Browsing by Subject "drylands"
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Item Convergent vegetation fog and dew water use in the Namib Desert(Wiley, 2019) Wang, Lixin; Kaseke, Kudzai Farai; Ravi, Sujith; Jiao, Wenzhe; Mushi, Roland; Shuuya, Titus; Maggs-Kölling, Gillian; Earth Sciences, School of ScienceNonrainfall water inputs (e.g., fog and dew) are the least studied hydrological components in ecohydrology. The importance of nonrainfall waters on vegetation water status in arid ecosystems is receiving increasing attention. However, a clear understanding on how common plant water status benefits from nonrainfall waters, the impacts of different types of fog and dew events on vegetation water status, and the vegetation uptake mechanisms of nonrainfall waters is still lacking. In this study, we used concurrent leaf and soil water potential measurements from 3 years to investigate the species‐specific capacity to utilize moisture from fog and dew within the Namib Desert. Eight common plant species in the Namib Desert were selected. Our results showed that both fog and dew significantly increased soil water potential. Seven of the eight plant species studied responded to fog and dew events, although the magnitude of the response differed. Plants generally showed stronger responses to fog than to dew. Fog timing seemed to be an important factor determining vegetation response; for example, night fog did not affect plant water potential. We also found that Euclea pseudebenus and Faidherbia albida likely exploit fog moisture through foliar uptake. This study provides a first comprehensive assessment of the effects of nonrainfall waters on plant water status within the Namib Desert. Furthermore, this study highlights the importance of concurrent leaf and soil water potential measurements to identify the pathways of nonrainfall water use by desert vegetation. Our results fill a knowledge gap in dryland ecohydrology and have important implications for other drylands.Item Dryland ecohydrology and climate change: critical issues and technical advances(2012-08) Wang, Lixin; D'Odorico, P.; Evans, J. P.; Eldridge, D. J.; McCabe, M. F.; Caylor, K. K.; King, E. G.Drylands cover about 40% of the terrestrial land surface and account for approximately 40% of global net primary productivity. Water is fundamental to the biophysical processes that sustain ecosystem function and food production, particularly in drylands where a tight coupling exists between ecosystem productivity, surface energy balance, biogeochemical cycles, and water resource availability. Currently, drylands support at least 2 billion people and comprise both natural and managed ecosystems. In this synthesis, we identify some current critical issues in the understanding of dryland systems and discuss how arid and semiarid environments are responding to the changes in climate and land use. The issues range from societal aspects such as rapid population growth, the resulting food and water security, and development issues, to natural aspects such as ecohydrological consequences of bush encroachment and the causes of desertification. To improve current understanding and inform upon the needed research efforts to address these critical issues, we identify some recent technical advances in terms of monitoring dryland water dynamics, water budget and vegetation water use, with a focus on the use of stable isotopes and remote sensing. These technological advances provide new tools that assist in addressing critical issues in dryland ecohydrology under climate change.Item Dynamic interactions of ecohydrological and biogeochemical processes in water-limited systems(Wiley, 2015-08) Wang, Lixin; Manzoni, Stefano; Ravi, Sujith; Riveros-Iregui, Diego; Caylor, Kelly; Department of Earth Sciences, School of ScienceWater is the essential reactant, catalyst, or medium for many biogeochemical reactions, thus playing an important role in the activation and deactivation of biogeochemical processes. The coupling between hydrological and biogeochemical processes is particularly evident in water-limited arid and semi-arid environments, but also in areas with strong seasonal precipitation patterns (e.g., Mediterranean) or in mesic systems during droughts. Moreover, this coupling is apparent at all levels in the ecosystems—from soil microbial cells to whole plants to landscapes. Identifying and quantifying the biogeochemical “hot spots” and “hot moments”, the underlying hydrological drivers, and how disturbance-induced vegetation transitions affect the hydrological-biogeochemical interactions are challenging because of the inherent complexity of these