Behaviour of carbon dioxide and water vapour flux densities from a disturbed raised peat bog

Measurements of carbon dioxide and water vapour flux densities were carried out for a disturbed raised peat bog in the north of the Netherlands during an 18 month continuous experiment. Tussock grass (sp. Molinea caerulae) mainly dominated the vegetation of the bog area. The maximum leaf area index (LAI) of the vegetation reached a numerical value of about 1.7 in mid-August. When the LAI is large enough (LAI > 0.2), a mean net uptake of carbon dioxide is observed with a clear daily pattern. The total evapotranspiration consists of a soil, an open water, and a plant transpiration part. When the LAI is large enough (LAI > 0.2), plant transpiration dominates the total evapotranspiration. The mean daily transpiration pattern, however, is not similar to the carbon dioxide uptake pattern. During the summer months, the daytime carbon dioxide uptake shows a single early morning maximum value followed by a decline in uptake during the rest of the day. The evapotranspiration, however, follows more or less the incoming short-wave radiation pattern. Effects of the vapour pressure deficit are suggested as a possible cause of this discrepanc

A simplified model to predict diurnal water temperature dynamics in a shallow tropical water pool

Water temperature is a critical regulator in the growth and development of malaria mosquito immatures, as they are poikilothermic. Measuring or estimating the diurnal temperature ranges to which these immatures are exposed is of the utmost importance, as these immatures will develop into adults that can transmit malaria. Recent attempts to predict the daily water temperature dynamics in mosquito breeding sites in Kenya have been successful. However, the developed model may be too complex, as the sophisticated equipment that was used for detailed meteorological observations is not widely distributed in Africa, making it difficult to predict the daily water temperature dynamics on a local scale. Therefore, we compared two energy budget models with earlier made observations of the daily water temperature dynamics in a small, shallow and clear water pool (diameter 0.96 m, depth 0.32 m) in Kenya. This paper describes (1) a complex 1-Dimensional model, and (2) a simplified second model, and (3) shows that both models mimic the water temperature dynamics in the water pool accurately. The latter model has the advantage that it only needs common weather data (air temperature, air humidity, wind speed and cloud cover) to estimate the diurnal temperature dynamics in breeding sites of African malari

The effect of water turbidity on the near-surface water temperature of larval habitats of the malaria mosquito Anopheles gambiae

Water temperature is an important determinant in many aquatic biological processes, including the growth and development of malaria mosquito (Anopheles arabiensis and A. gambiae) immatures. Water turbidity affects water temperature, as suspended particles in a water column absorb and scatter sunlight and hence determine the extinction of solar radiation. To get a better understanding of the relationship between water turbidity and water temperature, a series of semi-natural larval habitats (diameter 0.32 m, water depth 0.16 m) with increasing water turbidity was created. Here we show that at midday (1300 hours) the upper water layer (thickness of 10 mm) of the water pool with the highest turbidity was on average 2.8 degrees C warmer than the same layer of the clearest water pool. Suspended soil particles increase the water temperature and furthermore change the temperature dynamics of small water collections during daytime, exposing malaria mosquito larvae, which live in the top water layer, longer to higher temperatures

Sensitivity analysis of leaf wetness duration within a potato canopy

A description and analysis is given of a wetness duration experiment, carried out in a potato field in the centre of the Netherlands in September 2005. The observations are used to design and evaluate a within-canopy dew model which provides the leaf wetness distribution within the canopy caused by dew processes and by precipitation. This withincanopy dew model consists of three layers (bottom, centre, top) each with equal contribution to the leaf area index. The model results compared favourably with experimental evidence. The sensitivity of the dew and precipitation interception on the amount of free water and the duration of the leaf wetness was analysed by varying the leaf area index and some important weather variables. The findings suggest that the leaf area index affects the amount of free water, but is barely sensitive to leaf wetness duration. Wind speed has hardly any effect on the amount of free water collection as well as on leaf wetness duration. The net radiation, however, appears to be sensitive to the amount of collected free water as well as the leaf wetness duratio

