Evaporation from a large lowland reservoir – observed dynamics during a warm summer

<p>Distinct differences in surface characteristics between a water body and a land surface result in different drivers of evaporation and therefore its dynamics. It is essential to include and represent this difference in the parameterization of open water evaporation (E<sub>water</sub>) to improve operational hydrological models. Additionally, more accurate parameterization becomes even more crucial to predict potential changes in quantity and dynamics of E<sub>water</sub> in a changing climate in support of optimal water management now and in the future.</p><p>For this purpose, we performed a long-term measurement campaign to measure E<sub>water</sub> and related meteorological variables over a large lowland reservoir in the Netherlands. During the summer seasons of 2019 and 2020 eddy-covariance systems were applied at two locations at the border of lake IJsselmeer in the Netherlands. These high temporal resolution measurements gave us the opportunity to explore the dynamics and identify the underlying driving mechanisms of E<sub>water</sub>. Using the data collected during the summer of 2019 we were able to develop a simple regression model for both measurement locations. Combinations, both sums and products, of the following independent variables were considered: global radiation, wind speed, water skin temperature, vapour pressure deficit, and vertical vapour pressure gradient. The product of wind speed and vertical vapour pressure gradient best explained the observed hourly E<sub>water</sub> rates, which is consistent with the commonly used aerodynamic approach. The model was validated using the data of 2020. Additionally, we compared measured E<sub>water</sub> to E<sub>water</sub> computed with Makkink’s equation, which is currently used in the Dutch operational hydrological models to estimate E<sub>water</sub>. Although a correction factor is applied to account for the difference between land evaporation and E<sub>water</sub>, Makkink is not able to capture the dynamics of E<sub>water</sub>. This was reflected in the timing and shape of the evaporation peak at both daily and monthly scales. The disagreement of E<sub>water</sub> dynamics found between the measured and simulated E<sub>water</sub> even more demonstrates the value and need of a correct parameterization of E<sub>water</sub>.</p>

Drying Comparison of Nonhygroscopic and Hygroscopic Materials

This paper investigates and presents the simulation of drying for hygroscopic and nonhygroscopic materials. This present work used a coupled mathematical model of mass, heat and gas transfer that implemented to finite element method in two dimensional and numerically compute using Skyline solver to capture highly nonlinear transient process. Bound water contribution was taken into account in the drying of hygroscopic materials by incorporating constitutive equation of bound water. The results showed drying process can be divided into three periods named constant rate period (CRP), first falling rate period (FRP1) and second falling rate period (FRP2). Capillary action is dominated during CRP before vapour diffusion takes place in FRP1. Bound water movement is generated by vapour pressure gradient exists that represent hygroscopic material.

OBSERVATIONS ON THE PERMEABILITY OF HYGROSCOPIC MATERIALS TO WATER VAPOUR: I. OBSERVATIONS AT RELATIVE HUMIDITIES LESS THAN 75%

It is pointed out that experimental work has shown that for the transpiration of moisture through hygroscopic materials two distinct regions of relative humidity can be distinguished: (i) A region below some value between 70 and 80% where the moisture movement is proportional to the vapour pressure difference, and (ii) a region of high relative humidity where the moisture movement is not directly proportional to the vapour pressure. A theory is advanced that these two regions correspond to the two conditions in which water may be present in a hygroscopic material: (i) the water may be molecularly adsorbed, that is to say, the water is bound by the affinity of the molecules of water for those of the solid on which it is adsorbed; (ii) the water may be held in small fissures in the sub-microscopic structure of the sorbing material by capillary forces. The mechanism by which the moisture will move through the material would be different in the two cases and would result in the distinction between regions of high and of low relative humidity.A short discussion is given of Fick's law showing the form in which it might be expected to apply in the two cases. Measurements are given for fibreboards showing that below 75% relative humidity the resistance of the board to moisture transpiration is proportional to the thickness. The moisture content gradients through fibreboard samples have been determined. The diffusance through a board in which the moisture content gradient is opposed to the vapour pressure gradient shows that the latter is the important factor, and the determination of the moisture content gradient indicates that at these low humidities Fick's law is applicable.

THE DIFFUSION OF WATER VAPOUR THROUGH VARIOUS BUILDING MATERIALS

A method of measuring the diffusion coefficient of water vapour through solids is outlined, and a table of the coefficients for various materials used in building construction is given. The method of employing these coefficients to calculate the vapour pressure gradient through a typical wall is shown, and this is applied to estimate the resistance to water vapour necessary to prevent condensation.