interactions, thus requiring interdisciplinary approaches. At the same time, a holistic approach is essential to fully understand function and processes in water-limited ecosystems and to predict their responses to environmental change. This article examines some of the mechanisms responsible for microbial and vegetation responses to moisture inputs in water-limited ecosystems through a synthesis of existing literature. We begin with the initial observation of Birch effect in 1950s and examine our current understanding of the interactions among vegetation dynamics, hydrology, and biochemistry over the past 60 years. We also summarize the modeling advances in addressing these interactions. This paper focuses on three opportunities to advance coupled hydrological and biogeochemical research: (1) improved quantitative understanding of mechanisms linking hydrological and biogeochemical variations in drylands, (2) experimental and theoretical approaches that describe linkages between hydrology and biogeochemistry (particularly across scales), and (3) the use of these tools and insights to address critical dryland issues of societal relevance.Item The effects of short-term rainfall variability on leaf isotopic traits of desert plants in sand-binding ecosystems(2013-11) Zhao, Liangju; Wang, Lixin; Xiao, Honglang; Liu, Xiaohong; Cheng, Guodong; Ruan, YunfengSand-binding vegetation is effective in stabilizing sand dunes and reducing soil erosion, thus helps minimize the detrimental effects of desertification. The aim of this study is to better understand the relationships between water and nutrient usage of sand-binding species, and the effects of succession and rainfall variability on plants’ water–nutrient interactions. We examined the effects of long-term succession (50 years), inter-annual rainfall variability (from 65% of the mean annual precipitation in 2004 to 42% in 2005) and seasonality on water–nutrient interactions of three major sand-binding species (Artemisia ordosica, Hedysarum scoparium and Caragana korshinskii) by measuring foliar δ13C, δ15N and [N]. Long-term succession in general did not significantly alter δ13C, δ15N and [N] of the three species. Short-term rainfall variability, however, significantly increased foliar δ13C levels of all three species by 1.0–1.8‰ during the severely dry year. No significant seasonal patterns were found in foliar δ13C and δ15N values of the three species, whereas foliar [N] varied by season. For the two leguminous shrubs, the correlations between δ13C and δ15N were positive in both sampling years, and the positive correlation between [N] and δ13C was only found in the severely dry year. The results indicate that these sand-binding plants have developed into a relatively stable stage and they are able to regulate their nitrogen and water use in responding to environmental conditions, which reinforces the effectiveness of plantation of native shrubs without irrigation in degraded areas. However, the results also indicate that short-term climate variability could have severe impact on the vegetation functions.Item Enhanced coupling of light use efficiency and water use efficiency in arid and semi-arid environments(Wiley, 2021) Gao, Dexin; Wang, Shuai; Wang, Lixin; Li, Zidong; Pan, Ning; Liu, Yanxu; Fu, Bojie; Earth Sciences, School of ScienceBoth light use efficiency (LUE) and water use efficiency (WUE) play essential roles in ecosystem production. The extent to which ecosystem production is affected by the coupling between LUE and WUE remains unclear. In this study, we used data from flux measurements and weather stations in the Heihe River Basin, China, along a strong climatic gradient to quantify the relationship between LUE and WUE. Variations of LUE explained up to 85% of the variations of WUE. The contribution of LUE to WUE increased with increasing water stress. Pearson’s correlation coefficient between LUE and WUE increased from −0.12 to +0.63 with decreasing LUE. The coordination between LUE and WUE was essential to explaining the insensitive of WUE and GPP with increasing water stress. These results suggest that ecosystems enhance the coordination between light and water use when water stress is high. They enhance our understanding of the drought resilience of ecosystems and reduce uncertainties associated with the carbon cycle in drylands.Item The impact of fog on soil moisture dynamics in the Namib Desert(Elsevier, 2018-03) Li, Bonan; Wang, Lixin; Kaseke, Kudzai F.; Vogt, Roland; Li, Lin; Seely, Mary; Earth Science, School of ScienceSoil moisture is a crucial component supporting vegetation dynamics in drylands. Despite increasing attention