Burrows are animal-built structures that can buffer their occupants against the vagaries of the weather and provide protection from predators. We investigated whether the trapdoors of wolf spider (Lycosa sp.; temporary working name "L. hyraculus") burrows in the Negev Desert serve to maintain favorable environmental conditions within the burrow by removing trapdoors and monitoring the ensuing temperature and relative humidity regime within them. We also monitored the behavioral responses of “L. hyraculus” to trapdoor removal at different times of the day and in different seasons. “L. hyraculus” often spun silk mesh in their burrow entrances in response to trapdoor removal during the day, possibly to deter diurnal predators. The frequency of web-spinning peaked on summer mornings, but spiders began spinning webs sooner after trapdoor removal later in the day. In addition, we monitored temperature and relative humidity in artificial burrows in the summer during the morning and at midday. At noon, air temperature (T_a) at the bottom of open burrows increased by <1 °C more than in covered burrows, but water vapor pressure in burrows did not change. The relatively small increase in T_a in uncovered burrows at midday can probably be ascribed to the penetration of direct solar radiation. Thus, air temperature and humidity at the bottom of the burrow are apparently decoupled from airflow at the surface.

In arid and semiarid environments non-rainfall water inputs (NRWI) are an important source of water. In Israel's Negev desert direct absorption of atmospheric water vapor is the dominant NRWI and is strongly affected by soil properties, in particular clay content. The presence of a surface crust layer, whose physical and physico-chemical properties are substantially different from those of the underlying undisturbed substrate will likely affect the absorption patterns.  The objective of our study was to quantify the effect of soil type (loess vs. sand) and crust cover (crust vs. crust removed) on direct atmospheric water absorption.The loess soil samples were obtained in an open field adjacent to the Jacob Bluestein Institutes for Desert Research (BIDR), Ben-Gurion University of the Negev (30˚51’ N, 034˚46’ E, 470 m a.s.l); and the sand samples from the Nizzana Sand Dune area (30˚58’N, 034˚24’E, 226 m a.s.l.).  The loess crusts were physically induced while those present on the sand samples were of biological origin.A field experiment was carried out in the open field adjacent to the BIDR.  Four undisturbed 0.5 m depth soil samples (sand and loess with crust and with crust removed) were placed in micro-lysimeters and automatically weighed at 30 min. intervals.  This field experiment was carried during the dry season of May to October 2016.The field study was supplemented with a laboratory experiment in which undisturbed samples (1,3, 7 and 10 cm) obtained from the above mentioned sites were used. Oven-dry samples were exposed during 6 days to constant temperature and relative humidity conditions (25±1 oC and  85±5 %, respectively)  in sealed chambers.  Mass changes were recorded at varying time intervals.   The adsorption process in the field started in the late afternoon with the arrival of the sea breeze and ended with sun rise. On a daily basis the crusted loess sample adsorbed more water than the crusted sand sample, and the crust removed loess soil absorbed more water than the crust removed sand.  The crusted samples generally absorbed less water than the corresponding non-crusted ones.The results of the laboratory tests showed that loess samples with crust and with crust removed absorbed similar water amounts for all sample depths throughout the study period. The crusted sand samples however absorbed systematically more water than the crust removed samples for all sample depths.We conclude that the higher resistance of crusts to gaseous flux, a result of their higher bulk density and smaller pores, does not limit water vapor flux into the deeper soil layers and does not explain the field results.  

In arid and semiarid environments non-rainfall water inputs (NRWI) are an important source of water. In Israel's Negev desert direct absorption of atmospheric water vapor is the dominant NRWI and is strongly affected by soil properties, in particular clay content. The presence of a surface crust layer, whose physical and physico-chemical properties are substantially different from those of the underlying undisturbed substrate will likely affect the absorption patterns.  The objective of our study was to quantify the effect of soil type (loess vs. sand) and crust cover (crust vs. crust removed) on direct atmospheric water absorption. The loess soil samples were obtained in an open field adjacent to the Jacob Bluestein Institutes for Desert Research (BIDR), Ben-Gurion University of the Negev (30˚51’ N, 034˚46’ E, 470 m a.s.l); and the sand samples from the Nizzana Sand Dune area (30˚58’N, 034˚24’E, 226 m a.s.l.).  The loess crusts were physically induced while those present on the sand samples were of biological origin. A field experiment was carried out in the open field adjacent to the BIDR.  Four undisturbed 0.5 m depth soil samples (sand and loess with crust and with crust removed) were placed in micro-lysimeters and automatically weighed at 30 min. intervals.  This field experiment was carried during the dry season of May to October 2016. The field study was supplemented with a laboratory experiment in which undisturbed samples (1,3, 7 and 10 cm) obtained from the above mentioned sites were used. Oven-dry samples were exposed during 6 days to constant temperature and relative humidity conditions (25±1 oC and  85±5 %, respectively)  in sealed chambers.  Mass changes were recorded at varying time intervals.    The adsorption process in the field started in the late afternoon with the arrival of the sea breeze and ended with sun rise. On a daily basis the crusted loess sample adsorbed more water than the crusted sand sample, and the crust removed loess soil absorbed more water than the crust removed sand.  The crusted samples generally absorbed less water than the corresponding non-crusted ones. The results of the laboratory tests showed that loess samples with crust and with crust removed absorbed similar water amounts for all sample depths throughout the study period. The crusted sand samples however absorbed systematically more water than the crust removed samples for all sample depths. We conclude that the higher resistance of crusts to gaseous flux, a result of their higher bulk density and smaller pores, does not limit water vapor flux into the deeper soil layers and does not explain the field results.  