Flow around a line obstacle

Shear layer flows which are strongly disturbed, often occur in nature as well as in engineering practice. Up to now little is known about this class of flows. This is Partly explained by the complexity of these flows and partly by a lack of experimental data. The objective of this study is: first, to carry out a full-scale experiment around a two-dimensional barrier which is attached to the earth surface; second, to compare the obtained data with existing models which describe such strongly disturbed shear layer flows.In Chapter 1, the goal of this study is described in more detail. A review is given of the most important outdoor experiments that were carried out in the past. Besides, in Chapter 1 a qualitative description of these flows is given.In Chapter 2, more details are given about the measurement program and the terrain conditions. To ensure that the measured flow disturbances are caused by the erected barrier and are not caused by other terrain disturbances, special attention must be paid to the terrain conditions of the measuring site. The surface condition must be homogeneous and free of other obstacles to an upstream distance of about 100 times the highest measuring level from the most windward location. Such a terrain was found along an aircraft runway, located in the south east of the Netherlands.In Chapter 3 the data will be discussed. The data of the undisturbed reference location were analyzed and compared with those in the literature. Furthermore , with these data the homogeneity of the windward terrain was analyzed. From these results it appeared that the windward terrain had an overall roughness length of z o = 35 mm.The disturbed mean wind field was analyzed in: first, thermally neutral condition and perpendicular flow direction; second, non- neutral conditions and perpendicular flow direction; third, neutral condition and oblique flow directions. From the data obtained in oblique flow directions it appeared that the width of the barrier has an important influence on the sheltering effect near the surface. The disturbed r.m.s. values of the speed fluctuations are analyzed in order to gain more insight into the turbulence of the distorted flow field. From these results it appeared that the turbulence near the surface is strongly dependent on thermal stratification.In Chapter 4, special attention is given to the aerodynamic characterization of the barrier. A physically attractive way to characterize an obstacle is to express the effect of an obstacle in terms of its resistance to the fluid flow, or, in dimensionless form, in terms of a drag coefficient. This coefficient was estimated in two different ways. First, by direct force measurements. More or less in the center of the barrier a section was removed and replaced by a measuring plate. The drag on this plate, exerted by the fluid flow was measured with force sensors. Second, by application of momentum conservation to a control volume around the barrier. To use this indirect method, just around the barrier the velocity profiles and the static pressure profiles were measured. Moreover, in Chapter 4 we give an analysis of the influence of thermal stratification and flow direction upon the drag coefficient.In Chapter 5, the data in near-neutral conditions and perpendicular flow direction are compared with existing models. One of the earliest attempts to describe the disturbed shear flow behind a barrier, is the diffusive model of Kaiser (1959). Because of the weak physical foundation of Kaiser's model and the bad agreement with experimental evidence, we decided not to analyze this model any further in our study. A physically more realistic model describing the disturbed flow behind a barrier is the self-preserving model of Townsend (1965). The results of this model for the near wake region, i.e. the region between the barrier and the leeward reattachment point, do not coincide at all with our data. Beyond the reattachement point, Townsend's model overestimates the data considerably but with increasing distance from the barrier the differences become gradually smaller. The most sophisticated model up to now which describes the disturbed flow behind a barrier in the far wake region, is that by Counihan, Hunt and Jackson (1974). The far wake region is the region beyond the reattachment point. This model (referred to as the C.H.J. model) consists of three layers: first, the surface layer in which the flow is in local equilibrium;second, the mixing layer in which the flow is self-preserving; third, the external layer. We checked the C.H.J. model against our data and found an excellent agreement.In Chapter 6, the final conclusions are presented and suggestions are made for future research.<p/

Carbon dioxide and water vapour flux densities over a grassland area in the Netherlands

Measurements of carbon dioxide and water vapour flux densities were carried out for a grassland area in the centre of the Netherlands during a 12 month continuous experiment using the eddy-covariance technique. Lolium perenne and Poa trivialis mainly dominated the vegetation of the area. The leaf area index (LAI) of the grassland varied between 2.5 and 3.4, depending on the mowing scheme at the site. Fluxes are also calculated by using a canopy model in which the canopy resistance is related to the assimilation rate (A-g, model). The calculated evapotranspiration fluxes agree within 7% of the measured fluxes (R-2 = 0.93; SEE = 23 W m(-2)). The calculated net carbon dioxide fluxes agree within 12% of the measured fluxes (R-2 = 0.79;SEE = 0.16 mg m(-2)s(-1)). The net carbon dioxide fluxes consist of two opposite components: the uptake via assimilation and release via soil and plant respiration. It appears that nearly the whole year round the assimilation term dominates, which means an accumulation of carbon dioxide if no other exchange processes are present. For 2002 a net carbon uptake was found of 6.8 tonnes ha(-1)

Leaf Wetness within a Lily Canopy

A wetness duration experiment was carried out within a lily field situated adjacent to coastal dunes in the Netherlands. A within-canopy model was applied to simulate leaf wetness in three layers, with equal leaf area indices, within the canopy. This simulation model is an extension of an existing model. It appeared that in most cases leaf wetness started in the uppermost layer followed by the middle and bottom layer, respectively. The same occurred during the early morning drying process. Just after sunrise the upper layer started to dry, followed by the middle and bottom layer, respectively. The longest leaf wetness duration occurred in the bottom layer. The calculated leaf wetness durations were within 10 minutes of the results obtained using a leaf wetness senso