on fog in dryland ecosystems, the statistical characterization of fog distribution and how fog affects soil moisture dynamics have not been seen in literature. To this end, daily fog records over two years (Dec 1, 2014–Nov 1, 2016) from three sites within the Namib Desert were used to characterize fog distribution. Two sites were located within the Gobabeb Research and Training Center vicinity, the gravel plains and the sand dunes. The third site was located at the gravel plains, Kleinberg. A subset of the fog data during rainless period was used to investigate the effect of fog on soil moisture. A stochastic modeling framework was used to simulate the effect of fog on soil moisture dynamics. Our results showed that fog distribution can be characterized by a Poisson process with two parameters (arrival rate λ and average depth α (mm)). Fog and soil moisture observations from eighty (Aug 19, 2015–Nov 6, 2015) rainless days indicated a moderate positive relationship between soil moisture and fog in the Gobabeb gravel plains, a weaker relationship in the Gobabeb sand dunes while no relationship was observed at the Kleinberg site. The modeling results suggested that mean and major peaks of soil moisture dynamics can be captured by the fog modeling. Our field observations demonstrated the effects of fog on soil moisture dynamics during rainless periods at some locations, which has important implications on soil biogeochemical processes. The statistical characterization and modeling of fog distribution are of great value to predict fog distribution and investigate the effects of potential changes in fog distribution on soil moisture dynamics.Item Investigating Dew Trends and Drivers Using Ground-Based Metreological Observations at the Namib Desert(2024-08) Javanmardi, Sara; Wang, Lixin; Li, Lin; Jacinthe, Pierre-AndréIn arid environments such as the Namib Desert, sources of non-rainfall waters such as dew and fog are crucial for sustaining ecosystem functions such as vegetation dynamics and biogeochemical processes, yet they remain among the least studied features of the water cycle. There is a concern that dew may become increasingly scarce because of climate warming, a trend not yet documented in the Namib Desert. This study aims to examine how dew formation trends in this region are evolving, drawing on field observations and assessing the factors influencing these trends. Our analysis involved processing meteorological data gathered from SASSCAL (Southern African Science Service Center for Climate and Adaptive Land Management) between 2015 and 2022, supplemented by direct dew observations to estimate dew occurrence in Gobabeb, a research station in Namib Desert. We developed an empirical equation to calculate dew formation based on the meteorological data. We used field-based dew records to validate the empirical equation and applied this equation to calculate dew formation between 2015 and 2022. We found dew formation concentrated in December to May (wet season). We evaluated the trends of dew formation at both monthly and annual scales. The findings showed a decline in dew formation (13.74% in the annual dew days count), a trend that could significantly impact the ecosystems in the Namib Desert. We found that soil temperature, air temperature, and humidity are the major contributors of the observed dew trends. The inverse relationship observed between increasing temperatures and dew formation supports the notion that elevated temperatures accelerate evaporation rates and diminish the chances of dew formation. This association underscores the susceptibility of dew formation to the impacts of climate change, as rising temperatures and shifting precipitation patterns are anticipated outcomes.Item Investigating the role of evaporation in dew formation under different climates using 17O-excess(Elsevier, 2021-01) Tian, Chao; Jiao, Wenzhe; Beysens, Daniel; Kaseke, Kudzai Farai; Medici, Marie-Gabrielle; Li, Fadong; Wang, Lixin; Earth Sciences, School of ScienceWith increasing aridity in many regions, dew is likely to play an increasingly important role in the ecohydrological processes in many ecosystems, especially in arid and semiarid regions. Few studies investigated the role of evaporation during dew formation and how it varies under different climate settings. 