Direct atmospheric water vapor absorption by structureless soils in coastal deserts has been the subject of various field studies, usually carried out with micro-lysimeters (ML’s). In one of these studies the absorption patterns of loess and sand were studied. Given the larger surface area of the loess soil, it was hypothesized that the loess soil would absorb more water vapor than the sand. The results, when the natural crust present on both soils was removed, were surprisingly similar and contrary to expectation.We hypothesize that one of the reasons for these results was the different pore size distribution of both substrates, the larger pores of the sand allowing deeper penetration of downwelling eddies, thus directly exposing a thicker soil layer to the atmospheric water vapor concentration and hence to enhanced absorption or desorption.To test this hypothesis, it is necessary to obtain data on the soil water distribution dynamics within the soil profiles of MLs whose soils have different pore size distribution.In the present study we report the response of two aggregate sizes of two substrates (aggregated soil and quartz) to the daily fluctuations of atmospheric conditions.The field study was carried out at the Wadi Mashash Experimental Farm in the Negev Desert, Israel, using four MLs. The MLs were instrumented with six temperature and relative humidity (RH) sensors (MX2302A, HOBO) inserted at depths of 0.5, 2, 5, 10, 20 and 45 cm. Water retention curves were obtained using a vapor sorption analyzer (Aqualab, Addium) for the driest part of the curve and standard pressure plate for the wetter parts of the curve. Data from soil and meteorological sensors and scales were recorded every 15 minutes and collected for six successive days during late summer.The ML with large soil aggregates absorbed significantly more atmospheric water than the one with smaller aggregates, while the opposite trend was observed for the quartz particles. The absorption of both quartz MLs was, however, significantly lower than that of the small aggregate ML.The temporal changes in soil water content distribution with depth were estimated by using temperature and RH to compute the thermodynamic soil water potential and transforming the latter into water contents via the water retention curves obtained for each soil and size fraction. The computed total water absorption and release patterns of the soil profiles within each of the MLs corresponded very well with the total recorded mass changes.The depth of eddy penetration was indirectly estimated by comparing the fluctuations of water vapor concentration within the soil at various depths to the one measured simultaneously five cm. above the soil surface. Penetration depth was larger for the large quartz particles when compared to the small ones, but this effect was not so clear for the soil aggregates.These results highlight the importance of inter- and intra- pore size distribution in determining water vapor absorption and desorption patterns in bare soils.

Water vapor adsorption (WVA), a non-rainfall water input, is a poorly documented phenomenon despite its role in regulating water and energy fluxes in soils of coastal deserts. Water vapor movement towards the soil surface and its absorption by the soil occurs whenever the atmospheric water potential is higher than that of the air-filled soil pores. The latter is influenced by soil characteristics, in particular the soil surface area and pore connectivity. Thus, it is expected that under similar atmospheric conditions, absorption of water vapor will be determined by soil characteristics. We carried out a detailed field trial in which we compared two loamy soils with different salt content.Water vapor absorption was measured using micro-lysimeters (MLs) instrumented with relative humidity (RH) and temperature sensors at depths 0.5cm, 2cm, 5cm, 10cm, and 45cm in both MLs during the 2022 and 2023 summers. Total absorption was determined as the increase in mass from a minimum (obtained during late afternoon) to a peak observed on the next day before sunrise. Concurrent changes in soil water potential at each depth were computed by applying the Kelvin equation.Relative humidity in both soils was low during the entire season with the average computed water potential values being lower in the high salt content soil. The total daily water vapor absorption was lower in the low salt content soil, and the rate of absorption was different . The temperature and RH distribution patterns with depth also differed consistently throughout the measuring season for both soils. The effect of salt on water vapor absorption will be highlighted.