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Ben-Gurion University of the Negev (2005)

The role of dew in the water and heat balance of a bare soil in the Negev desert

Agam, Nurit Ninari

Titre : The role of dew in the water and heat balance of a bare soil in the Negev desert

Auteur : Agam, Nurit Ninari

Université de soutenance : Ben-Gurion University of the Negev,

Grade : Doctor of Philosophy (PhD) 2005

Résumé
The objectives of this research were (1) Examine the correlation between the amounts of dew deposition on artificial surfaces and the amounts deposit on bare soil ; (2) Describe the daily pattern of changes in water content in the upper soil layers and identify the mechanisms by which water is added to the soil (dew formation or direct adsorption) and their frequency ; and (3) Assess the relative magnitude of latent heat flux density throughout the dry season, and evaluate its importance in view of meso- and global-scale meteorological models. The research was carried out at the Wadi Mashash Experimental Farm in the Northern Negev, Israel, over a bare loess soil. Micro-meteorological parameters were measured in the vicinity of which a conventional Hiltner dew balance and a micro-lysimeter with an undisturbed soil sample were placed. During the first period, the depth of the micro-lysimeter was 15 cm while at the second period it was 55 cm. Complementary, nine 24-h field campaigns were carried out during the dry season of 2002, during which the 10cm topsoil was sampled hourly, and the gravimetric water content distribution at 1cm increments determined. The representativity of the micro-lysimeter was assessed by comparing its surface temperature to that of the surrounding surface using thermal images acquired on an hourly basis during several campaigns. A 15 cm deep micro-lysimeter was thoroughly tested and found unsuitable for dew measurements as significant differences between the soil surface temperature of the micro-lysimeter and the surrounding soil were detected. An improved micro-lysimeter, with a depth of 55 cm, was then built and tested again by comparing its surface temperatures to that of the surrounding soil. The differences were significantly smaller than those measured with the previous micro-lysimeter, and their magnitude was such that their effect on vertical fluxes is negligible. During all field campaigns, the soil surface temperature did usually not drop below the estimated dew-point temperature of the air mass and it was, therefore, to be expected that no dew would form on the soil surface. It was concluded that in the area in which this study was carried out, actual dew deposition on a bare soil surface is probably a rare occurrence. Nevertheless, a clear daily cycle of water content of the uppermost soil was detected. It was concluded that the main process responsible for the observed diurnal cycle is direct adsorption of water vapor by the soil. Moreover, a strong and significant correlation between the daily total adsorption of water vapor and the total potential evaporation between sunrise and sunset of the previous day was found. The water content of the uppermost soil layer was found to be significantly and systematically lower than the wilting point. Most of the commonly used meteorological models would, therefore, assume no latent heat flux. However, latent heat flux density was 20% of the net-radiation during the night and 10-15% of the net-radiation during the day. Hence models assuming that during the dry season there is no latent heat flux over deserts may lead to erroneous results. The results of this study bring about new and important knowledge regarding the diurnal changes in soil water content and the energy partitioning over loess soil, under very dry conditions. This knowledge, if applied, can improve the performance of meso- and global-scale meteorological models.

Mots Clés : Atmospheric thermodynamics — Israel — Negev. ; Dew — Israel — Negev. ; Energy budget (Geophysics) — Israel — Negev. ; Solar radiation — Israel — Negev. ; Water balance (Hydrology) — Israel — Negev.

Présentation (DISCCRS)

Page publiée le 19 mai 2012, mise à jour le 20 novembre 2018