17O-excess, as a new tracer, could be used to extract information of evaporation dynamics from natural water samples (e.g., precipitation, river, and lake). Therefore, to fill the knowledge gap in evaporation mechanisms during dew formation, we report the isotopic variation (δ2H, δ18O, δ17O, and 17O-excess) of dew and precipitation from three distinct climatic regions (i.e., Gobabeb in the central Namib Desert, Nice in France with Mediterranean climate, and Indianapolis in the central United States with humid continental climate). We examined whether dew formed in different climate settings was affected by different degree of evaporation using observed isotopic values and evaporation models during the formation processes, and modeled the effects of key meteorological variables (i.e., temperature and relative humidity) on 17O-excess variations. The results showed that dew in Gobabeb experienced kinetic fractionation associated with evaporation under non-steady state conditions during dew formation with enriched δ18O and low 17O-excess values. Dew formations with temperatures over 14.7 °C in Indianapolis were also influenced by evaporation under non-steady state conditions. However, dew formation in Nice did not experience significant evaporation. Evaporation processes (equilibrium or kinetic fractionation) occurring during nights with heavy dew under three climate settings were mainly related to the variation of atmosphere relative humidity. The 17O-excess tracer provides a new method to distinguish the different evaporation processes (equilibrium or kinetic fractionation) during dew formation and our result provides an improved understanding of dew formation.Item Massive crop expansion threatens agriculture and water sustainability in northwestern China(IOP, 2022-02-21) Lai, Jiameng; Li, Yanan; Chen, Jianli; Niu, Guo-Yue; Lin, Peirong; Li, Qi; Wang, Lixin; Han, Jimei; Luo, Zhenqi; Sun, Ying; Earth and Environmental Sciences, School of ScienceNorthwestern China (NWC) is among the major global hotspots undergoing massive terrestrial water storage (TWS) depletion. Yet driver(s) underlying such region-wide depletion remain controversial, i.e. warming-induced glaciermelting versus anthropogenic activities. Reconciling this controversy is the core initial step to guide policymaking to combat the dual challenges in agriculture production and water scarcity in the vastly dry NWC toward sustainable development. Utilizing diverse observations, we found persistent cropland expansion by >1.2 × 104 km2 since 2003, leading to growth of 59.9% in irrigated area and 19.5% in agricultural water use, despite a steady enhancement in irrigation efficiency. Correspondingly, a substantially faster evapotranspiration (ET) increase occurred in crop expansion areas, whereas precipitation exhibited no long-term trend. Counterfactual analyses suggest that the region-wide TWS depletion is unlikely to have occurred without an increase in crop expansion-driven ET even in the presence of glaciermelting. These findings imply that sustainable water management is critically needed to ensure agriculture and water security in NWC.Item A multi-scale analysis of Namibian rainfall over the recentdecade – comparing TMPA satellite estimates and groundobservations(Elsevier, 2016-12) Lu, Xuefei; Wang, Lixin; Pan, Ming; Kaseke, Kudzai F.; Li, Bonan; Department of Earth Sciences, School of ScienceStudy region Namibia. Study focus The lack of ground observations has long been a major obstacle in studying rainfall patterns in many dryland regions, particularly in the data scarce African continent. In this study, a continuous 6-year (2008–2013) daily record of ground observations collected from Weltevrede Farm at the edge of the Namib Desert was used to evaluate TRMM Multi-satellite Precipitation Analysis (TMPA, 0.25° resolution) daily rainfall estimates of this area. A Mann-Kendall trend analysis was conducted using all the available annual TMPA satellite data (1998–2015) to examine long-term trends in rainfall amount, intensity, frequency and seasonal variations over four locations across a rainfall gradient. New hydrological insights for the region The agreement between ground and satellite rainfall data was generally good at annual/monthly scales but large variations were observed at the daily scale. Results showed a spatial variability of rainfall trends across the rainfall gradient. We observed significant changes in frequency along with insignificant changes in intensity and no changes in total amount for the driest location, but no changes in any of the rainfall parameters were observed for the three wetter locations. The results also showed increased rainfall variability for the driest location. This study provided a useful approach of using TMPA data associated with trend analysis to extend the data record for ecohydrological studies for similar data scarce conditions. The results of this study will also help constrain IPCC predictions in this region.