag coefficients. The effective roughness was locally dominated by small-scale obstacles such as tree lines and dikes. Even at a regional scale (40,000 km2), the small-scale obstacle drag was of the same order of magnitude as the shear stress due to landuse. The neglect of those obstacles on a regional scale would result in approximately 10% overestimated averaged windspeed at 10~m above the surface. It was concluded that small-scale obstacles need to be taken into account to calculate the aerodynamic roughness of flat landscapes. Orography was of minor importance in this lowland country.